Development of High Resolution Counting Techniques for Body Radioactivity Measurements

[摘要] In vivo whole- and partial-body counting has been dominated by scintillation detectors because of their high counting efficiency for gamma-rays. However, large semi-conductor detectors are now becoming available and it is logical to apply them to body counting in order to take advantage of their very high resolution since they can give usable efficiencies. With the increasing interest in low-energy gamma and X- ray emitters such as plutonium and americium in the body, semi-conductor detectors namely the Low Axial - to diameter length- known as LOAX (a product name of EG&G Ortec), specifically designed for this energy region, can also be used for these measurements. Efforts to measure internally deposited transuranic elements in vivo have largely been concentrated on the accurate detection of the radionuclides in various parts of the body. This requires that the detectors are reliable and adequately sensitive. As an integral part of this study a great deal of effort has been spent on the modification of the shadow-shield whole-body counter at SURRC. The shadow-shield was reorganized to accommodate up to four n-type LOAX detectors and one 80% p-type HPGe-detector along with the two NaI(T1) detectors already present. This detector arrangement has transformed the monitor into a "hybrid" detection system capable of measuring low, intermediate and high energy gamma-emission from radionuclides. This research work has investigated the detection characteristics of the hyper-pure germanium detectors in the shadow-shield monitor as an improved method for quantitative measurement of internally deposited radionuclides. It provides a comprehensive background data analysis of the shadow-shield arrangement and discusses the various methods of background prediction for the HPGe-detectors used. The detection capability of the "hybrid" counter has been thoroughly examined by measuring standard sources and a number of different types of anthropomorphic phantoms. Specially made tissue equivalent phantoms labelled with various activities of isotopes of 241Am, 210Pb and 238U have been measured and used for the calibration. Combining the results of the calibration factors in terms of sensitivity, minimum detectable activity (MDA) and the scattering contribution of human subject measurements a number of optimum methods for data analysis have been presented for HPGe-detectors in the shadow-shield arrangement and discussed. To establish the usefulness of the equipment for body measurement, the MDA is what is of interest. The main criterion tried for carrying out data analysis of the small (low counts) peaks expected was to see whether the collected counts constituted a peak above established background or not. The utilisation of the available computer programs was not very helpful in this respect and the straight-forward method of visually comparing the spectrum with background from controlled subjects was examined. Using this method, the MDA taking 2sigma as the definition, corresponded to 3 Bq for 241Am 59.5 keV for a one hour count and to 10 Bq for 210Pb. Other methods were also used to define the exact region of interest for the low energy peaks, for example, the counts within each peak were manually calculated and their respective activity and MDA were tabulated. The results of special measurements of four human subjects that were known to contain various levels and types of the radioactivity of interest due to their various life styles, are presented. Also a number of whole dead sheep and various bones collected from decayed animals in the South West of Scotland area have been measured and found to contain a significant level of americium radionuclide mainly due to contamination by Sellafield discharges and lead radionuclide due to the presence of natural uranium decay products. The final reported activity levels were determined using calibration factors that were calculated using their appropriate phantom "internally" deposited activities. An activity of as low as 4 Bq was determined in one subject and as high as 100-150 Bq of 241Am in sheep and bone samples from the South West Scotland. These final results have proved the applicability and the considerable sensitivity of the detection system in the actual detection of real cases of internally deposited low-level and low-energy radioactivity. A number of interesting and valuable in vivo gamma-ray spectra obtained from the measurements of human subjects as well as soil samples showing various activities of 241Am, 210Pband other radionuclides are presented in the appendix A.

[发布机构] University:University of Glasgow

[关键词] Nuclear physics and radiation, Biophysics