[Paper Review] Aboveground test of an advanced Li$_2$MoO$_4$ scintillating bolometer to search for neutrinoless double beta decay of $^{100}$Mo

Tatyana B. Bekker, N. Coron+15 more|arXiv (Cornell University)|Oct 25, 2014

Neutrino Physics Research51 references3 citations

TL;DR

This study demonstrates the successful aboveground operation of a large Li₂MoO₄ scintillating bolometer (50 cm³) for neutrinoless double beta decay (0ν2β) search in ¹⁰⁰Mo. The detector achieved excellent energy resolution and α/β background suppression, with preliminary radiopurity indications, confirming Li₂MoO₄ as a highly promising material for next-generation low-background experiments.

ABSTRACT

Large lithium molybdate (Li$_2$MoO$_4$) crystal boules were produced by using the low thermal gradient Czochralski growth technique from deeply purified molybdenum. A small sample from one of the boules was preliminary characterized in terms of X-ray-induced and thermally-excited luminescence. A large cylindrical crystalline element (with a size of $\oslash 40 imes40$ mm) was used to fabricate a scintillating bolometer, which was operated aboveground at $\sim 15$ mK by using a pulse-tube cryostat housing a high-power dilution refrigerator. The excellent detector performance in terms of energy resolution and $α$ background suppression along with preliminary positive indications on the radiopurity of this material show the potentiality of Li$_2$MoO$_4$ scintillating bolometers for low-counting experiment to search for neutrinoless double beta decay of $^{100}$Mo.

Motivation & Objective

• To develop high-purity Li₂MoO₄ crystals using low thermal gradient Czochralski growth for use in scintillating bolometers.
• To test the performance of a large Li₂MoO₄ scintillating bolometer (40×40 mm) in an aboveground cryogenic setup at ~15 mK.
• To evaluate the energy resolution, α/β separation, and radiopurity of the detector for future 0ν2β decay experiments.
• To assess the feasibility of using Li₂MoO₄ as a source=detector material with high molybdenum concentration (55.2 wt%) and low intrinsic radioactivity.
• To provide a foundation for future underground operation and development of enriched ¹⁰⁰Mo-doped Li₂MoO₄ bolometers.

Proposed method

• Large Li₂MoO₄ boules were grown via low thermal gradient Czochralski technique from deeply purified molybdenum oxide.
• A cylindrical crystal (40×40 mm) was fabricated and used to construct a scintillating bolometer with combined heat and light detection.
• The detector was operated at ~15 mK using a pulse-tube cryostat with a high-power dilution refrigerator.
• X-ray-induced and thermally excited luminescence were measured to characterize the scintillation properties of the crystal.
• Energy resolution and α/β background suppression were evaluated using the heat-light scatter plot technique.
• Neutron capture from ⁶Li(n,t)α reaction was observed to assess neutron-induced background.

Experimental results

Research questions

• RQ1 Can high-quality Li₂MoO₄ crystals with high molybdenum content be grown using the Czochralski method for use in bolometric detectors?
• RQ2 Does a large Li₂MoO₄ scintillating bolometer achieve sufficient energy resolution and α/β rejection for 0ν2β decay searches?
• RQ3 What is the intrinsic radiopurity level of the Li₂MoO₄ material, particularly regarding ⁴⁰K, ²³²Th, and ²³⁸U contamination?
• RQ4 Can the scintillation light yield and luminescence efficiency be improved through defect control in the crystal?
• RQ5 How does the detector perform in an aboveground environment, and what is the potential for background suppression in underground operation?

Key findings

• The Li₂MoO₄ crystal exhibited a maximum luminescence emission at ~600 nm, increasing by a factor of 5 when cooled from room temperature to 8 K.
• The scintillating bolometer achieved excellent energy resolution and strong α/β separation, confirming its suitability for low-background experiments.
• A clear ⁶Li(n,t)α neutron capture peak was observed, indicating detectable neutron-induced background.
• The detector’s performance in terms of energy resolution and background suppression was comparable to or better than previous Li₂MoO₄ devices, despite being operated aboveground.
• Preliminary results suggest low radiopurity contamination, with ⁴⁰K activity at 170(80) mBq/kg and ²³²Th and ²³⁸U below 0.11 and 0.09 mBq/kg, respectively.
• The 50 cm³ detector volume is at least five times larger than any previously operated Li₂MoO₄ bolometer, approaching the size needed for a single module in a large-scale 0ν2β decay experiment.