NEUTRON ACTIVATION ANALYSIS
This experiment has a two-fold objective. One is to determine the percentage
copper in a brass sample from the amount of radioactivity that has been
produced by bombardment with a beam of neutrons, and the other is to
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determine the half-life of the 29 Cu
neutron-induced radioactivity decays.
isotope from the rate at which its
PRINCIPLES
Certain elements, when bombarded with a beam of neutrons, are made radioactive
by a reaction that occurs in the nucleus. In this experiment, you will expose copper and
brass samples to a beam of "slow" neutrons and the radioactivity that is produced will be
used to determine the percentage of copper in the brass sample.
The neutrons are produced by a sequence of nuclear reactions occurring in a small
neutron source encapsulated in stainless steel and immersed in a large tank of water (a
neutron howitzer) to protect the user from the neutrons. The source contains about 80
grams of plutonium-239
239
Pu
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which is a radioactive isotope that decays
spontaneously according to the following reaction:
239
235
Pu
92
94
+
U
4
2
He (an alpha particle)
The alpha particle is then absorbed by the beryllium that is mixed with the
plutonium, leading to the following reaction:
9
4
4
Be +
1
0n
He
2
+
12
6C
The neutrons that are produced have a very high energy (several million electron
volts). These neutrons are slowed down by collisions with hydrogen atoms in the water
and the plastic material surrounding the source.
They are therefore "slow" by the time they reach the samples being irradiated. The
neutrons which are not absorbed by a sample are ultimately absorbed by the water (via a
nuclear reaction with hydrogen atoms) before they have a chance to escape from the
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howitzer. Approximately 107 neutrons per second are produced by the 5 curie source in
our laboratory. Although the neutron source produces radiation at a level that can be
harmful, a consultant has confirmed with exhaustive testing that you will not be exposed
to significant levels of radiation above background. The water bath in which the source is
immersed provides very effective shielding. As an additional precaution, the neutron
source is stored in its own room separated from the counting room by a wall.
Natural copper is a mixture of two isotopes,
63
29
Cu
and
65
29
Cu ,
which interact
with these neutrons according to the equations:
63
Cu
29
65
Cu
29
+
+
1
64
n
29
0
1
0
+ 
Cu
66
n
29
Cu
+ 
The newly formed copper isotopes are unstable and undergo several types of
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decay. The half-life of 29 Cu is about 150 times longer than the half-life of
only the latter isotope that we are concerned with. The decay equation is:
66
Cu
29
0
–-
1
e
+
66
30
66
29
Cu
, and it is
Zn (stable)
The electrons that are produced in this decay process are detected by a radiation
counter and their rate of production is a measure of the number of
66
Cu
29
atoms that are
present at any given instant in the copper or brass sample. The samples used in this
experiment are so thick that any additional thickness would result in no increase in the
counting rate; the additional decay products would be self-absorbed by the sample before
reaching the Geiger-Müeller tube. The thickness or weight of the samples is thus
immaterial as long as the copper and brass samples have the same cross sectional area and
are counted in identical positions relative to the window of the Geiger-Müeller tube.
For analytical purposes it is necessary to find the number of counts per minute
(cpm) corresponding to the instant the samples were removed from the howitzer (i.e.,
corresponding to t = 0). Only this value of cpm will be proportional to the amount of
stable non-radioactive copper in the sample; any later value of cpm would be low, soon
vanishing to zero. Because of the statistical, fluctuating nature of radioactivity, this value
of cpm for t = O can be obtained reliably only by doing a least-squares plot of the data
and then calculating the desired value from the equation of the best-fit line for t = 0.
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You will be able to observe the samples while they are being irradiated but the
instructor will transfer the samples into and out of the howitzer. Since the half-life of
66
Cu
is so short, you must be prepared to begin counting soon after you receive your
radioactive sample.
29
One of the major constituents of brass is zinc, and with neutrons of the energy
used in the activation of your samples, there will be a small amount of activity resulting
from the radioactive decay of
65
Zn
30
65
Zn
30
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produced by the bombardment reaction 30 Zn (n, )
. However, the amount of activity from this source will be so small that it may be
neglected in your experiment.
The theory behind the relationship between half-life of an isotope and its rate of
decay is given in your text.
EXPERIMENTAL PROCEDURE
1.
Determining the Background. The Geiger counter should have been
switched ON in advance of the experiment and permitted to warm up so as to give stable
results. (The POWER should be turned on for one minute before switching ON the HIGH
VOLTAGE and setting it to the voltage indicated on the Geiger-Müller tube.) The
counting time in minutes is set by a combination of the setting of a two-digit rotary dial
and an attenuation switch with settings of 0.1, 1, and 10. For example, if the rotary dial is
set at 47 and the switch at 1, a counting period of 47 x 1 = 47 minutes is established. You
will want to measure the background for 15 minutes so set the dial at 47 and the switch at
1. Initiate the measurement by pressing the small red COUNT button. The red light
above the COUNT button will be illuminated during the measurement and will go out
once the counting period is over. You can halt the process by pressing the small red
HOLD button but if you do, you lose all information on time. Once the measurement
period is over, read the accumulated counts on the LED display. From the number of
counts and the elapsed time, calculate the average number of background counts per
minute (cpm). Once you have completed the measurement of the background, set the
time for 0.8 minutes (rotary dial at 08 and the attenuation switch at 0.1) in preparation for
the next step.
2.
Positioning the Samples for Counting. Two counting sessions are
involved, the first with pure copper and the second with brass. Both are handled in the
3
same way.
When the neutron-bombarded sample is taken out of the howitzer, pick it up with
tweezers, transport it to the Geiger counter with a small tray (provided), and finally
position it in the depression machined for this purpose in the sample holder. With both
samples, the paper label must be on the bottom. Be sure to record the number on the
brass sample label. The lab assistant will have a stopwatch and will count exactly 2
minutes from the time irradiation is stopped. Within this two minute period you must
have reset the timer, have the sample positioned under the window of the Geiger-Müller
tube at its highest position, and be prepared to press the COUNT button on your counter
(see next paragraph).
3.
Counting the Samples. When the two-minute period is up, the lab assistant
will start the lab timer (on the wall) and, simultaneously, you will press the COUNT
button on your counter to initiate counting for 0.8 min. As soon as the counting period
has elapsed, record the total counts along with the corresponding counting interval (0.8
min.) next to t = 2 min in your notebook. Note that when you press the COUNT button,
you reset the LED display to zero. When the minute hand on the lab timer sweeps past
"0" again (corresponding to t = 3 min after completion of irradiation), start another 0.8
min. count by pressing the COUNT button. This process is repeated until t = 25 min after
completion of irradiation.
The lab assistant will irradiate the brass samples while you are measuring the rate
of decay of the pure copper sample, using an irradiation time which is identical to that of
the copper. Count the brass sample in exactly the same way as you did the copper
sample. The same trap position (top tray) is used and the sample is inserted so that any
label is pointing down.
When you have finished counting each sample, use your tweezers to place it in the
container marked for this purpose near the counters. If you touch the sample, don't panic
as copper and brass are not water soluble. However, as a precaution, immediately wash
your hands. A small sink is located in the room.
DATA ANALYSIS
1.
For general discussion and examples, see the section "Curve Fitting and the
Method of Least Squares".
2.
Because the radioactive decay observed in this experiment is first order,
your data of counts per minute (CPM) versus time (T) will fit an exponential curve.
4
Therefore, a plot of the ln(CPM-B), where B is the background counts per minute, versus
T will yield a straight line. From the slope of this line we can determine the "half-life" of
Cu and from the intercept we can determine the initial CPM. The "half-life" is used to
identify Cu as the radioactive species and the initial CPM for the two samples is used to
determine the relative percent of Cu in the brass sample.
The following steps must be completed before leaving the laboratory session for
the day. Your laboratory assistant can help you if you need it.
a)
Data Preparation
Load the spreadsheet program Excel as you did in the Least Squares exercise.
Create two spreadsheets, one for pure copper and another for the brass sample.
Each spreadsheet should list in labeled columns the time at which each count
began, the recorder counts the counting interval in seconds, the observed CPM
(counts ÷ by counting interval in minutes), the CPM corrected for background, and
the natural log of the corrected CPM.
b)
Least-squares analysis of the data.
Using the Regression features of Excel obtain the best fit slope, yintercept, their standard deviations, the correlation coefficient, and the
standard error for both samples. Record these values in your notebook.
3.
The following tasks are to be completed by 5:00 p.m. the day after you have
completed the experimental work.
a)
Obtain a hard copy of the output from Excel. Obtain two hard copy plots,
one for pure copper and one for the brass sample, of ln (CPM-B) versus
time. Each plot should fill a full page.
b)
Once you have obtained a fit to your data, report the requested quantities
on your experimental data sheet. The items to be turned in with your
report sheets include a printout of your Excel spreadsheets.
WES, 30 July 2002, neutron_f02.doc
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Name____________________________ Lab Section________________
Counter Number________________Sample Number____________
Date Report Submitted__________________________
NEUTRON ACTIVATION ANALYSIS
1. Background Counting Rate
___________cpm
2. Pure Copper
Slope of line
____________________
Standard Deviation of the slope
____________________
Y-intercept
____________________
Standard Deviation of the intercept
____________________
Correlation coefficient, R
____________________
Standard error, S
____________________
Half-life, min.
____________________
Initial CPM
____________________
3. Brass Sample
Slope of line
____________________
Standard Deviation of the slope
____________________
Y-intercept
____________________
Standatd Deviation of the intercept
____________________
Correlation coefficient, R
____________________
Standard error, S
____________________
Half-life, min.
____________________
Initial CPM
____________________
4. Percent Copper in Brass Sample
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____________________%
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