BEEST FOR SCIENTISTS
Introduction
The search for sterile neutrinos is among the brightest possibilities in our quest for understanding the microscopic nature of dark matter. Sterile neutrinos, unlike the active neutrinos in the Standard Model (SM), do not couple to left-handed currents in the weak interaction and are best observed via their mass–generated effects that result from momentum conservation with SM particles. This can be done through high–precision measurements of electron capture (EC) nuclear decay where the final state only contains the neutrino and a recoiling atom. This approach is a powerful, model-independent method in the search for beyond SM scenarios since it relies only on the existence of a heavy neutrino admixture to the active neutrinos, which is a generic feature of neutrino mass mechanisms, and not on the model–dependent details of their interactions. The BeEST (beast) experiment employs the decay–momentum reconstruction technique to precisely measure the 7Be 7Li recoil energy spectrum in superconducting tunnel junctions (STJs).
Sterile Neutrinos on the keV Scale
Model-Dependent Cosmological Limits on keV Neutrinos
Present relic abundance, limits, and regions of interest in the mass-mixing space with νe assuming different pre-BBN cosmologies. As shown, cosmological limits are heavily model-dependent. Need: Model-independent search for keV neutrinos in the laboratory
Decay Momentum Reconstruction as a Neutrino-Mass Probe
Orbital Electron Capture Decay of Beryllium-7
Operational Principle of Superconducting Tunnel Junctions (STJs)
(left) A schematic of the Ta-based STJ detector layers [Ta (165 nm) – Al (50 nm) – Al2O3 (1 nm) – Al (50 nm) – Ta (265 nm)].
(right) Operational principle of STJs, where radiation breaks Cooper pairs, creates excess charge-carriers, which then tunnel across the insulating barrier generating the signal current.
In-Situ Calibration and Characterization
- Illumination from a pulsed 355 nm Nd:YVO4 laser provides simultaneous calibration and characterization of each device
- The response of an STJ pixel to the laser consists of a Poisson distributed set of peaks that correspond to integer numbers of absorbed 3.5 eV photons
- Calibration accuracy of the spectrum is ± 1.6 meV rms in the region of interest at 5 × 103 counts/s
Harvesting 7Be Sample at the TRIUMF-ISAC Facility
7Li Recoil Energy Spectra from Calibrated Phase-II Data
- First implantation of Phase-II (September 2019)
- Limited A = 7 to 30 minutes of irradiation due to high 7Li beam contamination in this specific source
- Generated low-rate data set with ≤ 10 counts/s per STJ
- Data from ~ 20 days of semi-continuous running in the ADR
- Detector resolution 1.4-2.0 eV in the ROI
- First evaluation of in-medium effects on peak shapes
- Limited A = 7 to 30 minutes of irradiation due to high 7Li beam contamination in this specific source
- Direct measurement of 7Be L/K capture ratio