Modeling Background Attenuation by SampleMatrix in Gamma Spectrometric Analyses

摘要

In laboratory gamma spectrometric analyses, the procedures for estimating backgroundusually overestimate it. If an empty container similar to that used to hold samples is measured, itdoes not consider the background attenuation by sample matrix. If a "blank" sample is measured,the hypothesis that this sample will be free of radionuclides is generally not true. The activity ofthis "blank" sample is frequently sufficient to mask or to overwhelm the effect of attenuation sothat the background remains overestimated. In order to overcome this problem, a model wasdeveloped to obtain the attenuated background from the spectrum acquired with the emptycontainer. Beyond reasonable hypotheses, the model presumes the knowledge of the linearattenuation coefficient of the samples and its dependence on photon energy and samples densities.An evaluation of the effects of this model on the Lowest Limit of Detection (LLD) is presented forgeological samples placed in cylindrical containers that completely cover the top of an HPGedetector that has a 66% relative efficiency. The results are presented for energies in the range of 63to 2614keV, for sample densities varying from 1.5 to 2.5g.cm~(-3), and for the height of the materialon the detector of 2 cm and 5 cm. For a sample density of 2.0g.cm~(-3)and with a 2cm height, themethod allowed for a lowering of 3.4% of the LLD for the energy of 1460keV, from ~(40)K, 3.9% forthe energy of 911keV from ~(228)Ac,4.5% for the energy of ~(609)keV from ~(214)Bi, and8.3 % for theenergy of 92keV from ~(234)Th. For a sample density of 1.75g.cm~(-3)and a 5cm height, the methodindicates a lowering of 6.5%, 7.4%, 8.3% and 12.9% of the LLD for the same respective energies.