Title: Absolute light yield measurement of NaI:Tl crystals for dark matter search

Abstract: The NaI:Tl crystals were early investigated and used for wide application fields due to high light yield and crystal growth advantages. So far, the absolute light yields of NaI:Tl crystal have typically been known to be 40 ph/keV. However, it varies widely, far from the theoretical estimation. Since the high light yield and better sensitivity of NaI:Tl crystal is important for low mass dark matter search. Therefore, it is necessary to use high light NaI:Tl crystal, and absolute light yield should be measured with accuracy. In this work, we use the single photoelectron technique for measuring the absolute light yield of 35 NaI:Tl crystals with various sizes from different vendors. There are several high-quality crystals from the COSINE-100 experiment and commercial companies in these crystals. The theoretical estimation and GEANT4 optical simulation have been studied to investigate the PMT optics. Results show the essential role of this correction in avoiding overrated light yield values. The SPE technique using different PMT was compared to the photodiode and avalanche photodiode methods. A 10% systematic error was obtained. Our results show the excellent absolute light yield of NaI:Tl, at 59.4 +- 5.9 ph/keV, while the theoretical predicted light yield is around 70 ph/keV. The evaluation with NaI:Tl crystals in the COSINE-100 experiment has been performed. The six crystals in the COSINE-100 experiment have a high light yield. Based on our results, the light loss of encapsulation needs to be improved, especially for the big-size crystals.