用铯-137γ射线衰减法测量土壤密度和含水量

本文介绍了用于土壤密度和含水量测量的¹³⁷Csr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。

MEASUREMENT OF SOIL DENSITY AND WATER CONTENT WITH Cs-137 GAMMA RAY ATTENUATION

The present paper describes 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 flatform.The source is a 60 mcl cesium-137 with a peak gamma ray energy of 0661 MeV.The detector 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 gamma radiation is essential to improve space resolution and measurement accuracy,and to warrant personal safety.Collimation is required on both source and detector siides.In our equipment,the holes in source-side and detectorside collimators are 8 crn in length and 1 cm in diameter and then stanard errors less than 0.001 g/cm³ may be obtained for a 10 cm thick soil with a bulk density of 1.4 g/cm³.By means of shielding 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 garruna ray attenuation,it is necessary to know mass absorption coefficients whnch depend 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 00814,0.0775,0.0746,0.0744 and 0.0732 respectively.For the soil samples mentioned above,measurements were made at several points and averaged due to nonuniform packing.