Answer to Question #9433 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:


I am attempting to do a direct measurement of uranium in field samples using gamma ray spectrometry. Can I use the 186 keV energy measurement from 226Ra to estimate the parent uranium?

The short answer to your question is "Yes," but with some cautions.

Naturally, the radium must be in secular equilibrium with the uranium in order to use the radium activity as a measure of the uranium activity, and the uranium concentration and sample size must be sufficient to allow statistically valid counting results. In addition, it may be necessary to correct the counts in the 186 keV region of interest to account for the contribution from naturally occurring 235U, which emits 185.7 keV gamma rays in about 54 percent of the 235U decays.

The 235U is present to the extent of only about 0.7 atom percent in natural uranium but, because of the shorter half-life of 235U (7.04 x 108 years) compared to 238U (4.5 x 109 years) and the higher 185.7 keV gamma yield (54 percent compared to the 3.3 percent yield of the 186.2 keV gamma ray from 226Ra), the 235U counts will typically be a significant fraction of the counts observed in the 186 keV region. It is possible to correct for this contribution under the assumption that the fraction of all uranium that 235U represents in the sample is known. You can find this discussed in various sources. 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.

Good luck.

George Chabot, PhD, CHP
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