Studies On Aspects Of Passive And Integrated Thoron Measurement Method And Effective Dose Evaluation Method Of Thoron Progeny And Their Preliminary Applications

The natural radiation is the main source of all radiation exposure to the public. United Nation Science Committee of Effect of Atomic Radiation (UNSCEAR) pointed out in its 1993 Report:The collective dose from nature radiation exposure is 76.6% of the total collective dose from all radiation exposure. With the development of society and science & technology,the effective dose from the nature radiation exposure to the public shows the tendency of increase. In the view of the effect of radiation exposure to human body,nature radiation is not different from artificial radiation. All people in the field of radiation protection acknowledge that it is necessary to investigate,evaluate and control the nature radiation exposure from the point of radiation protection.The evaluated results in UNSCEAR 1993 Report indicate that more than half of effective dose of nature radiation exposure to the human being comes from radon and its daughters,so the studies on radon and radon daughters have widely gotten attention in the field of radiation protection more than half century. Although thoron is one of isotopes of radon,and the characteristics of thoron and thoron progenies are similar to those of radon and radon progenies,because the half life of thoron is shorter (55.6s),the concentrations of thoron and thoron daughters in environment are lower,people usually think that the effective dose from thoron and its progenies can be eliminated,and the measurements of thoron progenies concentrations are difficulty,a little research work on the dose evaluation of thoron and thoron progenies in environment has been done.In recent years,more and more people pay attention to the investigation of thoron level in environment,the measured results indicate that there are a lot of regions with higher thoron concentrations. Thus,UNSCEAR pointed out in its 2000 report:the effective dose from thoron and thoron daughters is 20.5% of that of radon and radon daughters. In China,the content of thorium (232Th,the mother of thoron) in soil is higher,most of dwellings are made of brick and wood,or brick and concrete,or soil,and a certain number of people live in the cave dwellings,so the effective dose from thoron and thoron daughters may be higher than the world average's.The developments of passive and integrated thoron measurement methods and effective dose evaluation methods of thoron progeny are of important significance to compensate the shortage of studies on the evaluation methods of thoron and thoron daughters in environment,meet the technical needs of representative,comparability,reliability to the measurement results of the monitoring methods in the environmental evaluation,measure and evaluate usually thoron level in environment,protect ecological environment,ensure public health,and so on.The key problems on studies of passive and integrated thoron monitoring methods are to set up a thoron chamber for the conditional experiments and the calibration of thoron dosimeters,develop methods for discriminating thoron from radon and for the effective dose evaluation of thoron daughters base on thoron exposure. A simple thoron chamber,the measurement methods,the effective dose evaluation method of thoron daughters and their primary applications studied are described simply as the followings.The simple thoron chamber consists of three parts,they are the thoron accumulation box,the circuit of thoron source input and gas circulation,the circuit of thoron level measurement in the thoron accumulation box. The thoron chamber with the size of 350mm X 250mm X 180mm was made of the Perspex. Five holes were drilled on the two side with the size of 2 50mm XI 80mm for thoron source input or sampling. A Th-1025 thoron source made in Pylon Company of Canada was adopted. Thoron gas is mixed with circulation gas before they are input into the thoron accumulation box. The thoron level in the thoron accumulation box can be adjusted by changing the gas flow rate through the thoron source,or adding different volume of delay boxes in source input circu...