LiFE-SNS: LiF Experiment for keV-scale Sterile Neutrino Search

TL;DR

LiFE-SNS addresses keV-scale sterile neutrinos by calorimetrically measuring the total $^3$H beta spectrum with LiF-embedded tritium and MMC detectors at millikelvin temperatures. The work analyzes tritium generation, sensor technology, detector configuration, calibration, and systematic uncertainties, demonstrating energy resolutions around 200–500 eV and robust position calibration. It reports successful operation of neutron-activated LiF crystals over months and projects competitive sensitivities in the keV mass range, with clear paths to scaling to multi-channel, higher-activity configurations. Overall, the study provides a comprehensive calibration and performance foundation for LiFE-SNS as a complementary approach to endpoint-based sterile-neutrino searches.

Abstract

The LiF Experiment for keV-scale Sterile Neutrino Search (LiFE-SNS) aims to probe sterile neutrinos through precision measurements of the tritium $β$ spectrum. Tritium nuclei are produced and embedded in LiF crystals via the ${}^{6}\mathrm{Li}(n,α){}^{3}\mathrm{H}$ reaction, allowing thermal calorimetric detection of $β$ decays with magnetic microcalorimeters (MMCs) operated at millikelvin temperatures. We present the detector configuration, background studies, and calibration method, including modeling of position-dependent response and characterization of detector nonlinearity. We also discuss potential sources of systematic uncertainty relevant to the sterile-neutrino search. While the first phase of LiFE-SNS has been completed, this paper focuses on calibration and detector characterization. The achieved performance enables precision $β$-spectrum measurements, and projected sensitivities indicate competitive reach in the keV mass region.

Figures (12)

• Figure 1: Expected spectrum of $^3$H $\beta$ decay. The black dotted line represents the $\beta$ spectrum with no mixing with the heavy neutrino state. The blue solid line shows the expected decay spectrum of the $\beta^-$ electron from a $^3$H as the sum of the light (green dotted line) and heavy (red dotted line) terms. The rates are calculated with an assumption of $\sin ^2 \theta = 0.2$ and $m_\mathrm{heavy} = 7$ keV.
• Figure 2: Upper bounds of the experimental limits for sterile neutrino searches in keV scale from $\beta$ decay measurements. Their references are listed in Table \ref{['tab:beta_isotopes']}. The dotted lines are the expected sensitivities of the LiFE-SNS phases. Phase 1 corresponds to $6\times10^{8}$$\beta$ events from a two-channel four-month exposure, while Phase 2 assumes $10^{12}$$\beta$ events with increased channel count over a multi-year period.
• Figure 3: Expected $^3$H distribution in a 1$\times$1$\times$1 cm$^3$ LiF crystal exposed to a thermal neutron flux at the sample loading area of the KRISS neutron source storage. The 3D distribution is projected to the xy, yx and zx planes, as well as along the x, y, and z directions, with a bin size of 20 µ m.
• Figure 4: A LiFE-SNS detector setup composed of a LiF crystal and an MMC sensor. An $^{241}$Am source with a silver collimator and an $^{55}$Fe source were introduced for energy and position calibration. This picture shows the source configurations of Exp. 5.
• Figure 5: Simulated positions of 5.9 keV X-ray events (Mn K$_\alpha$) absorbed in the LiF crystal from various $^{55}$Fe source locations used in Exp. 2--10. Geant4 simulations were performed using the designed detector geometry, including the source, collimator, and crystal, to model the X-ray absorption in the crystal.