Application of the multiple isotope material basis set (MIMBS) method of isotope identification to low energy gamma emitters

The multiple isotope material basis set (MIMBS) method for isotope identification combines the material basis set (MBS) model of gamma spectrum attenuation with ordinary response function fitting to identify shielded gamma-emitting isotopes, using low and medium resolution gamma detectors such as NaI and LaBr3. Although MIMBS has been shown to outperform conventional isotope identification algorithms that do not correct for attenuation effects, it has difficulty identifying low energy emitters such as 57Co or 241Am. In this article we examine the use of optimized multiple attenuator thicknesses in generating basis spectra for each isotope to obtain better modeling of the low energy spectrum while simultaneously extending the range of the model to thicker attenuators. The effectiveness of the multiple thickness MIMBS algorithm in improving isotope identification rates compared with the original MIMBS method is demonstrated with analyses of simulated gamma spectra. The identification rates obtained with the MIMBS methods are compared to those obtained using the commercial peak-based ScintiVision NaI analysis software.