The Handling of K-Edge Effects in the Multiple Isotope Material Basis Set (MIMBS) Method of Isotope Identification

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(Tl) and LaBr₃. We recently improved the MIMBS algorithm to handle low energy gamma emitters such as ¹³³Xe or ²⁴¹Am. A concern with fitting the low energy region (below approximately 90 keV) is that the underlying MBS model assumptions fail because of the inherent K-edge discontinuities in the attenuation versus energy curve for the different atomic species. For example, with basis attenuators of Al (Z = 13) and Pb (Z = 82) , the MBS model for the attenuation curve of Sn (Z = 50) would have Al and Pb K-edge discontinuities at 1.55 and 88 keV, rather than at the Sn K-edge energy of 29.2 keV. Complex mixtures such as cargo would have a complex and unpredictable K-edge distribution. In this presentation we show that the effect of K-edge discontinuities on the improved multiple-thickness MIMBS algorithm for low energy isotope identification and spectrum simulation is small for common attenuator distributions.