| Airborne gamma-ray spectral survey system is made of enough efficient detectors,navigation,altimeter and corresponding software program,and so on.And it is mounted on suitable vehicles to measure gamma-ray irradiated by radioactive substance in surface layer and air.It has proven to be an effective means for monitoring regional radiation and geological exploration by analyzing the measurement spectrum.Spectral response of airborne gamma-ray spectral survey system is the key to scale those parameters associated with nuclear radiation,such as efficiency,the content of radionuclide,gamma-ray radiation dose,and so on.The standard calibration models of natural radionuclides have been established in China.It is far away from forming the effective calibration models for significant short-lived artificial radionuclides because of the short half-life periods of most artificial radionuclides,potential radiation hazards,high cost,and so on.And researchers tried to solve the above problem by numerical method.But the numerical model is based on a point detector,but it can't considered the geometry and materials of detector in AGS.This thesis is doing research on the spectral response of airborne gamma-ray spectral survey system in natural radiation environment.The achievement about the research can be able to supply the theoretical foundation and technical support for monitoring radioactive substance and geological exploration which have high pratical value and scientific significance.The main research and its results are as follows:1)The distributive characteristic and the main gamma radiation of cosmic gamma-rays,background of equipment,the daughters of radon in the air,natural and artificial radionuclides in environment are discussed in detail.And spectral response characteristics are carried out in-depth analysis and discussion when the airborne gamma-ray spectral survey system is radiated by the above sources,respectively.These results provide theoretical support for monitoring radioactive substance and geological exploration by measurement technique of airborne gamma-ray spectrum.2)When many of same-sized airborne gamma-ray spectrometry(AGS)are placed in the same height and the distances between the center of each AGS and the boundary of surface layer are greater than the detectable radius,the response laws of these AGSs are the same under infinite source particles.Based on the principle,a spherical shell model of Monte Carlo(MC)simulation for an airborne gamma-ray spectrum is proposed to reduce the variance and increase the precision for response spectrum by MC simulation.Based on the principle of media interchange,the basic thought of modular stratum and energy partitioning for characteristic gamma-ray,the combination model is used to obtain the AGS' response spectrum of any shape of stratum by MC simulation.And after a preliminary comparison we find two differences.In the first place,the computational speed of the latter is 500 times that of the former.In the second place,the difference of the response spectrum by the two models is found in the energy region of 40keV-350 keV.The results show that the combination model include more gamma rays coming from long-range stratigraphic than the spherical shell model.According to the response spectrum by the combination model,the least square model for solving airborne gamma-ray spectrum at the region of interest is established.And a hybrid algorithm based on particle swarm optimization and simulated annealing is adopted to solve the parameters in the least square model.By analyzing the eight airborne gamma-ray spectrum from the HuangBi reservoir in Shijiazhuang city,the results verify that the response spectrum from the daughters of radon in the open country can be replace by the response spectrum from 235 U and 238 U series in surface layer.At the same time,the least square model is accurate and reliable by analyzing the eight airborne gamma-ray spectrum from the dynamic scale model in Shijiazhuang city.3)Based on the basic thought in calculus,tiny source is divided from gamma radiation on arbitrary shape by a grid partition method and seen as a point source.And the full-energy peak efficiency formula of AGS for a point source is derived in this paper.According to the superposition principle,the sourceless efficiency calibration method of AGS is established.Firstly,the calculation values show an agreement within ±5% error in comparison with measurement values while a point source of 137 Cs is only a short distance away.Secondly,the calculation response law of different positions by this paper coincide with the experiment of Grasty et al.Finally,the calculation values show an agreement within ±10% error in comparison with measurement values while a point source of 137 Cs and 60 Co is put under the AGS-863 for 5-100 m.These prove that the numerical calculation is suitable for computing the full-energy peak efficiency of a point source.Meanwhile,the calculation values of a surface source and a volume source show an agreement within ±1.5% error in comparison with simulation values by MCNP5.And while AGS is 90-150 m away from infinite volume source,the calculation values of 1460.83 keV and 2614.533 keV are 8.33%-15.82% bigger than the measurement values.These results verify that the sourceless efficiency calibration method of AGS is accurate and reliable.4)A hybrid algorithm based on Adaptive Peak Removing Technology(APRT)and many Gaussian functions fitting the net counts in peak area is used to calculate the net area in airborne gamma-ray spectrum.The APRT is proposed to estimate the scattering background in full energy region,and the fitting method of the latter uses Levenberg-Marquardt algorithm.Two standard test spectrum supplied by the IAEA are analyzed by the hybrid algorithm.And the analysis results of net peak area is an agreement within ±5% error in comparison with recommended values by IAEA.These verify that the hybrid algorithm can provide reliable experiment data for other related studies.5)The calculation values of 10 km survey lines in BaiYunEBo ranch by the least square model coincide with the specification for AGS when the content of natural radioactivity nuclide is larger than 4.0×10-6g/g.Meanwhile,while the activity of artificial radionuclide is bigger than detection limit,the activity of artificial radionuclide calculated by the least square model is an agreement within ±10% error in comparison with the hybrid algorithm from the point 4).These verify that the least square model can provide reliable experiment data for other related studies.The results verify that the least square model is valid method to acquire the AGS' response spectrum of the main gamma radiation in stratum.6)The model is applied to calculate the detection limit of AGS for the point or infinite source.By analyzing the calculation values of detection limit of four artificial radionuclides,we can reach the following conclusions: 1.While the AGS is placed in the same height,detection limit is decreasing with the increasing the energy of emitting gamma ray in stratum.2.While the energy of emitting gamma ray in stratum is fixed,detection limit is increasing with the increasing the measuring height.The smallest activity of V category of pollution source can't be found even though the measuring height fall to 20 m.when the smallest activity of IV category of 241 Am,131I,137 Cs and 60 Co pollution source need to find,the minimum measuring height must be get up to 60 m,more than 150 m,more than 150 m,more than 150 m,respectively.Even if the measuring height is 150 m,the smallest activity of III category still can be discovered. |
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