| Objective As an isotope of radon (222Rn), thoron (220Rn) is a naturally occurring radioactive noble gas. Chronical inhalation of high concentration of 220Rn and its progeny may cause health risk such as lung cancer. On the other hand, it’s difficult to discriminate 220Rn and 222Rn as they always coexist with each other in the atmospheric environment. Therefore, high concentration of 220Rn may disturbe the accurate measurement of 222Rn. At present, only few of researchs on 220Rn measurement and its calibration equipment were reported, while many high 220Rn level environments exist in our country. Therefore, the objectives of this research are to establish a 220Rn calibration equipment, and study on the airflow-through method and passive accumulative methods for 220Rn measurement based on the calibration equipment. The result will not only provide methods for measuring the concentration of 220Rn accurately, exploring the distribution and transportation of 220Rn in the atmospheric environment, but also provide a new technique and dosimetry basis for evaluating the health risk contributed by inhaling 222Rn/22 Rn and their progeny.Methods As the basis of the calibration system, the resin granule thoron sources were developed based on the ion exchange technique, and the ion exchange efficiency and some other performances were studied through quantificational experiment. Based on the theoretic calculation and analyzing, a chamber in which the 220Rn concentration was relatively stable and uniform was designed for calibrating measurement equipment or detector of 220Rn combined with the characteristic of the 220Rn source, a commercial type of programmable constant temperature and humidity device and a fan system. The adjustable range, uniformity, stability and equilibrium time of 220Rn concentration for the chamber was validated through experiment research. Considering the correction of 220Rn decaying in the sample tube and scintillation cell, an airflow-through measurement method was provided based on the detection principle of scintillator for a particles and compared with the PTB. To expand the application of this method, the measuring timetables of grab sampling method for discriminating 220Rn/222Rn were optimized on the basis of the weighted least square method and simulation experiments. Besides, the software for inversing the concentration of 220Rn/222Rn was compiled based on visual C++. A new solid state nuclear track detectors (SSNTDs) with suitable ventilation rate and dimension was designed to accumulative measure 220Rn/222Rn concentration accurately. Combined with the thermodynamic theorem and Monte Carlo method, the effects of atmospheric parameters (such as air pressure, temperature, relative humidity and dew thickness) on radon measurements with several common typies of alpha-track detectors were discussed in this study. In addition, a new method for measuring concentration and spatial distribution of 220Rn was proposed on tha basis of the BaFX imaging plate (IP) technique and Monte Carlo simulation technique.Results (1) A suite of resin granule thoron sources were developed with high ion exchange rate and reliable performance. The Rn concentration in the developed thoron chamber with a volume of 218 L, is stability (fluctuation is less than 3%) and uniformity (spatial difference is less than 5.5%), and it could reach the equilibrium states rapidly with an adjustable range of 4.05 to 12.35 kBq·m-3. Based on these adventages, the chamber could be regarded as a calibration system for 220Rn concentration. (2) This calculated results showed that the decay effect could not be neglected in the flowing sampling and it was suggested that the sampling flow rate should be greater than 2 L/min to decrease the losses in the tube. The calibration factor of the airflow-through scintillation cell was 53.70 Bq m-3/cpm, and the lower limit of detection was estimated to be about 23Bq m-3 for a 60-min continuously sampling. The results showed that a measuring timetable of 1-min consecutive counting lasted for 10 times was an optimal routine for grab method of discriminating 220Rn/222Rn. Combined with the weighted least square method, the new measuring timetable could reduce the uncertainty of 220Rn measurements. (3) The optimized designed SSNTDs had an upper calibration factor and the ventilation rate was suitable for discriminating 220Rn/222Rn. The measuring results of the large dimensional SSNTDs were easily affected by the change of air pressure or temperature which should be corrected in the accurate measurement. Besides, the dew condensation phenomenon caused by high humidity or the sudden dropped temperature should be avoided as much as possible since it may destroy the detecting ability of the SSNTDs. (4) The new method for measuring 220Rn based on the IP technique has a calibration factor of3.05±0.08 count cm-2/(Bq m-3 h). This new method is adapted to measure the spatial distribution of 220Rn as its large detecting area.Conclusions A thoron chamber with the characteristics of stable, controllable and well-proportioned 220Rn concentration was developed and it could be used to calibrate the common measurement equipments of 220Rn concentration and provide an experimental platform for carrying out the research of 220Rn. The scintillation cell developed in this study could be used to measure 220Rn concentration and discriminate 220Rn/222Rn quickly and accurately. The new designed SSNTDs could be used to provide technique support for improving the accuracy of the 220Rn/222Rn concentration in large-scale survey and be contributed to the accurate assessment of public exposure suffered from 220Rn/222Rn. The new method based on the IP technique was developed which was propitious to measure the concentration distribution of 220Rn. |
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