Uranium Series Disequilibrium and Precision Measurement of Radionuclides Activity in Sediment Sample Using Low Background Gamma Spectrometry

Abstract

Low background gamma spectrometry was used to measure the radionuclides activity of 238 U, 232 Th, and 235 U series as well as 40 K and 137 Cs in a sediment sample. The goal of the study was to measure the 238 U (63.3 keV peak of 234 Th; 1001 keV peak of 234m Pa) and 235 U (143.76 keV, 163.33 keV, and 205.31 keV peaks) activity by low background gamma spectrometry in sediment sample. 235 U activity in environmental samples is difficult to accurately measure by gamma spectrometry due to its low abundance in nature and low gamma line intensities at 143.76 keV, 163.33 keV, and 205.31 keV. We have shown that by using low background gamma spectrometry, it is possible to accurately measure the 235 U activity in sediment samples. The 235 U activity was measured without using the major peak of 185.7 keV (I γ = 57.2%) which requires interference correction from 186.21 keV of 226 Ra. 226 Ra activity was determined by measuring 222 Rn daughters ( 214 Pb and 214 Bi). The precision and accuracy of the gamma activity measurement in the sediment sample were verified by using the HPGe detectors with Certified Reference Material (CRM) Irish Sea Sediment (IAEA-385). The results obtained for the 63.33 keV energy line of 234 Th are compared with the 1001 keV energy line of 234m Pa. The values of 238 U and 235 U activities, as well as 40 K, 137 Cs, and 226 Ra, agreed with the certificate values of CRM. The results show that the 238 U is in equilibrium with its daughters ( 234 Th, 234m Pa, and 210 Pb). 232 Th is also in equilibrium with its daughters ( 228 Ra, 212 Pb, 212 Bi and 208 Tl). 235 U/ 238 U activity ratio of 0.046 ± 0.