Research On Standard Method And Correction Factors Of Radon Measurement

Due to the hazards of radon and its progenies to public health and the important uses of radon measurements in earthquake forecast etc, the measurement accuracy causes more and more attention. This thesis studies the influence factors in measuring radon with internal gas ionization chambers, through analyzing the properties of radon and its decay products and their behavior in the electric field, some factors were theoretically calculated and tested by a few specially designed experiments. A conclusion was drew that the absolute radon measurement can be realized approximately through correction for the wall loss under special working conditions, and methods to improve the accuracy of absolute measurement were brought forward.Main contents and conclusions are: 1) The ionization current was measured continuously by a charge-integration method with a computer after the introduction of radon into the chamber. As the result of growth and decay of radon and its progenies, the precision course of current changing with time was got and compared with the theoretical curve. We also observed the counterdiffusion of radon from the material of chamber after flushing. 2) The chamber was calibrated with some standard sources of known activities. The calibration factor for relative measurement is 24.62 Bq/pA. Through many times of periodic measurements of the same radon source, the stability of system fits in with requirements. 3) The routine working condition was decided by measuring the relationship between the ionization current, voltage and gas pressure. 4) The method of cleaning the internal surface of the chamber with 0.1mol/L HCL acid is very effective for reducing the background current according to our experiment. 5) The uncertainty( k=2) of relative measurement is smaller than 5%, and the detection lower limit obtained from the fluctuation of background current is 0.5 Bq. 6) The correction for wall loss was theoretically calculated, and tested through a series of experiments. The main thought is that under the conditions of keeping the ionization chamber working in the saturation region, the rule of wall loss changing with the gas pressure can be got by measuring thechange of current with the pressure. From the experimental data, we can see that the relationship of the actual and theoretical ionization current with gas pressure is almost same, when the pressure is higher than 125 kPa, the total contribution of end effect and other influence factors is below 2%. so the radon activity can be approximately ciphered out with the actual ionization current corrected for the wall loss under these conditions. This is a sketchy absolute method for the measurement of radon, and the accuracy can be improved by several ways mentioned.In this work, the phenomenon of radon counterdiffusion from the material after flushing the ionization chamber with clean gas was observed for the first time, and the theoretical calculation and actual measurement of wall effect are the main creative contents.