Nonlinear scintillation response of thin NaI(Tl) crystals

Abstract

The nonlinear scintillation response, L, of two thin NaI(Tl) crystals, 1,5″ diameter × 3 mm and × 5 mm, was measured accurately by a spectroscopy system with and without coincidence technique. The measurements were made over the energy range from 6 to 1275 keV, using gamma-rays and K X-rays from radioactive sources. Two peak values 1.276 and 1.201 of scintillation efficiency, L/ E, were obtained for photon energies E = 11.5 and 45.5 keV, respectively, where the L/ E at 661 keV was normalized to unity. No distinct difference in L/ E was observed between the two crystals and also between gamma-rays and K X-rays at energies below 60 keV. The accuracy of L/ E was estimated to be ±0.25%− −0.6 +0%, depending on E. The contribution of the electron binding energies of the iodine atom to the nonlinear response was demonstrated clearly in the experiment. The average absorption energy per photon event in the crystals was also measured with a modified ADC system.