IMPLOVEiENTS IN METHODS OF EXTRACTION,
PURIFICATION, A1D 1ASUREIENT OF EADIO
GENIC ARGON IN MINEAiLS
by
LAWRENCE STRICKLLND
S.B., Massachusetts Institute of Technology
(1952)
SUBMITTED IN PARTIAL FULFILLIENT
OF THE REQUIREIENTS FOR THE
DEGREE OF DOCTOR OF
PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June, 1956
Signature
of
Author.
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4f
X,Of
Department
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....
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Geology and Geophysics
.I Septe;nber p3, 1955
Certified
Thepc SupgrvAsor
Accepted
by.\.........
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4..................
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Chairman, Departmental Committee
Students
on Gradua
-Ming==
A CINOWLEDGEMvENTS
The author is
indebted to the many people who helped
in the completion of this research.
He wishes to thank
Dr. Leonard Herzog, who was always available for consultation
when problems arose involving mass spectrometry and who
was an invaluable help in the early stages of this research.
Professor Patrick Hurley, who suggested the
author undertake this research and who was always willing
to take time from his busy schedule to help.
He was a
source of inspiration whenever forward progress was slow.
The author will remember his association with Professor
Hurley for many years.
Mr. Milo Backus, whose companion-
ship made the many hours spent on this research seem short.
The typist, Joan Whitehouse, for her untiring efforts to
complete this manuscriot in a tight schedule.
His wife,
Shirley, without her unlimited confidence in the author,
and unselfish devotion, the successful completion of this
research would have been impossible.
the Geology Department.
The entire staff of
The research presented in this
thesis was a part of a program supported
by the Atomic Energy Commission urAer
Contract AT(30-1)-1381.
ABSTRACT
Title: Improvements in methods of extraction, purification,
and measurement of radio genic argon in minerals.
Author: Lawrence Strickland
Submitted in partial fulfillment of the requirements
of the degree of Doctor of Philosophy at Massachusetts Institute of Technology
Age measurements by the A40 /K4 0 method have shown
promising results in recent tests. It is important that
the possibility of small errors in analysis be investigated
and that the techniques of analysis be simplified and shortened.
In this investigation new instruments and facilities were
constructed and tested to these ends.
It was planned that isotope dilution analysis would be
used to monitor experiments leading toward a possible reliable volumetric method of analysis. For this purpose a
mass spectrometer was constructed, after Nier's design,
with a 60 magnet sector and 6 inch radius, and with changes
made in the method of collection and measurement. The
method of measuring the isotopes of argon was a dynamic one,
in order to allow most of the sample to be used during an
analysis. Molecular flow conditions exist throughout the
entire gas-flow sheet. The isotope ratios measured at time
intervals were then extrapolated to the time the sample
started to flow into the ionization chamber.
Experimentation showed that argon could be lost if the
sample was absorbed on charcoal at too low temoeratures for
too long a time. It was also found that quantities of gas
containing argon c8uld be purified by selective adsorption
on charcoal at -78 c.
The mass spectrometric procedures were checked for
discriminatio]0and reproducibility by measuring the atmosratio and the radiogenic argon content of
pheric argon
a sample of llpidolite of known age. Results of these
tests were as follows:
(measured)
310
311
A T1OSPHERIC ARGON 40 / 36
Nier (1950)
2 96+l
308
Fractionation and
Discrimination
1.047
1.050
1.042
or approximately 2 percent discrimination per mass unit.
This value is different for each spectrometer. The value
obtained is reasonable.
LEPIDOLITE SAIIPLE
A4 0 / gm sample (x103 cm3 )
.79 + .08
.74 + .03
.73 + .03
age (m. y. )
Aldrich (1954)
This work
1610
1710 + 90
The volumetric analysis apparatus was checked by
analyzing air for its argon content with .993 percent,
.990 percent, and .992 percent the values obtained. This
is to be comoared with a value of .993 percent obtained
by Paneth.
'BLE
OF CONTENTS
.page
Acknowledgements......*00
Abstract..........................
Section I.
*
**
* .........
............
o.*eoo
.0
000
Introduction.................0.0......00.0.
00
......
0
i
11
1
Methods of Determining Geologic Age
Lead
Strontium-Rubidium
Argon-Potassium
Comparison of age Methods
Research Problems
Section II. Mass Spectrometer........................... 7
Introduction
Isotope Analysis of Argon
Theory of Mass Spectrometer
Refocusing of Divergent Beams
Causes of Ion Beam Spread
Resolution
Physical Arrangement of the Equipment
Section III. Vacuum Techniques and Gas Flow Conditions
in the Mass Spectrometer.................. 45
Introduction
Gas Flow Through the Mass Spectrometer
Cold Traps Background Mass Spectre
Gas Flow into the Mass Soectrometer
Section IV. Production of Positive Ions................
59
Introduction
Methods and Workmanship
The Orthodox Source
Mass Discrimination of Ion Source
Emission Regulator
Sensitivity
Stability
Section V. Collection and Measurement of Ion Beams.....
70
Collector Design
Preamxolifier
D-C Current Amplifier
Measurement of Ion Beams
Treatment of Data
Section VI. Isotope Dilution Techniques................
77
Tracer Introduction System
Calibration of the Tracer
Possible Errors in Tracer Calibration
Isotope Dilution Measurements
Possible Errors in the Determination of Radiogenic
Argon
Section VII. Volumetric Analysis of Argon..............
Introduction
Separation Procedure (Introductory Remarks)
Description of Equipment
85
Calibration of Volumes
Problems to be Solved
Loss of Argon
Extraction of Small Quantities of Argon from Minerals
Atmospheric Argon Contamination
Hydrogen Removal
Operation of the Barium Furnace
Gas Circulation System
Results of Volumetric Analyses
Section VIII. Standardized Procedures..................
104
Volumetric-Analysis
Isotope Dilution Analysis
Section IX.Measurement of Age by the Potassium-Argon
Method.................
..........
...
.. *.
110
Section X. Recommendations for Future Research.........
115
Appendix I. Use of Radio Frequency Induction Heater
Appendix II. Condensed Procedure Sheet
Biographical Sketch
I
__:i
LIST OF ILLUSTRATIONS
Figure 1. Refocusing properties of magnet sector.
11.
2. Effect on refocusing by shifting magnet .1
inch upward from correct position.
13.
3. Effect on refocusing by shifting magnet
.2 inch down and .1 inch towards source
from correct oosition.
14.
4. Bean spread due to various aberrations.
17.
5. Gas inlet system.
19.
6.
21.
Magnet poles.
7. Right side view of mass spectrometer.
23.
8. Left side view of mass spectrometer.
24.
9. Schematic diagram of high voltage supply .
25.
10. Front panel view of high voltage supply.
2.
11. Bottom view of high voltage supply.
27.
12. Rear view of high voltage supply.
28.
13. Schematic diagram of ion current amplifier.
29.
14. Front panel view of ion current amolifier.
30.
15. Bottom view of ion current amplifier.
31.
16. Rear view of ion current amplifier.
32.
17. Schematic diagram of magnet current supply.
33.
18. Schematic diagram of balancing panel.
34.
19. Front panel view of balancing panel.
35.
20. Rear view of balancing panel.
36.
21. Schematic diagram of regulated D.C. power supply. 37.
22. Front panel view of regulated D.C. power supply.
38.
Figure 23. Bottom view of regulated D.C. power supply. 319.
24. Rear view of regulated D.C. power supply.
40,
25. Schematic diagram of emission regulator.
41.
26. Front panel view of emission regulator.
42.
27. Rear view of emission regulator.
43.
28. Diagram of mass spectrometer tube.
44.
29. Schematic diagram of mass spectrometer
with possible appropriate pressures.
47,
30. Residual spectra using solid carbon dioxide
as coolant.
50.
31. Residual spectra using liquid nitrogen
as coolant.
51.
32. Increase of background spectra with
time.
52.
Variation in 4 ratio with time.
40
Variation in 40 ratio with time.
57.
35. Schematic diagram of ion source.
61.
36. Schematic diagram of electron gun.
66.
37. Schematic diagram of ion gun.
67.
38. Peak height vs. electron accelerating
voltage.
69.
39. Design of collector.
72.
40. Characteristics of CK5886 tube.
75.
41. Typical recorded ion beams.
76.
33.
34.
56.
42. Percent error in Qh for 1 percent error
43.
in Rmn.
83.
Furnace for extraction of gases.
87.
89.
Figure 44. Gas separation system.
45. Adsorption of argon on charcoal at liquid
nitrogen temperature.
Clean up of sma 1 quantities o
presence of 1.20 x 10-3 and cm
40
47. Decay scheme of K .
46.
gas in
argon.
93.
100.
112.
LIST OF TABLES
Table Ak Comparison of Argon ratios (mass discrimination) 64.
Table B Calibration of spike
79.
Table C Percent error in volume of pure tracer
determined per E percent error in ratio or
quantity
79.
Table D Percent error in the determination of radiogenic argon for a given percent error in
ratio and quantity
81.
Table E Results of volumetric analysis
103.
Table F Branching ratio
111.
Table G Comparison of ages
113.
Section I
INTRODUCTION
It was the purpose of this research to construct and calibrate
equipment and techniques for the extraction and quantitative separation
and isotopic measurement of the argon from potassium bearing minerals,
the ultimate objective being to contribute data toward the establishment
of the potassium-argon method of age determination.
It was also the purpose of this research to determine if it is
possible to make contamination-free volumetric analysis of argon in minerals.
The section that follows will acquaint the reader with the recent
developments which prompted the present research, and will present an
introductory statement of the research problems.
Methods of Determining the Geological Age of Rocks and Minerals
Natural radioactivities have provided a means of studying absolute
time in earth history.
28
232
U2 3 8 , U25 , Th23,
The more important of these are the breakdowns of
Rb 87 and K40
Several excellent reviews of the methods of age determination have
appeared in recent years.
A detailed account of the historical develop-
ment of the potassium-argon decay has been published in a paper by Birch
(1951), while Faul (1954) has an excellent review of all methods of age
determination.
fields.
Kohman (1954) presents the most recent developments in all
In order to present this research in its proper prospective a
brief review of the development of the methods of age determination is
presented.
Lead
The fact that the three heavy radioactive elements U2 38 , U 2 3 5 and Th 2 3 2
produce the three lead isotopes Pb2 0 6 , Pb207 and Pb2 0 8 has led to the
possibility of determining the age of uranium and thorium bearing minerals
be measuring their Pb/U+Th ratios.
Early lead age measurements were made by determining, chemically, the
total lead and uranium plus thorium content of the mineral.
Although
several hundred age determinations were made by the chemical lead method,
progress was slow until Nier's (Nier, 1939) work appeared in 1939.
It
was not until the development of a simple mass spectrometer (Nier, 1940)
for isotope abundance measurements that analysis of the lead isotope content of minerals became the practice.
It was then possible to make correc-
tions for primary lead contamination and to study the effect of losses of
parent and daughter elements.
The decay schemes involved allow computation of three ages for each
mineral.
Since it is evident that the three computed ages seldom agree,
loss of parent or daughter elements may be quite common.
Enough discrep-
ancies in lead age determinations, when compared with other methods of age
determination, have been indicated in recent years to warrant a concentrated program of investigation (Kulp 1954),(Kohman
1954).
However, when
the computed ages agree the "age" may be considered as reasonably accurate
and is being used as a common base point.
Strontium - Rubidium
The use of Rb8 7 decay as a method of age determination was first
suggested by Goldschmidt (1937) but, after the initial work by Hahn,
Strassman and Walling (1937) little was done until the late 1940's and
early 1950's.
Then work by Mattauch (1947), Ahrens (1949), Ahrens and
Gorfinkle (19501 Herzog (1952), Aldrich, Doak and Davis (1952), and others
increased the available information on the use of the Rb87 decay.
The requirement that there should be a high Rb8 7 /Sr8 7 ratio in any
mineral used made the method applicable, initially, only to such minerals
as lepidolites.
The increased use of isotope dilution techniques has
made the determination of small quantities of Rb 87 and Sr8 7 very accurate.
Other minerals such as biotites and potassium feldspars can now be utilized.
Rb/Sr ages at the present time seem to be as reproducible as lead ages,
although, some doubt exists as to the correct half-life to be used for the
Rb87 decay.
The method seems to be consistent within itself and the ages
derived agree with lead ages for the same region if a half-life of about
+10
4.9x10 is correct for Rb8 7
Argon-Potassium
Evans (1940), and independently Thompson and Rowlands (1943) first
suggested the use of the potassium-argon decay as a possible method of age
determination.
Birch (1951) in a review of previous literature stated:-
"Since 1930 over one hundred papers or
letters have
been published concerning the radioactivity of potassium-40."
In the same review he says,
"Few determinations of a es have as yet been made by
use of the radioactivity of K 0, but the existence of reasonably reliable constants should encourage efforts to obtain
ages of potassium minerals."
The large terrestrial abundance and ubiquitous nature of potassium,
and intermediate half-life value of potassium-40, has made age determination by the argon-potassium method appear very promising.
Research must
be undertaken in two separate areas if the potassium-argon decay is to
become reliable as a method of age determination.
One, it must be estab-
lished whether the contamination reported by other workers is primary argon
or contamination argon introduced during an analysis; and two, the decay
constant must be more firmly established.
4.
The problems associated with these areas of investigation have made
it difficult to obtain reliable analytical results.
The first problem is
the difficulty of quantitatively extracting small volumes of argon from
the mineral.
The second is associated with the necessity of determining
the isotopic composition of the argon.
A third problem not connected with
analytical procedures but of prime importance in determining the age of a
mineral, is the doubt that exists regarding the correct value of the branching ratio.
The first problem is discussed more completely in section VII. However,
the author now believes that by direct fusion or with the use of fluxes
quantitative extraction of argon is possible.
The second problem is not so easily handled.
The literature gives
no information regarding the quantity and isotopic composition of
"contamination" argon.
1% to 10%".
It is usually reported as "varying from less than
If it is not possible to obtain contamination free argon
and if it is true that all minerals contain some "common" argon (Kohman,
1954), then corrections must be made and mass spectrometric measurements
are necessary.
Since few laboratories are equipped with mass spectrometers
which can be devoted exclusively to the isotope analysis of argon, the
application of methods of age determination to the solution of geological
problems will be considerably curtailed.
The third problem, that of establishing a reliable branching ratio,
received most of the early research effort.
Table F) may be determined in three ways.
The branching ratio (see
The first is by observation
of the x-rays produced in the K-capture process; the second is by analysis
of the argon 40 produced during a known period of geological time; and the
third is by comparison of the quantities of A 4 0 and Ca4 0 produced in the
same mineral.
Comparison of Age Methods
It now appears possible to make accurate analysis of the parent and
daughter elements associated with the different methods of age determination.
Kulp (1954) lists a set of lead 207/206 ages that were determined by
three different laboratories on specimens from the same locality.
The
agreement is very good and regardless of the accuracy of the calculated
207/206 ages the mass spectrometric analyses must not be contributing any
variation in lead age determination.
Further a few samples from the same
mineral and same locality have been measured, for strontium and rubidium,
by both the Carnegie Institution in Washington and Nuclear Geophysics
Section of the Department of Geology and Geophysics at the Massachusetts
Institute of Technology.
In two samples the quantities of Rb and Sr deter-
mined agreed to withint 5% (Herzog, 1954 and 1955).
A third sample had
to be discarded because of large rubidium contamination corrections.
Thus,
the few existing interlaboratory checks indicate that quantities of lead,
rubidium, and strontium in a mineral can be accurately measured.
Therefore, it is now necessary to assure that radiogenic argon can
be measured with the same accuracy.
Although few interlaboratory checks
have been made, accurate measurements appear possible at the present time.
With assurance that quantities of rubidium, strontium, argon and lead can
be accurately measured a concentrated program of age determination is
possible.
Few laboratories in the world are equipped to make accurate
analyses for all these elements.
An interlaboratory program is necessary.
With such a program the discrepancies now evident in the ages determined
by the various methods may be resolved.
6.
Research Problems
A concentrated program of research devoted to the potassium-argon
method of age determination requires the use of a mass spectrometer and
a system to extract, purify, and measure the gases in a mineral.
Initial research effort was devoted to the construction of a mass
spectrometer that could be used for isotope analysis of argon.
The many
problems connected with the construction and calibration of a mass spectrometer are fully discussed in later sections.
A background of information concerning the extraction and purification of helium was available in the Department of Geology and Geophysics.
The problems associated with atmospheric contamination and the separation
of small quantities of gas in the case of argon analysis, however, are
more severe.
It was necessary to design and construct a furnace to
extract argon from minerals since the furnace currently in use for helium
analyses could not be sufficiently outgassed.
Argon, as well as other gases, is adsorbed on charcoal at liquid
nitrogen temperature.
Therefore, this method for the separation of gases,
commonly used in helium analysis, could not be employed.
The literature
contained little specific information regarding the separation and measurement of small quantities of argon.
Two papers, one by Soddy (1907) and
one by Arrol, Chackett, and Epstein (1949) provided the basic information
upon which the present separation system is built.
The many stages of
development through which this system went are discussed in Section VII.
7.
Section II
MASS SPECTROMETER
Introduction
The measurement of radiogenic argon in potassium bearing minerals
requires a knowledge of the isotopic composition as well as the quantity
of the gas.
A mass spectrometer was constructed similar to that described
by Nier (1947) with a six inch radius and 600 sector.
In selecting this
type of instrument thought had to be given not only to the problem of
argon analysis but to the availability of material, ease of construction,
and adaptability to other research problems that may arise after the
present research in completed.
Isotope Analysis of Argon
After the initial work of Aston and Dempster around 1918-1919, many
persons contributed to the development of mass spectrometry.
Several
excellent books have been written describing this development.
Among the
best are Ewald and Hinterberger (1954), Barnard (1952), and Aston (1942).
Aston (1920) first made use of the mass spectrometer to investigate
the isotopes of argon.
He gave 40.00
±
0.02 as the mass of the most
abundant argon isotope and reported the presence of "a faint line at mass
36, which may be about 3% of the total".
It was not until 1934 that
Zeeman and deGier (1934) announced the presence of an isotope of argon of
mass 38.
This was later confirmed by Nier (1936) when he also demonstrated
the lack of other isotopes with a high degree of precision.
the isotopes of 40 and 36 was estimated by Vaughn as 304112.
The ratio of
Nier (1936)
later gave 325 for this ratio and 5.1 for the ratio of the isotopes 38 and
36.
The latest determination of these ratios give 40/36 = 296t 1 and
38/36
= .188 t .001 (Nier 1950).
These later ratios are used throughout
this research.
Theory of the Mass Spectrometer
An analysis of the path of an ion beam in electrostatic and magnetic
fields has been carried out by Herzog (1934) and Stephens (1934).
Ewald
and Hintenberger (195 2 ) in their book "Methoden and Anvendungen Der Massenspectroscopic" have an excellent discussion of ion optics.
The discussion
here will be limited to applications of the general theory to the 600
sector spectrometer.
It is useful to write down, first, the equation for the passage of
a charged particle of mass m and charge e through a magnetic field of
intensity H.
velocity v.
The particle is projected into the magnetic field with a
The path of the particle will be a circle with the radius
dependent upon the velocity, mass, and charge of the particle and intensity
of the magnetic field.
The particle will experience a centrifugal force,
2
mv /r and for equilibrium this must be balanced by the force exerted by
magnetic field, Hev.
mv 2 /r = Hev
That is
or
r
= mv/eH
(1)
If (mv) and H are held constant then r is a constant.
particle has mass m (14-.m) and velocity v (1*
of my (1 + & m +- v).
If another
A v) it will have a momentum
If it is similarly projected into the magnetic field
it will have a radius of curvature of r = my (1+ A (mv))/eH
The magnetic field generates a momentum dispersion.
If it is now
assumed that the charged particle has acquired its velocity by falling
through an electrostatic potential V, the kinetic energy developed will be
equal to the potential energy of the particle eV, before acceleration, or
1/2 mv2 = eV/300
(2)
There will be a definite velocity associated with all particles having
the same energy (constant mV2 ) which is
root of their mass.
inversely proportional to the square
Each particle will, as a consequence,
describe a path
through the magnetic field with a radius of curvature proportional to the
square root of its mass.
The equations (1) and (2) can be combined into
a single equation eliminating v,
or
m/e = r2 H2 /2V.
(3)
If the radius is expressed in inches, the field intensity in gauss,
the mass in atomic mass units, the charge in terms of a single charge unit,
and V in volts, the equation (3) may be written,
m/e
=
3.09 x 10-4
r2 H2 /V
(4)
In the case of the mass spectrometer with a six inch radius this
equation becomes
m/e
= .0343 x H2 /V
Several points should be mentioned.
(5)
(1.) A more detailed discussion
of the focusing properties of a magnetic sector field follows, however,
it should be noted here that focusing is with respect to direction only.
(2.)
It has been assumed that each particle is monoenergetic.
The ion
source must be designed so that a small energy spread is achieved. (3.)
The mass spectrometer equation, (3),
is followed in the source region before
the ions have passed through the acceleration potential as well as in the
magnetic analyzer. (4.) More intense ion beams are possible if direction
focusing only is undertaken.
The mass spectrometer may be considered as
a constant deviation device.
Refocusing of Divergent Beams
The discussion may be extended now to include the refocusing properties
of the magnetic sector field.
Although the spectrometer was in use in 1918
10.
it was not until 1934 that a general discussion of the refocusing effects
of magnetic and electrostatic fields was published by Herzog (in Ewaldt&
Hintenberger, 1952).
A divergent beam of monoenergenic ions of one mass is directed into
the homogeneous magnetic field.
incident at an angle,6,
(See figure 1.)
The central ion ray is
, with the normal to the field boundaries and
emerges at an angle C. with the normal to the field boundary.
angle of divergence of the ion beam is
q
The semi-
The condition to be satisfied
.
in order that a beam divergent from point P1 should be refocused at point
P2 was given by Herzog (1934). (In Barnard, 1952).
r
sink
1
cos(-E 1)
12 cos(t2)
- 1,
sin( -6 -6 )
cos6 1
cos 2
r
cose 1
cosE 2 3
If a symmetrically arranged instrument has a radius of 6 inches, a
--
0(6)
magnetic field of 600, and an ion beam incident normal to the field
boundaries, the following equations are obtained,
+
rsin
with 4E
=6 2
ll(cos
)
+1,cos f
-
(11)
sinj: 0
(7)
0,
-
(1
= 12
1
r(cot4p 4- cosec#
)
(8)
with a symmetrically arranged instrument, and
1
=
6.00(
1
(3)6
2 )
(3)2
-
10.39 inches
with a six inch radius and 600 magnetic sector field.
The arrangement of
the source point, image point, and magnetic field for this case are shown
in figure 27.
An instrument of these dimensions was constructed for this
research.
Causes of Ion Beam Spread
Refocusing is not perfect even with correct alignment.
error if 2r(l-cosr).
of small e
.
The "focusing"
This may be written as rie 2 with the approximation
11.
K
P A= l1
P2B=12
General case of first-order focusing of ion beam
in homogeneous magnetic field with sharply defined
boundaries of any arbitrary shape.
12.
The minimum beam spread that can be achieved for first order focusing
2
is rCr .
To achieve this P
respect to Pm.
and P2 must be positioned correctly with
There are additional aberrations introduced if the source
and collector slits are not aligned parallel to each other and perpendicular
to the central plane of the magnet.
If the magnet is displaced from its correct position there are further
aberrations.
R. A. Davies (in Barnard, 1953) has derived the following
equations for beam spread due to three possible directions of misalignment.
(1)
Magnet displaced distance x1 along x axis.
Spread
(2)
= rr
2
1+
r4C (
2
~
2~*
2
2/2 - gxI/r + 4x /r2
Magnet displaced distance g, along the g axis
Spread = r d2
(3)
rC
+ 4
y + re (
-
42/2 - ffy/2r + 2y
2
.)
-
(10)
Magnet notated angle i about its nominal apex, Pm
Spread
= r 42
.r
e
+-re
(
2/2
+
rR
-
9/4 e
2
+
.)
(11)
Figures 3, 4 show diagrammatically the effect of the refocusing
properties of the magnet sector a displacement of the magnet from its
correct position.
The figures show that a displacement in the x direction,
for any given a and r, causes the greatest beam spread.
It has been assumed throughout the above discussion that the magnetic
field has well defined boundaries.
Barnard (1954) has a discussion of
fringing flux corrections and states that additional adjustments are
necessary after the magnet has been positioned on a theoretical basis.
A
good approximation is obtained then by considering that the boundaries of
the magnetic fields extend out to a distance of one gap width.
It has
been further assumed throughout the above discussion that the ions of each
mass are monenergetic with a velocity characteristic of that
mass.
However,
A
Ii'
ill
'II
'II
I'
.1-100I
-*100
-'-0
-*9
-
0l
100I
00
-K;Figure 2
'00,
/
'0
Effect on refocusing by shifting the magnet .1
inch upward from it- correct position.
I
III
IIII
I'i
I|
~/
/
//
//
--
,9000
0-000
--000
9
00
//
100
.00,1000
-.-
10000,-
'000,
-:: , .0001
Figure 3
-9
-;--
-O,
Effect on refocusing by shifting the magnet .2
inch down and .1 inch towards source from its
correct position.
15.
a small energy spread is unavoidable.
spread.
(1.)
There are three causes of energy
When ions are obtained by electron bombardment, a potential
gradient across the electron beam is necessary to withdraw the ions from
the beam.
Even with a beam of small cross-sectional area some ions acquire
potential of
6V
in excess of that acquired by other ions.
AR - 2r
introduced is 2
The aberration
SV/V. (2.) a broadening of the ion
S/%= r
beam can arise because of energy changes associated with collisions
between similar ions.
The pressure must be below 10-6, if collisions are
not to cause excessive broadening.
(3.)
Ions formed from molecular dis-
sociation products have associated with them a varying amount of energy
Aberrations arising from this source are a cause for
of dissociation.
concern in hydrocarbon analysis.
However, the gases encountered in this
research were monatomic and consequently have no energy of dissociation
associated with them.
Resolution
Ewaldtand Hintenberger (1952) have derived the following expression
for the resolution:
M
------
2(12)
M
v
K"
where, theoretically, for the spectrometer used in this research
=
K"
and
2r
G
=
-1.
The expression for the revolution then becomes
M
=
1
A V
V
+
.
Si f.. S2
r
(13)
16.
For the spectrometer used in this research values of 6.00, .008, .050
are observed for r, S, and S2,
for
dV/V.
respectively.
A value of 15/2300 is obtained
The resolution is
M/ 4A M
=
l/.0013 + .0096
92.
That is to say there is a separation of one mass unit at mass 92.
The resolution is sufficient for this research and can be improved, if
necessary, by reducing the width of the collector slit.
A picture of possible aberrations introduced is shown in figure
(Barnard).
.
This figure shows the effects of misalignment of source and
collector slits; of spherical aberration for r = 150 mm, r
of chromatic aberrations of
&V
=
0.5V, V
=
100OV, and r
=
1/30 radian;
150 mm; and
of non-uniformity in the z direction of the magnetic field for a pole gap
of 20 mm in relation to an ion ribbon width of 10 mm symmetrically disposed
abput the central plane.
Physical Arrangement of Equipment
The equipment used in this research to produce, analyze, and collect
positive ions consisted of:
(1)
Ion Source
(2)
Collector
(3)
Regulated high voltage supply
0
-5000V with taps for drawing out
and focusing potentials.
(4)
Regulated power supply 225V
(5)
Regulated magnet current supply 0-300ma.
(6)
Magnet
(7)
D-C Current Amplifiers
(8)
Preamplifier
(9)
Emission Regulator
17.
a
b
C
d
e
f
Figure 4
Beam spread due to various aberrations.
(a) Exact reproduction of source slit (0.2 x 10 mm) with
collector slit. Accurately aligned; no aberrations.
(b) Spherical aberration added.
(c) Chromatic aberration added.
(d) Curvature of image due to variation of magnetic flux
density across pole gap and to ion rays passing obliquely
through central plane.
(e) Image broadening due to oscillatory component in
acceleration potential.
(f) Distortion due to misalignment by 10 of collector slit
with source slit.
(g) Superposition of aberrations; for clarity each flank
considered separately; one flank shown extended by
aberrations (b). (c), (e) and (f); the other flank shown
extended by aberrations (d) and (f) only.
18.
(10)
Vacuum System
(12)
Spectrometer tube
(13)
Source magnets and aligning mechanism
The ion source (1) is discussed in Section IV.
The collector (2),
D-C current amplifier (7), and preamplifier (8) are discussed in section V.
Pictures and schematic diagrams of the electronic equipment are shown on
the following pages.
All the electronic equipment with the exception of
the magnet current supply and preamplifier were built by Dunn Engineering
Associates of Cambridge.
(10)
Vacuum System
The vacuum system consisted of an umbrella-type diffusion pump, a cold
trap and fore pump.
The diffusion pump was designed by Homer Priest of the
Research Laboratory for Electronics, and built by Ryan, Velluto and Anderson
who also did most of the glass work necessary on the spectrometer and the
gas extraction and analysis system.
two-stage rotory pump.
The fore pump was a Welch Duo-seal
It was possible to attain vacua of 3-5x10~ mm of
mercury after prolonged heating of metal parts and with liquid nitrogen
as a cold trap coolant.
It was necessary to use liquid nitrogen as a cool-
ant to reduce hydrocarbon background
(11)
(see section III).
Gas Inlet System
The gas inlet system consisted of an inlet to which the sample container could be attached, a small calibrated volume, a 50 cm3 sample
reservoir, a 5 liter sample reservoir, a cold trap, evacuating system and
variable gas leak.
A diagram of the inlet system is shown in figure 5.
All glassware was Pyrex and all stopcocks were mercury seal stopcocks with
a 4 mm bore.
- F j IMMOMMOOM
10,11M I- M
Gas Leak
Sample tube
To spectrometer
ITo
fore pump
Cold trap
Cold trap
Diffusion pump
Figure 5
Gas inlet system.
WIN
20.
The evacuating system consisted of a nozzle-type mercury diffusion
pump and a cold trap.
attainable.
Vacuums of less than 2-3x0~7 mm of mercury were
It was necessary to maintain liquid nitrogen on the cold
trap during sample analysis because of the presence of hydrocarbons in
the background spectra.
The variable leak was variable over a wide range.
With a small open-
ing it was possible to analyze air for atmospheric argon, although some
distortion of peak shape was observed due to the presence of large oxygen
and nitrogen beams.
It was also possible to accurately control the flow
rate of samples of the size encountered in this research.
All parts of the
leak were made of stainless steel and nickel-plated to prevent outgassing.
Some difficulty was experienced with air leaks developing around the pressfit connectors.
It is recommended that in a permanent installation these
be replaced by silver soldered connections.
(6)
Magnet
A diagram of the magnet poles is shown in figure 6.
were made of Armco ingot magnet iron.
The magnet poles
Each core was wound with 20,000
turns of #22 magnet wire covered with double formex coating.
With a gap
width of .625 inches, 100 ma magnet current produced a field intensity of
3060 gauss.
A plot of field intensity vs magnet current shows that the
field variation is linear in the region of interest.
Ion beams can be located approximately by the use of the mass spectrometer equation,
m/e
=
3.09 x 10~4
=
k
I2 /V
where I is in milliamperes and V in volts.
r2 H2 /V
(14)
The constant k is equal to 8.b6.
If one papameter is fixed the other may be found using equation (14) and
the ion beam located.
ALL dimensions in inches
Figure 6
Magnet poles.
||||||Il li
AI
-
~
22.
(12)
Spectrometer tube
The spectrometer tube was made from 2-inch copper tubing with all
joints being silver soldered.
stainless steel.
All flanges were made of non-magnetic
The diagram on page 44 shows the location of the source,
collector and vacuum outlets.
Copper is very gassy and it was necessary
to sandblast the inside of the tube and maintain periodic heating periods
to obtain an adequate vacuum.
In order to obtain satisfactory recorded
ion beam shapes the source and image points must be accurately located.
The tube was assembled and swedged by R. Thorness, machinist for A. 0.
Nier.
After the tube had arrived it was necessary to position the source
and collector flanges relative to each other and to the central plane of
the tube.
With the aid of a competent machinest and a larger optical
flat it was possible to locate these flanges accurately to within .01
inch.
Figures 7 and 8 show right and left sideviews of the spectrometer,
showing especially the method of mounting the spectrometer tube.
(13)
Source magnets and aligning mechanism
The source magnets were two 2xl inch rods of Alnico V magnetic
material.
They could be correctly aligned with the aid of the alignment
mechanism shown in figures 7 and 8.
Once correctly aligned it was
possible to lock them securely in position.
It was necessary to lock
them in position as any change in their position would make a redetermination of the discrimination value necessary.
Figure 7.
---
Right side view of the Mass Spectrometer
="W -
-WO
24.
Figure 8.
Left side view of the Mass Spectrometer
25.
Figure 9.
Schematic diagram of high voltage supply.
115V 60,,v
5000V REGULATED DC SUPPLY
26.
Figure 10.
Front panel view of high voltage supply.
27.
Figure 11.
Bottom view of high voltage supply.
28.
Figure 12.
Rear view of high voltage supply.
29.
Figure 13.
Schematic diagram of ion current amplifier.
CK5886
ION CURRENT AMPLIFIER
30.
Figure 14.
Front panel view of ion current amplifier.
31.
Figure 15.
Bottom view of ion current amplifier.
32.
A
a VP
ftftftwmw
Figure 16.
Rear view of ion current amplifier.
33.
Figure 17.
Schematic diagram of magnet current supply.
ZP
T, CHICAGO F-65
T2 STANCOR P-6134
5A
60oT3 T HORDARSON T-21F04
T4 UTC CG - 301
NEON
6.3V
T2
I Meg
50K
IPWW
8
TRIAD C-ISA
S
T3
6SF5
5V_
90V
I0K
15KW
8A-iJi
P3K1
MOTR
2______
1W
COILSERI
5
B
10 SEC TIME
7W
DELAY
TD
0.25
600V
V1
5U4
D
-~
5U
0
C05-
_ _ _j
-
2
9
lOK
50K
50K
50K
I~wCURN
C C
12P
3
IW
V2PW
*00
50K
2
51.14SU2_>30K
50W LINE ADJ.
20W
5 V5
_MICA
20WR
ID
--
COCL
C C
47K
loW
30K
15K
20W
.
R
WHITEUT
NEODN
0
NEON
TREEN
3AE
BODEi5A
ALLIED BUU 12A
MOMENTARY CONTACT
REVERSING
D
SWITCHES
SNHNSO
o____!______
REMOTE CONTROL
UNIT
REGULATED D.C.
MAGNET SUPPLY
2N
34.
Figure 18.
Schematic diagram of balancing panel.
INPUT
FROM AMPLIFIERS
*2
I
OFF
R17
1000
25K
RI6
500
15K
R13
200
Rii
Rg
Ry
7
R5
R3
-
-
R,
I
+
100
50
20
10
5
2
1
5K Io2.5K
5001
2500
50f
50i1
0-2MA DC 0-20MADC
50fl%
50f1±1%
ALL RESISTORS 1% PWWSW5
1.5K
A
-
-
INPUT *1
R2
R4
50n1
150nl
R6
Re
Rio
25010 500n0 1.5K
ALL RESISTORS
R12
2.5K
R24
5K
R516
15K
1% PWW
SW4
B
ODIRECT
10
2
2
DECADE +
3
DECADE
3
SW
R3 4
4
O BAT+
99K
R3 2
25BAT-
9K
R24
IOOK
2 2.5 V E
PUT S TAKE
DECADE
RESISTANCES
1%PWW
2
R33
R29
99n
04
OFF
I
nD2
BALANCING PANEL
Ri
25K
OFF
35.
Figure 19.
Front panel view of balancing panel.
36.
Figure 20.
Rear view of balancing panel.
37.
Figure 21.
Schematic diagram of regulated D.C. power supply.
LI STANCOR C-1003
TI UTC
R- 102
OUTPUT A5
Al
A2
A3
A4
GROUND CONNECTORS
REGULATED D.C. SUPPLY
225V, 180 M. A.
60-%
38.
Figure 22.
Schematic diagram of regulated D.C. power supply.
39.
Figure 23.
Bottom view of regulated D.C. power supply.
40.
Figure 24.
Rear view of regulated D.C. power supply.
trap
sheitd
filoment
Figure 25.
Schematic diagram of emission regulator.
Rl
150 ohm 50 watt rheostat
R2
150 ohm 50 watt adjustable
R3
250,000 ohm wire would potentiometer
R4
20,000 ohm precision wire wound
R5
50,000 ohm wire wound potentiometer
B1 , B2 , B, B
45 volt B batteries
M
mete', 0O10 amps. a.c.
meter, 0-1.5 milliamps d.c.
M3
meter, 0-500 microamps d.c.
T, filament transformer, secondary 2.5 volts
10 amps. 7500 ras volts insulation
SWi SPST toggle switch
SW2 two position selector switch, 2500 voLts
insulation
42.
Figure 26.
Front panel view of emission regulator.
43.
Figure 27.
Rear view of emission regulator.
44.
Figure 28.
Diagram of mass spectrometer tube.
NOTE:
2'Copper Tubing used throughout
unless otherwise specified
* 7 Drill(.201) thru
V4-20 N.C. Top 6 holes
equally spaced on
3.5" B.C.
* 25 Drill - 2 holes
'/"deep for i x 'W
long Dowel Pins
125
5"
.375
2.75
DETA I L OF 0OUTER FACE
OF FLI ANGES
.9"
0.D.
but central I"
I.D. ="
6"R
to be held parallel
to ±0.005"
li"Copper Tubing
Copper reducing nipple 2 "-l1"
(shave nipple O.D. and Tee
I.D. to snug fit for silver brazing)
Cut off"Tee"oand silver
91"
l7 g
-(
solder 2"-60* "Ell"
Stainless Steel
Flange
600 MASS SPECTROMETER TUBE
scale
j". i"
Scale tolerance on tube
dimensions except where
otherwise stated
45.
Section III
VACUUM TECHNIQUES AND GAS FLOW CONDITIONS IN THE MASS SPECTROMETER
Introduction
The gas handling system
required for introduction of the sample into
the mass spectrometer depends upon the type of gas to be analyzed.
If the
sample is a single gas introduced for isotope assay, fractionation may,
in general, be ignored and a simple handling system is sufficient.
If,
however, the gas to be analyzed is a complex mixture consisting of many
isotopes, it is necessary to meet several requirements in so far as possible.
First, there should exist a known relationship between the partial
pressures of each isotope in the sample reservoir and the ionization
chamber.
Second, the composition of the sample should not change during
the analysis.
Third, the total peak height at any mass should be the
linear sum of all contributing isotopes of the gas mixture.
Fourth, the
rate of gas flow should remain constant during the analysis.
Fifth, no
gas striking the filament should be allowed to re-enter the ionization
chamber.
Sixth, erratic behavior of the diffusion pump should not have
any effect on analysis.
Seventh, and last, it is desirable that gas enter-
ing the ionization chamber should have reached temperature equilibrium.
The above requirements are of prime importance in hydrocarbon analysis.
In the present research, however, the problem is one of introducing a
single gas for isotope analysis, complicated by the smallness of the
sample, so that detailed discussion of the above requirements is not
included.
The problems of gas flow can be grouped into two headings.
One,
gas flow through the spectrometer and two, introduction of the sample into
the spectrometer.
These problems are discussed in the present section.
46.
Gas Flow Through the Spectrometer
It is necessary that the pressure in the spectrometer be maintained
such that the mean free path of molecules is greater than
of any part of the spectrometer.
the dimensions
Figure 29 shows a schematic diagram of
the spectrometer with possible appropriate pressures for the various parts
of the spectrometer.
The rate of molecular flow between any two points is given by
Q
=
Km
dP/MA
(15)
where Q is the rate of flow, dP is the pressure difference, and M1 is a
constant depending on the geometry and temperature of the system.
the gas flow is proportional to 1/M
Since
fractionation must occur in the
source from which the gas is being withdrawn, once steady state conditions
have been established.
The peak height of any isotope is dependent directly
upon the partial pressure of that isotope in the ionization chamber.
To
determine the steady state partial pressure in the ionization chamber it
is necessary to know the rate at which the sample flows into the ionization chamber, v, expressed in litres/sec; the rate at which the sample
is withdrawn from the chamber, S, expressed in litres/sec; and the volume
of the ionization chamber, V, expressed in litres.
The ionization chamber gains v dt standard litres of gas in time dt,
and loses pSdt litres in time dt, where p denotes the partial pressure
of the gas expressed in atmospheres, in the ionization chamber.
The net
gain in gas then is
d(pV)
=
(v -
pS)dt
or
V
dp/dt
=
v -
pS.
(16)
Integration of this equation gives the partial pressure of the gas
entering the ionization chamber or also the partial pressure of the gas
10-1
mm of mercury
10-2
J
10~4
10-6
10- 7
10-8
Figure 29.
Schematic diagram of mass spectrometer with possible appropriate pressures.
48.
intersecting the electron beam at a time t after entering the ionization
chamber.
Pt
v/S
(1
exp(-St/V))
-
(17)
The steady state partial pressure then is v/S.
The time required
to reach steady state conditions for a given rate of inflow, v, is
dependent upon S and V.
when f/S is large.
The most efficient use of a gas sample is obtained
In order to reach steady state conditions within a
reasonable length of time S/V should be as large as possible.
The necessary
information regarding S is not known for the spectrometer used in this
research but some idea of the partial pressures attained may be gained from
an examination of the available information.
For example, for one particular
analysis, the sample size was 2.04 x 10-3 cm3S.T.P. and the rate of inflow
was .5 x 10~9 litres
sec~
1.
Experience has shown that steady state flow
conditions for mass 38 are reached in about 60 seconds.
The volume of the
ionization chamber is approximately
(2.54 cm x 1.27 cm x 1.27 cm)/1000
or 4.1 x 10-3 litres.
The factor exp(-St/V) should have reached a small
value, say .01, before steady state conditions are reached.
Therefore,
e-x is equal to .01 when x is equal to 4.6,or St/V is equal to 4.6.
S = 4.6 x (4) x 10-3
60
60
=
Then,
3 x 10~4 litres sec~1.
This is the pumping speed at the ionization chamber slit.
The partial
pressure of mass 38 then is
.5 x 10~9/s x 10~4
or
.16 x 10-5 atmosphere.
At this pressure the mean free path is 7.30/1.6, (Dushman 1949), approximately 4.6 cm. or about twice the longest dimension of the ionization chamber.
It should be noted that this is a minimum value since t = 60 secs is a
maximum value.
Although this is an approximation of the conditions existing
49.
in the ionization chamber, it is seen that molecular flow conditions do
exist in the spectrometer for samples of the size used in this research.
One important feature should be noted.
If any appreciable volume exists
between the leak and the ionization chamber, and if the conductance of this
volume is comparable with that of the leak, the time constant in attaining
steady-state pressures will become very large.
The leak must be the only
controlling factor finally in operation.
Cold Traps:
Background Mass Spectra
The diffusion pump used on the spectrometer is a mercury diffusion
pump designed by Homer Priest of Research Laboratory of Electronics at
M.I.T.
3
Since mercury has a vapor pressure of .185 x 10- mm mercury at 0QC
it is necessary to prevent mercury from entering the spectrometer.
It is
further desirable to keep mercury from diffusing into the interior of the
spectrometer to prevent deterioration of the silver soldered joints.
Diffusion of mercury and hydrocarbons into the spectrometer can be prevented
by cooling a trap with solid carbon dioxide in alcohol or with liquid
nitrogen.
A comparison of the residual background spectra using either
coolant is shown by comparing figures 30 and
31.
That hydrocarbons
do diffuse into the spectrometer can be seen by comparing figures 30 and
31..
(See also section II ). The hydrocarbons are probably vapors from
the oil used in the forepump and from the stopcock lubricant used on
stopcocks in the gas inlet system.
The forepump oils and stopcock lubric-
ant used have vapor pressures of 10-4
-
10- 6 mm of mercury at 20 0 C.
However,
the temperature of the oil is probably much higher than this due to
continuous operation in a hot room.
It is necessary, therefore, to insert
a cold trap immediately adjacent to the gas leak and mercury diffusion pump.
50.
10
w
5
C.
35
36
37
38 39 40 41
MASS NUMBER
42
43
44
Figure 30.
Residual spectra using solid carbon
daxide as a coolant.
51.
0
Y-O.5
A
Lu
0-
.
lI
.
35
36
37
.
I
38 39
40 41
MASS NUMBER
II
42
43
Figure 31.
Residual spectra using liquid nitrogen as coolant.
44
52.
36
37
38
39
40
41
42
43
44
10 MIN
36
37
38
39
40
41
42
43
30 MIN
Figure 32.
Increase of background spec-tra with tim,
44
53.
Figure 32 shows another source of background mass spectra.
The increase
in background mass spectra is probably due to outgassing of the filament
and electron bombardment of ionization chamber walls.. Note particularly
the increase in the carbon dioxide (44) peak.
Recommendations for Further Work
If further work is planned that requires a more sensitive instrument
it will be necessary to reduce the background spectra.
This can be achieved
by plating the spectrometer tube with chromium and by vigorous torching or
prolonged baking of glass parts.
It would also be desirable to degas all metal parts in the source and
collector by heating them in a vacuum furnace with an industion heater.
Control of Gas Flow Into the Ionization Chamber
In an earlier section reference was made to the existence of molecular
flow conditions in the ionization chamber.
It is necessary now to consider
how these flow conditions are established.
The physical arrangement of
the leak was discussed in section II.
Only the effect of the leak upon
the gas flow conditions is discussed here.
Suppose that the volume of the sample reservoir is V litres, and the
gas is withdrawn from the reservoir at molecular flow rates.
Since the
sample reservoir is a closed system there is a steady loss of gas.
Let
S be the rate of withdrawal of the gas at the pressure in the reservoir.
Then for any particular gas the loss per time dt is PS dt where P is the
partial pressure of the gas.
d(PV)= - PS dt
Therefore:
or
dp/p
=
- S/V dt.
54.
is the initial partial pressure in the system and Pt the
Hence, if P
partial pressure at time t,
Po
Pt
(18)
exp(-St/V)
Thus, the pressure time characteristic is different for each gas simply
because S
is proportional to 1/M.
If the reservoir contains a binary mixture, say argon 40 and argon
38, the relationships become
In
Pil
Pt
.948
(It
P
4
In- Pt 384
n( Pt)
Po0)40
(19)
-P
\/38
With molecular flow conditions, the gases in the mixture are mutually
independent and each ion current decays by a factor exp(-Smt/V) in
consequence of the pressure decay.
If measurements of the unknown mixture
and calibration mixture are taken at exactly the same time t, then the
percentage decay will be the same for each isotope in the unknown and
calibration mixture.
to be t = 0.
In this research the time of comparison was taken
This is the most easily reproducable time.
For most all
samples of the size encountered in this research the initial decay is
very approximately linear.
It is, therefore, easier and more accurate to
extrapolate to zero time than interpolate between measured points on an
exponential curve.
Fractionation Patterns
Molecular flow exists where the mean free path of molecules is long
with respect to the diameters of the tubes through which flow takes place.
The pressure in the ionization chamber of a mass spectrometer is always
low enough so that this condition prevails.
If the pressure in the sample
reservoir is also low enough to allow molecular flow, then molecular flow
55.
prevails throughout the spectrometer and regardless of the nature of the
leak the composition of the gas in the ionization chamber is the same
as that of the sample (Inghram 1954).
However, with this low pressure
in the sample reservoir, the leak must be fairly large to keep the pressure
in the analyzing region at an acceptably high value.
Thus there is a
fairly rapid depletion of the sample in the reservoir and because the
flow rate of a gas component in molecular flow varies inversely as the
square root of the mass of this component the sample reservoir, and hence
the ionizing region becomes in time depleted in the lighter components.
From equation 18 the following relations can be derived.
(1)
40
Pt
(2)
P38
=
40
Po
40
exp(-Sm t/V)
P3 8
e
38
(19)
t/V)
40
Dividing (1) by (2) and setting Sm '
(20)
38)
m equal to 1/40
and 1/382
respectively, we have:
P4 0 /P 3 8
(3)
t)
P40 p38
exp
/pf(8)'
(40)2
(40)2
_
-
3
(38)-
t/v
V
(21)
Therefore, the ratio argon 40/argon 38 increases with time at a
definite rate.
The variation of the ratio 40/38 in most analyses resembled
that shown in figure 33.
Since the peak height is proportional to the partial pressure in the
ionization chamber, the peak height measurement may be considered representative of
40
t
38
40
and P
0
,P
38
,
0
or
40 38
ht/ht
From the graph on page 56, we have 1.0525
=
40
h
38
/h0
ext(
t)
1.0400 exp( g t) and
2x 10- 5 sec-1.
The value of d
, theoretically is
(
1/(38)
-
1/(40)1) 1/V or 4.2x10-3/V.
A computation of the volume of the gas inlet system gives as the volume
approximately 200 cm 3 or theoretically
0- 2.lxlO-5sec~1 .
3
1.061
0
5
10
Figure 33.
Variation in 40/38 ratio with time.
15
MINS.
SNIH
SL
,
- -
-
OL
-
-
-I
,
,
,
E
CD
S
,
,
,
,
,
,O
LE
58.
A similar calculation can be made for the ratio 40/38 in atmospheric
argon.
40 36
ht /ht
-
40 38
ho0 /h8 exp( U
t)
From the graph in figure 34 we have 321.5
of
f
is 5.5x10-5sec~ .
(1/(36)1 - 1/(40)1)
The value of
1/V
f
=
311 exp(d
t).
The value
, theoretically, is
= 8.6x10-3/200
=
4.3x10-5sec -1
The agreement here is not as good but definitely indicates that molecular
flow conditions are established.
59.
Section IV
PRODUCTION OF POSITIVE IONS
Introduction
In section II it was shown that the mass spectrometer is a constant
deviation spectrometer in which focusing is in respect to direction only.
A spectrometer with adequate resolution is possible only if the positive
ions have a small energy spread.
be used.
This limits the type of source that may
For example, the gaseous discharge type of source has an ion
energy spread of 1000 ev.
There are two main types of sources; (1) the hot anode or solids
source and (2) the electron bombardment or gaseous source.
Use of hot
anode source requires that the sample can be applied to a filament in a
solid form, while the electron bombardment source requires that the sample
be introduced in a gaseous or vapor form.
The electron bombardment source was first used by Dempster (1922) and
subsequently developed by Bleakney (1932), Tate and Smith (1934), Nier
(1940, 1947), and others.
The Nier-type source has been called the orthodox
electron-bombardment source because of its almost universal use in routine
mass spectrometric application.
Materials and Workmanship
Careful selection of materials for construction of the ion source is
necessary.
Metals should be used which do not corrode or oxidize easily,
which have a permeability less than 1.005 and which are not gassy.
non-magnetic nichromes and tantanlum are very satisfactory.
The
Tantalum, how-
ever, should not be used in the presence of hydrogen since it becomes
brittle and weak.
Adequate insulation as well as mechanical stability have
60.
to be considered in selecting insulators.
Fused silica, glass or hydrogen
fired lavite have the best insulation and stability characteristics.
Three features should have careful consideration; (1) maintenance of
design geometry, (2) elimination, in so far as possible, of edges, and
(3) a surface finish.
In (1) where requires, alignment, parallelisms,
and squarenesses, should be held to
.001 inch.
In construction, elimin-
ation of sharp edges (2) is necessary to prevent the intense electrostatic
field disturbances that sharp edges exhibit.
Uncontrolled cold field
emission due to these high fields may give rise to background peaks in
the mass region of interest.
not fully considered.
A source will function if these points are
A more carefully constructed source, however, will
give more satisfactory over-all performance.
The Orthodox Source
An ion source of this type may be said to consist of four parts;
(1) a device for introducing the gas into the source; (2) an ionization
chamber; (3) an electron gun and (4) an ion gun.
In this section it is
assumed that the gas has been properly introduced into the ionization
chamber and is representative of the original sample.
Figure 35 shows the physical arrangement of the source used in the
present research.
(3)
The Electron Gun
In the source, the electron gun (see figure 36 ) consists of a heated
tungsten filament and an anode.
The potential applied to the ionization
chamber, the thermal energy, and the potential disturbances in the chamber
determine the energy of the electrons.
the use of source magnets.
The electon beam is collimated by
The poles of the magnet are aligned so that
the major component of the electron velocity is parallel to the lines of
61.
GA S
IONIZATION
CHAMBER
ELECTRON
BEAM
17
JJFILAMENT
CONTROL
TRAP
FOCUSING
PLATE
COLLiMNATING PLA TES
PLATESI
Figure 35.
Schematic diagram of ion source.
62.
force.
Those electrons with a velocity component transverse to the magnetic
lines of force experience a force causing them to rotate in circles whose
plane is perpendicular to the magnetic lines of force.
electron, therefore, is in a circular helix.
(1)
be noted.
The motion of each
Two important features should
The electron beam should be aligned so as to pass through
the ionization chamber and be collected without bombarding any slit edges.
A wider slit at the collecting end of the chamber does not help, since
excessive penetration of the collecting voltage into the ionization chamber
may cause serious deflections of the ion beam.
A larger source magnet is
the only solution.
(2)
Correct alignment may be made empirically from scale drawings,
but final small adjustments are necessary.
The best position is indicated
by a compromise between maximum trap current and maximum ion current.
Even this is no guarantee that secondary electrons do not contribute an
important percentage of the ionization.
(4)
The Ion Gun
The ion gun (see figure 37 ) consists of a drawing-out potential,
accelerating potential, and collimating system.
The drawing-out potential is variable up to 14% of the accelerating
potential.
In adjusting the drawing-out field care must be taken to avoid
extreme penetration of the field into the ion chamber.
This will cause
deviation of the electron beam with a resultant spreading of the ion beam.
The accelerating potential is variable from 0-5000 volts, with 2500
volts the voltage most commonly used.
The collimating system consists of two plates with eight-thousandths
inch slits.
2.25
0
The half angle of divergence of this system is approximately
63.
Mass Discrimination
Incorrect isotope abundances can arise from two main causes (1) fractionation in the gas handling system and (2) mass discrimination in the ion
source.
The former is discussed fully in the section III on gas flow in
the mass spectrometer.
The latter is caused by the presence of a magnetic
field in the source region.
Mass discrimination has also been observed
when electrostatic scanning is used.
Since magnetic scanning and not
electrostatic scanning is used, the latter is not a factor in this research.
The source magnets used in aligning the electron beam are a source of
mass discrimination.
Ions of lighter masses are made to move in circular
paths more easily than the heavier masses, hence the lighter masses will
appear in less than their true abundance.
The energy of the ion before
it has passed through the accelerating potential is low and as a consequence
the ion is easily made to move in a circular path.
Since the energies of all ions of the same mass may not be equal it is
impossible to predict the mass discrimination.
must be determined empirically.
The mass discrimination
It was determined by measuring the
atmospheric A4 0 /A36 ratio.
A comparison of the ratios of 40/38, 40/36, 48/36 by Nier (1950) and
the ratios obtained using the mass spectrometer employed throughout this
research are shown in Table A
.
The difference between the two is due to
the mass discrimination of the ion sources.
The discrimination values for
the ratios 40/38, 40/36, 38/36 may be computed by knowing only the 40/36
ratio.
This is standard procedure used by mass spectrometrists.
ratio is related to the 40/38 ratio as follows:
(40/36)A/(40/38)i
:
310/296
40/38
1575
The 40/36
64.
In the same manner the value for
The value 40/38 is computed as 1604.
the ratio 38/36 is computed as .192.
Table
RATIO
A
40/36
40/38
38/36
296 t 1
1575
.188
Nier (1950)
310 ±-3
1604
.192
This work
Emission Regulator
The physical arrangement of the emission regulator used in this research
has been discussed in Section II.
and Nier (1949).
figure
25.
It was patterned after a design by Winn
A schematic diagram of the emission regulator appears in
Regulation is achieved by control of the electric field at
the filament by a control plate placed in front of the filament.
known as a space-charge-controlled regulator.
This is
This is electrically
analogous to running a common triode vacuum tube with a positive grid.
Voltage for the control plate is obtained from battery B1 .
The electron
current to the control plate flows through the battery B1 and the resistors
Any variation in the electron emission current causes the
R3 and R 4 .
control plate voltage to vary which tends to oppose the change in electron
current.
The filament is a seven mil tungsten wire bent into the shape of a
hairpin.
It was necessary to use a filament of this shape to obtain an
intense electron beam.
With a flat or straight filament most of the electron
emission would go to the control plate.
The emission density from a hairpin-
shaped filament is considerably greater at the point than elsewhere.
When
the point is placed close to the hole in the control plate a considerable
65.
portion of the emission goes through the hole in the control plate while
still being subjected to the controlling field.
It was found by experimentation that the filament should be placed about
one-half millimeter from the control plate hole.
With too great a filament-
control plate spacing too much of the electron emission goes to the control
plate and not enough goes through the hole as ionizing electrons.
With too
close spacing more electrons go through the control plate hole, but not
enough current goes to the control plate to maintain good stability.
Sensitivity
In ion production two efficiencies are considered.
to the gas molecules and one with respect to electrons.
One with respect
The problem of gas
flow in the spectrometer and efficient use of gas molecules are discussed
in section III.
It should be mentioned here that the total gas flow through
the spectrometer, Q
,
is expressed in liters-micron-sec
1.
That is, the
number of liters of gas at one micron pressure flowing through the source
per sec.
The over-all sensitivity, then, is expressed as the number of
liters-micron-sec
collector.
1-needed to produce a given number of amps at the
The sensitivity of the spectrometer used in this research is
6.3xl0~ 4lit-micron-sec~
for 10-12 amps at mass 40.
This sensitivity is
limited only by the background at mass 40 which is generally below 10-12
amps.
(see section III for a more complete discussion of background).
The number of ions produced may be expressed as
i
-
noQisie
(21)
where n0 is the density of the gas molecules,
Q is the collision cross-
section of the molecules for a given electron energy, s is the path length
of the electron in the gas, and i
is the electron current.
small quantity and is taken to be the ionization probability.
n0sQi is a
(Barnard 1952).
66.
IONIZATION
CHAMBER
CONTROL
TRAP
PLATE
FILAME NT
45V
40"
454
Figure 36.
Schematic diagram of electron gun.
67.
CHAMBER
GAS
ELECTRON
BEAM
DRAWING-OUT
VOLTAGE
ACCELERATING
VOLTAGE
I
ION BEAM
-
I
.9-
1 I-10
if,"
Figure 37.
Schematic diagram of ion gun.
68.
A plot of the observed peak height vs. electron energy is shown in figure 38.
Stability
In all ion sources, adequate electronic equipment must be provided to
stabilize the voltages applied to the electrodes in the ion and electron
guns.
Of consideralbe importance is the use of proper insulation.
section II).
(see
A leaky insulator can result in an unstable ion beam.
This
type of instability is difficult to locate and can best be prevented by
adequate attention to cleanliness in the source.
If in equation (21) Qisn 0 is a constant for any given set of conditions
(as it usually is) the stability of the ion current depends upon the stability of the electron current, i e .
Since secondary electron emission is,
to some extent, always present, it is desirable to control the total
electron current immediately adjacent to the filament.
As discussed in
an above subsection this is the method used in the present research.
The density of gas molecules no, is directly proportional to the
rate of gas inflow Qgffl and inversely proportional to the pumping speed
S, or no
Cc Q9f 1/S.
It is necessary, therefore, to control not only the
rate of gas inflow but also the rate at which the gas is pumped from the
ionization chamber.
The pumping speed can be controlled by proper control
of the heating element in the diffusion pump and by proper design of the
ionization chamber.
1.0
+
++
.8-
. Os
5n
m
J0
+
4
.2-
0
10
20
30
40
50
60
70
80
90
ELECTRON ACCELERATING VOLTAGE
100
(VOLTS)
70.
Section V
COLLECTION AND MEASUREMENT OF ION BEAMS
Collector Design
Several different collector designs have been considered in the present
research.
The main decision to be made being that of selecting a single
or multiple collecting system.
The wide mass separation of the isotopes
of argon made null method measuring impractical without multiple collection.
This, however, would introduce unwanted mass discriminations. (Barnard 1952).
Further because of the smallness of the sample usually encountered in this
research, the decay of ion beams was rapid and null method measurement
would be impossible.
The design of the first single collector in use was
similar to one used on a solid source instrument in the same laboratory.
This collector, however, had two major faults.
One, it was mechanically
unstable and consequently gave rise to large background noises when the
instrument was subjected to any small vibration; and two, several negative
peaks were noted, the one occuring at mass 36 being the more important.
The exact reason for the presence of these peaks in unknown.
The most
probable reason being that the ion beams of other mass spectra would be
glancing off the sides of the tube, picking up electrons and being
collected as negative ions at the time mass 36 was being collected.
design of the collector now being used is shown in figure 38 .
lector has given excellent results,
The
This col-
It is mechanically stable, and negative
peaks have not been observed.
Preamplifier
The ion currents measured in mass spectrometers range from a maximum
of the order 5 x -10~9 A to a minimum determined by the limits of present
day techniques.
Measurement of these low currents requires the use of a
71.
stage of preamplification before they can be effectively used and measured.
Several considerations are of importance in preamplifier design and tube
selection.
One, grid insulation of tube should be satisfactory; two,
adequate shock mounting is necessary; three, maintainance of a dry atmosphere
surrounding the preamplifier is desirable; four, adequate electronic shielding must be provided; and, five, proper adjustment of condensor and resistor
values are necessary such that the time constant of the recorder and not
the preamplifier is the determining factor in recorded peak shape.
A schematic diagram of the preamplifier used in the present research
is shown in figure 13 .
The tube used is a Raytheon CK5886.
ier as it is now designed gives very satisfactory service.
The preamplifThe grid leak
current of the tube used in this preamplifier is listed by the manufacturer
as being less than 2x10-13
amps.
This is extremely satisfactory since
the current is at leact a factor of 500 below the ion currents that it
would be desirable to measure.
figure 40
.
Complete characteristics are shown in
The tube is mounted on its own leads, as are the leads from
the collector box itself and the grid resister.
The aluminum housing is
gas tight, so it is possible to maintain a dry helium atmosphere around
the preamplifier.
preamplifier.
This housing also serves to electronically shield the
The current from the preamplifier and voltages necessary
to run the preamplifier are carried in a shielded cable from the power
supply and D-C amplifiers housed in the main electronic contact panel.
D-C Current Amplifier
The physical arrangement of the D-C amplifier has been discussed in
Section II.
A schematic diagram of the amplifier appears in figure
13
No attempt will be made here to discuss the theory of D-C amplifiers.
Instead, the interested reader is referred to Aiken (1947)
72.
springs
TANTALUM OR NICHROME
NO. 303 STAINLESS STEEL
FUSED
SILICA
SCALE
I1
-
I
1/500
0
I
0.100 a 5/"
39
C040
K
SLIT
K41
B
/7
\
0.042%1/2"
SLIT
0.060x 5/9"
SLIT
0.100x5/9~
CAGE
II
Figure 39.
Design of collector
73.
and the many other excellent works on the subject.
The amplifier is simple in construction and maintainance time is
negligible.
Two features warrant mention.
One, it is necessary to
adjust the zero step-adjust resistor whenever a new tube is inserted;
and, two, the amplifier is extremely sensitive to microphonics.
Tubes
which are microphonic can be eliminated by tapping them lightly.
The
resultant noise increase in immediately apparent.
Measurement of Ion Beams
After the ion current has been amplified the resulting current is
passed through a meter and series of scaling resistors
(See figure 18 ).
The voltage thus set up is recorded by a Brown recorder.
A typical set
of ion peaks for isotopes argon 38 and argon 40 are shown in figure 41
The peaks are measured, after drawing in zero and peak top lines, to the
nearest .01 inch.
The peak measurements are then plotted on semi-log
graph paper and the ratios calculated.
Section V contains a more complete
discussion of the ratios thus obtained and their interpretation.
Treatment of Data
When a sufficient number of peaks have been recorded, the record is
removed from the recorder and the peak heights measured and timing-lines
added.
Peaks are measured to the nearest .01 inch and times recorded to
the nearest .1 minute.
The 40, 38 and 36 peak heights are plotted on
semi-log graph paper and the decay extrapolated to t
=
0.
In all analyses
the plotted peak heights have plotted as straight lines on semi-log graph
paper.
The values of the peak height extrapolated to t
any background correction subtracted.
38/36.
=
0 are read and
Ratios are obtained for 40/38 and
The raw data is then corrected for discrimination.
used to correct each ratio are given in section IV.
The factors
The quantity of radio-
74.
genic argon and atmospheric contamination observed can then be computed.
75.
DESCRIPTION
The CK5886 is an electrometer pentode of subminiature construction having extremely low filament current, high
emission stability and low microphonics. Operated as a triode, the tube has an unusually high ratio of transconductonce to control grid current for single stage circuits. As a pentode, the amplification factor is high enough to afford
considerable voltage gain in the electrometer stage of a multi-stage circuit. The flexible terminal leads may be soldered or welded directly to the terminals of circuit components without the use of sockets. Standard subminiature
sockets may be used by cutting the leads to 0.20" length.
MECHANICAL DATA
ENVELOPE:
BASE:
D
T-2X3Glass
None (0.016" tinned flexible leads. Length: 1.5" min.
Spacing: Leads 4- 7 0.150" center - to - center;
Other Leads 0.050" center -to - center.)
TERMINAL CONNECTIONS:
(Red Dot is adjacent to Lead 1)
Lead 1 Plate
Lead 2 Screen Grid
Lead 3 Filament, Positive; One Deflector
MOUNTING POSITION: Any
FFilament Negative; One Deflector
Lead 4
Lead 7
ontrol drid
Press
Width
0.4 10"
max.
ELECTRICAL DATA
DIRECT INTERELECTRODE CAPACITANCES: (wufd.)
Control Grid to Filament
Control Grid to Screen Grid and Plate
DESIGN CENTER MAXIMUM RATINGS:
F iloment Voltage (dc)*
Plate Voltage
Screen Grid Voltage
CHARACTERISTICS AND TYPLCAL"OPERAT ION:
1.25 volts
45 volts
45 volts
Pentade
1.25 volts
10 ma.
12 volts
4.5 volts
-2 volts
6 uo.
3.6 ua.
1.25
10
10.5
Filament Voltage (dc)
Filament Current
Plate Voltage
Screen Grid Voltage
Control Grid Voltage
Plate Current
Screen Grid Current
Amplification Factor
Transconductance
Plate Resistance
Max. Control Grid Current
7
.3
200
2.0
160
14
umhos
II
2 X 10~
meg.
3 X i1
amp.
* For use nigh-batteries having an initial voltage of 1.55 voltes max.
* Screen Grid connected to plate.
AVERAGE TRANSFER CHARACTERISTICS
C425v
-4 I
T
Tff~~
fltM-
TW-
-5
.6
-4
4
44(
.
;
Y
7FT7~t
---
-7
*
:
; ;
L
.
1
4t
t
-8
Ec - VOLTS
4
4 32 1
Q
0
-43-
/f]
/8
Figure 41.
Typical
recorded ion beams.
II
77.
Section VI
ISOTOPE DILUTION TECHNIQUES
The Atomic Energy Commission has made available quantities of material
artificially enriched in rare isotopes.
of Rb
87
, Sr
84
, Ca
48
, Ca
42
, and A
38
Materials enriched in the isotopes
are available.
The use of these
enriched samples, commonly called "spikes" or "tracers" have made it possible
to greatly improve the absolute accuracy in the measurement of small quantities
of these elements.
The isotope dilution technique consists of the addition
of an accurately known quantity of a tracer, T, artificially enriched in
isotope X, in which the isotope abundance ratios Xl/X2 are accurately
known, to a known total quantity of sample, S, in which the quantity of the
isotope, X2 to be determined is unknown.
By determining the isotope ratio (Xl)T/(X2)S V (X2)T in the mixture
of sample and tracer, and by knowing the ratio (Xi/X2)T and quantity of
the tracer added, it is possible to determine the amount of isotope X
2
in the original sample.
The section that follows explains the use of
isotope dilution techniques in determining small quantities of radiogenic
argon in minerals.
Tracer Introduction System
The tracer used in this research was enriched in argon 38.
It was
introduced into a 3 liter bulb from which it could be withdrawn when
needed.
The bulb was previously prepared by flushing with hydrogen and
evacuating for 24 hours.
Any small quantity of gas left in the bulb was
evacuated by adsorbtion on charcoal cooled with liquid nitrogen.
The tracer
was then added from the break seal tube in which it had been stored.
The
tracer could be withdrawn into a small volume when needed and expanded
into the McLeod gauge for measurement.
The tracer was added to the sample
78.
before the purification procedure.
Calibration of the Tracer
Since the tracer was not pure argon but contained unknown quantities
of hydrogen, nitrogen and carbon dioxide, it was necessary to determine
the quantity of tracer per unit volume of total gas in the bulb.
This
quantity can be determined if the isotope abundance ratios of the tracer
and of spectroscopic argon, and a mixture of the two, are known.
If the following notation is used,
R
=
(4 0 / 38 )x
Qx
=
quantity of material x
where x may stand for S for spectroscopic argon, m for mixture of the
two, and T for tracer, the following ratios are determined.
40
QT
40T
T
-
QT/l
38
T
QS
t- (3 8 /4 0 )T
=
RT QT/ 1 +
=
40S 4-
38
4-
36
S
RT
(22)
40S
=
38T
=QT/1
QS/l+
(38/40) -4- (36/40)S
RT
Using the 40/38 ratios measured in the tracer, spectroscopic argon,
and mixture of the two, we obtain
(40/38)m
=
40T +
40
s/ 38 T
RTQTA1 + RT) * QSl + (36/40)
Rm
+ (38/40)S)
QT/1 + RT
QT (Rm - RT/l + RT)
QT
=
Q/1+
(36/40)s + (38/40)s
S/l + (36/40)S + (38/40)Si f(1
- RT/Rm - RT
The quantity of tracer in the mixture is then
QT
=
(40/36)S
QS/1
t (40/36)S I
(1 -+RT/Rm- RT
(23)
79.
Table B shows the ratios measured and the final results.
The gas in
the bulb was found to contain 93.6% tracer per unit volume of gas.
Possible Errors in the Calibration of Tracer
The determination of the quantity of tracer per unit volume of gas in
the bulb is subject to error dependent upon the error in measurement of
the ratio Rxard quantities Q .
Table
C shows a tabulation of the errors
introduced in the determination of the volume of tracer for a given errcr
in the determination of Rs, RT, Rm' QS, and QT
Table
B
CALIBRATION OF SPIKE
Trace r
40/38
.0825
40/36
161
38/36
Q
Spectroscopic
Argon
1965
5
2.04 x10-cm3
Mixture
1
1575
1.039
296
281
.188
272
6
1.80 x10
3
cm3
3.99 x 10-3 cm3
Contamination in tracer 6.4% per unit volume
Table
C
Error In Volume Of Pure Tracer Determined Per E% In Ratio Or Quantity
RATIO OR QUANTITY
S
Rm
T
QS
T
-. 1% per -10%
-t.7%per -1%
-.1% per -4%
-. 8% per -1%
-h.8% per -1%
80.
The main concern for sources of errors in the determination of this
value is in the measurement of Rm' QS'
and the discrimination value
of the ion source.
The discrimination of the ion source, that is the percentage difference
between the actual and recorded ratio introduced in the ratio measurement
of two masses, is caused by the presence of the electron beam aligning
magnets.
A complete discussion of the discrimination of the ion source
is given in section IV.
This factor should not introduce any large error
since the reproducibility of the discrimination value was within
Measurement of Q
and QT,
.3%.
if in error, would be most probably in error
by the same percentage in the same direction.
An error in the determin-
ation of these two quantities would tend to cancel each other.
An error
in Rm is likely to be small since QS and QT were so chosen that the value
of Rm would be nearly 1.000.
The error in the determination of peak
heights on the same scale and of approximately equal heights is likely to
be small.
Any discrimination or non-linearity that may exist in the
amplification system would then have no effect on this ratio.
As a con-
sequence the determination of the value of 93.6% tracer per unit volume
of gas in the bulb is believed to be accurate to within
t 1%.
Isotope Dilution Measurements
The quantity of radiogenic argon in any sample of gas may be determined
by isotope dilution techniques if the quantities QT and ratios 40/38, 40/36,
and 38/36 in the tracer gas mixture of gases are known.
If use is made
of the notation
40/38X = Rx; (4 0 / 36 )x = Px, (38 / 3 6 )x
=
Tx and Qx is the quantity of x,
where x may be T for tracer; C for contamination; m for mixture, or R for
81.
radiogenic, we have the following ratios:
40
402 38
C
40
40
T -4
+ QR *
/ 38 m
38/36m
36
(24)
(24a)
38
QT/l -
40
O/
4o
G/l 4
=
R
Q'/1.082
+4/8
38
/4 0T
=
RQT/l + R
.082
1.082
From(249)we have the volume of contamination QC
38
/ 3 6m
(r6
QC/l t ( 4 0 /
3
3 6 )C
=
=)
8
- (38/36)c
38
3
1-T
or QC=
36
1 38/36m
or
1
38/36)
3
)
36
r
_
36
38
/3 6 m
(25)
4M
Combining 24 and 25 and simplifying, we have as the quantity of radiogenic argon
QR=
QT
(.924
(4 0 /3 8 )m
~
(38/36)
(274)
t
.0642).
(26)
Errors in the Determination of Radiogenic Argon
Aside from the error introduced by improper technique in handling the
sample, the quantity of radiogenic argon determined may be in error due
to errors in the determination of the ratios
the quantities of tracer QT.
4 0 38
/
m,
40
/38 T, and 38/36m and
Table D shows the errors introduced in the
determination of the quantity of radiogenic argon for a given error in the
ratios
4 0 38
/
m,
40
/ 38 T' and Q,
Table D
Error In QR Per E% Error In Ratio Or Quantity
40
Ratios
/38 T
40/38m
t .5%/ ±- 2% (1)
>
1.25 and Qc/h
(1)
Rm
(2)
Qc/QT -.~ 0, Rm
Qc/Q
:P 0 -
^/
4
1%/ ±
4r
1% (2)
t
1%/
1%
0.
> 1.25, see figure 42 and discussion below when
82.
The errors were determined by use of the following equations:
,)OQR/
) QR
--
924
O
QR
/
QR
QT
Rm
R
40
/3 8 m
-
Rm
.924
40 38
.924
/ m
40 275
+ .0642
38
Further;
38
/ 3 6m
QT
Tm
.0642
3/6
( .924)
) Rm
+
275
38
m
/ 3 6m
=
(.924)
QT 4
2127
QV
297
.924
1&
2127
Qc . 1
0
T
297
Figure 42,showing the per cent error in the determination of the
quantity of radiogenic argon for a given percent error in
4
0/ 38 m,was
derived using the above equation and the following relationship between
QR, Qc, and QT'
OR/Qc
.924
Rm
QT
-
Qc
For any value of
40
.07589Q
Qc
-
1
/ 38 T there is an optimum value of the ratio,
tracer argon to radiogenic argon.
If the quantity of atmospheric argon contamination is small, the
assigned percent error in QR has the same value as the error in Rm'
However, if the ratio QR/Qc is less than 5,more accurate results are
obtained if the value of QR/QT is greater than 1.
This is shown particul-
arly clearly in figure 42 where it will be noted that to keep the percentage error in QR near one, for any value of Qc/QT the value of the ratio
QR/QT must be greater than one.
3.
0T/0
m
C1
0
2.0
1.0
0
m
02.
1.-
--
10
agaC
1.0
ligure 42.
7kError in Q
for 1% error in Ra'
84.
Each analysis can be in error due to errors in
38
/ 3 6 m.
/ 3 8 T' QT' Qc, and
Further, a 1% error in the (4 0 / 38 )T ratio would introduce a
very small error in QR,
The error in the determination of QT should be
small since the value of Q~T was quite reproducible.
error then is due to Qc and 38/36 .
m
/ 3 6 m is very small.
The main source of
When the value of the ratio 38/36m
is large, approximately 1900, the error in QR,
38
40
due to a 1% error in
85.
Section VII
VOLUMETRIC ANALYSIS OF ARGON
Introduction
As stated earlier in this work, it is also the purpose of this research
to determine if it is possible to make volumetric analysis of radiogenic
argon without atmospheric argon contamination.
If this is possible, and
the mineral contaiisno primary argon, then it would be possible to make
volumetric analyses of radiogenic argon without continuing mass spectrometric analysis.
Isotope dilution and mass spectrometric analysis require
techniques and equipment not available in all laboratories.
The volumetric analysis equipment described below, with exception of
the R. F. Induction heater, although a far less expensive model would
surfice, are easy to obtain, less expensive than mass spectrometer equipment, and more easily manipulated.
Separation Procedure
A detailed description of the standardized argon separation procedures
in included in section VIII.
A brief description is included here to
facilitate understanding of the detailed discussions that follow.
The mineral sample, after being weighed and wrapped is placed in the
vacuum furnace and the furnace flushed and evacuated.
degassed at a high temperature.
The crucible is
The crucible is allowed to cool and the
pumps and the crucible is heated until the mineral has melted and the gas
sample extracted.
The gas sample is transferred to the separation system.
The hydrogen present is removed by converting it to water.
If a large
quantity of water is present it is adsorbed on a charcoal trap cooled to
-780 C.
The charcoal is heated to room temperature.
The remaining small
86.
sample is transferred to the gas circulating system for final purification.
The volume of gas in the system is measured periodically until a constant
reading is obtained.
The sample is then transferred to a break-seal tube
and transported to the mass-spectrometer for isotopic analysis.
Description of Equipment
In order to facilitate any discussion concerning techniques of volumetricanalysis a section is included here describing the equipment used.
Radio-Frequency Induction Heater and Furnace
The induction heater used in this research is a 10 KW induction heater
manufactured by the Lepel High Frequency Laboratories.
of operating procedures see Appendix
(For a full discussion
I ).
The furnace as is shown in figure 4 3 consisted of a graphite crucible,
2" x 2V" with
j" walls and base.
This was supported on two tungsten rods
on the inside of an alundum cylinder 2" x 8" with {" walls.
and alundum
The crucible
cylinder were surrounded by a bell jar which was surrounded
by a water jacket.
The bell jar and assembly were supported on an aluminum
plate.
The bell jar seal was made with high melting point vacuum grease and
apeizon Q sealing wax.
Ideally this assembly would be connected directly
by glass to the rest of the system.
For experimental purposes it was
connected to the system through a ground ball and socket joint.
Such an
arrangement would ordinarily hold
It was
a vacuum for 24 to 36 hours.
not, however, entirely dependable and might leak air at any time.
Since
an average heating lasted no more than one or two hours this assembly was
satisfactory for early experimental procedures.
With such an arrangement
0
temperatures of 2000 C or more could be maintained without raising the
temperature of the cooling water more than a few degrees.
8mm tubing
sample
/
3/4 in. tubi ng
N11
LLway
Steel ball
-/in. Glass pipe
Graphite
crucible
2 X 1 1/2
-100mm Glass pipe
Water inlet
Alundum
2XBX1/4in.
Apiezon Q
Ground
0 Ring
Aluminum__
6.0 in.
Figure 43.
Furnace for extraction of gases.
-
U w-
88.
The sample was admitted without breaking vacuum.
wrapped in aluminum foil and held in a side tube.
The sample was
It was admitted to
the crucible at the proper time by pushing it ahead with a steel ball
activated by a magnet.
The steel ball was then returned to the sample tube.
Charcoal Traps
The charcoal traps used in this research contained Cenco activated nut
charcoal.
The traps contained varying amounts of charcoal depending on the
purpose for which they were used.
Traps for removal of contaminating
gases contained approximately 40 gms. of charcoal, while traps for transfer
of small quantities of gas contained approximately 6 gms.
of charcoal.
It was necessary to bake all charcoal traps for 48 hours before initial
use.
This time was necessary to completely activate the charcoal.
Calcium and Barium Furnaces
Several different furnace designs were tried.
Originally furnaces
were vertically mounted with the metal in the bottom of the furnace.
In
order to provide more surface for adsorption of gas horizontally oriented
furnaces were used.
The final design was a horizontal furnace through
which the gas could be circulated.
(see figure 44.).
Careful control of the temperature of the barium furnace was necessary.
If the furnace was allowed to become too hot and a thick barium mirror formed, the active barium would react with the quartz.
When the furnace was
cooled severe strains would develop in the quartz.
In most instances
the quartz would crack and a new furnace would have to be installed.
Calcium turnings were used initially since they were easy to obtain
and not easily oxidized.
However, barium was used in later research because
calcium was found to be very gassy at elevated temperatures.
Pipette
210cc
Transfer
U L
tube
CuO
3 4
2
1
CharcoaL traps
5
Cold
tCop
raps Diffusion
pump
533cc STP
Manometer
McLteod
Figure 44.
Gas separation system.
90.
A complete discussion of the operation of the barium furnace for
cleanup of gases is given below.
McLeod Gauge
The McLeod gauge used in this research was built by Ryan, Velluto,
and Anderson, glass blowers.
by the author (see figure
The capillary and volume was calibrated
).
The capillary was calibrated by weigh-
ing and measuring the length of a mercury column at various positions in
the capillary.
All length measurements were made with a vernier caliper
readable to .0005 inches using a jewelers glass.
Three determinations
of the volume per centimeter length of the capillary were made.
appear in the Table H.
They
The volume of the McLeod gauge bulb was determined
by weighing and measuring the quantity of water in the bulb.
Temperature
equilibrium was attained and bottles used which were calibrated to deliver
a known volume.
All volumes except those of the McLeod and capillary were measured
two times each by two methods, the high pressure helium method and low
pressure helium method.
cm. of mercury were used.
In the high pressure helium method, pressure of
In the low pressure method pressures of 10-2
to 10-3 mm mercury were used.
Problems to be Faced
Early experimentation showed that four major problems would have to
be solved if volumetric analysis was to be quantitative.
One, loss of
argon; two, incomplete extraction of argon from the mineral; three, the
possibility of atmospheric argon contamination; four, incomplete removal
of contaminating gases other than atmospheric argon.
be discussed in the following sections.
These problems will
91.
Table H
Volume of Gas Separation System
Volume of capillary
.002101 cm 3/cm
.002108
.002101
Volume of McLeod gauge
533 +
Volume of gas circulating system
168 cm 3
1 cm3
92.
Loss of Argon
There are three ways in which argon may be "lost".
One, semi-permanent
adsorption on charcoal; two, semi-permanent adsorption on glass; three,
solution of argon in the molten mineral.
Three experiments showed that semi-permanent adsorption on charcoal is
possible (see figure 45).
The argon was adsorbed on the charcoals at -189.5 0C
for varying lengths of time.
In experiment A (Curve A) and experiment B
(Curve B) the adsorption time was 30 minutes.
time was 60 minutes.
In experiment C the adsorption
In experiments A and B the charcoal was allowed to
reach room temperature slowly.
In experiment C the charcoal was heated to
room temperature by immersion in water.
In all cases the quantity of argon
lost was proportional to the quantity of charcoal in the trap.
In another
experiment the argon was adsorbed on several charcoals in sequence at -189.50C.
In this experiment 20% of the argon sample present in the system was lost.
Other adsorption temperatures were tried and it was found that at -780 C
(dry ice and alcohol mixture) 82.5% of the argon sample present in the system
was adsorbed on the charcoal.
However, when the charcoal was heated to room
temperature the argon was completely desorbed.
No direct confirmation of these results has appeared in the literature.
Paneth (1953) states that,
"For separation of argon from krypton and xenon the charcoal
should be kept at -780 C. In one complete fractionation 95% of
the argon is removed (recovered)1 *".
Wetherill (1954) states, in a mass spectrometric investigation of argon and
neon that "most" of the argon comes off the charcoal at -100 0 C. (Liquid N
2
and acetone mixture).
(1)
Inserted by the author.
U-
--
/
I
-z--
AuA
B.*
I..
I
I
0
5
10
I
I
20
I
I
30
MINS.
Figure 45.
Adsorption of argon on charcoal at liquid nitrogen temperature.
94.
It may be that in the first instance the xenon preferentially occupies
the adsorption sites, for Wetherill further states that he makes use of
xenon in his argon 38 spike containers to assure that the argon 38 is not
adsorbed on the glass.
No experimental evidence was offered, however,
that argon 38 is adsorbed on glass to any appreciable extent.
Loss of Argon by Adsorption On Glass
As stated above no experimental evidence was offered by Wetherill
that argon was adsorbed on glass.
noted by Paneth (1953).
Solution of helium in glass has been
The quantity, however, was not important until
analysis of quantities of He of approximately 10~9 cm
were undertaken.
It is probable that no appreciable quantity of argon is soluble in glass
at room temperature.
Loss of Argon by Solution In the Molten Mineral
No evidence can be offered that argon is soluble in the molten mineral
sample under the conditions of a temperature 15000c and pressures less
than 1 or 2 mm.
(Naughton (1953) has experimented with molten pyrex
glass and found that argon is not soluble at pressures of 10 cm or more.
It is improbable therefore, that solution of argon in the molten mineral
can cause any error in quantitative determinations.
Extraction of Small Quantities of Argon from Minerals
The difficulty of extracting small quantities of argon has been
reported by many workers.
Most of the earlier papers published give no
mention of whether the yield was quantitative or not.
have been employed.
Several methods
Aldrich and Nier (1948) heated the mineral to 100000
with no length of time specified in the publication.
Thode and Fleming
(1953) in their work on argon 38 in pitchblende minerals
95.
heated the sample in an inconel tube to 250-3000C for one hour to drive
off adsorbed gases, after having evacuated the tube for 24 hours.
Follow-
ing this, the temperature was slowly raised to 12500C (maximum attainable).
While there were no tests to determine if the yield was quantitative,
tests did prove that no structural argon was lost in heating to 3000C.
The Use of Fluxes
Most recent work has made use of Na fluxes in one form or another.
An example of incomplete removal of argon was found in the work of Russell,
et al (1953) who obtained a branching ratio
K/
2/
of 0.06.
The sample
had been fluxed with metallic Na which subsequently was found to yield
35% less than a NaOH flux.
Most workers (Wasserburg, 1955; Wetherill,
1954; Shillibeer, 1954) now make use of NaOH as a flux.
Thomson and Mayne
(1955) have experimented both with and without flux, and report that the
use of sodium peroxide is superior to either sodium hydroxide or sodium
carbonate.
This is mainly because of the large quantities of water and
carbon dioxide, respectively, released by the two fluxes.
In a fusion
without flux the atmospheric argon contamination was found to be lower
by 40%.
In the present work, tests have been made on the extraction of argon
from biotite, lepidolite.
A high frequency induction heater available in the Department of
Geology and Geophysics was used to heat the samples.
With proper
furnace design it was possible to maintain temperatures of 1500-20000C.
The use of fluxes is unnecessary when such temperatures are used.
The
mineral structure in all cases was completely destroyed and in most
instances the iron oxide in the sample was partially reduced to iron.
96.
Atmospheric Argon Contamination
Atmospheric argon may be introduced into the gaseous sample during two
stages in the analysis procedure.
(1.)
If the graphite crucible is not
completely degassed, some quantity of atmospheric argon which may be
adsorbed during the sample loading procedure, is collected along with the
gaseous sample upon reheating.
Dushman (1949) states,
"At 21500C (according to Norton and Marshall) it is
possible to degas graphite so that subsequent heating at
a higherl* temperature given no further gas. It is very
interesting to note that the gas evolved in the range
1700-2200 0 C is predominantly nitrogen."
Atmospheric argon may be introduced into the extraction or separation
system through a leak in the glassware or around grease-sealed ground
joints.
(2.)
Early in the research use was made of ground glass joints
to attach the transfer tube to the extraction and separation systems.
It was not possible to depend on these seals and several analyses were
discarded because of large leaks.
The use of grease seals was discontinued
and the transfer tubes were sealed onto each system by glassblowing it
in position.
It was then possible to make analyses with less than 5%
atmospheric contamination.
In the new system being constructed it is not
necessary to use transfer tubes and all grease seals have been eliminated.
Mercury seal stopcocks are used throughout the system.
These stopcocks
will not leak, although some attention is necessary to ensure that the
bulb is periodically evacuated.
In one analysis there was less than .10xlO
-3 3
cm of argon contamination.
If it were always possible to keep the contamination at this level, or lower,
other problems not considered, volumetric analyses would be possible.
1.
The emphasis is by the author.
97.
Contaminating Gases Other Than Atmospheric Argon
The gases extracted from a mineral consist mainly of large amounts
of water, hydrogen if a graphite crucible is used, and smaller quantities
carbon dioxide and carbon monoxide.
The radiogenic argon is only a very
minor percentage of the total quantity.
The volume of gas collected
depends upon the type and weight of the mineral sample.
Hydrogen Removal
Hydrogen can be removed by oxidation to water.
The hydrogen is
readily oxidized in the presence of copper oxide heated to 450 0 C.
water formed is then adsorbed on charcoal at -780C.
The
Only a small percentage
of water will be desorbed on heating to room temperature.
However, it
is not possible to remove the last small quantity (less than .2cm3STP) of
water by adsorption.
It is important to note the water should not be removed by freezing.
When the water is frozen argon is trapped. (Smits 1953).
In one experi-
ment 30% of the radiogenic argon was lost when the water was removed
by freezing.
When the water is adsorbed on charcoal it may condense and
freeze thereby trapping argon.
It is thought, however, that the water is
in the gaseous form at room temperature and remains adsorbed while the
argon is' liberated.
Cleanup of Final Traces of Gas
After treatment with copper oxide it is necessary to admit the gas
to a barium furnace where the remaining water and most of the remaining
contaminating gases are removed.
98.
Operation of the Barium Furnace
The use of calcium for the production of high vacua was first proposed
by Soddy (1907).
Soddy observed that Co, C02, H2 0, C 2 N2 , SO2 , NH3 and
oxide of nitrogen were readily cleaned up.
any appreciable quantity.
Hydrogen was not adsorbed in
He states,
"There is no doubt that a low initial pressure not exceeding a few millimeters of mercury is as essential in causing
calcium to combine with gases as a high temperature. For rapid
and continuous adsorption, volatilization is necessary. Argon,
helium, and the other rare gases were not adsorbed by calcium."
Since the original paper by Soddy intensive investigation has shown
that other metals such as barium and magnesium are effective as adsorption
agents.
The order in which metals may be rated as cleanup reagents for
most gases corresponds roughly to the chemical activity of the alkalineearths.
Barium is the most efficient but is difficult to use because it
is readily oxidizable.
as efficient.
Calcium is not so readily oxidizable and is almost
Calcium was used initially as a cleanup reagent.
However,
because calcium was found to be very gassy at high temperatures attempts
were made to make use of barium metal.
The difficulty encountered due
to the rapid oxidation of barium was overcome by storing the clean barium
metal in an atmosphere of helium.
When a fresh supply of barium was needed,
a small hole was blown in the quartz furnace, the barium inserted, and the
hole closed.
The system was toen evacuated.
not more than 10 minutes.
The total time necessary was
In this way, it was possible to have a fresh
unoxidized supply of barium metal in the furnace.
The gas separation system contains two barium furnaces.
used as a rough pump to remove large quantities of gas.
The first is
The second is
part of the gas circulation system (see discussion on next page) and is
used for the removal of small traces of gases.
99.
In order to clean up large quantities of gas it was necessary to
450 0 C was the most
volitalize an appreciable quantity of barium metal.
efficient temperature for rapid volatilization, (variac setting 36).
After
one-half hour a mirror of barium formed and the temperature was reduced
to 250 0 C,(variac setting 20).
At the end of one hour from the start
of volatilization the pressure had usually been reduced from an initial
value of a few (3-5) mm of mercury to 10~1-10-2mm of mercury.
The
gaseous sample was then transferred to the gas circulating system.
quantity of contaminating gas remaining is small.
The
The barium furnace in
the circulating system was operated at a lower temperature (250 0 C).
A
mirror was not formed when the furnace was operated at this temperature.
The amount of barium vapor formed, however, was adequate for efficient
sample cleanup.
Figure 46
illustrated an experiment which involved
cleanup of a small quantity of gas (8x10-3cm3 ) in the presence of
2x10-3cm 3 of argon.
The contaminating gas consisted mainly of carbon
dioxide and oxygen.
It will be noted that the system is quite similar
to a differential-thermal-analyzer.
The reaction between the vaporous
barium metal and gas is thought, because of the shape of temperature
curves, to be endothermic.
Gas Circulating System
It is necessary, for efficient cleanup, that the gases come in contact
with the barium vapor or metal.
If the pressure of argon is approximately
equal to or greater than the pressure of contaminating gases cleanup is
considerably slowed if the gas in the barium furnace is not changed
periodically.
That is, the ratio of argon to contaminating gas in the
furnace becomes very high.
When the pressure in the system is below 10
the gases can only move about by diffusion processes.
4
mm
The thermal agitation
400.-
f -
231,
5,
19
17
1 X10-2
CM
3
Figure 46.
Cleanup of small juantities of pe in presence of I.9xIO
X10
0120
120
180
10
cm3 argon.
21.0
240
300
MINS
300
MINS
101.
of the gas in the furnace and the small opening in the stopcock through
which the gas must pass makes the diffusion of gases into the barium
furnace an extremely slow process.
In order to remove this difficulty
a system was constructed consisting of a barium furnace, a copper oxide
furnace, a mercury diffusion pump, and isolating cold traps.
A diagram
of the system is shown in figure 44 .
The gas sample was brought into the gas circulating system by
adsorption on charcoal using liquid nitrogen (T -189.5 0C).
The circul-
ating system was isolated from the rest of the system and the sample
desorbed by heating the charcoal to 150 0 C.
at this temperature.
All the argon is desorbed
The diffusion pump and barium furnace was heated.
The gas circulated in this system was efficiently cleaned up since the
gas samples could be repeatedly exposed to the barium vapor.
Further,
no gas was allowed to remain in the barium furnace for an appreciable
length of time so that the ratio of contaminating gas to argon was the
same throughout the system.
Calibration of the System
The volumes of the various parts of the separation system are shown
in a diagram of the entire system (figure
44).
The entire system was
calibrated by analyzing air for the volume of argon present.
obtained in three analyses were .993%, .990%, .992%.
The values
The gas circulating
system was not included in these analyses.
Results of the Volumetric Analysis
As stated above, it is possible to make analyses free of atmospheric
argon to within 5%.
The new system being constructed should further reduce
the level of contamination.
The graphite crucible will remain the main
102.
source of atmospheric argon.
However, if sufficient degassing procedures
are observed this difficulty may be eliminated.
In any analysis other
than for atmospheric argon in air it has not been possible to completely
remove all of the contaminating gases.
With correct cleanup procedures
(see section VIII describing standardized procedures) it is thought that
-3
the quantity of contaminating gases can be kept below .05x10 .
The
results of several analyses and an explanation of the reasons for failure
(if necessary) is given in table
E
rlq
7-
Table E
Sample
Total Gas Determined
Volumetric
Isotope Dilution
A40/gm. Sample x 10-3
Volumetric
Isotope Dilution
Bob Ingersoll
ID #1
4.64
4.78
ID #2
12.00
13.80
ID #4
ID #5
(1)
.83
.79
(3)
.74
3.67
(2)
4.43
.50
(2)
(4-)
.73
It is thought that the total volume of gas as determined by isotope dilution
technique is higher than that determined by a volumetric determination
because of (1) an extremely small leak in the break seal tube, or
(2)
outgassing of the glass when the break seal tube is
removed from the separation system.
(2)
Not determined.
(3)
Not computed since it would have no meaning.
(4)
In this experiment water was frozen in a cold trap.
104.
Section VIII
STANDARDIZED PROCEDURES
Volumetric Analysis
The procedure for determining the volume of the argon in
a mineral
has been changed many times during the experimental development, and
changes may be made if
future work indicates such changes are necessary.
Two separate procedures are included in the following subsections.
Procedures are included for making volumetric analyses and isotopic
dilution analyses of the argon content of a mineral.
Included in the
sections are not only the immediate steps necessary but also such
cautionary and advisory notes as are deemed expedient to a successful
analysis.
A condensed procedure sheet has been added in Appendix II.
The procedure for volumetric analysis of argon occupies the present subsection.
Preparation of the Gas Analysis System
Preparatory to making an analysis any gases in the system should be
evacuated.
All charcoal traps should be baked and evacuated for periods
of not less than 2 or 3 hours.
The transfer charcoal trap and break seal
tubes should be baked and evacuated overnight if time permits.
The
barium furnace, copper oxide furnace, and gas circulating system diffusion
pump should be heated and evacuated.
The gas circulating system should
be prepared separate from the remainder of the system.
The copper oxide
furnace should be heated to 450 0 C, filled with oxygen, and heating continued
for 2 more hours.
This procedure convertsto copper oxide any copper that
may have formed during the previous run.
The barium furnace should be
heated to 350 0 C (variac setting 30) and evacuated along with the McLeod
gauge and gas circulating diffusion pump (variac setting 60) for a period
105.
of 12 hours if time permits but not less than 2-3 hours.
Although very
little gas is admitted to the gas circulating system, the barium in the
furnace becomes poisoned after 2 or 3 runs and cleanup time is considerably
lengthened if adequate pre-analysis treatment is neglected.
Weighing and Wrapping the Sample
All samples should be weighed to the nearest .0005 gms. and wrapped
in aluminum foil.
foil around a
The sample container is made by wrapping the aluminum
1/2 oz. sample bottle leaving one end open.
After weigh-
ing, the sample is put into the aluminum foil container and the open end
twisted closed.
At this stage a series of very small pin holes may be
made in the aluminum foil container to allow any air to escape more
readily when the sample is admitted to the vacuum furnace.
Preparation of Gas Extraction Furnace
The crucible should be cleaned of slag from the previous run and
the sample and steel ball inserted into sample holder.
The bell jar
ground glass joint should be cleaned of any old vacuum lubricant (cylvaceneheavy is recommended) and a thin coating of fresh lubricant applied.
The
alundum radiation shield and crucible are placed in position and the bell
jar assembly placed in position over them.
The assembly should be rotated
to thoroughly lubricate and seal the ground glass joint.
Apeizon
A wedge of
Q is applied to the outside of the ground glass joint.
The glass
seal between the bell jar and the rest of the system is made at this time.
The water circulation is started and the water seal at the ground glass
joint is made.
The transfer tube is now sealed on to the system and the
crucible is ready for degassing.
In the new system under construction
transfer tubes are not necessary so this step can be omitted.
The crucible
106.
is degassed at approximately 2000oC-22000 C (150 amperes current through
the induction coil).
of 2 or 3 minutes.
The system is closed from time to time for periods
The change in the pitch of the fore pump indicates
the quantity of gas coming off the crucible.
When no change in the pitch
is noted, upon opening the system, the crucible is considered as degassed.
The time to degas the crucible is usually 15 minutes to one-half hour.
This is a weak point in the analysis that will be removed in the permanent
system where the evacuating apparatus consists of diffusion pumps and
cold traps and an ion gauge or simple McLeod gauge to measure the pressure
in the system.
The evacuation and degassing used, however, was found to
be sufficient for experimental purposes.
The crucible is allowed to
The sample is admitted to the
cool and the lead to the fore pump closed.
crucible and heating of the sample started.
It is extremely important
to heat the sample slowly in the initial stages.
A large quantity of gas,
0
probably water vapor comes off the sample at approximately 300-500 C
(temperature of crucible).
The aluminum foil has melted by this time
and the sample will as a consequence spill over the system if care is
not taken to ensure that the gas does not leave the furnace too rapidly.
The heating is continued until the temperature has reached 1500 0 C (130
amperes current through the induction coil), a temperature slightly cooler
than was used to degas the crucible.
until all the gas has been collected.
The temperature is then held constant
In the experimental system difficulty
was sometimes experienced in pulling the entire sample of gas,extracted
from the mineral, into the charcoal trap.
The pressure noticed, however,
was thought to be due to continuous reduction of the iron oxide in the
sample.
slag.
Many magnetic particles are fould to be present in the reduced
In the permanent system the charcoal will be replaced with an automatic
107.
toepler pump and the gas will be evacuated from the furnace region.
After
all the gas has been pulled into the transfer tube, the tube is removed
from the system and sealed onto the analysis system.
The section in
between the stopcock and the transfer tube and the analysis system is
evacuated by adsorption of any remaining gases on charcoal at liquid
nitrogen temperatures.
Separation of Argon
The sample is admitted to the separation system and copper oxide furnace.
The copper oxide furnace has been preheated to 4500C (variac setting 43).
The transfer tube is heated to 1500C (variac setting 23) to ensure that all
the argon is desorbed.
The gaseous sample is left in the copper oxide
furnace until all or most of the hydrogen has been converted to water.
If
the pressure of water exceeds a few millimeters of mercury, the sample
should be adsorbed on one of the charcoal traps at -780 C for one-half hour.
At this time the coolant is removed and the charcoal trap heated to room
temperature.
If the pressure is not below a few millimeters of mercury,
the process is repeated on another charcoal trap.
When the pressure in
the system is below a few millimeters, usually after treatment on one
charcoal trap, the roughing barium furnace is opened to the system and the
temperature of the furnace raised to 5000C (variac setting 36).
When a
mirror has formed the temperature is lowered to 2000c (variac setting 20),
and the sample left to stand for one-half hour.
Throughout all the above
procedures the copper oxide furnace has remained hot and all charcoal
traps are left open to the system.
be below .2 mm.
The pressure in the system should now
The sample is adsorved on the charcoal in the gas circul-
ating system and the circulating system isolated from the rest of the
system.
The adsorption time usually lasts for two hours.
Previous to
108.
isolating the gas circulating system, the entire system may be evacuated
along with the charcoal trap on which the sample is adsorbed.
It is
thought that in this manner it may be possible to remove any hydrogen
that had not been previously converted to water.
tried only once.
This procedure was
Thomson and Mayne (1955) state:
"Ventil P was closed and the tap Q opened to the vacuum
for two minutes, and thus any helium and neon in the gas sample
were removed."
"Trial experiments showed that argon was not lost in this
process."
The cleanup barium furnace is heated to 300 0 C (variac setting 30),
the diffusion pump heater is turned on, and the isolating cold traps
covered with solid carbon-dioxide and alcohol mixture (temperature -780 C).
The gas is left circulating over night.
The volume of the gas is
measured periodically until a constant volume is reached.
The heaters are
turned off and the cold traps heated to room temperature and the volume
measured again.
The actual volume of gas in the system can be computed
using the constants listed in another part of this section.
Isotope Dilution
The procedures followed in an isotope dilution analyses are similar
to those observed in a volumetric analysis.
The system is prepared in the
same manner and the gaseous sample extracted from the mineral sample by
similar procedures.
The traced is added when the sample is admitted to
the separation system.
If it is desired to make a volumetric analysis of
the sample the procedures to follow are the same as would be observed if
the tracer had not been added.
When the volume has been measured the gaseous
sample is adsorbed on the charcoal in a break seal tube.
time enough to adsorb 99% of the sample.
One hour is usually
The break seal tube is sealed off
and blown onto the gas inlet system of the mass spectrometer.
109.
Preparation of the Mass Spectrometer
In order to make satisfactory and accurate mass spectrometric analyses
it is necessary that certain proceduresbe carefully followed.
One half to
one hour before an analysis is to be made the filament should be turned so
that the source area will have reached temperature equilibrium.
Previous
to this the spectrometer tube should be baked out for 2-3 hours and allowed
to cool.
A background spectrum of the mass spectrometer with and without
the leak open should be obtained.
A comparison of the two will determine
if there is an air leak in the gas inlet system.
If there is no air leak
in the gas inlet system the gas leak is closed and the break seal on the
sample tube broken.
The sample is allowed to cool for one-half hour,
with liquid nitrogen on the cold trap, to condense any hydrocarbons that
may be in the sample.
The spectrometer is set to record a mass 40 or
mass 38 ion beam and the gas leak is opened to the desired setting.
The
scale changer is set to the appropriate scale and manual recording started.
The recording is continued until a sufficient number of peaks have been
recorded.
110.
Section IX
MEASUREMENT OF AGE BY THE POTASSIUM-ARGON METHOD
The age of a mineral may be determined, by the potassium-argon method,
if the quantities of argon and potassium in the mineral are known.
It is
necessary that the argon produced by the decay of potassium 40 remain in
the mineral, that is the mineral must be a closed system.
Potassium 40 decays by beta emission to calcium 40 with a decay constant
and by K capture to argon 40 with a decay constant
40*
atoms of K40 present at any time t is
40
KO
40
(Kt
where
-
K #
.
exp(-
A
The number of
t))
The ratio of the number of atoms of argon 40 that
decay per unit time to the number of atoms of calcium 40 that decay in the
is
same length of time,
called the branching ratio,R
/
.
The
40
number of atoms of argon produced from K0 atoms of potassium
A4
or
0
40
A
=4040
R -=
l/R
K
R + 1/R
K
(1
0
40
-At
-d
)
(ea -1)
It is then possible to determine the age of the mineral, or,
t ;+
1R
+ 1
R
A4 0
K4 0
The Branching Ratio
Table F
past years.
.09-.130.
shows that the branching ratio has varied widely over the
It can be said that the ratio probably lies in the range
This variation introduces a considerable uncertainty in age
determination by the potassium-argon method.
Several methods have been
employed to measure the branching ratio, the physical method and the geological method.
The physical method makes use of the beta emission
associated with the calcium 40 or
If emission associated with the argon 40.
111.
Table F
Year
Authority
Method
b
k/
1943
Thompson and Rowlands
X-Rays
1947
Bleuler and Gabriel
X-Rays
Horteck and Suess
Argon
Ahrens and Evans
Calcium
1.4
Aldrich and Nier
Argon
0.02 to 0.09
1948
1950
3 to 4
1.9
±
0.4
0.1
± 0.02
Ceccarelli, Quarcini,
and Rostagni
X-Rays
Graf
and X-Rays
0.07
0.127 to 0.67
Inghram, Brown, Patterson
A40 /Ca40
and Hess
Sawyer and Wiedenbeck
Auger Electrons
from X-Rays
1955
0.126 t 0.003
0.135
Wasserburg and Hayden
A40/Pb ages
Moljk
(1)
0.124 to 0.136
Wasserburg (2)
(2)
0.102!: 0.01
(3)
0.128 ± 0.02
A40 /Ca40
0.150
(3)
Backus and Strickland
.085
t.02
(1)
No mention made of method of determination.
(2)
Using Sawyer and Weidenbeck's data and a redetermination of the
42
gamma emission of K
(3)
.
(Kahn and Ryan (1953)).
Computed by the author using the average of the best determinations
of the beta and gamma emission.
(4)
(4)
Based on the determination of the quantities of argon and calcium
in one lepidolite.
112.
/2p is
All measurements indicate that the ratio
.
equivalent to
(see figure 47).
40
Decay scheme for K 40,
after Sawyer and Wiedenbeck; Energies
after Alburger.
The geological method makes use of the fact that known quantities of
argon and calcium are produced in a known length of time.
If the quantity
of argon produced in a mineral is known, and if it is possible to determine the age by some other method, as for example with lead, the branching
ratio then is
R
=
K'U/A 40
1
(
e-
1)
-1
If it is possible to measure the quantities of radiogenic argon and
radiogenic calcium in the same mineral, the branching ratio may be determined directly regardless of the age of the mineral.
That is
A 4 0 /Ca4 0
=
It is necessary to know the abundance of potassium 40 in order to
measure the age by the potassium-argon method.
Herzog (1955) has a com-
plete listing of all the published measurements of the potassium 40
abundance.
The value determined by Nier (1950) (.0122%
probably the most accurate.
by weight) is
~-
Table G
Sample
Best Geological Age
Bob Ingersoll
1450
(1)
Rb/Sr
A 4 0 /K 4 0
(.085)
2050 (2)
1610 (8)
2060 (3)
850 t 200 (4)
1710f 90 (7)
Best Lead Age
1600 ±
50
1500
(1)
Kulp (1955).
(2)
Aldrich, et al (1955).
(3)
Herzog (1955), not yet published.
(4)
(5)
.
300 (5)
Ahrens (1951).
(6)
Backus (1955), not yet published.
(7)
This work.
(8)
Presented by Wetherill at the American Geophysical Union meeting,
Washington, 1965.
Ca
(.125)
1180
+
90 (6)
"-
114.
The Bob Ingersoll Lepidolite was used for calibration purposes.
age is presented so that some comparisons may be made.
Its
It can be seen
from the table that the age as determined by the Rb/Sr method is 17% higher
than the age determined by the argon-potassium method, and 20% higher than
Several explanations of the differences observed may be
the lead method.
offered.
(1)
would be high.
There may be rubidium leaching
(2)
age would be low.
There may be loss of argon in which case the A
(3)
40
40
/K
There may be loss of calcium, in which case the
calcium age would be low.
be made.
in which case the Rb/Sr age
It is necessary that many more measurements
Two laboratories in the United States are admirable suited for
these measurements, Massachusetts Institute of Technology, Department of
Geology and Geophysics, and the Carnegie Institution in Washington.
115.
Recommendations
q
Futu
Research
It is always possible, after the completion of a
research problem, to think of better ways to approach
the problem, and other problems that should be attacked.
This research is no exception.
Mass Spectrometery
For more accurate and sensitive analysis the spectrometer tube should be plated, the gas inlet system should
be converted to an all metal system to eliminate hydrocarbon background, and the ionization chabber should be
made gas tight to reduce the residual mass spectra.
Volumetri. Analysis
With a single one piece entire glass system it
should be possible to make volumetric analysis of argon
without the presence of atmospheric contamination.
Such
a system should be constructed.
Gezteral
The problem of "common'
argon has not been solved.
This problem could be solved if the Mass Spectrometer
was placed so that gases attracted from the mineral could
be measured directly.
A system consisting of a radio
frequency heater, furnace gas separation system and mass
116.
Spectrometer, all directly connected could form the
foundation of a gas analysis system which would aid
immeasureably in solving a wide variety of problems.
Appendix I
Use of the R.F. Induction Heater
The induction heater is a very useful instrument, however, several
precautions should be observed if no one is to be injured.
included with the heater gives the basic information.
The booklet
However, several
points should be carefully observed.
1.
Become very familiar with the location of all dials and switches,
especially the 100 ampere circuit breaker.
2.
Be sure to turn on the main power switch before turning on the
switch for the water circulating pump.
3.
Allow at least 20 minutes warmup period after "start" buttom
has been pushed.
4.
When the preliminary adjustments have been made step on "step-
switch" for a "split second" and observe readings.
5.
If the 100 ampere circuit breaker opens, the water pressure will
go up to 60 p.s.i.
To avoid damage to the water pump this switch
must be closed immediately or the water by-pass valve should be
opened.
6.
Since the induction heater makes no distinction between metals,
extreme care should be taken not to wear rings too near (6 inches)
the coil or leads.
7.
A familiarity with the note-book that came with the induction
heater is an invaluable aid in learning the correct operating
procedures.
Appendix II
Condensed Procedure Sheet
1.
Bake all charcoal traps and evacuate for period of 2 to 3 hours.
2.
Bake and evacuate transfer tube overnight.
3.
Prepare the copper oxide furnace and the barium furnace.
4.
Weigh and wrap sample.
5.
Prepare the gas extraction furnace.
6.
Bake out the crucible (150 amperes through induction coil) for
one-half hour or until no gas is being given off.
7.
Let the crucible cool.
8.
Push the sample into the crucible with the steel ball and magnet.
9.
Heat the sample, taking care to heat very slowly in the initial
stages.
10. Put liguid nitrogen on the transfer tube.
11. Continue heating at 140 amperes for at least one-half hour, or
until no more gas is coming off.
12. Close the stopcock to the transfer tube and break it off.
13.
Seal on to the gas separation system.
14. Evacuate the section between the transfer tube and the gas
separation system.
15. Heat the copper oxide and transfer tube.
16. Allow the gas to remain in the copper oxide tube for at least
two hours or until the pressure has stopped decreasing.
17. If the pressure is greater than a few mm adsorb the gases on
charcoal in a trap cooled with solid carbon dioxide for one-half hour.
18.
Let this trap heat to room temperature for one-half hour.
19. If the pressure is now below a few mm heat up the rough barium
furnace.
20. After a mirror has formed turn the heat down to variac setting 25.
2L After one-half hour the pressure should have decreased below a
readable pressure on the manometer.
22. Adsorb the gas on the charcoal in the gas circulating system.
23. Isolate the gas circulating system from the rest of the system.
24. Turn on the barium furnace and the diffusion pump.
25. Let the gas circulate for a period of twelve hours or over night.
26. Measure the volume of gas in the system.
27. Continue circulating the gas until the volume read reached a
constant value.
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Biographical Sketch of the Author
The author was born in Providence, R. I. in 1925,
the first of a family of two.
After graduation from
Hope High School he enlisted in the U. S. Navy, spending
five years as a pilot.
During this time he attended
Trinity College in Hartford, Conn. for one year.
He was
separated from the Navy in 1948 and came to M.I.T. in
that year.
He received an S. B. from M.I.T. in>1952.
His professional experience includes summer work
with Geophysical Services Inc., Atlantic Refining Co.,
and California Company in the field of geophysics.
Part time experience has been varied including employment as a teacher and electronic trouble shooter.
He was elected to the Sigma X1 in 1955 and is a
member of the A.G.U. and E.A.E.G.
Upon graduation he
plans to work with Geophysical Services Inc.
I