The Study Of Gamma Radiation Field And Its Environmental On Gas-ground Interface In Chengdu Plain

Natural radioactivity when Becquerel discovered it in 1896,has been filling in human survival and the areas of nuclear geophysical radiation field. we know and use it day by day. it includesγradiation field,βradiation, neutron radiation field,αradiation field,μradiation field, as well as other particles in the cosmic ray field. Andγradiation field occupies a very important position.γradiation field was calledγfield in short. It Shows the the spatial distribution ofγray. When we need evaluate the environmental radioactivity or nuclear contamination caused by leakage evaluation, theγfield must be researched on ground. This way can obtained the content of natural radio nuclides include uranium, thorium and potassium, and the absorbed dose rate which caused by the natural radio nuclides.The paper studies the natural conditions, the topography, the geological features, the geochemistry, and the soil types and its distribution of the Chengdu Plain. It established the forward mathematical expression ofγ-ray exposure rate on over-infinite gamma radiation body. According to this expression,the content of soil medium uranium, thorium and potassium, the specific activity and the conversion factors ofγ-ray absorbed dose rate,which 1 m height from the ground, it presented the correction methods of Chengdu Plain uranium - radium equilibrium coefficient, radon emanation coefficient from the theory and practice. It provides the theoretical basis and technical approach for evaluating the radioactivity level for the Chengdu Plain Wilderness. According to the theory of diffusion and convection, It discusses the mathematical model of the radon concentration in soil pore medium and the steady-state radon concentration in the atmosphere and the mathematical expression between Uranium and the soil medium. It provides the theoretical evaluating basis of the Radon potential fields for the Chengdu Plain and the indoor radon concentration distribution.Meanwhile, the paper collection and analysis of a wide range of physical geography,geological structure, geochemistry and soil types of Chengdu Plain, use the Plain's content data of 1:250 000 soil geochemical sampling of uranium, thorium and potassium and the measurement results ofγ-ray spectrometry of soil radon measurements and radon concentration in the Chengdu Plain,get the characteristics of the natural gamma radiation spectrum and environmental on ground-gas in the Chengdu Plain and make a explanation from the geological structure and the environment.①Through statistical analysising the soil samples'chemical analysis data and theγ-ray spectrometry data which obtained uranium - radium equilibrium coefficient, the paper showed that the frequency of large takes the shape of normal distribution. When we measure the Chengdu Plain'sγfield, we can take the uranium - radium equilibrium coefficient Kp = 1.92.②In Chengdu Plain, the radionuclide activity values of 40K is between 300.5 Bq/Kg and 1273.9Bq/Kg, the vast majority is between 350 Bq/Kg and 800Bq/Kg,and its average is 659.8Bq/Kg, lower than the national average.232Th is between 18.5Bq/Kg and 127.2Bq/Kg, distributing in normal and its average is 56.8Bq/Kg.The variance is 15.5Bg/Kg ,as the same as the national average.238U is between 11.6 Bq/Kg and 84.8Bq/Kg, distributing in normal and its average is 31.5Bq/Kg. The variance is 7.3Bg/Kg, lower than the national average. The geological structure is a great impact on the specific activity of radioactive 238U.The specific activity 238U in fault zones is higher than others.③In Chengdu Plain, the irradiation index is between 0.06 and 0.42, its average is 0.158.The external exposure index is between 0.24 and 0.81, its average is 0.461, all lower than the national mandatory requirements. Theγradiation absorption dose rate is between 41.10 nGy/h and 140.74 nGy/h, its average is 79.516 nGy/h, its square deviation is 13.7922 nGy/h, as the same as the national and the world. The annual effective dose is between 0.02 mSv and 0.07mSv, its average is 0.039mSv, its square deviation is 0.0068mSv,far lower than the world's average annual effective external dose limits.④In Chengdu Plain,the soil radon measurement of the background value is between 2.098×10~4Bq/m~3 and 15.332×10~4Bq/m~3, the average is 5.710×10~4Bq/m~3, and the standard deviation is 1.329×10~4Bq/m~3. The radon content in the northern such as Lijiazhan, Heqing, etc. and the southern regions such as Zhongxinchang, Qinglongchang of the Chengdu Plain is higher than other places, reaching 8.369×10~4Bq/m~3, a handful of area of more than 9.699×10~4Bq/m~3, but all below in 15.332×10~4Bq/m~3,most areas of the radon concentration is lower than 5.710×10~4Bq/m~3.⑤In the main regions of the Chengdu Plain,the radon concentrations ranged from 8 ~ 177 Bq/m~3, average 39.4 Bq/m~3, MSE 22.9 Bq/m~3, gamma dose rate range of 75 ~ 169nSv / h, average to 122.1 nSv / h, the mean square 116.3nSv / h. the indoor's radon concentration measurements were Poisson, the gama dose rate measurements were normally distributed,the indoor radon concentration of Wenjiang, Qionglai,Chengdu High-tech Zone and Pengzhou is higher than other areas, other's level are 35~45 Bq/m~3 .Except Wenjiang, the gamma dose rate is between 110 nSv/hand140 nSv/h.The natural geological structure impacts the specific activity of radio nuclides greatly through studied theγfield and its environmental effects on the gas-ground interface in Chengdu Plain. The distribution of radioactivity showed that the regional geological structure of the Chengdu Plain impacts radiation levels greatly. The level of radioactivity, such as theγray exposure rate in fault zones, the internal and external exposure index ,were higher than other areas. The migration or enrichment of quaternary alluvial , flood sediment and surface water in Chengdu Plain also impact the radioactive level in some areas.The paper supported by the National Fund which is "The Research of Radon Radiation Field and Its Environmental Radiation Effects on Gas-ground Interface" (40374051) ,the 7th-project of the National 863 Program ,which is "The System of Air Geophysical Exploration" ,"The Research of airborne gamma spectrometry system" (2006AA06A207) in the field of Resources and Environment and the geological survey project of the Geological Survey Bureau which is " The Urban's Evaluation of Eco-geochemical in Chengdu Economic Zone" (200314200015).