[Paper Review] Light Collection in the Prototypes of the ANAIS Dark Matter Project

J. Amaré, S. Cebrián+11 more|arXiv (Cornell University)|Aug 25, 2015

Dark Matter and Cosmic Phenomena5 references3 citations

TL;DR

This paper presents a detailed analysis of light collection efficiency in the ANAIS dark matter experiment's prototype detectors, using Single Electron Response (SER) measurements from in-situ operations at the Canfranc Underground Laboratory. It confirms an average light collection of 15 phe/keV, enabling sub-keV energy thresholds and significantly enhancing energy resolution for detecting annual modulation in WIMP signals.

ABSTRACT

The ANAIS experiment aims at the confirmation of the DAMA/LIBRA signal using the same target and technique at the Canfranc Underground Laboratory (LSC) in Spain. ANAIS detectors consist of large NaI crystals coupled to two photomultipliers (PMTs). In this work we present Single Electron Response (SER) data for several units of the Hamamatsu R12669SEL2 PMT model extracted from normal operation data of ANAIS underground prototypes and we compare them with PMT SER characterization previously done at surface lab before coupling them to NaI crystal. Moreover, total light collection for different ANAIS prototypes has been calculated, producing an excellent average result of 15 phe/keV, which has a good impact in both energy resolution and threshold.

Motivation & Objective

• To validate the Single Electron Response (SER) of PMTs used in ANAIS prototypes through in-situ measurements at the Canfranc Underground Laboratory.
• To compare SER data from underground operations with surface laboratory measurements to ensure consistency and reliability.
• To quantify total light collection efficiency across ANAIS-25 and ANAIS-37 detector configurations for improved energy resolution.
• To assess the impact of light collection on achieving a low energy threshold (below 2 keVee) for detecting annual modulation in WIMP signals.
• To support the ANAIS experiment’s goal of confirming the DAMA/LIBRA signal using identical target and technique at low background conditions.

Proposed method

• Single Electron Response (SER) was extracted from normal underground operation data by applying a peak identification algorithm to PMT pulses with low photoelectron content to avoid pile-up.
• SER charge distributions were obtained by integrating the pulse area in a fixed time window around the peak maximum, ensuring accurate charge measurement per photoelectron.
• Light collection was calculated by dividing the mean charge from a 22.6 keV ${}^{109} ext{Cd}$ calibration line by the mean SER charge, yielding phe/keV values per PMT.
• Measurements were cross-validated between surface lab SER characterization and in-situ data, showing full agreement between both methods.
• Energy resolution at 22.6 keV was evaluated using the full width at half maximum (FWHM) of the Gaussian fit to the ${}^{109} ext{Cd}$ line, reported as σ/E (%) for each detector.
• The ANAIS-37 setup used higher PMT voltages to optimize performance in the low-energy region, with new detector D2 selected for high-gain operation (10⁶ gain) to explore high-energy responses.

Experimental results

Research questions

• RQ1 Can the Single Electron Response (SER) of Hamamatsu R12669SEL2 PMTs be reliably extracted from normal underground operation data without dedicated surface calibration?
• RQ2 How does the light collection efficiency of ANAIS prototypes compare between in-situ measurements and pre-installation surface characterization?
• RQ3 What is the total light collection efficiency (in phe/keV) achieved by the ANAIS-25 and ANAIS-37 detector configurations?
• RQ4 To what extent does the measured light collection impact the energy resolution and energy threshold of the ANAIS experiment?
• RQ5 Can the light collection performance support a sub-keV energy threshold for detecting annual modulation in WIMP signals?

Key findings

• The SER charge distributions extracted from in-situ underground operations show full agreement with those measured at the surface lab, validating the reliability of in-situ SER extraction.
• The average light collection efficiency across all ANAIS prototypes is 15 phe/keV, with individual PMTs achieving values between 6.81 and 8.21 phe/keV.
• ANAIS-25 detectors achieved a mean light collection of 15.24 phe/keV (D0) and 15.19 phe/keV (D1), with energy resolution of 8.51% and 8.59% respectively at 22.6 keV.
• ANAIS-37 detectors showed consistent light collection, with D0 and D1 yielding 15.26 phe/keV and 14.44 phe/keV respectively, and resolution of 8.73% and 8.80% at 22.6 keV.
• The new D2 detector in ANAIS-37 achieved 16.31 phe/keV (PM T20) and 16.31 phe/keV (PM T21), indicating excellent light collection performance at high gain.
• The high light collection and low energy resolution (σ/E ~ 8.5–11.5%) demonstrate strong potential for achieving a 1 keVee energy threshold, critical for WIMP annual modulation detection.