TITLE:
Uranium Series Disequilibrium and Precision Measurement of Radionuclides Activity in Sediment Sample Using Low Background Gamma Spectrometry
AUTHORS:
Abdul J. Khan, Umme-Farzana Syed, Amy L. Roselan, Cynthia A. Costello
KEYWORDS:
Low Background Gamma Spectrometry, HPGe Detector, Sediment, 238U, 235U, Secular Equilibrium
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.11 No.10,
October
23,
2023
ABSTRACT: Low background gamma spectrometry was used to measure the radionuclides
activity of 238U, 232Th, and 235U series as
well as 40K and 137Cs in a sediment sample. The goal of
the study was to measure the 238U (63.3 keV peak of 234Th;
1001 keV peak of 234mPa) and 235U (143.76 keV, 163.33
keV, and 205.31 keV peaks) activity by low background gamma spectrometry in
sediment sample. 235U 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 235U activity in sediment
samples. The 235U activity was measured without using the
major peak of 185.7 keV (Iγ = 57.2%) which requires interference correction from 186.21 keV of 226Ra. 226Ra activity was determined by measuring 222Rn
daughters (214Pb and 214Bi). 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 234Th
are compared with the 1001 keV energy line of 234mPa. The values of 238U
and 235U activities, as well as 40K, 137Cs, and 226Ra, agreed with the certificate values of CRM. The
results show that the 238U is in equilibrium with its daughters (234Th, 234mPa, and 210Pb). 232Th is also in
equilibrium with its daughters (228Ra, 212Pb, 212Bi
and 208Tl). 235U/238U activity ratio of 0.046
± 0.007 in the sediment is constant in nature
but fluctuates due to geological processes. A value of 0.055 ± 0.008 was
found in our sediment sample.