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

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?

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.

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 man-made 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, E_s, is given by

E_s = E_g/[1+(E_g/0.511)(1 – cos T )]

where E_g is the initial gamma energy, and T is the scattering angle. For T = 180^o and E_g = 0.662 MeV, E_s = 0.1843 MeV. The backscatter peak includes energy deposition associated with photons scattered over a range of angles close to 180^o 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.