[Paper Review] Slow-Fluor Scintillator for Low Energy Solar Neutrinos and Neutrinoless Double Beta Decay

Jack Dunger, E. J. Leming+1 more|arXiv (Cornell University)|Mar 2, 2022

Neutrino Physics Research17 references7 citations

TL;DR

This paper proposes using slow-fluor liquid scintillators with acenaphthene as a primary fluor to enable time- and direction-separated detection of Cherenkov and scintillation light in large-scale detectors. Simulations show that with just 30% photocathode coverage, such a detector can measure the CNO solar neutrino flux with <10% precision in a few kiloton-years, while achieving ~10× suppression of solar neutrino backgrounds in 0νββ searches via directional rejection along the solar axis.

ABSTRACT

The potential for using slow-fluor liquid scintillators to study low energy solar neutrinos and neutrinoless double beta decay (0nbb) is explored through a series of simulations. The fluorescence model assumed for the primary fluor has characteristics similar to acenaphthene, recently used to demonstrate Cherenkov separation at energies around 1 MeV. Results here indicate notably better directional reconstruction in large-scale detectors than has previously been suggested by other approaches, allowing better identification of low energy solar neutrinos. These studies indicate that a detector with as little as ~30% coverage using currently available photomultiplier tubes could be able to make a measurement of the CNO solar neutrino flux to a precision of better than 10% (enough to distinguish metallicity models) with a few kiloton-years of exposure. In terms of 0nbb studies here suggest that the ability to separate mechanisms based on angular distributions is weak, but that the rejection of solar neutrino backgrounds with such a technique might potentially approach a factor of 10 for endpoint energies near 2.5 MeV in the angular hemisphere defined by the solar direction.

Motivation & Objective

• To evaluate the feasibility of using slow-fluor scintillators with acenaphthene for detecting low-energy solar neutrinos and neutrinoless double beta decay (0νββ).
• To assess the directional reconstruction performance of Cherenkov light in large-scale liquid scintillator detectors using time-separated signals.
• To quantify background suppression in 0νββ experiments by exploiting directional information from solar neutrinos.
• To determine the optimal scintillator composition (with/without secondary fluor) for maximizing directional sensitivity and energy resolution.
• To evaluate detector performance under practical constraints, including realistic PMT coverage and timing response.

Proposed method

• Simulations were performed using the SNO+ GEANT4-based RAT software package, including full photon transport with reflection, refraction, scattering, and absorption.
• A spherical 8.8 m radius acrylic vessel with 8" Hamamatsu R5912 PMTs (77% effective photocathode coverage) was modeled, with water-filled gaps between PMTs and the vessel.
• The scintillator was modeled as linear alkyl benzene (LAB) doped with 4 g/L acenaphthene as a slow fluor with ~45 ns decay time, favoring Cherenkov light separation.
• Secondary fluor bis-MSB (0 or 1 mg/L) was tested to improve light yield and shift emission to longer wavelengths, reducing absorption.
• A maximum likelihood algorithm was used to jointly reconstruct vertex position, time, and direction by fitting 2D probability density functions of PMT hit times and angles relative to particle direction.
• Background suppression in 0νββ was evaluated by comparing angular distributions of solar neutrino events in the solar hemisphere versus the rest of the detector.

Experimental results

Research questions

• RQ1 Can slow-fluor scintillators with acenaphthene enable sufficient time separation between Cherenkov and scintillation light to allow directional reconstruction in large-scale detectors?
• RQ2 What level of photocathode coverage is sufficient to achieve sub-10% precision in measuring the CNO solar neutrino flux with a few kiloton-years of exposure?
• RQ3 To what extent can directional information from Cherenkov light suppress solar neutrino backgrounds in 0νββ experiments?
• RQ4 How does the inclusion of a secondary fluor like bis-MSB affect directional reconstruction and background rejection?
• RQ5 Can angular distributions of Cherenkov light distinguish between left-handed and right-handed Majorana neutrino exchange mechanisms in 0νββ?

Key findings

• A detector with only ~30% photocathode coverage using commercially available HQE PMTs can measure the CNO solar neutrino flux with a precision better than 10% after a few kiloton-years of exposure.
• The use of acenaphthene alone (without secondary fluor) maximizes unscattered Cherenkov photon detection and improves directional resolution by reducing the scintillation component.
• Solar neutrino backgrounds in 0νββ can be suppressed by a factor of approximately 10 in the angular hemisphere aligned with the Sun, improving signal-to-background ratio by ~1.6 in high-background regimes.
• Directional discrimination between left-handed and right-handed Majorana neutrino exchange mechanisms (LNE vs. RHC) is weak, with less than 1σ separation even for 100 observed 0νββ events when vertex reconstruction is used.
• The time separation of Cherenkov and scintillation signals—demonstrated on a benchtop scale—remains effective in a 10m-scale detector, enabling robust directional reconstruction.
• Vertex and direction reconstruction via maximum likelihood fitting achieves unbiased estimates by simultaneously solving for time, position, and direction using 2D PDFs of PMT hit times and angles.