Nuclear Spectroscopy

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0.8
which is coincident with the gamma from the 226 Ra
decay. An enriched source will have a much larger
peak at 185 keV than you have observed for the natural
uranium ores.
Activity
0.6
0.4
Pb-214
Bi-214
Exercise 13.2
Gamma Spectra from Materials
Containing Radium and Radon
0.2
ratio
(Bi/Pb)
0
0 50 100 150 200 250 300
Time (min)
Figure 13.3 Uranium daughter activities as a function of
time.
photopeaks with energies above 400 keV in your spectra?
Refer to Figures 13.1 and 13.2 to guide your
analysis.
If the isotopes of a decay series, such as uranium, are
left undisturbed, then the series approaches an equilibrium
(see Figure 13.3) with the property that the
activities of each isotope are the same (See the discussion
in Appendix C). Using your measured net counts,
the gamma decay factors from Appendix E, and the
detector efficiency of Figure 10.1, calculate the ratio of
measured activities relative to the activity of the 352
keV photopeak from 214 Pb. The detector’s solid angle
cancels in all ratios, so the use of the efficiencies from
Figure 10.1 is valid for these calculations. Are the
activity ratios of the gamma intensities from 214 Bi
consistent with each other? Are the activity ratios from
gammas emitted by 214 Pb and 214 Bi (and any other
observed isotopes) consistent with each other, and
should they be so? What are the statistical limitations
to your net photopeak counts? If any photopeaks lie
on Compton edges or overlap possible backscattering
peaks, then your measurements of the net counts in
those peaks is very suspect.
If you located and observed an enriched uranium
source, the signature gamma for 235 U decay series is
the 185 keV gamma (from 54% of all 235 U decays),
OBJECTIVE
Observe gammas from sources containing radium and
radon. Radon, an inert gas, can diffuse easily through
many materials, disrupting the equilibrium activities
of the rest of the decay series. Not all radium sources
will show good series equilibrium, even if the sources
are old enough for equilibrium to exist.
SUPPLIES
•Radon charcoal canister
•An old clock or watch with a luminous (radium) dial
•Brazil nuts (two pounds, shelled)
SUGGESTED EXPERIMENTAL PROCEDURE
1. Start the MCA and check your calibration, as in
#13.1.
2. Acquire and save a good spectrum from each
radium/radon sample.
3. Correct the spectra for background radiation.
4. Place the charcoal canister in a basement with
poor or no ventilation for a few days. Retrieve
the canister and acquire a spectrum.
5. Finely chop the Brazil nuts in a blender and store
them in a plastic bag. Remove the detector from
its shielding and cover it with a plastic bag. Surround
the detector with the nuts, and surround
the entire assemblage with lead or iron shielding.
Acquire a spectrum from the nuts.
DATA ANALYSIS
Record the photopeak energies and net counts for all
significant peaks that you observe in your spectra. The
Brazil nuts and the clock contain the radium isotope,
226
Ra. Are the spectra of the two radium sources
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