POSITRONFIT: A versatile program for analysing positron lifetime spectra

Abstract

TlPbI3 is a promising semiconductor material for fabricating room-temperature radiation detectors, which have wide applications in national security, medical imaging, astrophysics research, industrial process monitoring and environmental survey. TlPbI3 has a large energy bandgap at 2.3 eV, a high density (6.04 g/cm3) and high concentrations of the high atomic number elements Tl and Pb. Such physical properties offer great potential to use TlPbI3 to detect gamma-ray at room temperature with high detection efficiency. In this work, we used the positron annihilation lifetime spectroscopy (PALS) measurement and infrared transmission microscopy to study the material defects in bulk TlPbI3 crystals. These crystals were grown with Bridgman method. For the PALS measurements, we used the positron experimental setup at North Carolina State University’s PULSTAR reactor facility. A 15 μCi Na-22 positron source sealed with 7.6μm thick Kapton films was sandwiched between two identical pieces of TlPbI3 samples. Two cylindrical plastic scintillators (1 inch diameter by 1 inch long) combined with Hamamatsu H3378-50 photomultiplier tubes (PMT) were used to detect the 1.27 MeV gamma-rays in coincidence with the 511 keV annihilation gamma-rays as the start and the stop signals, respectively. A LeCroy Wavepro 7300A digital oscilloscope was used to digitize the raw PMT pulses and acquire the PALS spectra. The dominating positron lifetime in TlPbI3 is 393 ps and its intensity is more than 92%. This component is typically attributed to some vacancy type (or more likely, vacancy cluster) positron trapping sites. The first component of ∼ 140 ps could be related to mono-vacancies or positrons annihilate in a delocalized lattice state. Compared with MAPbI3, the higher average lifetime, τav, and the higher intermediate lifetime (τ2) in TlPbI3 indicate the presence of more anion-type vacancies and imply an increase in ionic conductivity. Using infrared transmission microscopy, we also observed the formation of large volume TlPbI3 single crystal even in the transition portion between the conical seeding pocket and the normal growth chunk.