[Paper Review] Liquid Argon Ionization Detector for Double Beta Decay Studies

V. D. Ashitkov, A. S. Barabash+8 more|arXiv (Cornell University)|Sep 2, 2003

Neutrino Physics Research3 citations

TL;DR

This paper presents a multisection liquid argon ionization detector developed by the DBA collaboration to study double beta decay in 100Mo at the Gran Sasso Underground Laboratory. The detector achieved the first observation of 100Mo's two-neutrino double beta decay with a half-life of (7.2 ± 0.9(stat) ± 1.8(syst)) × 10^18 yr, and set new limits on the neutrinoless modes, establishing world-leading upper bounds on 222Rn activity (1.2 × 10^{-3} Bq/kg) and 42Ar content (4.3 × 10^{-21} g/g) in liquid argon at 90% C.L.

ABSTRACT

A multisection liquid argon ionization detector was developed by the DBA collaboration to study the double beta- decay of $^{100}$Mo. The experiment was carried out in the Gran Sasso underground laboratory in Italy. The detector design and main characteristics are described. The $ββ(2ν)$ decay of $^{100}$Mo was observed and its half-life measured: $T_{1/2}=[7.2 \pm 0.9(stat) \pm 1.8(syst)] imes 10^{18}$ yr. Limits on the 0$ν$ and 0$νχ^{0}$ modes of the decay were obtained: $T_{1/2}> 8.4(4.9) imes 10^{21}$ yr and $T_{1/2}> 4.1(3.2) imes 10^{20}$ yr at 68% (90%) C.L., respectively. In addition the upper limits on the $^{42}$Ar content and $^{222}$Rn activity in liquid Ar were found to be $4.3 imes 10^{-21}$ g/g and $1.2 imes 10^{-3}$ Bq/kg, respectively.

Motivation & Objective

• To search for neutrinoless double beta decay (0νββ) in 100Mo using a liquid argon ionization detector.
• To measure the two-neutrino double beta decay (2νββ) half-life of 100Mo as a benchmark for nuclear matrix element calculations.
• To set stringent limits on radioactive impurities in liquid argon, particularly 42Ar and 222Rn, which are critical background sources for future large-scale liquid argon detectors.
• To demonstrate the feasibility and performance of a high-purity liquid argon ionization detector for rare decay searches in a deep underground environment.

Proposed method

• A multisection liquid argon ionization detector with titanium and fluoroplastic components was constructed, using 14 cathodes, 15 anodes, and 28 screening grids to define the sensitive volume.
• The detector was cooled with liquid nitrogen via a vacuum-insulated system to maintain stable liquid argon temperature and reduce boil-off to 6 L/h.
• High-voltage electrodes (−4.8 kV on cathodes, −2 kV on grids) and charge-sensitive preamplifiers enabled precise ionization charge collection from electron tracks.
• Background suppression was achieved through passive lead shielding, radon protection, and energy thresholds set at 1 MeV per electron for 0νββ and 0νχ⁰ searches.
• Data acquisition used a 16-bit ADC system with 100 ns sampling to record charge pulses, enabling energy and shape discrimination for event identification.
• Monte Carlo simulations were used to calculate detection efficiencies for 2νββ (2.2%), 0νββ (6.9%), and 0νχ⁰ (5.7%) decay modes, with assumptions on source distribution in liquid argon or on cathodes.

Experimental results

Research questions

• RQ1 What is the half-life of the two-neutrino double beta decay of 100Mo in liquid argon?
• RQ2 What are the lower limits on the half-life of the neutrinoless double beta decay (0νββ) of 100Mo?
• RQ3 What are the upper limits on the 42Ar content and 222Rn activity in liquid argon, respectively, as determined from single-electron background spectra?
• RQ4 How effective is the combination of passive shielding and energy thresholding in suppressing background for 0νββ and 0νχ⁰ decay searches?
• RQ5 To what extent do radioactive contaminants in the molybdenum foils contribute to systematic uncertainty in the half-life measurement?

Key findings

• The two-neutrino double beta decay of 100Mo was observed with a half-life of (7.2 ± 0.9(stat) ± 1.8(syst)) × 10^18 yr.
• A lower limit on the half-life of the 0νββ decay mode of 100Mo was set at T₁/₂ > 8.4 × 10^21 yr at 68% C.L. and > 4.9 × 10^21 yr at 90% C.L.
• A lower limit on the half-life of the 0νχ⁰ decay mode was set at T₁/₂ > 4.1 × 10^20 yr at 68% C.L. and > 3.2 × 10^20 yr at 90% C.L.
• The upper limit on 222Rn activity in liquid argon was determined to be 1.2 × 10^{-3} Bq/kg at 90% C.L., representing the world's best experimental limit.
• The upper limit on 42Ar content in liquid argon was found to be 4.3 × 10^{-21} g/g at 90% C.L., also the world's best limit.
• The detector achieved a liquid argon purity of 1.9 × 10^{-9} equiv.O₂, confirming the feasibility of high-purity operation for rare process searches.