Absolute light yield measurement of various NaI:Tl crystal scintillators

We report on investigating the absolute light yield of various NaI:Tl crystals, which have been used for a variety of applications such as medical imaging, homeland security, and pure science research due to their high light yield, low cost, and crystal growth advantages. Even if it was known that the absolute light yield is known to be approximately 40,000 photons/MeV, the absolute light yields of these crystals have varied widely from typical values. We focus on using the PMT single photoelectron technique to measure the absolute light yield of many NaI:Tl crystals of various sizes, including high-quality crystals grown by laboratory and commercial companies. We also investigate the PMT optic using theoretical estimation and GEANT4 optical simulation to correct overrated light yield values. The results show the essential role of PMT optic correction in obtaining accurate absolute light yield values. We also compare the single photoelectron technique using different PMT to the method using a photodiode and avalanche photodiode and find a 10% systematic error. Our study shows that light yield varies from 30,000 photons/MeV – 60,000 photons which is close to the theoretical predicted light yield of around 70,000 ph/MeV. Overall, our study provides valuable insights into the absolute light yield of NaI:Tl crystals and the importance of correcting for the PMT optic to obtain accurate absolute light yield.