A 0νββ search using large scintillating crystal with metallic magnetic calorimeter

Abstract

Observations of neutrino oscillation phenomenon imply that neutrinos have non-zero masses and there is a mixing between weak and mass eigenstates. Mixing angles and square mass differences of mass eigenstates have been obtained from neutrino oscillation experiments with various baseline, but origin of neutrino masses (Dirac or Majorana) and their absolute mass scale remain as fundamental questions. Search for the neutrinoless double beta decay (0νββ) is a key experiment to reveal Majorana nature of neutrinos. 0νββ is a very rare transition that two beta decays occur at the same time without neutrino emission, which is possible if the neutrinos are Majorana particles. Probability of this decays is proportional to square of effective Majorana neutrino mass, and extremely small masses of neutrinos result in very long half-life that makes it hard to be observed. Low temperature detectors with large scintillating crystals are the promising detection technique for rare event search experiments because of its high energy resolution, low energy threshold, and particle discrimination performances. The AMoRE (Advanced Mo-based Rare process Experiment) project employs metallic magnetic calorimeters (MMCs) and large {\CMO} scintillating crystals for a 0νββ search of the 100Mo. It uses a simultaneous measurement scheme of heat and scintillation light signals at tens of millikelvin temperatures. Overcoming of low thermal coupling between large dielectric crystals and MMCs by efficient athermal phonon collection using large gold film phonon collector provides significant improvement on detector performances. The detectors show better than 10 keV FWHM energy resolution at 2.6 MeV (close to the Q-value of 0νββ) and 20 σ separation power for alpha-induced background events with relatively fast rise-time of around 1~ms, which satisfies requirement for the zero-background experiment with tens of kg scale detectors. In this thesis, detector development procedure based on the thermal model study and details on detector performances obtained from the characterization measurement are described. Also, test of 1.5 kg 40Ca100MoO4 detectors at an underground laboratory for AMoRE-pilot experiment and its preliminary results are discussed.