| Refers to the field gamma ray spectroscopy surve directly by gamma spectrometer to measure the surface rock,soil etc. We can get the activity,content,related information of nuclide from the gamma spectrum.The method is the geological exploration and measurement of radioactivity in the economical, accurate and efficient nuclear geophysical methods.It is widely used in the field of environmental monitoring, exploration of uranium resources, nuclear emergency treatment. accurate efficiency calibration is one of the most importa nt parts of the application of gamma spectrometer.In particular, when nuclear emergency incident happens, the staff need to select the appropriate method of efficiency calibration and complete the scene detection according to the measurement environment.This paper studies the absolute measurement and relative measurement of the gamma spectrometer, including numerical analysis method, the efficiency of passive calibration method and effective calibration method.We study the principle and characteristics o f the three methods, analyze the factors that affect the efficiency calibration, and propose a suitable method for the correction. Accurate efficiency calibration is the important prerequisite for quantitative analysis of radionuclides.The factors that affect the measurement of the gamma spectrum are as follows, density, humidity of surface soil, topography, the uneven distribution of the radionuclides, the surface coverings, the media self absorption, soil or rock porosity, uranium radium imbalance, etc. The effect of surface humidity and terrain on the results of measurement is greater, so revising the results according to the measurement environment is necessary. In this paper, the standard model of national measurement station is used to calibrate Na I(Tl) γ spectrometer accurately, using theoretical calculation method and Monte Carlo method to determine the effective detection range of γspectrometer and calculate the impact of factors above on the measurement results. The research can also provide a theoretical basis formodifyingresults of the quantitative analysis.In order to calibrate γspectrometerand select appropriate locations of γ spectrometry and correct measurement results, especially in the field measurement,users need to change the environment of measurements to complete the efficiency calibration or correction efficiency calibration timely and accurately, and stability testing of γ spectrometer.Passive efficiency calibration method based on the following Lab Windows / CVI development environment is used to write efficient efficiency calibration software, and verified by the experiment(in addition, if usersadopt numerical analysis, active efficiency calibration method to calibrate γ spectrometer, the software can also be used to achieve some theoretical calculation of relevant parameters).According to the analysis of the influence factor of the gamma spectrum, the following conclusions are drawn:(1) In the semi- infinite measurement conditions,We can get The effective detection range of Na I(Tl) detector, and the variation of the efficiency of gamma spectrometer with the height of the ground by Monte Carlo.(2) If the gamma spectrometer is affixed to the ground, the detector may tilt when the ground survey is uneven. The experimental results show that the influence of the angle of the detector tilt angle can be negligible in the range of 10.(3) The detection efficiency of γspectrometer is different according to different incoming directions of γ-ray. If the incident gamma rays in different directions,.the greater the incidence angles, the higher the detection efficiency. Experiments show that detection efficiency is relatively higher on the lateral of cylindrical.(4) The detection efficiencyof γ-ray withdifferent energydecreases linearly with humidityincreasing of surface medium, but the degree of change are different. The humidity of medium has more affect for low energy γ-ray.(5) The absorption effect of γ-ray weakens graduallywithincreasing density of different medium in soil.When soil density increased from 1.3g ?cm-3to 2.2 g?cm-3,decreasing amplitude o f efficiency to detect gamma rays of 0.662 Me V, 1.46 Me V, 1.76 Me V, 2.62 Me V are 41.17% and 38.78% and 42.98%, 40.56%, and the difference of the detection efficiency of gamma ray is also gradually decreased.(6) When the soil thickness of bedrock surface layer increases, the attenuation of high-energy gamma ray is obviously lower than that of low energy gamma ray, and the total detection efficiency is in accordance with the law of polynomial attenuation.(7) The detection efficiency of the gamma ray decreases grad ually with the increase of the soil particle size in the range of.005 cm to 0.2cm.Soil particle size has more significant effects of low-energy gamma rays, the maximum deviation is 70%;the detection efficiency of the gamma ray is the same as the particle size ismore than 0.1cm.(8) According to the field measurement environment, analyzes the influence of the steps such as terrain, terrain slope, gully topography on the the measurement. |
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