Answer to Question #9431 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

When performing gamma spectroscopy on beach sand, the activity of 226Ra is overestimated because of the interference from 235U. How can we obtain the correct activity? After making a correction, if the activity of 226Ra is still high compared to its daughter products, are there other reasons that cause high radium activity? In one sample, could it be possible that the gamma-ray energy about 186 keV comes from three sources; i.e., 238U, 235U, and 226Ra itself?

A

To make the correction for the contribution of 185.7 keV gamma rays from 235U to the counting region for the 186.2 keV gamma rays from 226Ra, you must know the fractional amount of 235U among all the uranium in the sample. For natural uranium it is commonly assumed that 235U is present to the atom fraction of 0.0072 among all the uranium, 0.99275 being the atom fraction of 238U in “natural” uranium. While these fractions are typical, depending on the origin of the samples of concern, there may be variations in the relative fractions of 235U and 238U. Here is a link to a paper from the Idaho Environmental and Engineering Laboratory. Attachment 1 near the end of the paper describes the 235U correction for samples containing natural uranium with a given nominal percentage of 235U.

The 186 keV gamma ray energy region is limited to rather few radionuclides, 226Ra and 235U being the only significant naturally occurring ones. There are a handful of manmade radionuclides that emit gamma rays around this energy, but they are not nuclides one would expect to find in typical beach sand.

As you are aware, the 238U decay chain includes several high-energy gamma rays, especially those from the 214Pb-Bi progeny. These contribute Compton-scattered photons that result in a continuum of counts under the 186 keV photopeak. You should be sure that whatever software you are using to perform the gamma ray pulse height analysis routines is performing adequately to subtract the Compton continuum counts from the region of interest. Also confirm that the region of interest includes most of the photopeak in the region of interest, but do not make the region of interest overly wide so as to avoid extraneous contributions to the photopeak region. This is usually not a problem if you are using well-tested and accepted gamma ray spectrometry software.

One notable potential interference that I am aware of might occur in some samples that contain measurable 137Cs. Cesium-137, a fission product produced in abundance when atmospheric bomb testing was popular, is present in many environmental samples. Some regions contaminated by specific accidental releases, such as those associated with the Chernobyl incident, may have much higher environmental levels than other areas farther away from the vicinity of deposition resulting from the incident. In samples of notable 137Cs contamination it is possible that 662 keV photons emitted by 137mBa, the daughter of 137Cs, may contribute to counts in the 186 keV region of interest by way of backscattered photons that are produced in the sample and find their way to the detector. The Compton-scattered photon energy, Es, is given by

Es = Eg/[1+(Eg/0.511)(1 – cos T )]

where Eg is the initial gamma energy, and T is the scattering angle. For T = 180o and Eg = 0.662 MeV, Es = 0.1843 MeV. The backscatter peak includes energy deposition associated with photons scattered over a range of angles close to 180o and is rather broad and may contribute counts to the analytical 186 keV region of interest.

There are other possible sources of elevated counts in the region of interest, but they generally require the presence of unlikely radionuclides and involve processes such as sum peaks associated with the simultaneous measurement of two or more gamma rays and/or x-rays whose energies sum to about 186 keV. These do not warrant further speculation here.

I hope this discussion is helpful to you.

George Chabot, PhD, CHP

NOTE: See also Question #9057 submitted to “Ask the Experts” under the category of Environmental and Background Radiation — Soil and Fallout

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