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SCINTILLATION COUNTING
ADVANCE READING: Knoll, pp. 219-225, 231-243, 247-250, 265-272, 276-278, 283-287,
322-323.
HIGH VOLTAGE: Do not exceed 1300 V.
PROCEDURE:
During this experiment you will use a photomultiplier tube (PMT). Never expose the
PMT to room lighting with the high voltage turned on. The tube can be destroyed this way!!
Inorganic Scintillators
1. With the help of the lab instructor, assemble the 2" x 2" NaI(Tl) crystal and PMT using optical
coupling compound between the crystal and tube. Enclose the assembly in a light-tight container
and test for light leaks at a low voltage (about 600 V) by switching the room lights off and on
while examining the system output on the scope. Look for changes in the preamp output baseline
noise level as a sign of light leakage. Do this each time you switch to a different scintillator.
2. Observe both the preamp and amplifier output on the scope. Place a 137Cs source near the
crystal and observe the bright trace corresponding to the photopeak. Observe and sketch the
preamp output pulse and measure its rise and fall times. Record the amplifier output amplitude as
a function of tube voltage. Do not exceed the rated maximum voltage. Carefully watch both the
preamp and amp outputs on the scope as the voltage is raised, and be alert for the point at which
the pulses begin to saturate (flat top). Plot the relative tube gain vs. applied voltage.
The PMT has 10 stages (N=10). Assuming that the number of electrons at the anode per
photoelectron (tube gain) is given by G=N, and that the secondary electron yield per dynode, ,
is proportional to Vn, where V is the applied voltage, find the value of the exponent n from a plot
of ln(G) vs. ln(V).
3. Set up the system to record the pulse height spectrum from the scintillator. Display the
spectrum obtained from a 137Cs source, and adjust the amplifier gain so that the photopeak is less
than half scale on the MCA. Increase the gamma ray interaction rate as much as possible by
turning the rod so that the source end is adjacent to the scintillator. Set the MCA display to the
logarithmic scale, and note the appearance of the sum peak at the upper end of the spectrum.
Identify all other recognizable features of the spectrum. From the net areas of the photopeak and
sum peak, estimate the system resolving time from Eq. 10.13 of the text.
4. At a fixed operating voltage (around 800-900 V), adjust the amplifier gain so that the 137Cs
photopeak is at about 45% of the full scale. With the source and detector in an easily
reproducible geometry (~2 inches from the detector face), measure:
a. Average pulse amplitude (channel number) of the photopeak
b. Energy resolution of the photopeak
c. Total number of counts under the spectrum
d. Net counts under the photopeak
REV: MAR 06
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NOTE: For steps 5-8, use the same system settings (high voltage, live time, and source
geometry) as in step 4. Before changing detector crystals, turn off the high voltage!
5. Replace the 2" x 2" NaI(Tl) with the 1" x 1" NaI(Tl). Repeat steps 4a - 4d.
6. Disassemble the system and remove all of the coupling compound (c.c.) from the PMT face
and crystal. Reassemble the system. Repeat steps 4a - 4d.
7. Replace crystal with the 1" x 1" BGO scintillator (with c.c.). Repeat steps 4a - 4d.
8. Replace crystal with the 1" x 1" CsI(Tl) scintillator (with c.c.). Repeat steps 4a - 4d.
9. Make a table comparing the pulse heights, percent resolution, peak-to-total ratio, photopeak
area ratio (normalize to the 1" x 1" NaI(Tl)), and the full spectrum area ratio (again, normalize to
the 1" x 1" NaI(Tl)) for all of the scintillator crystals (including the configuration without
coupling compound). Explain the differences in these table values for each detector crystal.
Consider crystal density, Z, size, light yield, and index of refraction; use of coupling compound;
and spectral compatibility of the crystal with the PMT. Hint: there is a very helpful table in the
reading for this analysis.
Organic Scintillators
10. Now use the 2" x 2" plastic (organic) scintillator (with c.c.) to repeat step 4 (do not adjust
amplifier gain). Why is there no photopeak? Compare the total area (gross area) under the
spectrum with the 2" x 2" NaI(Tl) and estimate the ratio of the detection efficiencies at the energy
of 137Cs for these two scintillators.
11. Record the spectra from several beta sources and estimate the channel number corresponding
to the endpoint of each beta spectrum (see Fig. 1.1 of the text). Note that you will have to enclose
the beta source inside the light-tight housing, and orient the source towards the crystal, in order to
count the betas with a reasonable efficiency. Plot the endpoint channel number vs. the published
endpoint energy and comment on the meaning of this plot. Estimate the activity of at least one
beta source from the recorded data by calculating the solid angle subtended by the detector and
assuming that its intrinsic efficiency is near unity.
12. Observe and sketch the preamp output pulse shape you observe on the scope for any
convenient source. Measure the rise and fall times of the pulse and compare with those measured
earlier for the NaI(Tl) crystal. Comment on the results.
REV: MAR 06