Measurement of caesium-137 in the human body using a whole body counter
Elessawi, Elkhadra Abdulmula
Gamma radiation in the environment is mainly due to naturally occurring radionuclides. However, there is also a contribution from anthropogenic radionuclides such as 137Cs which originate from nuclear fission processes. Since 1986, the accident at the Chernobyl power plant has been a significant source of artificial environmental radioactivity. In order to assess the radiological impact of these radionuclides, it is necessary to measure their activities in samples drawn from the environment and in plants and animals including human populations. The whole body counter (WBC) at the University Hospital of Wales in Cardiff makes in vivo measurements of gamma emitting radionuclides using a scanning ring of six large-volume thallium-doped sodium iodide (Nal(Tl)) scintillation detectors. In this work the WBC was upgraded by the addition of two high purity germanium (HPGe) detectors. The performance and suitability of the detection systems were evaluated by comparing the detection limits for Cs. Sensitivities were measured using sources of known activity in a water filled anthropomorphic phantom and theoretical minimum detectable count-rates were estimated from phantom background pulse height spectra. The theoretical minimum detectable activity was about 24 Bq for the combination of six Nal(Tl) detectors whereas for the individual HPGe detectors it was 64 Bq and 65 Bq, despite the much improved energy resolution Activities of 137Cs in the human body between 1993 and 2007 were estimated from the background Nal(Tl) spectra of 813 patients and compared with recent measurements in 14 volunteers. The body burden of Cs in Cardiff patients increased from an average of about 60 Bq in the early and mid 1990s to a maximum of about 100 Bq in 2000. By 2007 it had decreased to about 40 Bq. This latter value was similar to that of Cardiff residents at the time of the Chernobyl accident and to that of the volunteers measured in 2007 (51 Bq). However, it was less than the mean activity of Cardiff residents in 1988 (130 Bq) indicating an overall decrease over a period of about 20 years. The variation in the in vivo activity is probably due to complex inter-relationships between a number of factors such as the removal of deposited 137Cs into the sea by rainfall, individual dietary choices, the imposition and removal of restrictions on foodstuffs from Chernobyl-affected areas and travel to countries that suffered greater initial fall-out than the UK.
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