Study On Radon Influence Estimation In Airborne Gamma-ray Spectrometry

Radon is the unique natural radioactivity gas, its decay daughter suspend in the atmosphere, spontaneous decay and release the gamma-ray. Therefore, airborne gamma-ray spectrometry is under influence of the gama-ray released by the radon decay daughter. The study on correction method of atmospheric radon will improve of airborne gamma measurement technique, and it will make the airborne gamma-ray spectrometry has more application.In this thesis, author discusses the sources of atmospheric radon and its distribution. By the new boundary conditions has been put forward, mathematical expressions of the steady-state concentration of radon in the atmosphere and soil has been established based on the diffusive and convection theory. Based on the theoretic analysis, auther explain the distribution of radon on the ground-air interface.At the support of the radon distribution theory and rule of height-attenuation coefficient, the influence of atmospheric radon has been analysised. Based on the component of airborne gamma-ray spectrum and the height-attenuation coefficients of different energy windows, author has established an advanced radon correction method. This method is avoiding scattered ray influence using the FFT scattered rays estimate technology rather than tripping ratios by convention. Since the introduction of this technology, the spectral-ratio radon correction method will have broader application.In the process of the method study, author design and accomplish two experiments: (1) Atmospheric radon effects on the airborne gamma-ray spectrometry on the ground. The experiment validates the influence of the atmospheric radon, and makes a preliminary estimate of the atmospheric radon effect to U energy window. (2) Airborne gamma ray spectrometry measurement at different height. By this experiment, height-attenuation coefficients of different energy windows under the influence of atmospheric radon had been obtained and made comparing with the height-attenuation coefficients without atmospheric radon influence.The correction factor is the key of Atmospheric radon correction, and it decides the precision of radon background estimation. Author constructs the relation between correction factor of U component and correction factor of atmospheric radon component. Combine with the height-attenuation coefficients of different energy windows, Auther designs the correction factors calibration by standard saturation model measurement. The calibration technology avoids the possible errors of correction factor calibration which use of matrix computations and mathematical fitting based on spectrum shape. Further more, the coefficient calibration method is easier to implement and making calibration more efficient.Instead of numerical analysis and experimental measurement, author has study the radon correction method of airborne gamma ray spectrometry using the Monte Carlo method. In order to improve simulation efficiency, the the Monte Carlo simulation process has been decomposed. The simulation process is described as below: (1) The characteristic peak intrinsic detecting efficiency of different characteristic gamma-ray detector had been simulated. For the sake of describe the relation between the reference gama-ray and characteristic peak full-energy-peak intrinsic detecting efficiency, author define the conception of relative characteristic peak intrinsic detecting efficiency about U and Th components. (2) The flux ratio of reference gama-ray to characteristic gama-ray of atmospheric radon component, uranium and thorium components had been simulated. (3) Using Monte Carlo method, author had simulated height-attenuation coefficients and correction factors in different air density.Finally, the correction factors had been calculated by standard saturation model measurement technology and Monte Carlo simulation. In addition, error in the correction factor calibration has been analysis. Using the two sets of correction factors which calibrated by standard saturation model measurement technology and Monte Carlo simulation, the radon background of spectrum measured in different height had been estimated. Comparing the height-attenuation coefficients caculated by numerical analysis, the relative error of the corrected height-attenuation coefficients is less than 6%. Moreover, using the two sets of correction factors, the relative error of the corrected uranium energy window count is less than 30% in the 80~110m.