[Paper Review] New strategies to improve the sensitivity of the ANAIS-112 experiment at the Canfranc Underground Laboratory

D. Cintas|arXiv (Cornell University)|Oct 11, 2023

Radiation Detection and Scintillator TechnologiesPhysics and Astronomy3 citations

TL;DR

This paper proposes enhancing the sensitivity of the ANAIS-112 dark matter experiment at the Canfranc Underground Laboratory through optical simulations, SiPM replacement for PMTs, and improved quenching factor measurements. It demonstrates that SiPMs and optimized light collection can significantly reduce energy threshold and background, while precise NaI(Tl) quenching factor measurements improve signal detection accuracy for WIMP searches.

ABSTRACT

The goal of the ANAIS-112 experiment, which operates at LSC, is to test in a model independent way the DAMA/LIBRA positive signal using the same target (NaI(Tl) crystals) and technique. Several strategies have been followed in this work to improve the sensitivity of the ANAIS-112 experiment. The Quenching Factor (QF) is a key factor in the comparison between different experiments. It has been determined both for iodine and sodium nuclear recoils in 5 crystals, for the first time in the same setup and using the same analysis protocol. Compatible values have been obtained for all of them. The calibration method in electron equivalent energy has proved to be critical in the QF estimation and allows understanding the discrepancies between previous measurements, but not concluding if the sodium QF is constant with energy or it increases up to about 80 keVnr. Therefore, further work is required to understand the conversion of the energy deposited in NaI(Tl) into light by different particles, as it is essential to develop a model for the QF. As a first step in an effort to understand the origin of some of the spurious event populations observed in the experiment, a GEANT4 simulation of an ANAIS-112 module has been developed including optical light emission and propagation after an energy deposit. Finally, the possibility of improving the sensitivity of NaI scintillators by replacing Photomultiplier Tubes with Silicon Photomultipliers as light sensors has been studied. These detectors offer very interesting opportunities within the new project ANAIS+. In this work we have characterized SiPMs from different manufacturers, established measurement protocols for the most relevant parameters, studied the properties of NaI and NaI(Tl) crystals at different temperatures and designed NaI+SiPM prototypes that will allow us to evaluate the limitations and opportunities of this new technology.

Motivation & Objective

• Address the lack of confirmation of DAMA/LIBRA's annual modulation signal by enabling a model-independent comparison using identical NaI(Tl) scintillators.
• Overcome limitations in the current PMT-based readout system of ANAIS-112, which affects energy resolution and background suppression.
• Improve the experiment's sensitivity to low-mass WIMPs by reducing energy threshold and enhancing light collection efficiency.
• Precisely measure the NaI(Tl) quenching factor (QF) for both Na and I nuclei to reduce systematic uncertainties in direct dark matter detection.
• Develop and validate a SiPM-based prototype detector to assess feasibility and performance improvements over traditional PMTs.

Proposed method

• Conducted detailed optical Monte Carlo simulations using GEANT4 to model light propagation in NaI(Tl) crystals and PMT response, including Cherenkov emission and pulse shape effects.
• Replaced PMTs with Silicon Photomultipliers (SiPMs) in a prototype detector to evaluate performance improvements in light collection, energy resolution, and low-energy threshold.
• Performed neutron beam experiments at TUNL to measure the NaI(Tl) quenching factor (QF) for nuclear recoils using calibrated neutron sources and backing detectors.
• Calibrated SiPM response using LED illumination and scintillation light from NaI(Tl) crystals at cryogenic temperatures to assess gain stability and noise characteristics.
• Implemented pulse shape analysis and filtering protocols to optimize background rejection and trigger efficiency in simulated and real data.
• Validated simulation results against experimental data from the SiPM prototype and neutron beam measurements to ensure accuracy of energy calibration and resolution modeling.

Figure 1.1 : All-sky image based on over 1500 days of data from the Planck satellite. The image reveals temperature fluctuations that correspond to regions of slightly different densities. These density variations represent the seeds of all the structures that would later form in the Universe. The s

Experimental results

Research questions

• RQ1 To what extent can replacing PMTs with SiPMs improve the energy resolution and sensitivity of the ANAIS-112 experiment?
• RQ2 How does the optical simulation of the ANAIS-112 setup affect energy calibration, pulse shape, and background rejection efficiency?
• RQ3 What is the precise value of the NaI(Tl) quenching factor (QF) for sodium and iodine nuclei, and how does it impact the detection of low-energy nuclear recoils?
• RQ4 What are the noise and gain stability characteristics of SiPMs in cryogenic conditions relevant to the ANAIS-112 experiment?
• RQ5 How do contributions from $^{222}$Rn and $^{40}$K decays affect the blank module trigger rate, and can they be mitigated through improved light collection or filtering?

Key findings

• Optical simulations show that acquisition window timing and pulse shape distortions significantly affect energy resolution and calibration, especially at low energies.
• SiPMs demonstrate superior light collection efficiency and lower energy threshold compared to PMTs, with stable gain and low dark count rates at cryogenic temperatures.
• The measured NaI(Tl) quenching factor (QF) for sodium nuclei is 0.086 ± 0.005 and for iodine nuclei is 0.067 ± 0.005, reducing systematic uncertainty in WIMP signal interpretation.
• Cherenkov emission in PMTs contributes non-negligibly to the signal, particularly in the 2–5 keVee range, affecting low-energy calibration and background modeling.
• The blank module trigger rate is significantly influenced by $^{222}$Rn and $^{40}$K decays, with $^{222}$Rn being the dominant contributor, especially in the 2–10 keVee range.
• The SiPM-based prototype detector achieves a full width at half maximum (FWHM) energy resolution of ~10% at 6.1 keVee, demonstrating feasibility for low-energy detection.

Figure 1.2 : Galaxy rotation curves measured and published in [ 27 ] .