Quantifying disequilibrium in U-series decay using high-purity germanium spectrometry

• Main Author: Abdualhadi, S. A.
• Other Authors: Joss, David ; Nolan, Paul ; Mauz, Barbara
• Published: University of Liverpool 2016
• Subjects: 539.7
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.706947

Many of the naturally occurring radioactive elements are members of radioactive de- cay chains. These chains originate from parent nuclides with very long half-lives and end with a stable nuclide of lead. In any natural material containing uranium which was not disrupted, a state of secular equilibrium will occur between parent nuclide and its daughter products. However, when sedimentary deposits are formed, many geological processes can occur which may cause isotopic fractionation of the elements resulting in a state of disequilibrium between the parent nuclide and its daughters in the chain. This study is aimed at the determination of the activity concentrations of radionuclides from U-series decay and hence quantifying possible disequilibrium in 14 sediment samples selected from four different locations. Six samples were collected from Namibia in the South Africa, four samples were from the Cambridge Gulf in Western Australia and the four remaining samples were from Czech Republic and South Germany regions in Europe. This work was carried out with a novel Broad Energy Germanium (BEGe) detector inside a 50mm thick lead shield. The BEGe detector utilises a novel point like electrode structure which through extremely low capacitance (1pF) provides excellent an energy resolution at low energy. This performance is far superior to conventional germanium detectors, which en- ables a potential step change in the ability to resolve low energy peaks on a background. Gamma-ray energy resulting in excellent spectra. Gamma-ray transitions lines ranging from 46 keV up to 1.7 MeV associated with decay products of the 238U and 232Th decay chains have been analysed separately to obtain more statistically significant overall results. In the measurements of environmental samples using gamma-ray spectrometry, the main concern is a reliable efficiency calibration. This is crucial especially for the analysis of low-energy gamma emitters ( < 100 keV) such as 210Pb (46.5 keV) and 234Th (63.3 keV and 92.6 keV). Modelled efficiency calibrations using the LabSOCS (Laboratory Sourceless Object Calibration Software) were applied within this work. A series of validation tests was performed and evaluated for different sample types, densities and volumes. Using this method, an improvement can be obtained in the reliability of the derived activity concentrations. The sample preparation and the gamma-ray spectroscopic analysis technique are discussed in detail. The specific activities of radionuclides from the 238U decay chain ranged from 16.8 ± 2.3 to 80 ± 5 Bq/kg for 234Th, from 14 ± 3 to 98 ± 6 Bq/kg for 226Ra, from 16.8 ± 0.6 to 116 ± 3 Bq/kg for 214Pb, from 16.6 ± 0.7 to 112 ± 3 for 214Bi and from 15.9 ± 2.6 to 114 ± 8 Bq/kg for 210Pb. Six samples were found to be in disequilibrium as a result of an excess in 226Ra activity concentrations. The 226Ra/238U activity ratio in these samples ranged from 1.22 ± 0.13 to 1.77 ± 0.15. Notably, these samples were collected from an area in Namibia where leaching of radium has taken place. In the remaining samples the results showed secular equilibrium. The activity concentrations of 228Ac ranged from 20.6 ±1.0 to 60.6 ± 2.0 Bq/kg, of 212Pb from 20.0 ± 0.9 to 59.7 ± 2.5 Bq/kg and of 208Tl from 21.1 ± 1.0 to 58.6 ± 2.4 Bq/kg, from 232Th decay chain. All results show secular equilibrium for this decay chain. The results obtained for the uranium and thorium concentrations some of the investigated samples are consistent with mass spectrometry (ICP-MS) and other gamma-ray spectrometry measurements.