环境样品自吸收因子的计算和实验测定

CALCULATION AND EXPERIMENTAL DETERMINATION OF SELF ABSORPTION FACTORS OF ENVIRONMENTAL SAMPLES BY MONTE CARLO METHOD

用γ谱仪分析大体积环境样品时,需要作自吸收修正。本文介绍用蒙特卡洛方法,计算以水、土壤、铝、铁为基质,直径为7.5cm,高为2、4、6、8cm的圆柱形体源在50—3000keV能区内的自吸收因子。在100—3000keV能区,考虑了Ge(Li)晶体影响的自吸收因子f_1与考虑了晶体及其死层和包装材料影响的自吸收因子f_2相等,它们均大于不考虑上述影响因素时的自吸收因子f_0;对于不同的基质,f_1、f_2与f_0之间的差别小于4.4％(不包括铁时小于2.1％)。在50—100keV能区,f_1与f_0近似相等,其偏差小于1.56％(不包括铁时小于0.93％);f_2与f_0之间的偏差可达10％(不包括铁时小于5％)。用NaI(Tl)代替Ge(Li)时,自吸收因子的变化小于0.23％。为了检验计算结果,用^(152)Eu点源模拟测量有基质和无基质时的体源效率,得到自吸收因子的实验值f_实,与计算值f_计相比,在2.6％之内一致。文中给出了几种不同成分、不同密度基质的体源在50—3000keV能区的自吸收因子f与线减弱系数μ_s的关系曲线,以及用实验方法确定待测样品基质的线减弱系数,以便用于环境样品测量中进行自吸收修正。

For γspectrum analysis of bulk environmental sample, itis necessary to find out its self absorption factor f_0, which is called f_1 ifthe additional effects from Ge(Li) crystal are considered, and f_(2计) the addit-ional effects from crystal and its-dead layer and packaging materialconsidered. In 50-3000keV energy range the factors Were calculated throughMonte Carlo Method in this paper for cylindrical sources with matricesof water, iron, aluminium, Soil (NBS matrix reference material) indiameter of 7.5cm and heights of 2, 4, 6, 8cm respectively. For γrayenergy ranging from 100 to 3000keV, the results showed that: f_1 wasapproximately equal to f_2, and both of them were larger than f_0;the differences between f_1 or f_2 and f_0 were less than 2.1% for all conside-red matrices with an exception of 4.4% for iron. For 50-100keV energyrange, f_1 was approximately equal to f_0 with difference less than 1.56%(0.93% when not including iron), while the differences between f_2 andf_0 might be as hight as 10% (less than 5% when not including iron).Substituting Nal(T1) for Ge(Li) crystal, the change of self absorptionfactors was less than 0.23%. To order to check the calculated results,analogue measurements of efficiencies of bulk sample with and withoutmatrix were carried out seperately using ^(152)Eu point source. The resultssuggested that the calculated self absorption factors were consistent withthat experimentally obtained self absorption factors within the range of2.6%.In the paper, in 50-3000keV the curve of self absorption factors versuslinear attenuation coefficients are given through calculation for samplematrices of different composition and density, and linear attenuationscoefficients of some matrix materials experimentally measured are alsopresented so as to faciliate the use of above calculated results in γspectrumanalysis.