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CANDLES experiment in Kamioka Underground Observatory aims to obtain theneutrino-less double beta decay ($0\nu\beta\beta$) from $^{48}$Ca. Thismeasurement is a big challenge due to the extremely rare decay rate of$^{48}$Ca. Thus, in order to obtain 0\nu\beta\beta, it is needed to reducebackground as much as possible. Series of alpha and beta decays originated fromradioactive impurities can remain as background in the energy region of$0\nu\beta\beta$. Because they are sequential decays, we can remove them bytagging preceding and following events. This tagging method requires minimizeddead-time of DAQ system. A new DAQ system was introduced in CANDLES with newFADC (Flash Analog-to-Digital Converter) modules using 8 event buffers andSpaceWire-to-GigabitEthernet network for data readout. To reduce the dead-time,we developed our DAQ system with 4 parallel reading processes. As a result, theread-time is reduced by 4 times: 40msec down to 10msec, which is in equivalentto a half of previous DAQ's read-time. With reduced read-time accompanied bymultiple event buffers, the new DAQ system is realized with efficiency is veryclose to 100$\%$ (no event lost at 20cps, which is CANDLES trigger rate, after63 hours of data taking). With improved performance, it is expected to achievehigher background suppression for CANDLES experiment.

Parallel data readout for background rejection in CANDLES experiment