Bulk NaI(Tl) scintillation low energy events selection with the ANAIS-0 module

Abstract

Dark matter particles scattering off target nuclei are expected to deposit very small energies in form of nuclear recoils (below 100 keV). Because of the low scintillation efficiency for nuclear recoils as compared to electron recoils, in most of the scintillating targets considered in the search for dark matter, the region below 10 keVee (electron equivalent energy) concentrates most of the expected dark matter signal. For this reason, very low energy threshold (at or below 2 keVee) and very low background are required to be competitive in the search for dark matter with such detection technique. This is the case of Annual modulation with NaI Scintillators (ANAIS), which is an experiment to be carried out at the Canfranc Underground Laboratory. A good knowledge of the detector response function for real scintillation events in the active volume, a good characterization of other anomalous or noise event populations contributing in that energy range, and the development of convenient filtering procedures for the latter are mandatory in order to achieve the required low background at such a low energy. In this work we present the characteristics of different types of events observed in large size NaI(Tl) detectors, and the event-type identification techniques developed. Such techniques allow distinguishing among events associated with bulk NaI scintillation, and events related to muon interactions in the detectors or shielding, photomultiplier origin events, and analysis event fakes. We describe the specific protocols developed to build bulk scintillation events spectra from the raw data and we apply them to data obtained with one of the ANAIS prototypes, ANAIS-0. Nuclear recoil type events were also explored using data from a neutron calibraa e-mail: ccuesta@uw.edu tion; however pulse shape cuts were found not to be effective to discriminate them from electron recoil events. The effect of the filtering procedures developed in this nuclear recoils population has been analyzed in order to properly correct cut efficiencies in dark matter analysis.

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

“…ciency was experimentally measured for previous ANAIS prototypes in set-ups having two or three modules and a slightly different readout: all the modules traces were stored independently on the module triggering. 22 Na and 40 K high energy events allowed to identify real coincidence events with a very low energy deposition in one of the modules, and by selecting those effectively triggering our DAQ to calculate the trigger efficiency [39]. Very high trigger efficiency was observed then, and as far as the same DAQ hardware/software is used in ANAIS-112, we do not expect differences in trigger efficiency in the present set-up.…”

The ANAIS-112 experiment at the Canfranc Underground Laboratory

“…Effective filtering protocols for rejecting non-bulk scintillation events, similar to those described at [3] and optimized for each detector, have been applied. Multiparametric cuts based on the number of peaks in the pulses, the temporal parameters of the pulses and the asymmetry in light sharing between PMTs are considered.…”

“…This goal imposes strong experimental requirements: an energy threshold at or below 2 keV ee 5 , background as low as possible below 10 keV ee and very stable operation conditions. Since the nineties, NaI(Tl) detectors from different suppliers have been in operation in Canfranc [2,3]; Alpha Spectra Inc. (AS) detectors have shown best performance and radiopurity (characterized in set-ups with two or three detectors, referred as ANAIS-25 [4] and ANAIS-37 [5], respectively) and then, they have been selected to be used in ANAIS. In 2017, ANAIS-112 experiment has been commissioned: the ANAIS-112 set-up consists of a 3×3 matrix of 12.5 kg NaI(Tl) modules and data taking is underway since August, 2017 for at least the next two years in the same conditions.…”

Assessment of backgrounds of the ANAIS experiment for dark matter direct detection

“…Several prototypes have been developed and operated at LSC using BICRON and Saint-Gobain crystals; all of them were disregarded due to an unacceptable K content in the crystal at the level of hundreds of ppb. Among them, the so-called ANAIS-0 detector [18][19][20][21], a 9.6 kg Saint-Gobain crystal similar to those of DAMA experiment, has to be highlighted because its successful background model [18] has been the starting point for the present work. Some other interesting results, as very slow scintillation in NaI(Tl) [20] or an anomalous fast event population attributable to quartz light emission [22] were also obtained from first prototypes.…”