We describe an undergraduate physics laboratory experiment that uses a 2-inch diameter NaI gamma detector to measure natural radiation in soils. Students first calibrate the detector for the source-detector geometry, energy-dependent detection efficiency, and sample self-absorption. Then, the activity of the 238U and 232Th decay series, as well as the activity of 40K in the soil, are measured. The results are comparable in accuracy to those obtained using a high-resolution germanium gamma detector.
REFERENCES
1.
P. B.
Siegel
, “
Gamma spectroscopy of environmental samples
,” Am. J. Phys.
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, 381
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(2013
).2.
Data acquisition with the 2-inch NaI gamma detector is done with Digibase from Ortec. The acquisition software produces a 1024 channel *.Chn file, which we read and analyze with our own software.
3.
The isotope decay data and disk source data were obtained from the National Nuclear Data Center at Brookhaven National Laboratory. The data are available at the following web site: <https://www.nndc.bnl.gov/nudat2/>.
4.
The link to obtain the exponent b from the disk source data is found on our laboratory web page and is <https://www.cpp.edu/~pbsiegel/javascript/disksources.html>. The app initially gives the linear regression result, then an option to do a chi-square analysis.
5.
Byron
Curry
,
Dave
Riggins
, and
P. B.
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Data analysis in the undergraduate nuclear laboratory
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John R.
Taylor
, An Introduction to Error Analysis, 2nd ed.
(
University Science Books
,
California
, 1997
). After taking the logarithm of both sides of Eq. (3), we use the methods of Chap. 8 to solve for the common slope b and the three
values.7.
We solve for the minimum of the function
to determine b and the three
values. The software the students use is found and explained on our laboratory manual for the Radiation Laboratory Course at <https://www.cpp.edu/~pbsiegel/phy432/labman/manual.html>.
8.
G. F.
Knoll
, Radiation Detection and Measurement
, 3rd ed. (
Wiley
,
New York
, 1999
). See Fig. 2.18 for a plot of the energy dependence of gamma-ray interaction processes in sodium iodide.9.
M. S.
Al-Masri
,
M.
Hasan
,
A.
Al-Hamwi
,
Y.
Amin
, and
A. W.
Doubal
, “
Mass attenuation coefficients of soil and sediment samples using gamma energies from 46.5 to 1332 keV
,” J. Environ. Radioactivity
116
, 28
–33
(2013
).10.
Typical values for 232Th and 238U in soils range between 15 and 60 Bq/kg. See
Kekelidze
et al, “
Radioactivity of soils in Mtskheta-Myianeti region (Georgia)
,” Ann. Agrarian Sci.
15
, 304
–311
(2017
) for a detailed analysis of soils in Georgia.11.
Alberi
et al, “
Training future engineers to be ghostbusters: Hunting for the spectral environmental radioactivity
,” Educ. Sci.
9
(1
), 1–14
(2019
).© 2021 Author(s). Published under an exclusive license by American Association of Physics Teachers.
2021
Author(s)
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