井型γ谱仪土壤样品探测效率刻度方法研究

Study on efficiency calibrating method of well-type γ ray spectrometer for soil samples

井型γ谱仪能大幅缩短测量时间和样品用量,但在实际使用前,须对其进行效率刻度。介绍了一种采用MCNP(Monte Carlo N Particle Transport Code)通用软件包模拟计算与单能量点源实测修正相结合的井型γ谱仪土壤样品效率刻度方法。基于MCNP计算了γ射线不同能量、土壤样品不同质量的井型NaI(Tl)γ谱仪探测效率值,拟合了效率曲线数学模型,拟合值与MC模拟值最大相对偏差<±7%,并通过实测标准土壤样品源验证了所提方法的可行性,建立了基于MCNP模拟计算与体源实测修正相结合的井型γ谱仪效率刻度方法。

[Background]Putting samples into the well-typeγray spectrometer can greatly improve the activity measurement efficiency,therefore it can reduce the measurement time and the mass of samples.[Purpose]This study aims to accurately obtain detection efficiencies of well-type NaI(Tl) detector as a function of gamma ray energy and the well depth before using the well-typeγray spectrometer.[Methods]First of all,the Monte Carlo N Particle Transport Code (MCNP) was employed to simulate energy deposition spectrum ofγray in well-type NaI(Tl) detector according to mathematic model based on the characteristic parameters of detector and standardγsource.Based on simulating a well-type NaI(Tl) detector and soil samples,the peak efficiency of differentγray energy and depth of soil samples were calculated by MCNP4C program and corrected by single energy point source measurement.Meanwhile the efficiency calibration functions were fitted accoeding to detection efficiency of well type NaI(TI)gamma spectrometer with different energy and quality of soil samples.Finally,137Cs point source was used for experimental verification for main standard soil samples of SiO2,Al2O3,Fe2O3,Fe O,Mg O and Ca O.[Results]The maximum relative deviation between the function fitting value and the MCNP simulation value is less than±7%.[Conclusions]The efficiency calibrating method of well-type is established based on the combination of MCNP simulation calculation and volume source measurement correction.