<sup> 220 </ Sup> In Rn Gansu Indoor Radon Measurements And Radon Survey Data Correction Method

Radon and its progenies are the main contribution to the effective dose from natural radiation for human.It is critical to accurately measure radon exposure level for studying the relationship between indoor radon and lung cancer.Recently,it was a controversial issue for the result of the epidemiological study on radon and lung cancer collaboratively conducted by the Laboratory of Industrial Hygiene(LIH), China,and the U.S.National Cancer Institute(NCI) in Gansu province,China. Because the Radtrak detector used in the study is known to be affected by the presence of thoron.Thus the measured radon concentration might be overestimated. The primary purpose of this study is to propose the correction methods on the overestimated radon data of LIH-NCI study to reduce the influence of thoron.The resolution and sensitivity for the radon and thoron of the LD-P detector, which was remodeled from Radtrack detector was researched.The result demonstrated that the resolution for the radon cup of the LD-P detector was 0.99 for radon,and 0.03 for thoron,under the pure radon and thoron conditions.Under radon and thoron mixed condition with the ratio of thoron to radon ranged from 0.4 to 2.9, the sensitivity for radon and thoron remained stable.LD-P detector participated in the intercomparision organized by NIRS(National Institute of Radiological Sciences) in Japan.The relative percent deviation(RPD) from the reference value was 12%for radon,and 5%for thoron,within the receptable control limit.The specification of LD-P detector achieved the level of other detectors of the same kind.LD-P detector was used to study the radon and thoron levels,their spatial distribution characteristic as well as their influential factors in traditional dwellings in east of Gansu province.Three counties,such as Jingchuan County,Zhenyuan County and Ning County,which was included in the investigated area of the former epidemiological study,were selected,with one village chosen in each county.The Xigouquan village chosen in Zhenyuan County was the quality control point for radon measurement in LIH-NCI study.6 types of dwellings were surveyed,and a total of 175 dwellings for radon and thoron were obtained.The result showed the radon concentration measured by Radtrak detector in LIH-NCI study was overestimated by 1.5 times higher,due to the interference of thoron.The thoron concentration of soil structured dwellings showed normal distribution.Statistic analysis demonstrated that there were no significant difference(P＞0.05) among three regions for both indoor radon and thoron,while both of them were significant different(P＜0.001) among types of dwellings.According to the difference for radon and thoron levels of different types of dwelling,the number of types of dwellings was integrated into three. These results created the advantage for the correction of the radon data influenced by thoron.According to the relationship between the Radtrak and LD-P detectors,it can be deduced that the ratio of track density between radon cup and radon plus thoron cup of LD-P detectors equals to the ratio of the radon concentration measured by Radtrak (C₀) and the radon concentration observed by LD-P detector(C_Rn).Thus calibration of thoron for the detector was avoided,and the error would decline.LD-P detectors were suspended at different distances from wall,and the relationship between C₀ and C_Rn with distance from wall was deduced through the ratio of track density between radon cup and radon plus thoron cup.According to the location of the Radtrak detectors in the LIH-NCI study,the correction parameter can be obtained for the detectors in each type of dwelling at 2.5cm,5cm and 8cm distance from wall.By preliminary verification,comparing the corrected value(C_修) and the measured value(C_Rn) of each type of dwelling,the ratio of C_修to C_Rn ranged from 0.86 to 1.51,the mean value was 1.14.The relative deviation of radon concentration significantly declined after correction.By using the correction method and parameter in this work,a large number of the overestimated radon data can be corrected,and thoron interference can be excluded at a large extent.