Response of NaI(Tl) to X-rays and electrons

Abstract

Measurements of the light output response of NaI(Tl) to X-rays were taken as part of the pre-flight calibrations of the High Energy X-ray Timing Experiment (HEXTE), currently in orbit aboard NASA's Rossi X-ray Timing Explorer. These measurements cover the energy range 12–159 keV. Radionuclides and fluorescent sources were used to provide a coarse sampling of this energy range, and a tunable X-ray monochromator was used to provide a fine scale mapping around the iodine K-edge (33.17 keV), where the light output response of NaI(Tl) varies rapidly with energy. The light output response to X-rays is measured to an accuracy of ± 0.7%. In addition to presenting the results of these X-ray calibrations, we use these data to infer the response of NaI(Tl) to electrons in the energy range 0.1–125 keV. This is done using the procedure of Collinson and Hill [Proc. Phys. Soc. 81 (1963) 883], modified to take into account systematic effects that have been noted in the literature and to improve the model that is used to describe the cascade liberation of electrons that follows the absorption of an X-ray in the crystal. The Collinson and Hill procedure is particularly well suited to work with X-ray data, such as ours, that include detail at energies just above the iodine K-edge; the data here are used to determine the response of NaI(Tl) to low energy electrons, a point of contention in the literature. Our measurements of the electron response extend down to lower energies than have been examined in the past. The light output response to electrons is measured to an accuracy of ±90% at 0.1 keV, ±50% at 1 keV, ±5% at 10 keV, and ±4% at 100 keV.