Abstract
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 over1000 kg.The response of the central and other 18 MTAS modules to γ-radiation was
simulated using the GEANT4 tool kit modified to analyze the nonlinear light output of NaI(Tl).A detailed
description oftheGEANT4modifications madeisdiscussed.SimulatedenergyresolutionofMTAS
modules is found to agree well with the measurements for single γ transitions of 662keV (137Cs) with
8.2% full width half maximum (FWHM),835keV (54Mn) with FWHM of 7.5% FWHM, and 1115keV (65Zn)
with FWHM of 6.5%.Simulations of single and multiple γ-rays from 60Co are also discussed.
