Nuclear Spectroscopy

SUGGESTED EXPERIMENTAL PROCEDURE
1. Start the MCA, using the calibration of the previous
lab exercise. Check the calibration using the
137
Cs source. The photopeak should be centered
near 661.6 keV. If it is not within 10 keV of the
accepted value, you may need to recalibrate your
system. Be certain that your amplifier gain and/
or detector bias voltage are such that the energy
scale extends about 500 keV past the expected
energy of the sum peak.
2. Place the 22 Na source on the third shelf under the
detector and acquire a spectrum until the net
count in the sum peak is greater than 1,000.
Record the gross count and net count of the sum
peak and each gamma photopeak along with the
live time.
3. If there is enough time, repeat step 2 for 60 Co.
DATA ANALYSIS
From your data of photopeak counts and live time
calculate the net count rate and the statistical error
for each measurement made. Calculate the ratio of
observed activities, as in equation 10.4, to determine
the absolute activity of the source. If the source is
previously calibrated, compare your measurements
to that of the manufacturer’s, after correcting for
decay from the date of calibration. Even uncalibrated
sources have a nominal activity noted by the manufacturer.
Assuming this activity to be correct, determine
the date of the source’s preparation. If you have used
the same 22 Na source in exercises 10.1 and 10.2, your
values should agree within statistical errors.
Determining the absolute activity of extended
sources, especially environmental samples, is
very challenging. The source to detector distance
varies over the sample, as does the effective
detector area. This difficulty is often compounded
when a thick source self-shields itself
from the detector. The problem is challenging
but not impossible. Exercise 11.1 asks you to
solve these problems for a very interesting measurement.
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