Improved Isotopic Identification for NaI Spectroscopic Portal Monitors Using the Material Basis Set (MBS) Method

This report describes our application of the multiple isotope material basis set (MIMBS) method for isotope identification to the analysis of simulated spectra of Nal logs used in spectral portal monitors. The MIMBS method couples attenuation corrections for shielding materials with an ordinary isotopic response function fit. Spectrum simulations of twenty-four isotopes were generated using the Speculator software (with MCNP for attenuation calculations) for a 3 x 5 x 16-inch Nal log detector, through a series of attenuating absorbers representing a Z range from Al through Pb. Two-isotope combinations of these were analyzed using the MIMBS algorithm, with and without added 4 K background and Poisson noise. We found the MIMBS method to be highly accurate in identifying both the component isotopes and their relative amounts, even at high noise and background levels.