A neutron tube with constant output 10E10 n per sec for activation analysis and reactor applications

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N U C L E A R
I N S T R U M E N T S
AND
M E T H O D S
33 ([965)
283-288;
©
N O R T H - H O L L A N D
P U B L I S H I N G
CO.
A NEUTRON TUBE WITH CONSTANT OUTPUT (101° n/see)
FOR ACTIVATION ANALYSIS AND REACTOR APPLICATIONS
J. E. BOUNDEN, P. D. LOMER and J. D. L. H. WOOD
Services Electronics Research Laboratory, Baldock, Herts, England
Received 22 September 1964
A compact source of fast neutrons capable of producing well
over 1010 n/see has been developed. The tube, which uses the
D-T reaction consists of an r.f. ion source from which a mixture
of deuterons and tritons are accelerated onto a loaded erbium
target. A life of at least 100 hour has been achieved. The design
features which lead to this high output and long life are discussed.
The application of the tube to reactor instrumentation and
activation analysis is briefly described.
1. Introduction
C o o n 5) with very thick t i t a n i u m targets having a halflife o f 600 p A h o u r / c m 2.
A large i m p r o v e m e n t can be o b t a i n e d by using a
mixture o f d e u t e r i u m and t r i t i u m in b o t h target a n d
ion source. Then the percentages o f t r i t i u m a n d
d e u t e r i u m in the target should remain u n c h a n g e d by
ion b o m b a r d m e n t over very long periods. Experiments
confirming this were carried o u t by C o o n 5) using the
D(d,n) reaction ; the n e u t r o n o u t p u t r e m a i n e d c o n s t a n t
t h r o u g h o u t e x p e r i m e n t s in which a total o f 3 C / c m 2
b o m b a r d e d the target. The use o f a mixture o f deuterium a n d t r i t i u m is p r e c l u d e d in a c o n t i n u o u s l y
p u m p e d accelerator because o f the a t t e n d a n t health
h a z a r d a n d the large a m o u n t o f t r i t i u m used. In a
The present p a p e r describes a sealed-off fast n e u t r o n
source which gives a c o n t i n u o u s yield o f 1010 n e u t r o n s /
sec for over 100 hour. T h e source, which uses the D-T
r e a c t i o n (D + T ~ n + He 4 + 17.6 MeV) is simple
a n d c o m p a c t c o m p a r e d with an a c c e l e r a t o r a n d has the
m a j o r a d v a n t a g e o f a c o n s t a n t o u t p u t over a long
period. The stable n e u t r o n o u t p u t a n d reliability in use
are p a r t i c u l a r a d v a n t a g e s in the a p p l i c a t i o n o f the
source to i n d u s t r i a l p r o b l e m s r e q u i r i n g routine activation analysis, such as the d e t e r m i n a t i o n o f oxygen in
steel, a n d to the p r o d u c t i o n o f short-lived isotopes.
T h e n e u t r o n o u t p u t m a y also be m o d u l a t e d over a
wide range o f pulse lengths a n d repetition frequencies,
as required for m o r e s o p h i s t i c a t e d reactor experiments1). F o r these p u r p o s e s it m a y be assembled into a
6" d i a m e t e r c o n t a i n e r for insertion in the reactor.
The main a d v a n c e offered by the present source over
previous sealed-off sources 2-4) is an increase in neut r o n o u t p u t by a factor o f from 20 to 100 to b r i n g it
within the levels o f interest for activation analysis.
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2. Design factors
T h e design c r i t e r i a for a neutron source using the
D - T reaction are d e t e r m i n e d largely by the efficiency
o f the reaction as a function o f the ion accelerating
voltage and by the a s s o c i a t e d target p r o b l e m s o f life
and p o w e r d i s s i p a t i o n . These c r i t e r i a are the same for a
c o n t i n u o u s l y - p u m p e d accelerator or a sealed-off
n e u t r o n tube except in c o n s i d e r a t i o n o f target life. In
o r d e r to achieve n e u t r o n o u t p u t s o f I0 ' ° n/see, ion
c u r r e n t s o f at least 200-300 ttA are needed. This represents a flow o f gas at a rate o f several cc at N T P
per hour. Thus in a t i m e o f the o r d e r o f an hour, a 1
c m 2 t r i t i u m - l o a d e d target will be d i l u t e d with d e u t e r i u m
from the ion source, sufficiently to halve the n e u t r o n
o u t p u t . This has been confirmed by all users o f accelerators, the best results being those o b t a i n e d by
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ENERGY
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(keV)
Fig. 1. Calculated neutron yield from a thick erbium target as a
function of the ion beam energy. Ion beam composition 50 ~ D! ~
5 0 ~ Tl +. Target composition Er D I T 1.
283
J.E. BOUNDEN eta].
284
diameter and an effective target area of s,, diameter
has been designed for use with this.
3. Source configuration
The neutron tube, fig. 3, operates at a uniform pressure ( ~ 1510 of a gas mixture of deuterium and
tritium and consists essentially of a 1 mA ion source
and a 100-150 kV accelerating gap. The gas is supplied
from a heated titanium replenisher and the pressure
measured by a pirani gauge. The ions are accelerated
onto an erbium target and although most secondary
electrons are suppressed, there are sufficient backstreaming electrons to require an electron "backstop"
in the ion source to absorb the power dissipated by
these electrons.
In order to produce a high proportion of atomic
rather than molecular ions of hydrogen isotopes and
so obtain maximum efficiency, an r.f. ion source is
used operating at 15 Mc/s. lons are extracted from the
plasma through the central hole in the extractor by
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120
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ION BEAM ENERGY (keY)
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Fig. 2. Neutron yield per joule of ion beam power dissipated at
the target plotted vs ion energy for the conditions given in fig. 1.
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sealed-off tube, however, this solution can be adopted
without difficulty.
Erbium has been selected as the target material in
view of its high thermal stability6). The calculated yield
from a thick erbium target containing a 50/50 D-T
mixture, when bombarded by monatomic ions of the
same mixture is plotted as a function of ion energy in
fig. 1. In addilion to yield efficiency there is another
consideration in determining the optimum acceleraling
voltage, namely heat dissipation at the target. In fig. 2
the ratio of neutron yield to ion beam po ~¢er dissipated
in the target is plotted against ion beam energy for
the same conditions. The efficiency is a maximum at
210 keV but only falls off appreciably below 150 keV.
Thus the accelerating voltage can be limited to 150 kV
in ordcr to minimise the insulation problems without
significant loss in efficiency. In prac:ice a neuCron
source giving 10 ~° neutrons/sec will require an ion
current of from 0.5 up to 1 mA at 150 kV.
The size of target used is determined by the requirements of activation analysis in which the maximum
flux is required through the sample being investigated.
The tube to be described is based on a sample of 5 ir
INSULATOR
EXTRACTOR
DISC ~
SHIELD
SHIELD
INSULATOR
EXTRACTOR
ION SOURCE
ENVELOPE
VITREOUS
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Fig. 3. Neutron generator type L.
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Fig. 4. H.T. circuit.
applying a high negative potential to the target shield
with respect to the extractor. A high proportion of
atomic ions can only be achieved with an r.f. ion source
if it is designed with inside surfaces having low coefficients for the recombination of neutral gas atoms
impinging on them. This is in order that the dissociation
of the gas by the discharge may lead to the build-up
of a high concentration of neutral gas atoms, which
when subsequently ionized, will result in a high yield
of atomic ionsT). With the exception of aluminium,
metals in general have high recombination coefficients, whereas glasses have low recombination co-
CONSTANT
OUTPUT
285
efficients, hence in the present design every effort has
been made to keep the plasma away from metal
surfaces. The extractor has therefore been screened
with an extractor shield and also a coating of vitreous
enamel has been fired onto its conical surface facing
the plasma. Another reason for screening the metal
surfaces is that they will be rapidly sputtered by ion
bombardment from the plasma. The sputtered material
would not only increase the atom recombination rate
but would also act as an ion pump.
The present ion source does not rely on probe and
canal type extraction of the ionsS), instead the electron
backstop and the extractor are at the same potential
and the plasma is allowed to diffuse into the central
hole in the extractor. The target shield and extractor
geometry have been designed to eliminate vacuum
sparking 9) and yet not enter the region of Paschen
breakdown 1o) as previously discussed2). The extractor
and the target shield fit fairly closely inside the glass
shield insulator in order to shield glass to metal seals
from voltage stresses and also to ensure that there are
no long path lengths between electrodes along which
Paschen breakdown would preferentially occur.
A negative potential of about 400 V is applied to the
target shield, which is insulated from the target (fig. 4).
This target bias voltage serves to suppress secondary
electrons liberated at the target by the impinging ions
and some of the electrons from the ionization which is
produced in the region between the target and the
target shield by the ion beam. Electrons, which are
drawn from this ionized region and accelerated back
Fig. 5 (a) Neutron generator type L.
286
J.E. BOUNDEN et al.
into the ion source are stopped at the end of the ion
source by a molybdenum pressing, which is liquid
cooled to absorb the power dissipated. The main components of the tube are constructed of Nilo K and
Kodial glass. The tube is illustrated in fig. 5a.
plasma increases. The yield also increases with the
accelerating voltage, at the rate which would be
expected from the theoretical yield voltage curves.
Sources of the present type have produced yields
of 10 l° DT neutrons/sec for periods of the order of
100 hour. The neutron output remains constant
4. Operating characteristics
The neutron source is operated in a sealed container,
an experimental version of which is shown in fig. 5b.
The main purpose of the container is to surround the
high voltage region of the tube with insulating oil, and
also to immerse the ion source in a suitable fluid
which is used to conduct away the heat dissipated from
the ion source plasma. The container also prevents the
escape of tritium should the tube envelope fracture.
At the target end, the container has facilities for liquid
cooling the target. Owing to the high voltage of the
target this fluid must be a good electrical insulator and
the halogenated hydrocarbon Arcton 113 fluid has
been used. Normal operating conditions are as follows:
TABLE I
Pressure:
Ion source power:
Target voltage, VT:
Tube current, 1A:
Target current, IT
Shield current, Is:
Target bias voltage:
Neutron output:
15 # Hg
500 W into r.f. oscillator
100-120 kV
1.6 mA
0.2 mA
1.8 mA
400 V
1010 neutrons/sec
The r.f. power coupled into the discharge is about
200 W. The directions of the positive currents under
normal operating conditions are as shown by the
arrows in fig. 4. The target current I T corresponds to a
negative current going to the target from the ion
beam. This means that in fact the target is collecting
more electrons produced by ionization of the gas inside
the target shield than positive ions from the ion beam.
The slow positive ions produced concurrently with
the electrons by ionization inside the shield are collected by the shield and constitute most of the shield
current I s . The ion beam which has a diameter of
approximately ~5 t! at the target is not intercepted appreciably by tile target shield.
The effect of the target bias voltage is shown in
fig. 6. The decrease of the X-ray yield from the tube
as the bias voltage is increased plainly demonstrates
the suppression of high voltage backstreaming electrons. The neutron producing efficiency of the source
increases as the power delivered to the oscillator is
increased, as shown in fig. 7. This is due to an increase
in the relative atomic ion yield as the power in the
Fig. 5 (b) Experimental container for the neutron source.
287
A N E U T R O N TUBE WITH CONSTANT O U T P U T
throughout the life of the tube and a typical 6.5 hour
continuous run is shown in fig. 8. During activation
analysis work rapid switching of the tube is also possible as illustrated in fig. 9. When filled solely with
deuterium and operated under the conditions specified
in table 1 the source will produce 108 D D neutrons/see.
cies ranging from 0.0167 c/s to 5 kc/s, corresponding to
pulse lengths ranging from 30 sec to 100/~s. Examples
of the pulse shapes are shown in fig. 10. The fluctuations in pulse height more specially noticeable at the
higher modulation frequencies, are due to statistical
fluctuations in the detector.
For modulation frequencies less than 1 c/s, restriking
of the discharge at the beginning of the r.f. pulses is
intermittent when a 100~ depth of modulation is used.
This can be overcome by reducing the depth of modulation to 9 0 ~ , i.e. by maintaining a weak r.f. discharge
in between the main r.f. pulses.
Since the neutron yield is a sensitive function of the
proportion of atomic ions formed by the ion source,
the neutron yield risetime will be determined by the
rate at which the gas in the ion source can be redissociated at the beginning of each pulse. This time is
normally 1-2 ms ~~). Rise times of this order have been
obtained, for modulation frequencies less than 0.5 c/s
in the present case.
As the modulation frequency is increased from 0.5
c/s, this effect will become steadily smaller since there
is insufficient time between pulses for appreciable recombination. Thus at the higher modulation frequencies the rise time is limited to a few microseconds by
the breakdown characteristics of the r.f. discharge.
5. Modulation of the neutron output
6. Applications
For some applications a modulated neutron output
is required. In particular for some nuclear reactor
studies square wave modulation of the neutron output
with risetimes less than one tenth of the neutron pulse
length are required.
By modulation of the r.f. power to the ion source, it
has been possible to achieve this with the present
neutron source for square wave modulation frequen-
Compared with accelerators the present neutron
source has the major advantage of providing a constant
(~)IT
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X-RAY
/
YIELD
(ARBITRARY
UNITS)
XIRAY
YIELD
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PRESSURE 2 0 M I C R O N S
R F O S C I L L A T O R POWER
300
20'
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×
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400
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TO T A R G E T S H I E L D ( V O L T S )
-20
Fig. 6. Effect of the target bias voltage on Is,I T and the X-ray yield
due to backstreaming electrons.
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4
5
OPERATING TIME (HOURS)
Fig. 8. Illustration of output stability when operating for long
periods.
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200
POWER
1
300
SUPPLIED
I
400
TO
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500
RE
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600
oSCrLLATOR
I
700
I
800
(WATTS)
Fig. 7. Relation between neutron source efficiency and power
supplied to the r.f. oscillator.
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6
5
4
3
'r ~M F. (MINUTES)
Fig. 9. Illustration of rapid switching of neutron output.
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288
J.E. BOUNDEN et al.
uniform over a ~ inch disc and is efficiently used.
T h e neutron o u t p u t r e m a i n s c o n s t a n t for long periods
so t h a t the a c t i v a t i o n analysis technique is no longer
confined to i m p u r i t i e s which p r o d u c e short-lived isotopes b u t can be e x t e n d e d to isotopes with h a l f lives
o f m a n y hours. This provides an i m p o r t a n t and considerable extension o f the fast neutron activation
analysis technique.
t ~
I
c/s
- -
r
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Fig. 10. Neutron pulse shapes for square wave modulation.
N = neutron yield/sec; t = time.
stable o u t p u t over a p e r i o d o f 100 h o u r or more. T h e
source is also c o m p a c t a n d easily p o r t a b l e being
suitable for i n d u s t r i a l use. I n its sealed-off f o r m there
is no t r i t i u m h a z a r d .
T h e source has f o u n d two a p p l i c a t i o n s , one for
r e a c t o r s a n d the o t h e r for a c t i v a t i o n analysis. The m a j o r
a d v a n t a g e o f the source in r e a c t o r use is its size, which
permits its insertion into a 6" d i a m e t e r hole, a n d the
high o u t p u t , which m a y be m o d u l a t e d . This o u t p u t is a
factor o f 100 greater t h a n t h a t p r o v i d e d by previous
sealed-off n e u t r o n tubes for r e a c t o r a p p l i c a t i o n s . T h e
a p p l i c a t i o n to activation analysis is o f wider significance a n d the main a d v a n t a g e in this case is the
c o n t i n u o u s stable o u t p u t coupled again with the c o m pactness o f the source. This p e r m i t s the use o f relatively
simple shielding facilities such as an eight foot diameter water t a n k , o r a hole in the g r o u n d c o n t a i n i n g
the tube. The tube has been used for a c t i v a t i o n
analysis with samples in the f o r m o f g5 t! dia, discs
which can be inserted to within 3" o f the back face o f
the target by means o f a p n e u m a t i c transfer system.
Since n e u t r o n s are p r o d u c e d over a ~" d i a m e t e r section
o f the centre o f the target, the n e u t r o n flux is fairly
7. Conclusions
A c o m p a c t sealed-off n e u t r o n tube has been developed c a p a b l e o f p r o v i d i n g a c o n s t a n t stable o u t p u t
of 101° neutrons/sec for over 100 hour. The neutron
tube is being a p p l i e d to reactor i n s t r u m e n t a t i o n and
also to activation analysis problems. The tube is
expected to find p a r t i c u l a r a p p l i c a t i o n where routine
analysis is required.
Initial e x p e r i m e n t s on this tube were carried out by
Dr. W. S. Whitlock.
T h e c o n t r i b u t i o n s o f Mr. D. W. D o w n t o n in the
testing a n d Mr. D. J. T a y l o r in the cons ruction o f
these tubes are gratefully a c k n o w l e d g e d . This p a p e r is
published by permission o f the M i n i s t r y o f Defence.
References
1) T. E. Stern, A. Blaquiere and J. Valat, Reactor Sci. and
Techn. 16 (1962) 499.
2) p. D. Lomer, J. D. L. H. Wood and R. C. Bottom[ey, Nucl.
Instr. and Meth. 26 (1964) 7.
3) O. Reifenschweiler, Nucleonics 18 (1960) 69.
4) B. J. Carr, Nucleonics 18 (1960) 75.
5) j. H. Coon in Fast Neutron Physics, Part I (Interscience 1960)
p. 677.
6) R. Redstone and M. C. Rowland, Nature 201 (1964) 1115.
7) C. C. Goodyear and A. von Engel, Proc. Phys. Soc. 79 (1962)
732.
8) C. D. Moak, H. Reese and W. M. Good, Nucleonics 9 (1951)
18.
9) W. D. Kilpatrick, UCRL Report No. 2321 (1953).
10) A. von Engel, Ionized Gases (Oxford, 1955), p. 172.
l J) A. C. Riviere, private communication.