Spectrum shape analysis applied to radioactive waste gamma-ray spectroscopy

Gamma ray spectroscopy of radioactive waste drums is often difficult at low energy (less than 200 keV) because of high photon attenuation in the matrix. This paper deals with the assay of 225 liters packages (about 60 US gallons) filled with radioactive salts mixed with bitumen. The presence of high-Z elements, such as barium (Z=56) or uranium (Z=92), has a great impact on the low energy photon attenuation. As these elements appear in various quantities in the waste, the attenuation corrections required for activities calculations is quite delicate. We propose to take this effect into account through the analysis of the spectrum shape. The low energy continuum due to scattered photons (partly in the matrix and partly in the germanium detector), is indeed very related to the heavy elements content, whereas this is not the case of the 661.7 keV peak of 137mBa (137Cs radioactive daughter), main gamma emitter in this waste. (orig.)