Reliability of using 238U/235U and 234U/238U ratios from alpha spectrometry as qualitative indicators of enriched uranium contamination

Alpha spectrometry is a commonly used technique for the measurement of uranium isotopes in environmental samples because it is widely available at a relatively low cost. For natural uranium the U-234 to U-238 activity ratio should be 1 and the U-238 to U-235 activity ratio should be 21.7. However, a lower U-238 to U-235 ratio is usually observed in alpha spectrometric analysis of environmental soil samples. This observation has led to the conclusion that soils from nuclear weapons facilities were contaminated with highly enriched uranium. This study was undertaken to test the reliability of using activity ratios from alpha spectrometry to infer the presence of highly enriched uranium in soil samples. The results of these experiments indicate that the U-238 to U-235 activity ratio is not a reliable indicator, but that the U-234 to U-238 activity ratio can be used to qualitatively indicate the presence of highly enriched uranium at concentrations near 10 ng g(-1) and above.