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

Abstract

A large effort has been carried out to characterize the background of sodium iodide crystals within the Annual modulation with NaI Scintillators (ANAIS) project. In this paper, the background models developed for three 12.5 kg NaI(Tl) scintillators produced by Alpha Spectra Inc. and operated at the Canfranc Underground Laboratory are presented together with an evaluation of the background prospects for the full experiment. Measured spectra from threshold to high energy in different conditions are well described by the models based on quantified activities. At the region of interest, crystal bulk contamination is the dominant background source. Contributions from 210 Pb, 40 K, 22 Na and 3 H are the most relevant. Those from 40 K and 22 Na could be efficiently suppressed thanks to anticoincidence operation in a crystals matrix or inside a liquid scintillator veto (LSV), while that from 210 Pb has been reduced by improving crystal production methods and 3 H production could be reduced by shielding against cosmic rays during production. Assuming the activities of the last characterized detector, for nine crystals with a total mass of 112.5 kg the expected background rate is 2.5 counts/(keV kg day) in the region from 1 to 4 keV, which could be reduced at 1.4 counts/(keV kg day) by using a LSV.

Background assessment for the TREX dark matter experiment

“…The measured activity at hall A of LSC of (63±1) Bq/m 3 [40] has been considered; the implementation of a N 2 gas flux system inside the shielding to avoid radon intrusion providing a factor ∼100 reduction in the air 222 Rn activity allows to reduce this contribution to a non-relevant level (see table 3). Even if radon inside the shielding has not been directly quantified, the reduction factor assumed here and in other experiments operated in Canfranc with similar shielding and nitrogen flux is supported by the good agreement between models and measured data for peaks due to 222 Rn emissions [21]. In any case, this assumption is not very relevant because if TREX-DM operates using the radon-free air facility available at the LSC a bigger radon suppression factor at the level of about 1000 would be at reach.…”

“…Operating deep underground at ultra-low background conditions is a must in experiments looking for rare events like the direct detection of WIMPs. In this kind of experiments, the construction of reliable background models, based on an accurate assay of background sources and on a careful computation of their contribution to the experiment, is essential; they provide guidance and constraints for design and allow robust estimations of the experiment sensitivity (some recent examples can be found at [15,16,17,18,19,20,21,22,23]). The preliminary background model of TREX-DM for operation at LSC presented in [11,24] has now been completed and updated, including as inputs the activities from a dedicated material screening program together with the measured fluxes of different backgrounds at LSC (gamma-rays, neutrons and muons).…”

ANAIS-112 sensitivity in the search for dark matter annual modulation

“…This high light collection, possible thanks to the excellent crystal quality and the use of high quantum efficiency PMTs, has a direct impact in energy threshold. Triggering below 1 keV ee is confirmed by the identification of bulk 22 Na and 40 K events at 0.9 and 3.2 keV ee , respectively, thanks to coincidences with the corresponding high energy photons following the electron capture decays to excited levels [2].…”

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

“…In the following we will refer as PMT-0 (PMT-1) to the units placed on the East (West) side of the shielding. All PMT units, as well as many of the materials used in the building of the detectors, have been screened for radiopurity using germanium detectors at the LSC low background facilities and their contribution to the experiment background has been estimated [20,21]. The ANAIS-112 shielding consists of 10 cm of archaeological lead, 20 cm of low activity lead, an anti-radon box (continuously flushed with radon-free nitrogen gas), an active muon veto system made up of 16 plastic scintillators designed to cover top and sides of the whole ANAIS set-up (see Figure 1) and 40 cm of neutron moderator (a combination of water tanks and polyethylene blocks).…”