Calculation of the relative counting rate of a well-type sodium iodide scintillation crystal in isotopic neutron source activation analysis

Abstract

Calculations of the relative counting rates of ∅ 5″'×5″' sodium iodide well detectors have been made for samples activated with an isotopic neutron source. The study was undertaken in order to determine the most efficient source, sample and detector combination for a NA analyzer, which will be used to assay the Si-contents of the output from a ferrochrome smelting furnace. Hence only one energy of the gamma rays, 1.78 MeV, corresponding to the energy of 28Al which is formed by the reaction 28Si(n,p) 28Al in the bombardment with fast neutrons, has been considered. The counting rate has, however, been studied as a function of various parameters, e.g. the neutron source dimensions and the emission rate of the source, sample dimensions, and the radius and depth of the crystal well. The calculation is divided into two parts; calculation of the activity of the sample and evaluation of the detection efficiency of the crystal. The sample activity is calculated using Romberg's method for numerical quadrature. No absorption of the neutrons, in the neutron source, its steel encapsulation or in the sample holder and the sample material, is taken into consideration. The detector efficiency is evaluated using a Monte Carlo program, and the attenuation of the gamma photons by the sample is considered.