Segregation effects of Tl and Li ions on the scintillation properties of NaI:Tl,Li single crystals

In this work, high quality NaI:Tl and NaI:Tl,Li single crystals with a size of 11 mm in diameter and 135 mm in length were grown by the multi-ampoule Bridgman method. The concentrations of Tl and Li ions along the solidification direction were measured by the inductively coupled plasma optical emission spectrometer. The effective segregation coefficients of Tl and Li in NaI single crystals were estimated as 0.26 and 0.35, respectively. The radioluminescence spectra under X-ray excitation, pulse height spectra and scintillation decay kinetics under γ-ray excitation of the samples taken from different solidification positions in NaI:Tl and NaI:Tl,Li single crystals were systematically studied. For RL spectrum and scintillation decay time, there is a negligible position-dependent difference for both NaI:Tl and NaI:Tl,Li single crystals. In contrast, the energy resolution at 662 keV gradually deteriorates along the solidification direction, and the light yield maintains almost the same until reaching the last-to-freeze region.