Studies Of Radon Radiation Field In Soil And Development Of Radon Measure Instruments

As we know, natural radon is released by radiative elements Uranium and Radium as they decay in geological object underground. Therefore, it is very important to study on the radiative field of radon in soil. One is to study on the hot spot, so as to find solution for vary application regions, such as mine exploration, oil and gas exploration, geological disaster monitoring, earthquake forcast, archaeology, environmental engineering, etc. Another is to study on the radon transportation law to improve environmental radon evaluation, by study environmental radon along with geological background. The motivation of this thesis is the last one.In this thesis, the fixed solution of radon concentration on ground-air interface has been established based on diffusive and convection theory. The new boundary condition of the fixed solution also has been put forward, namely, the airflow density of radon on ground-air interface is continuous. Based on this postulate, the distribution of radon concentration on the both sides of the ground-air interface is change with the soil depth, height and the concentration of radium and uranium in surface soil and rock. Furthermore, the change is expressed by mathematical equation. The relationship and change between radon concentration in soil and the concentration in atmosphere is explained adequately by defining a physical concept, Radon emission rate.Based on the characteristics of experimental research, three kinds of radon measure instrument are designed and manufactured. (1) A soil radon monitor can be buried in the underground to measure the radon concentration in the soil. Designed based on CAN bus, multiple radon measure systems can be used to built up a radon detecting area network. . Thus, the soil radon concentrations can be measured at the same time in the region of several square kilometers. The detecting sensitivity can meet the requirements of GB50325-2001 (the minimum detecting activity is no more than 10Bq·m-3) under the measuring cycle of 2 hours. (2) The uncertainty of gas flow rate brings error in a measurement while pumping gas. Using a popular radon measure instrument FD3017, the average value of relative deviation is 39% according to the data surveyed in Jinghua Town Section in Chengdu city. IED3000, a new design in this thesis, can correct the result by using the air pressure calibration coefficient. Comparing to FD3017 in the measurements with radon chamber, the performances of IED3000, such as detecting sensitivity and relative deviation, are improved. (3) The new radon measure system with automatic tole sliding can switch the measuring tole without manual manipulation. It is the first time that the automation of switching tole is applied in radon collected by non-contact. The new design avoids the influence caused by manual manipulation. The collection chamber and detection chamber can be designed integrative and the connect cable will be retrenched. Therefore, the performance of the new design is better than IED3000.The experimental study on soil radon radiation field is described as below. (1) In order to prove the accuracy of mathematical expression about radon concentration distribution on the both sides of the ground-air interface, the radon concentration in the soil (atmosphere) at various depths (height) had been measured. The result of the measurement was coincident with theoretic values and the maximal value of relative deviation is only 55% in underground experiment. (2) It was found that the variety range of radon concentration in the end of a month was four times higher than variety range at the beginning of the month at 0.28m depth underground in the experiment. Moreover, both the soil temperature and air pressure affect the soil radon radiation field, the soil radon radiation field has a negative correlation with air pressure and positive correlation with soil temperature. (3) The parameters, such as radon concentration, specific activity of radium, moisture content and porosity, are acquired by experiment in two section planes. Based on error analysis, multiple regression processing and inversion fitting computation, the moisture content correction model have been established corresponding to four different soil properties. In section plane of CDF1, about 60% theoretic values of the sample are coincident with the observed values. After moisture content adjustment, the average relative deviation decreases from 97% down to 50% in position of observation. Comparing the average relative deviation in section plane of CDF1, the average relative deviation of CDF3 between theoretic values and observed values is 70% by validity calculation. (4) Using the moisture content correction model, regional radon concentration distribution can be calculated, the isograms of radon concentration paralleling to the horizon and the relationship between depth and radon concentration are acquired at the same time.