Fluorescent Response of NaI (Tl) to Nuclear Radiations

Abstract

The dependence on thallium concentration of the fluorescent response of NaI (Tl) to gamma rays and charged particles has been examined for crystals containing known mole fractions of Tl in the range from 0.00006 to 0.008. Scintillations were detected with a photomultiplier tube and pulses were displayed on an oscilloscope and recorded photographically. Plots of integrated pulse height vs energy for alpha-particle excitation show a region of nonlinearity which decreases with increasing Tl concentration. At sufficiently high alpha-particle energies a linear relation is approached for all crystals. No deviations from linearity were observed in plots of pulse height vs energy for deuterons or protons in crystals containing a Tl mole fraction of 0.0013. The fluorescent efficiency increases sharply with Tl concentration for mole fractions smaller than about 0.0015 and decreases for higher concentrations, this behavior being more pronounced for deuteron than for alpha-particle excitation. Four separate decay processes characterize the pulse shapes. In addition to the main part of the pulse, which has a rise time (mean life) of about 5.9×${10}^{-8\mathrm{}}$ sec and a long, concentration dependent, exponential decay (decay constant 2.3 to 3.5×${10}^{-7\mathrm{}}$ sec, independent of exciting radiation), there are two faster decays: 1, an emission with a decay constant of 1.2×${10}^{-8\mathrm{}}$ sec from crystals containing Tl mole fractions less than about 0.0002; and 2, a 1.5×${10}^{-8\mathrm{}}$ sec decay from high Tl content crystals excited by particles having a large specific energy loss. The emission spectrum consists of two bands centered at about 3500 A and 4100 A, their exact positions depending upon the Tl concentration and shifting toward longer wavelengths with increasing Tl concentration. The 1.2×${10}^{-8\mathrm{}}$ sec emission is from the short wavelength band, and the other emissions are from the band at 4100 A. The dependence of the integrated pulse height on the Tl concentration and specific energy loss of the exciting particle are discussed, and possible explanations of the various decay processes are suggested.