Feasibility study of pure NaI and CsI directly coupled to PMTs at 77K for dark matter search

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A vacuum chamber hosting two photomultiplier tubes (PMTs) and a pure NaI or CsI crystal was built and operated at both room and liquid nitrogen temperatures (RT & LNT).The ratio of the quantum efficiency of PMT R8778(AR) at RT & LNT was measured to be very close to 1 using 2 LED with peak intensities at 470 and 300 nm, respectively. The light yields of the NaI and the CsI crystal at RT & LMT were measured using Ba133, Cs137, Co60 and K40 gamma-ray sources. Significant increase of the light yield at LNT compared to that at RT was observed in both crystals. A yield of ~18 photoelectrons/keV was reached in the CsI crystal at LNT. Such a high yield makes it possible to reach an energy threshold 3 times lower than that of the KIMS dark matter experiment, which utilizes CsI(Tl) at RT, and also to increase the signal/background discrimination power of scintillation pulse shape analysis, one of the key techniques to suppress the background for dark matter or neutrino-nucleus coherent scattering detection.The light yield of the NaI crystal at LNT was not as high due to its unpolished side surface and imperfect optical coupling to the PMT. The dimension of the NaI crystal was also too small to capture gamma-rays above 100 keV. This made it difficult to calibrate the system using common gamma-ray sources. The glove box was large enough to assemble the crystal and PMTs, but too small to house the whole vacuum chamber. Part of the assembly had to be done outside of the glove box, leading to partially moisturized surface of NaI crystal. All of these can be easily improved in the future.