The Fluorescent Response of NaI(Tl), CsI(Tl), CsI(Na) and CaF2(Eu) to X-Rays and Low Energy Gamma Rays

The fluorescent response functions of NaI(Tl), CsI(Tl), CsI(Na) and CaF2(Eu) have been determined with thin crystals over the energy range from 3.3 keV to 840 keV. A pronounced nonlinear dependence on photon energy has been observed in this region for all four types of crystals investigated. Qualitatively the response functions for NaI(Tl), CsI(Tl) and CsI(Na) are similar down to a photon energy of about 20 keV, but major differences are observed at lower energies. The response function of CaF2(Eu) is very different from the response functions for the other three crystals over the entire energy range examined. Both K-shell and L-shell absorption edge discontinuities are evident in the fluorescent response curves for Na(Tl) and CsI(Tl). Only K-shell absorption edge discontinuities could be observed in the response curves for CsI(Na) and CaF2(Eu). A nonlinear fluorescent dependence on electron energy is indicated for all four types of crystals. Discontinuities in the spectral resolution function are observed in NaI(Tl) at energies corresponding to both the K-shell and L-shell absorption edges of iodine. A shift toward longer wavelengths in the light spectrum emitted by NaI(Tl) in response to excitation by X-rays with energies below about 10 keV may also be indicated by photomultiplier-dependent differences observed only at the lowest energies.