Tests on NaI(Tl) crystals for WIMP search at the Yangyang Underground Laboratory

Abstract

Among the direct searches for WIMP-type dark matter, the DAMA experiment is unique in that it has consistently reported a positive signal for an annual-modulation signal with a large (9.3σ) statistical significance. This result is controversial because if it is interpreted as a signature for WIMP interactions, it conflicts with other direct search experiments that report null signals in the regions of parameter space that are allowed by the DAMA observation. This necessitates an independent verification of the origin of the observed modulation signal using the same technique as that employed by the DAMA experiment, namely low-background NaI(Tl) crystal detectors. Here, we report first results of a program of NaI(Tl) crystal measurements at the Yangyang Underground Laboratory aimed at producing NaI(Tl) crystal detectors with lower background levels and higher light yields than those used for the DAMA measurements.

Pulse-shape discrimination between electron and nuclear recoils in a NaI(Tl) crystal

“…We have achieved a background level of approximately 3 dru at 6 keV with a below 2 keV energy threshold [44]. Because we already identified the significant backgrounds of the NaI(Tl) crystals as being due to 210 Pb and 40 K contaminations, the background levels of future crystals that are produced as part of this program will be significantly reduced.…”

“…In the past, the discrimination power of NaI(Tl) crystals has been much poorer than that of CsI(Tl) crystals [49][50][51]. However recently developed NaI(Tl) crystals grown by Alpha Spectra Inc. provide higher light yields [42,44], and this makes it interesting to study their PSD performance and evaluate their sensitivity.…”

“…The trigger condition for the NaI(Tl) crystal was one or more photoelectrons in each PMT within a 200 ns window, which is the same condition that was used for the underground data described in ref. [44]. We additionally required a time coincidence between the NaI(Tl) crystal and one of the neutron detectors to occur within a 2 µs time window.…”

“…A small 2×2×1.5 cm 3 NaI(Tl) crystal made from the same, Alpha Spectra-grown, ingot as a large crystal (NaI 002) used for background measurements in the Yangyang underground laboratory [44] was used as a target. Two 3-inch photomultiplier tubes (PMTs) that were selected for high quantum efficiency (R12669SEL, Hamamatsu Photonics) were attached to each end of the NaI(Tl) target crystal, which was located at the exit of the collimated aperture of the neutron source.…”

“…To reproduce the DAMA/LIBRA observation, several experimental groups have started developing ultra-low-background NaI(Tl) crystals [42][43][44] with target background levels at or below those measured in the DAMA/LIBRA experiment. If this background…”