Abstract
Limits on the cross section for weakly interacting massive particles (WIMPs) elastic scattering on nuclei in NaI(Tl) detectors at the Yangyang Underground Laboratory are obtained from a 2967.4 kg·day data exposure. The nuclei recoiling from the scattering process are identified by the pulse shape of the scintillation light signals that they produce. The data are consistent with a no nuclear-recoil hypothesis, and WIMP-mass-dependent 90% confidence-level upper-limits are set on WIMP-nuclei elastic scattering cross sections. These limits partially exclude the DAMA/LIBRA allowed region for WIMP-sodium interactions with the same NaI(Tl) target material. The 90% confidence level upper limit on the WIMP-nucleon spin-independent cross section is 3.26×10−4 pb for a WIMP mass of 10 GeV/c2.
Highlights
The Korea Invisible Mass Search (KIMS) collaboration has developed low-background NaI(Tl) crystals for the purpose of reproducing the DAMA’s modulation results [19, 25]
This paper presents results from a search for weakly interacting massive particles (WIMPs)-nucleon interactions with two, low-background NaI(Tl) crystals using a 2967.4 kg·day exposure that uses a pulse shape discrimination (PSD) analysis to directly extract NR-induced events
Data fits are performed for each assumed WIMP mass value
Summary
The two crystals are cylindrical in shape but with different dimensions; one is 12.7 cm in diameter and 17.8 cm in length with a mass of 8.26 kg (NaI1); the other is 10.7 cm in diameter and 27.9 cm in length with a mass of 9.15 kg (NaI2), as described in table 1. The background levels of the NaI1 (NaI2) crystals are approximately 9 (5) counts/day/kg/keV in the 2–6 keV recoil energy region and decrease to 6 (4) counts/kg/day/keV in the 6–10 keV energy region, as shown in figure 1 We show the background spectra of the crystals after the application of the event selection criteria described below. To isolate a calibration sample of SR events, we cut an 8 cm diameter, 10 cm long cylindrical NaI(Tl) crystal into two, 5 cm-long pieces and contaminated the exposed surface of one of them (referred to as NaI-SR) with 222Rn. After waiting two weeks to allow for the build-up of surface 210Po, we reconnected the contaminated and uncontaminated cylinders with a thin light barrier, and tagged recoil 206Pb signals in NaI-SR by 5.3 MeV 210Po α signals in the uncontaminated cylinder [43]
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