Test spectra experimental construction for evaluating gamma-spectrometry computer codes for the U-235 determination

The determination of ²³⁵U in environmental samples from its 185.72 keV photons may require the deconvolution of the multiplet photopeak at ~186 keV, due to the co-existence of the 186.25 keV photons of ²²⁶Ra in the spectrum. Successful deconvolution depends on many parameters, such as the detector characteristics, the activity concentration of the ²³⁵U and ²²⁶Ra in the sample, the background continuum in the 186 keV energy region and the gamma-spectrometry computer code used. In this work two sets of experimental test spectra were constructed for examining the deconvolution of the multiplet photopeak performed by different codes. For the construction of the test spectra, a high-resolution low energy germanium detector was used. The first series consists of 140 spectra and simulates environmental samples containing various activity concentration levels of ²³⁵U and ²²⁶Ra. The second series consists of 280 spectra and has been derived by adding ¹³⁷Cs, corresponding to various activity concentration levels, to specific first series test spectra. As the ¹³⁷Cs backscatter edge is detected in the energy region of the multiplet photopeak at ~186 keV, this second series of test spectra tests the analysis of the multiplet photopeak in high background continuum conditions. The analysis of the test spectra is performed by two different γ-spectrometry analysis codes: (a) spectrum unix analysis code, a computer code developed in-house and (b) analysis of germanium detector spectra, a program freely available from the IAEA. The results obtained by the two programs are compared in terms of photopeak detection and photopeak area determination.