Radiation Dectection Properties Of A BaFX Imaging Plate And Its Application For Measurements Of Gross Alpha

Objective Public exposure to the natural and artificial ionizing radiation has been increasing due to the social and sciencific developments, and the increased human activities. Furthermore, occasionally occurred serious nuclear accidents which polluted our living environments had caused much stronger public concerns. For accurate evaluations of the health effects of ionizing radiation, the effective monitoring of ionizing radiation is an important prerequisite. As a part of radiation monitoring, measurements of the grossα-activity in our environmental media which might be intaken by human beings and evaluations of the internal doses have been important and difficult tasks in the fields of radiological health and radiation medicine. However, as the limits of adequate measuring methods and equipments, it was still impossible to quickly and accurately measure the gross a-activity in large numbers of environmental media with present measures. Based on the obvious advantages of BaFX imaging plate (IP), such as large detection area and its sensitivity to a-radiation, a new practical method for identification and quantitative measurement of a-radiation by using the IP techniques was studied in this research, in order to meet the increasing demands of gross a-radiation measurements in environmental media.Methods Different spieces of standard radiation sources were used to irradiate the IP in order to grasp the sensitivity and dose linearity of the IP through a series of experiments. By simulating different environmental temperatures, the fading characteristics of different radiation signals stored on the IP were experimentally studied. And a universal formula, which can be used for correcting the fading effecet at various temperatures and elapsed time, was established through the further analyses and the least-squares method. Based on the analysis of photo stimulated luminescence (PSL) spectra on the IP irradiated by different spieces of radiation as well as the fading characteristics of different radiation signals, a new process with an optimal discrimination threshold was determined and a software was programmed for automatic identifying and counting of a-signals in the IP. A spatial model of the IP and a mathematical model of the PSL value were established, and the conversion factor of the absorbed radiation energy to the IP’s PSL value was determined through the comparisons of Monte Carlo simulation data and the air kerma measured in the condition of X-ray exposure. Further simulations were performed to acquire the quantitative variations of detection efficiency of the IP irradiated by different incident energies and angles ofα-radiation, and the results were also conducted to explain several experimental phenomena. By using the IP-based method established in this study and other common measurement methods, several comparison experiments for measuring the grossα-activity in some atmospheric and mineral samples were performed in order to evaluate the qualities and advantages of the new method in practical applications. Results The detection efficiency of the IP irradiated by the a-radiation of 241Am source was about 0.206 PSL-Bq1-·-s-1(0.218 cts-Bq-1·-s-1), and an excellent dose linearity was observed over 5 orders of magnitude. In the same temperature and elapsed time conditions,α-signals stored in the IP faded faster than the signals ofβ/γradiation. However, the fading effect can be well corrected by using the empirical formula established in this study. The track identification method (A threshold of 0.25 was set for the value of PSLpeak which was defined as the sum of PSL value in a peak and its adjacent four pixels) proposed in this study was superior to the traditional method using the total PSL value for identifying a-signals. By using the identifying and counting method proposed in the study, the lower detection limit of a-activity was estimated to be 3.5×10-3 Bq·cm-2 for 30 min irradiation. The conversion factor of absorbed energy to the IP’s PSL value was determined to be 7.71×1011 PSL·J-1. The lower detection limit of energy and the critical angle of incident a-particles were estimated to be 3 MeV and 80°, respectively. The PSL value increased with the increasing energy of incident a-particles (an additional of about 0.04 PSL value will appear when the energy increases 1MeV), while the PSL value decreases with the increasing incidence angle. Through the comparison measurements of gross a-activity in the environmental media, the IP-based measurement method proposed by this study was proved to be reliable and accurate. The advantages of this new method, such as the ability to simultaneously measure multiple samples without sample-sized limit, the high geometric detection efficiency, the rapid and simple process of measurement, and the ability to map the spatial distribution of radioactivity and so on, were obvious.Conclusions The new IP-based method for identification and quantitative measurement of gross a-activity in the environmental media was established in this study and proved to be reliable and accurate. By compared with other traditional methods for measuring the gross a-activity in environmental media, the new method is able to simultaneously measure multiple samples without sample-sized limit, and it has higher geometric detection efficiency and lower lowest detection limit. Additionally, the new method has the capability to map the spatial information of radioactivity in the environmental media. It is believed that the new method can be used for both quick and accurate measurements of the grossα-activity in large amount of environmental media. However, further studies are still needed to quantify the effect on identifying and counting for different energies of incidentα-particles in new method. The applicability of the new method to different types of IP and their imaging readers is also hoped to be studied.