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.
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.
A new µTCA DAQ system was introduced in CANDLES experiment with SpaceWire-to-GigabitEthernet (SpaceWire-GigabitEthernet) network for data readout and Flash Analog-to-Digital Converters (FADCs). With SpaceWire-GigabitEthernet, we can construct a flexible DAQ network with multi-path access to FADCs by using off-the-shelf computers. FADCs are equipped 8 event buffers, which act as de-randomizer to detect sequential decays from the background. SpaceWire-GigabitEthernet has high latency (about 100 µsec) due to long turnaround time, while GigabitEthernet has high throughput. To reduce dead-time, we developed the DAQ system with 4 "crateparallel" (modules in crates are read in parallel) reading threads. As a result, the readout time is reduced by 4 times: 40 msec down to 10 msec. With improved performance, it is expected to achieve higher background suppression for CANDLES experiment. Moreover, for energy calibration, "event-parallel" reading process (events are read in parallel) is also introduced to reduce measurement time. With 2 "event-parallel" reading processes, the data rate is increased 2 times.
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