| Radon is a kind of natural radioactive inert gas, and almost exists at everywhere, and also, a main source of natural radiation exposure.222 Rn and 220 Rn are a couple of common isotopes of Radon, 220 Rn almost appears with 222 Rn. The contribution of 222 Rn progenies, especially220 Rn progeny for environmental radiation dose has been underestimated. Because of the difference between solid metal granule of 222Rn/220 Rn progenies and the inert gas of 222Rn/220 Rn, those granule mostly deposit at the lung of human body and further cause the contribution of 222Rn/220 Rn progenies for effective dose equivalent of human body much more higher than 222Rn/220 Rn. In order to accurately evaluate the radiation dose of environmental 222Rn/220 Rn and their progenies, especially facilities such as high thorium background 、tombarthite factory workshop, and also for the need of better estimation of222Rn/220 Rn and their progenies radiation dose, it has to monitor the exposure dose of 222Rn/220 Rn and their progenies. Therefore, it needs to focus on the rapid and continuous measurement method of 222Rn/220 Rn progenies level and measurement devices.Commonly used measurement instruments of 222Rn/220 Rn progeny concentration such as WLx(Pylon,Canada), BWLM-PLUS(Tracerlab,Germany) and EQF3200(SARAD, Germany), and all belong to the alphaspectroscopy-based radon(or thorium) measurement instrument, and only continuously issued the equilibrium equivalent concentration(EEC) or potential alpha energy concentrations(PAEC) of 222Rn/220 Rn progeny, and the activity concentration of Ra A(218Po), Ra B(214Pb), Ra C(214Bi) which are short life progenies of 222 Rn through measuring the alpha paticle counts in specific time and basing on simple balance relationship. Obviously, through directly measuring the 222Rn/220 Rn progeny concentration to estimate the radiation dose caused by 222Rn/220 Rn progeny in environment is more accurate and more more meaningful on dosimetry. This article systemically analyzed all kinds of222Rn/220 Rn progeny measurement methods, and established the alpha spectroscopy-based continuous measurement method(denoted as continuous measurement model) which can reliably and continuously determine the222Rn/220 Rn progenies activity concentration without frequent changing filter membrane. Based on that, this article analyzed all issues which might effect the accuracy of continuous measurement device, then raised the design scheme of high sensitive continuous measurement instrument of 222Rn/220 Rn progenies, solved related critical technical problems, finally, continuous measurement instrument of 222Rn/220 Rn progenies was developed, and the related testing and calibration of instruments were completed. The main contents are as follow.(1)Formulating α spectroscopy-based continuous measurement method of222Rn/220 Rn progenies(operating about 7 days for each single membrane)through introducing the progeny cumulative activity function and Bateman decay equation which mainly describes the enrichment and decay process of222Rn/220 Rn progenies.(2)The three key technical problems which impact the accuracy and reliability of continuous measurement instrument has been solved through theoretical analysis and experimental research. Including the implementation of adjustable 220 Rn progeny measuring environment, the effect of alpha radioactive aerosols deposition on microfiltration membrane self-absorption,and the determination of the peaks overlap factors of hybrid 222 Rn, 220 Rn progeny’s alpha energy spectrum.(3)The contrast experiment was carried out between the continuous measurement method and reference method in pure 220 Rn, hybrid 222 Rn and220Rn progenies environment. The results show that: The continuous measurement method for measuring concentration of 222Rn/220 Rn progenies has higher accuracy and reliability. When the level of 220 Rn progenies is as low as 100Bq/m3, the uncertainty of method is less than 5%. The way of introducing the cumulative activity function to eliminate the impact of previous measurements on present measurements is reasonable and feasible without leading significant error propagation.(4) Based on the key technologies that have been solved, six tasks which are the overall instrument design, mechanical design and processing, circuit control system development and software design and compile, performancetesting, and calibration of instrument efficiency have been completed. Finally,practical progeny continuous measurement instrument of 222Rn/220 Rn have been developed.(5)The continuous measurement instrument have been applied to measure the concentration of 222Rn/220 Rn progeny in the measuring environment of pure 222 Rn progeny, and indoor. And the tability and reliability of instrument have been demonstrated by measurement results.In a word, this paper introduced a cumulative activity function and established an alpha spectroscopy-based continuous measurement method of222Rn/220 Rn progeny which doesn’t need to change filter membrane frequently.The experimental result indicated the continous measurement method had relatively high accuracy and reliability in the 80-3000 bq/m3 measuring environment, and can eliminate the Method error of iterative deduction method. Based on the key technologies that have been solved, the practical progeny continuous measurement instrument of 222Rn/220 Rn have been manufactured, which has simple structure, operation and maintain is convenient, and dosn’t require vacuum and frequent changes of the filter membrane. So, this instrument completely suits for continuous measurement of 222Rn/220 Rn progeny in rare production areas, high thorium background region and other places. |
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