The nonlinear light output of NaI(Tl) detectors in the Modular Total Absorption Spectrometer

A new detector array, the Modular Total Absorption Spectrometer (MTAS), was commissioned at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Lab (ORNL). Total absorption gamma spectra measured with MTAS are expected to improve beta-feeding patterns and beta strength functions in fission products. MTAS is constructed out of hexagonal NaI(Tl) detectors with a unique central module surrounded by 18 identical crystals assembled in three rings. The total NaI(Tl) mass of MTAS is over 1000 kg. The response of the central and other 18 MTAS modules to γ-radiation was simulated using the GEANT4 toolkit modified to analyze the nonlinear light output of NaI(Tl). A detailed description of the GEANT4 modifications made is discussed. Simulated energy resolution of MTAS modules is found to agree well with the measurements for single γ transitions of 662 keV (137Cs) with 8.2% full width half maximum (FWHM), 835 keV (54Mn) with FWHM of 7.5% FWHM, and 1115 keV (65Zn) with FWHM of 6.5%. Simulations of single and multiple γ-rays from 60Co are also discussed.