Measurement of muon annual modulation and muon-induced phosphorescence in NaI(Tl) crystals with DM-Ice17

Abstract

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 µ/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.PACS numbers: 95.35.+d, 29.40.Mc, 95.85.Ry

Performance of ANAIS-112 experiment after the first year of data taking

“…The role played by the veto scintillator system in ANAIS-112 data selection and annual modulation analysis is very important. It is well known that the underground muon flux has an annual modulation [23][24][25][26] which could cause muon related events in dark matter detectors to mimic the dark matter modulation. However, different phases from DAMA/LIBRA and underground muon flux modulations have been determined and DAMA/LIBRA collaboration completely discard muons as possible explanation of their signal [27].…”

The COSINE-100 data acquisition system

“…When muons pass through a NaI(Tl) crystal, they cause a very high trigger rate (greater than 1 kHz) which exceeds the capacity of the DAQ system. Both the KIMS-NaI [2] and DM-Ice experiments [12] have measured elevated trigger rates of muon-induced events within NaI(Tl) targets. To prevent this, we apply a 1 ms hardware dead time to the FADC channel that generates a trigger.…”

First search for a dark matter annual modulation signal with NaI(Tl) in the Southern Hemisphere by DM-Ice17

“…Under the assumption of the standard dark matter halo model, the constraints for many of the WIMP and WIMP-like models set by other experiments are inconsistent with DAMA's claim for discovery [10][11][12][13][14][15][16][17][18][19][20][21][22]. Many alternative explanations coupling the modulation to seasonal effects have been proposed [23][24][25][26], but to date, none have been able to fully explain the DAMA signal [27][28][29][30].…”

“…Three types of noise trigger the detector: electromagnetic in-terference (EMI) with the detector monitoring queries, coincident SPE pulses, and "thin peaks" that exhibit a decay time an order of magnitude quicker than scintillation [36]. SPE events mainly come about from accidental coincidence of PMT dark noise and phosphorescence from crystal interactions [30]. Thin peaks are suspected to originate from Cherenkov light emission in the quartz light guides or PMT windows [44,45], and frequently exhibit asymmetric pulse heights in the two PMTs.…”