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Register a new userFirst search for a dark matter annual modulation signal with NaI(Tl) in the Southern Hemisphere by DM-Ice17
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We present the first search for a dark matter annual modulation signal in the Southern Hemisphere conducted with NaI(Tl) detectors, performed by the DM-Ice17 experiment. Nuclear recoils from dark matter interactions are expected to yield an annually modulated signal independent of location within the Earths hemispheres. DM-Ice17, the first step in the DM-Ice experimental program, consists of 17 kg of NaI(Tl) located at the South Pole under 2200 m.w.e. overburden of Antarctic glacial ice. Taken over 3.6 years for a total exposure of 60.8 kg yr, DM-Ice17 data are consistent with no modulation in the energy range of 4-20 keV, providing the strongest limits on weakly interacting massive particle dark matter from a direct detection experiment located in the Southern Hemisphere. The successful deployment and stable long-term operation of DM-Ice17 establishes the South Pole ice as a viable location for future dark matter searches and in particular for a high-sensitivity NaI(Tl) dark matter experiment to directly test the DAMA/LIBRA claim of the observation of dark matter.
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We report the measurement of muons and muon-induced phosphorescence in DM-Ice17, a NaI(Tl) direct detection dark matter experiment at the South Pole. Muon interactions in the crystal are identified by their observed pulse shape and large energy depositions. The measured muon rate in DM-Ice17 is 2.93 +/- 0.04 muons/crystal/day with a modulation amplitude of 12.3 +/- 1.7%, consistent with expectation. Following muon interactions, we observe long-lived phosphorescence in the NaI(Tl) crystals with a decay time of 5.5 +/- 0.5 s. The prompt energy deposited by a muon is correlated to the amount of delayed phosphorescence, the brightest of which consist of tens of millions of photons. These photons are distributed over tens of seconds with a rate and arrival timing that do not mimic a scintillation signal above 2 keVee. While the properties of phosphorescence vary among individual crystals, the annually-modulating signal observed by DAMA cannot be accounted for by phosphorescence with the characteristics observed in DM-Ice17.
We present new constraints on the dark matter-induced annual modulation signal using 1.7 years, of COSINE-100 data with a total exposure of 97.7 kg$cdot$years. The COSINE-100 experiment, consisting of 106 kg of NaI(Tl) target material, is designed to carry out a model-independent test of DAMA/LIBRAs claim of WIMP discovery by searching for the same annual modulation signal using the same NaI(Tl) target. The crystal data show a 2.7 cpd/kg/keV background rate on average in the 2--6 keV energy region of interest. Using a $chi$-squared minimization method we observe best fit values for modulation amplitude and phase of 0.0092$pm$0.0067 cpd/kg/keV and 127.2$pm$45 d, respectively.
DM-Ice is a phased experimental program using low-background NaI(Tl) crystals with the aim to unambiguously test the claim of dark matter detection by the DAMA experiments. DM-Ice17, consisting of 17 kg of NaI(Tl), has been continuously operating at a depth of 2457 m in the South Pole ice for over five years, demonstrating the feasibility of a low-background experiment in the Antarctic ice. Studies of low and high energy spectra, an annual modulation analysis, and a WIMP exclusion limit based on the physics run of DM-Ice17 are presented. We also discuss the plan and projected sensitivity of a new joint physics run, COSINE-100, with upgraded detectors at the Yangyang Underground Laboratory in Korea.
A dark matter search project needs and extremely low background radiation detector since the expected event rate of dark matter is less than a few events in one year in one tonne of the detector mass. The authors developed a highly radiopure NaI(Tl) crystal to search for dark matter. The best combination of the purification methods was developed, resulting $^{mathrm{nat}}$K and $^{210}$Pb were less than 20 ppb and 5.7 $mu$Bq/kg, respectively. The authors will construct a large volume detector system with high-purity NaI(Tl) crystals. The design and the performance of the prototype detector module will be reported in this article.
The dark matter search project by means of ultra high purity NaI(Tl) scintillator is now underdevelopment. An array of large volume NaI(Tl) detectors whose volume is 12.7 cm$phitimes$12.7 cm is applied to search for dark matter signal. To remove radioactive impurities in NaI(Tl) crystal is one of the most important task to find small number of dark matter signals. We have developed high purity NaI(Tl) crystal which contains small amounts of radioactive impurities, $<4$ ppb of $^{nat}$K, 0.3 ppt of Th chain, 58 $mu$Bq/kg of $^{226}$Ra and 30 $mu$Bq/kg of $^{210}$Pb. Future prospects to search for dark matter by means of a large volume and high purity NaI(Tl) scintillator is discussed.
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