[Paper Review] The ANAIS-112 experiment at the Canfranc Underground Laboratory

J. Amaré, S. Cebrián+12 more|arXiv (Cornell University)|Oct 10, 2017

Dark Matter and Cosmic Phenomena1 references3 citations

TL;DR

The ANAIS-112 experiment at the Canfranc Underground Laboratory uses 112.5 kg of high-purity NaI(Tl) scintillators to test the annual modulation signal reported by DAMA/LIBRA, aiming to confirm dark matter detection. With a 1 keV energy threshold and ultra-low background, the experiment achieved 15 phe/keV light collection and measured 40K and 210Pb activities at 1.0 mBq/kg and 1.5 mBq/kg on average, respectively, demonstrating sensitivity to detect the DAMA signal if it is due to weakly interacting massive particles (WIMPs).

ABSTRACT

The ANAIS experiment aims at the confirmation of the DAMA/LIBRA signal at the Canfranc Underground Laboratory (LSC). Several 12.5 kg NaI(Tl) modules produced by Alpha Spectra Inc. have been operated there during the last years in various set-ups; an outstanding light collection at the level of 15 photoelectrons per keV, which allows triggering at 1 keV of visible energy, has been measured for all of them and a complete characterization of their background has been achieved. In the first months of 2017, the full ANAIS-112 set-up consisting of nine Alpha Spectra detectors with a total mass of 112.5 kg was commissioned at LSC and the first dark matter run started in August, 2017. Here, the latest results on the detectors performance and measured background from the commissioning run will be presented and the sensitivity prospects of the ANAIS-112 experiment will be discussed.

Motivation & Objective

• Confirm the annual modulation signal reported by DAMA/LIBRA using the same target material (NaI(Tl)) but in a different underground environment.
• Achieve a low energy threshold (≤2 keV ee) and background levels below 10 keV ee to ensure sensitivity to potential WIMP signals.
• Minimize instrumental and environmental backgrounds through radiopure materials, active shielding, and radon suppression.
• Establish a stable, long-term data acquisition system with redundant triggering and monitoring for systematic control.
• Enable a blind annual modulation analysis and public data release after scientific exploitation to ensure objectivity and reproducibility.

Proposed method

• Deployed nine 12.5 kg NaI(Tl) detectors from Alpha Spectra Inc., each with two PMTs and OFE copper encapsulation, in a 3×3 array at 2450 m.w.e. depth in the Canfranc Underground Laboratory.
• Achieved 15 phe/keV light collection across all detectors, enabling triggering at 1 keV visible energy, with full characterization of light output and energy resolution.
• Employed a multi-layer shielding system: 10 cm archaeological lead, 20 cm low-activity lead, 40 cm neutron moderator, radon-free air flushing, and an active muon veto with plastic scintillators.
• Conducted radiopurity screening of all PMTs using germanium detectors to minimize background from intrinsic radioactivity.
• Used pulse shape analysis and BiPo sequence identification to quantify 232Th and 238U activities via alpha rates, and measured 40K and 210Pb via gamma and coincidence techniques.
• Simulated background spectra using Geant4, incorporating measured crystal activities and detector geometry, to model contributions from 40K, 210Pb, cosmogenics, and 22Na.

Experimental results

Research questions

• RQ1 Can the ANAIS-112 experiment confirm the annual modulation signal observed by DAMA/LIBRA using identical NaI(Tl) scintillators in a different underground location?
• RQ2 What is the intrinsic background level in the 2–6 keV ee energy range, particularly from 40K and 210Pb, and can it be reduced to allow detection of a potential WIMP signal?
• RQ3 Does the detector performance—specifically light collection and energy threshold—meet the requirements for a sensitive annual modulation search?
• RQ4 How do cosmogenic radionuclides (e.g., 3H, 22Na) contribute to the low-energy background, and can their time-dependent decay be used to validate the background model?
• RQ5 What is the projected sensitivity of ANAIS-112 to an annual modulation signal after five years of data taking, under the dark matter hypothesis?

Key findings

• The ANAIS-112 experiment achieved a light collection of 15 phe/keV across all nine detectors, confirming high performance and enabling triggering at 1 keV visible energy.
• The average 40K activity in the crystals was measured at 1.0 mBq/kg, with individual values ranging from 0.6 to 1.4 mBq/kg, consistent with low intrinsic radioactivity.
• The average 210Pb activity was 1.5 mBq/kg, with all detectors showing out-of-equilibrium 210Pb, indicating contamination from prior exposure; this was reduced in later-produced crystals.
• Background spectra from the commissioning run (March–July 2017) showed a clear decay of cosmogenic activities, particularly in the last detectors, validating the time-dependent background model.
• Preliminary background modeling for detector D8 showed good agreement with data, with 210Pb contributing significantly around 50 keV and cosmogenics still relevant due to short underground exposure time.
• Sensitivity projections after five years of data taking indicate that ANAIS-112 can detect the annual modulation signal at 90% confidence level in the 3σ region compatible with the DAMA/LIBRA result, under the WIMP hypothesis.