Analysis of backgrounds for the ANAIS-112 dark matter experiment

Abstract

The ANAIS (Annual modulation with NaI(Tl) Scintillators) experiment aims at the confirmation or refutation of the DAMA/LIBRA positive annual modulation signal in the low energy detection rate, using the same target and technique, at the Canfranc Underground Laboratory (LSC) in Spain. ANAIS-112, consisting of nine 12.5 kg NaI(Tl) modules produced by Alpha Spectra Inc. in a 3×3 matrix configuration, is taking data smoothly in "dark matter search" mode since August, 2017, after a commissioning phase and operation of the first detectors during the last years in various set-ups. A large effort has been carried out within ANAIS to characterize the background of sodium iodide detectors, before unblinding the data and performing the first annual modulation analysis. Here, the background models developed for all the nine ANAIS-112 detectors are presented. Measured spectra from threshold to high energy in different conditions are well described by the models based on quantified activities independently estimated following several approaches. In the region from 1 to 6 keV ee the measured, efficiency corrected background level is 3.58±0.02 keV −1 kg −1 d −1 ; NaI crystal bulk contamination is the dominant background source being 210 Pb, 40 K, 22 Na and 3 H contributions the most relevant ones. This background level, added to the achieved 1 keV ee analysis threshold (thanks to the outstanding light collection and robust filtering procedures developed), allow ANAIS-112 to be sensitive to the modulation amplitude measured by DAMA/LIBRA, and able to explore at three sigma level in five years the WIMP parameter region singled out by this experiment. a

ANAIS-112 status: two years results on annual modulation

“…After the first year of data taking we unblinded 10% of those events, chosen randomly distributed days along the data taking, for fine tuning the events rejection procedures and carry-out general background assessment and sensitivity estimates. All of this information was published [17,18,19]. After 1.5 years, using the designed analysis protocol, the unblinding of the ROI and annual modulation analysis allowed the confirmation of the sensitivity projections, while producing some tension with DAMA/LIBRA annual modulation result [20].…”

“…The ANAIS-112 shielding consists of 10 cm of archaeological lead, 20 cm of low activity lead, an anti-radon box (kept under overpressure with radon-free nitrogen gas) and 40 cm of a combination of water tanks and polyethylene bricks. An active muon veto made up of 16 plastic scintillators covers the top and sides of the set-up [17]. ANAIS-112 DAQ hardware and software is robust and it has been fine-tuned in the operation of several prototypes.…”

“…Triggering below 1 keV 1 with almost 100% efficiency is guaranteed by the observation of the 22 Na events population. However, analysis threshold is set up at 1 keV because of the PMT-related events rejection, which forces to establish events selection criteria whose acceptance efficiencies for bulk scintillation events decrease down to about 15% at 1 keV [17].…”

Small scale direct dark matter search experiments

“…The best fit modulation amplitude derived for the 2-6 keV ee region is S m = (0.0083 ± 0.0068) cpd/kg/keV. ANAIS (Annual modulation with NAI Scintillators) is operating 9 NaI(Tl) modules also built by Alpha Spectra (112.5 kg in total) at the Canfranc Underground Laboratory in Spain [19,20]. The dark matter run is underway since August 2017, with an outstanding light collection of ∼15 phe/keV for all modules allowing an energy threshold at 1 keV ee .…”

“…They work with CHF 3 +CF 4 +C 4 H 10 and have observed 3D tracks of radon progeny nuclear recoils [34]. First observation of 19 F ion tracks at ion beam facilities with angular resolution better than 10 o has been reported [35] and quenching factors of He and F with an ion source in Grenoble have been measured. NEWAGE (NEw generation WIMP search with an Advanced Gaseous tracker Experiment) uses a simplified system with amplification structure and readout in a monolithic detector with a TPC and a micro-pixel chamber.…”