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

초록

NaI:Tl crystals have been extensively investigated and widely applied due to their high light yield and advantageous crystal growth properties. Historically, the absolute light yield of NaI:Tl crystals has been reported around 40 ph/keV, although this value often deviates from theoretical estimations. In recent decades, the high light yield and enhanced sensitivity of NaI:Tl crystals have become crucial for low-mass dark matter searches, necessitating the accurate measurement of their absolute light yield. In this study, we employed the single photoelectron (SPE) method using a photomultiplier tube (PMT) to measure the absolute light yield of 35 NaI:Tl crystals of various sizes from different vendors. Among these crystals, several high-quality samples were sourced from the COSINE-100 experiment and commercial suppliers. Our GEANT4 optical simulations underscore the critical importance of precise PMT optics in accurately measuring absolute light yield, consistent with optical theory. To prevent significant overestimations of light yield values, we strongly recommend the implementation of PMT optical corrections when using the PMT method. This correction is essential for ensuring the reliability and accuracy of measurements. The SPE measurement was compared to other methods, revealing a systematic error of approximately 10% for the SPE method. Our results demonstrate an excellent absolute light yield for NaI:Tl crystals, measuring 59.4 +/- 5.9 ph/keV under 661 keV gamma-ray excitation at room temperature. The light yield results range from 50 ph/keV to 60 ph/keV, setting a new target for NaI:Tl absolute light yield to exceed 60 ph/keV, approaching the theoretical prediction of around 70 ph/keV. Evaluation of NaI:Tl crystals in the COSINE-100 experiment suggests that these crystals exhibit high light yields but experience significant light losses.

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