Precision response estimation, energy calibration, and unfolding of spectra measured with a large NaI detector

Abstract

Methods are evaluated for determining the energy calibration and the response of a 20 cm by 25 cm NaI detector, and for unfolding the detector response from measured spectra of 10 to 30 MV bremsstrahlung. A set of measured γ-ray and electron spectra with energies ranging from 1 to 30 MeV were fit to response functions calculated using Monte Carlo techniques (EGS4) to determine the energy calibration of the spectrometer and the degree of broadening required for the calculated response functions. For electrons, the broadened response was in excellent agreement with the measurements. For γ-rays, there was evidence of collimator-degraded photons in the measured spectra. A matrix of broadened response-functions was generated for use in unfolding bremsstrahlung spectra in the energy range of 0.2 to 35 MeV. The unfolding routine FERDO was shown to provide reliable estimates of the spectrum of photons in the bremsstrahlung beam when this response matrix was used for unfolding. Collimator effects were ignored without significantly affecting the shape of the unfolded spectrum.