摘要
本文介绍了用于土壤密度和含水量测量的^(137)Csr射线衰减装置的研制、校准及其测试方法的研究。通过充分地屏蔽和准直,适当地控制计数率和计数时间等途径,使测量误差达到1%之内。本文还设计了多格容器逐格加水的实验方法,简便而准确地校准和标定了测量系统,证明了计数系统具有较好地线性。最后,本文讨论了分辨时间校正问题和测量精度问题,给出了测量误差的估算公式。
The present paper deseribes a nondestructive method in which gamma ray attenuation is used to determine the soil density and water content in a laboratory column.The gamma ray apparatus consists of a radiation source,a detector,a counting system and a mounting flat form. The source is a 60 m_(cl) cesium-137 with a peak gamma ray energy of 0.661 MeV.The derector contains a sodium iodide (NaI) scintillation crystal of 4 cm diameter and 4 cm thick,a photomultiplier tube and a preamplifier.Sufficient shielding and collimating of gamnla radiation is cssential to improve space resolution and measurement accuracy,and to warrant personal safety.Collimation is required on both source and detector sides.In our equipment,the holes in source-side and detectorside Collimators are 8 cm in length and 1 cm in diameter and then stanard errors less than 0.001 g/cm^3 may be obtained for a 10 cm thick soil with a bulk density of 1.4 g/cm ~3.By means of shieIding the source,Collimating the beam and controlling the counting rate and time,percentage errors for the system may reach 1%and even less.To determine the soil density and water content with gamma ray attenuation,it is necessary to know mass absorption coefficients which dedend only on material if the source is specified.The calibration experiments were conducted in plexiglas containers.Measured mass absorption coefficiens for water, plexiglas,sand,loam and clay were 0.0814,0.0775,0.0746,0.0744 and 0.0732 respectively.For the soil samples mentioned above,measurements were made at several points and averaged due to nonuniform packing.Two experimental procedures were designed where water is successively filled into a compartmented box.This method provides a convenient and accurate approach to calibrate the system.The results of calibration measurement indicate that the system possesses a good linearity of 0.99999 and measuement deviation of 0.0025 g/cm^3.ResolVing time correction is an important problem,especially at high count rates.To avoid the correcltion,the resolving time of Counting system should be less than 1 μs. According to error analysis,errors in the soil density and water content measurements is primarily due to the counting error which results from the random nature of gamma ray emission。The ormula for estimation of measurement error is given in this paper.
出处
《土壤学报》
CAS
CSCD
北大核心
1991年第1期58-65,共8页
Acta Pedologica Sinica
基金
国家自然科学基金
