A minimum biased shell-source method for the calibration of rad waste assay systems

This article outlines a simple experimental procedure to obtain the minimum biased detection efficiency calibration for gamma waste assay systems using the so called shell-source method in cylindrical waste drums. The conventional use of the shell-source method to simulate a homogeneous activity distribution in a cylindrical waste drum results in a systematic underestimation of the detection efficiency. The bias may be more then 30% at gamma energies of 59 keV and a waste density of 2 g/cm3. Experimental evidence of this bias is given for the low level waste assay system, Q2, and a simple measurement procedure to eliminate this bias is presented. The proposed method does not rely on data from computer simulations and is applicable to all types of waste matrices.