CANDLES(CAlcium fluoride for the study of Neutrinos and Dark matters by Low Energy Spectrometer) is the experiment to search for the neutrino-less double beta decay(0vββ) of 48Ca with CaF2 scintillator. 48Ca has the highest Q ββ -value (4.3 MeV) among all isotope candidates for 0vββ. It enables us to measure signals with very low background condition. After rejection analysis with 131 days × 86 kg data for background events from radioactive contaminations in the CaF2 scintillators, no events are observed in the Q ββ -value region. As a result, the half-life of 48Ca is greater than 6.2 × 1022 yr (90% confidence level). For further high sensitive measurement of 48Ca 0vββ search, we have been developing the 48Ca enrichment and CaF2 scintillating bolometer techniques. In this paper, the latest result for CANDLES and the status of scintillating bolometer development are described.

Scintillating bolometer is a powerful tool to search neutrinoless double beta decay. We established a large scintillating bolometer using a 312 g CaF2(Eu) crystal with a readout technology of metallic magnetic calorimeters. A set of successful measurements were carried out for simultaneous detection for heat and light signals at 10-40 mK in an above-ground laboratory. We found large light signals with clear difference in scintillation yields between electron- and alpha-induced events. The comparison of relative amplitudes of heat and light signals obtained about 10 σ discrimination power. We also found the heat signals experiencing strong position dependence from the event location. This position dependence can be interpreted by the spin-lattice interaction of paramagnetic Eu ions in the CaF2 crystal.

A new data acquisition (DAQ) system for the CAN-DLES experiment was developed using the SpaceWire protocol and the DAQ-Middleware framework. The CANDLES experiment uses a trigger board and Flash Analog-to-Digital Converters (FADCs). The SpaceWire helps us construct the DAQ system with a flexible and multi-path access to FADCs and the trigger board. FADCs have a ring buffer with three buffers to detect sequential decays from backgrounds. We developed the DAQ system with parallel read-out to reduce the dead time at high trigger rates using three read-out personal computers (PCs) that are connected to the three buffers in FADCs one by one. Each PC connects all FADCs and the trigger board and gathers a complete data set of one event without any event builder. The maximum DAQ speed of parallel read-out by three PCs was 2.4 times higher than that of single read-out. In order to collect event data sets from PCs, we built the network distributed DAQ system through Ethernet, which is naturally introduced with the DAQ-Middleware. To realize remote monitoring and histogram modification while the DAQ running, we also developed a dynamic online monitor system independent with the DAQ-Middleware.

The neutrino-less double beta decay is acquiring great interest after the confirmation of neutrino oscillation which demonstrated nonzero neutrino mass. In order to search for neutrino-less double beta decay of 48 Ca, we proposed the CANDLES project by using CaF 2 (pure) scintillators. Now we installed the CANDLES III system at the Kamioka underground laboratory. The CANDLES III system realizes the low background condition by a characteristic structure and data analyses for background rejection. Furthermore we installed new shielding system in order to reduce γ-ray backgrounds from neutron capture reaction. Here we report performances of the CANDLES III system.

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