Nuclear Spectroscopy

Figure 5.3. Spectrum for the β − decay of
24 Na. Note the escape peaks from the
2,754 keV gamma and the 511 keV
gammas from the positron creation and
annihilation.
OBJECTIVE
Observe the 511 keV gammas from positron annihilation
and observe the escape peaks after pair production.
SUPPLIES
•NaI(Tl) detector with MCA
•Radioactive sources: 22 Na, 65 Zn, 88 Y/ 88 Zr, 137 Cs,
144
Ce, 228 Th/ 232 Th
SUGGESTED EXPERIMENTAL PROCEDURE
1. Start the MCA and check the calibration using
the 137 Cs source. Your energy scale must extend
at least out to 2,800 keV.
2. Place another source on the fourth shelf under
the detector. Acquire a spectrum for a period
long enough to observe the annihilation gammas
and any escape peaks from the high-energy gammas.
Record the energies of the observed
photopeaks.
DATA ANALYSIS
The 22 Na, 65 Zn sources all have positron decay channels,
while the 88 Y/ 88 Zr, 144 Ce, and Thorium sources
emit high-energy gammas with energies greater then
1,700 keV. What energies do you observe for the
annihilation photons and the escape peaks? Do your
results make good physical sense of the situation?
Why are there no escape peaks visible in the 22 Na,
spectrum? The answer lies in the cross section graph
of Figure 6.2.
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