Current status and future prospects of the CANDLES experiment

Neutrino-less double beta decay(0νββ) is acquiring great interest after the confirmation of neutrino oscillation which demonstrated nonzero neutrino mass. Measurement of 0νββ provides a test for the Majorana nature of neutrinos and gives an absolute scale of the effective neutrino mass.

 

In order to search for 0νββ of ⁴⁸Ca, we proposed the CANDLES project and a detector system using pure CaF₂. The CANDLES III system, which is one of the CANDLES projects, aims for a high-sensitive measurement with a characteristic detector system. The system achieves a complete 4p active shield by immersing the CaF₂ scintillators in liquid scintillator. The active shield leads to a low background condition for the measurement.

 

We have developed the CANDLES III system at the Kamioka underground laboratory. The system incorporates a shielding system to reduce background events caused by high-energy gamma rays emitted from neutron capture reactions on surrounding materials. Additionally, we are preparing a new analysis to reduce background events originating from ²⁰⁸Tl and ²¹²Bi, which are present within the CaF₂ scintillators. In this talk, we will report on the analysis for background reduction and provide the latest status of the search for 0nbb after this upgrade.

 

Furthermore, we have started the development of a next-generation detector system. In this system, we will utilize a CaF₂ scintillating bolometer and enriched ⁴⁸Ca. Laser isotope separation (LIS) will provide the feasibility for enriching ⁴⁸Ca through the calcium isotope shift. Currently, we are working on developing a stable atomic beam generator and a high-power semiconductor laser system using master and slave lasers for the production of ⁴⁸Ca. In this talk, we will also provide a report on the current status of the CaF₂ scintillating bolometer and the enrichment of ⁴⁸Ca.