野外γ能谱仪效率刻度方法研究及软件设计

发布时间：2018-09-13 10:16

【摘要】：野外γ能谱测量是指通过γ谱仪直接测量地表岩石、土壤等,从得到的γ谱特征中分析出地层中核素的活度浓度、含量及相关信息。该方法是地质勘查和放射性测量中经济、准确、高效的核地球物理方法,广泛应用于环境监测、铀资源勘探、核应急处理等方面,而准确的效率刻度是伽马能谱仪应用的重要环节之一。尤其是处理核应急事件,需要工作人员根据测量环境,选择适合的效率刻度方法,快速准确的完成现场检测。分别从γ能谱仪绝对测量和相对测量两个方面进行研究,包括数值计算法、无源效率刻度法、有效刻度法等三种主要的效率刻度方法,对比三种方法各自的原理、特点,并分析影响效率刻度的因素以及提出合适的修正方法,方便使用者因地制宜选择适合的刻度方法,是γ能谱仪能准确完成环境放射性核素定量分析的重要前提。现场γ能谱测量往往受地表土壤密度、湿度、地形、核素分布不均匀、表层覆盖物、介质自吸收、土壤或岩石孔隙度、铀镭不平衡等因素的影响,尤其是地表湿度和地形的影响,导致测量结果存在较大的误差,这些问题都需要测量人员依据测量环境对结果进行适当的修正。以国家计量站的标准模型为研究对象,为准确的完成对Na I(Tl)数字化γ谱仪的效率刻度,通过理论公式及蒙特卡罗方法,确定γ谱仪的有效探测范围、计算上述因素对测量结果的影响,并研究其变化规律,为修正定量分析的测量结果提供理论依据。此外,为方便用户对野外γ能谱仪进行效率刻度、选择合适的测量位置以及修正测量结果,尤其是在核应急事件中,用户需要根据现场测量环境的变化,及时准确完成对γ能谱仪的效率刻度或校正效率刻度的结果以及γ谱仪的稳定性检测等,采用无源效率刻度方法,基于Lab Windows/CVI开发环境下,编写高效的效率刻度分析软件,并通过实验进行验证(此外,用户若采用数值计算法、有源效率刻度法对γ谱仪进行效率刻度,也可采用该软件实现部分理论相关参数的计算)。通过对现场γ能谱测量影响因素的研究,得出如下结论:(1)在半无限测量条件下,通过蒙特卡罗方法可较准确的计算出特定型号的Na I(Tl)γ谱仪贴地测量时的有效可探测范围为?180×60cm,且随着γ能谱仪距离地面高度的增加,其探测效率降低。(2)当现场测量的地表起伏,如γ能谱仪贴地测量时,可能发生倾斜,实验表明,探测器倾斜角度在±10°范围内对测量结果的影响较小,可忽略不计。(3)γ能谱仪对不同入射方向的γ射线探测效率不同,随入射角度(偏离晶体正前方的角度)的越大,探测效率越高,表明圆柱形探测器侧面的探测效率相对越高。(4)不同能量γ射线的探测效率与地表土壤的湿度呈线性负相关关系,且线性变化的趋势不同。其中,低能γ射线受介质湿度的影响最大。(5)土壤对不同能量γ射线自吸收效应随介质密度的增大而逐渐减弱。土壤的密度从1.3g?cm-3增加到2.2 g?cm-3,0.609 Me V、1.46 Me V、1.76 Me V、2.62Me V特征γ射线的探测效率分别减小了41.17%、38.78%、42.98%、40.56%,且不同能量特征γ射线间探测效率差异也逐渐减小。(6)当基岩表层覆盖物土壤厚度增加时,高能γ射线的衰减量明显小于低能γ射线,且总的探测效率符合多项式衰减规律。(7)土壤颗粒半径由0.005cm至0.2cm变化时,随着粒径的增加,γ射线的探测效率逐渐降低,且对测量低能γ射线的影响最大,其最大偏大可达70%;当粒径大于0.1cm时,其γ射线的探测效率基本不变。(8)针对野外测量环境,分析了台阶地形、山坡地形、冲沟地形等三种地形对测量结果的影响,并提出了模型简化的理论计算方法。
[Abstract]:Field gamma-ray spectrometry is an economical, accurate and efficient nuclear geophysical method for geological exploration and radioactivity measurement, which is widely used in environmental monitoring and uranium resource exploration. Accurate efficiency calibration is one of the important links in the application of gamma spectrometer in nuclear emergency treatment. Especially in dealing with nuclear emergency, it is necessary for staff to select appropriate efficiency calibration methods according to the measurement environment and complete the field detection quickly and accurately. Three main efficiency calibration methods, including numerical calculation method, passive efficiency calibration method and effective calibration method, are studied. The principle and characteristics of the three methods are compared. The factors affecting the efficiency calibration are analyzed and the appropriate correction methods are put forward. It is convenient for users to choose the appropriate calibration method according to local conditions. The gamma spectrometer can complete the ring accurately. Field gamma-ray spectrometry is often affected by factors such as surface soil density, humidity, topography, uneven distribution of nuclides, surface cover, self-absorption of media, porosity of soil or rock, imbalance of uranium and radium, especially surface humidity and topography, resulting in errors in measurement results. In order to calibrate the efficiency of Na I (Tl) digital gamma spectrometer accurately, the effective detection range of the gamma spectrometer is determined by theoretical formula and Monte Carlo method, and the above factors are calculated. In addition, in order to facilitate users to calibrate the efficiency of field gamma spectrometer, select the appropriate measurement position and correct the measurement results, especially in nuclear emergency, users need to be timely and accurate according to the changes of the field measurement environment. Accurately complete the efficiency calibration or calibration efficiency calibration results of the gamma spectrometer and stability testing of the gamma spectrometer, using passive efficiency calibration method, based on Lab Windows/CVI development environment, write efficient efficiency calibration analysis software, and verify by experiments (in addition, if users use numerical calculation method, active efficiency calibration method) The efficiency calibration of gamma spectrometer can also be carried out by this software. Through the study of the influence factors of gamma spectrometer measurement, the following conclusions can be drawn: (1) Under the semi-infinite measurement conditions, the Monte Carlo method can accurately calculate the validity of a specific type of NAI (Tl) gamma spectrometer when it is applied to the ground. The detection range is 180 The detection efficiency of the cylindrical detector is higher with the increase of the incidence angle (the angle away from the front of the crystal), which indicates that the detection efficiency of the cylindrical detector is higher. (4) The detection efficiency of the different energy gamma rays has a linear negative correlation with the surface soil moisture, and the trend of the linear change is different. Among them, low-energy gamma rays are most affected by medium humidity. (5) Soil self-absorption effect of different energy gamma rays gradually weakens with the increase of medium density. Soil density increases from 1.3 g? Cm-3 to 2.2 g? Cm-3, 0.609 Me V, 1.46 Me V, 1.76 Me V, 2.62 Me V. The detection efficiency of characteristic gamma rays decreases by 41.17%, 38.78%, 42.98%, 40.56%, respectively. (6) The attenuation of high-energy gamma rays is obviously less than that of low-energy gamma rays when the soil thickness of bedrock surface mulch increases, and the total detection efficiency conforms to the polynomial attenuation law. (7) When the radius of soil particles varies from 0.005 cm to 0.2 cm, with the increase of particle size, the attenuation of gamma rays is obviously smaller than that of low-energy gamma rays. The detection efficiency decreases gradually, and the influence on the measurement of low-energy gamma rays is the greatest, and the maximum is 70%. When the particle size is larger than 0.1 cm, the detection efficiency of gamma rays is basically unchanged. (8) Aiming at the field measurement environment, the influence of terrain, such as step terrain, hillside terrain and gully terrain on the measurement results is analyzed, and the simplified model is proposed. Theoretical calculation method.
【学位授予单位】：成都理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TH842

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