Studies On The Measurement Method Of Thoron By Passive Solvent Absorption And The Equilibrium Factor Of Thoron

The natural radioactivity is the main component to human exposure, it is recognized that the most important contributors to the committed dose received by general population due to natural sources are the short-lived decay products of radon (including ²²²Rn and ²²⁰Rn). Comparing with ²²²Rn, the health risk due to the presence of ²²⁰Rn indoors is easily neglected because of its generally low concentrations in indoor environment, which is essentially caused by its short half-life. Nevertheless, under some situations, the exposure of ²²⁰Rn and its daughters may produce a dose comparable to that of ²²²Rn and its daughters, and it is possible to become an important radiological pollutant. According to UNSCEAR 2000 report, the contribution from ²²⁰Rn and its daughters can represent a significant fraction of the total exposure; enough attentions must be paid to the issue of ²²⁰Rn radiation.²²⁰Rn is the sole gaseous nuclide of the thorium radioactive decay chain, its ancestor, i.e. ²³²Th, exists widely in the crust. According to some investigations, the content of ²³²Th in soil and building materials in China is higher than the average level all around the world, so is the dose induced by ²²⁰Rn and its daughters to inhabitants in our country.The lack of attention to ²²⁰Rn in the past has enlarged the knowledge gap between ²²²Rn and ²²⁰Rn, both in the measurement methodology and in the dose assessment. So, it is necessary to study the passive solvent absorption measurement method and the equilibrium factor of ²²⁰Rn.1 Study on the measurement method of ²²⁰Rn by passive solvent absorptionBased on the relatively high dissolubility in some organic solvents and short half-life of ²²⁰Rn, it has been established the passive solvent absorption measurement method for ²²⁰Rn concentrations in the ambient air.The research contents on the measurement method mainly include four aspects:(1) Probe into the mechanism of passive absorption of ²²⁰Rn by organic solvents, and then find out the relevant factors.(2) Stipulate the technical courses for the measurement method, and finish the design of apparatus and the selection of materials for the measurement.(3) Experiment on the relevant factors, i.e., kind of solvents, sampler size, the impact of wind speed and temperature, the interference of ²²²Rn during the sampling process, and then do some quantitative analysis.(4) Compare with other measurement methods in the laboratory and in-situ after relevant calibration.Analyzing the dynamic process of passive absorption of ²²⁰Rn in organic solvents qualitatively, the sampled quantity of ²²⁰Rn is influnced by the concentrations of ²²⁰Rn in the ambient air, the kind of solvents used, sampling time, sampler size, and the temperature during the sampling process.Because ²²⁰Rn decays soon after it is absorbed by organic solvents, it is possible to finish the accumulation of ²²⁰Rn progeny (²¹²Pb mainly) in the solvents within a pre-set period. After having been selected the suitable sampling time, for which the progeny accumulated in the solvent decays too, the measurement protocol is set down. The procedure includes three steps, first sample ²²⁰Rn with the organic solvents, then process the samples with suitable methods (such as combustion), and finally detect the radiation (alpha particles) to get the ²²⁰Rn concentrations indirectly without the transferring of samples.By the use of ²²⁰Rn metrological check device, school of nuclear science and technology, Nanhua University, a great amount of conditional experiment has been done on each factor. Quantitative analyses have been done on the factors mentioned above. The results indicate, the selection of solvent is very important. The sampled quantity of ²²⁰Rn is proportional to sampler size, but it is reverse proportional to the temperature during the sampling process. In the presence of ²²²Rn, there is almost no influence on the absorption of ²²⁰Rn, but the ²²²Rn progeny could affect the detection of ²²⁰Rn progeny. During the sampling process, the impact of wind speed upon the absorption of ²²⁰Rn could be neglected.According to the measurement procedure, select the absorbent (glycol) and other materials, and finish the related calibration. Then, undertake the comparison measurement with other methods (continuous and nuclear track detecting measurement) in laboratory and in-situ, and get a series of comparable results.The results indicate that, it is feasible that the passive solvent absorption method, which is convenient, practicable, and low-cost, is used for the determination of ²²⁰Rn concentrations in the ambient air, and it could meet the need of measurement in-situ with low limit detection that is 18 Bq·m^-32 Study on the equilibrium factor of ²²⁰RnThe goal of studying on the equilibrium factor of ²²⁰Rn (Fm for short) is to assess the dose of its progeny. So, we have tried to get ²²⁰Rn progeny concentrations from average ²²⁰Rn concentrations by the use of F_Tn reasonably. Through a series of experiment, the F_Tn values have been determined under some environmental conditions.The study on F_Tn includes three aspects:(1) Try to determine the F_Tn values in the special environment. For the sake of trying to prove the practicability of Fm, based on the theoretical analysis, measure the F_Tn values inside the ²²⁰Rn,²²²Rn mixed chamber and the underground copper mining tunnel, and then analyze the relevant factors.(2) Try to determine the Fm values of indoor environment. Considering the uneven distribution of ²²⁰Rn in ordinary living space, define the Representative ²²⁰Rn Concentration (RTC for short) as the average concentration of indoor environment and provide methods to obtain its value, and then determine the F_Tn values of indoor environment (especially the countryside earthen-wall dwellings) experimentally by the use of RTC, and research on the distribution status of ²²⁰Rn progeny indoors at the same time.(3) Estimate the dose of ²²⁰Rn progeny by the use of F_Tn.The results indicate that, because both ²²⁰Rn and its progeny have the relatively even distribution in the special environment, the F_Tn values maintain stable, so it is feasible that the F_Tn is used for the dose assessment of ²²⁰Rn progeny. But the F_Tn value will be changed with the ventilation rate and the concentration of aerosol in the ambient air, the higher the ventilation rate, the lower it is, and the higher concentration of aerosol, the greater it is. In a typical underground copper mine, the F_Tn value is 0.08 at the stope, and 0.03 at some connecting tunnels.In the living space, the ²²⁰Rn concentrations decrease exponentially with the distance to the wall (i.e. the source), so the ²²⁰Rn concentrations indoors show very large spatial distribution, it means that it is difficult to get the average ²²⁰Rn concentration by measurement and assess the exposure of ²²⁰Rn progeny to the residents by the use of F_Tn directly. By using of average ²²⁰Rn concentration in one-dimension (i.e. RTC) to represent the average indoor concentration, it is helpful to solve this problem. Under the different ventilation situations inside the countryside earthen-wall dwellings, it has been found that the typical F_Tn values are 0.008 (waiting room and kitchen) and 0.012 (bedroom), respectively.The results also indicate that, the indoor ²²⁰Rn progeny maintains a relatively even distribution, though there exist some fluctuations. It is further proved that the use of F_Tn for dose assessment of ²²⁰Rn progeny is reasonable.The passive solvent absorption measurement method of ²²⁰Rn, developed by this study, provides a technical mean for the assessment of health hazard caused by ²²⁰Rn and makes the methodology of ²²⁰Rn measurement enriched. The study on the ²²⁰Rn equilibrium factor lays the foundation for dose assessment of ²²⁰Rn progeny.However, the efficiency and sensitivity of measurement method by passive solvent absorption should be improved. The quantitative impact of associated factors on the ²²⁰Rn equilibrium factor should be studied further, for promoting the quality of the dose assessment.