The CANDLES Trigger System for the Study of Double Beta Decay of $^{48}{\rm Ca}$

The CANDLES experiment is a study of the double beta decay of $^{48}{\rm Ca}$ nuclei. The detector consists of two scintillators. One is the ${\rm CaF}_{2}$ crystal that acts as a source of double beta decay and a scintillator. The other is a liquid scintillator, which is used for gamma-veto. The total light collected from the liquid scintillator is on the same order of magnitude as that of the ${\rm CaF}_{2}$ scintillator, but the decay constant of the liquid scintillator is one or more orders of magnitude shorter than that of the ${\rm CaF}_{2}$ scintillator. Therefore, if we trigger the signal by pulse height, the energy threshold of the liquid scintillator light will be one or more orders of magnitude lower than that of the ${\rm CaF}_{2}$ scintillator. A high counting rate in the liquid scintillator causes deadtime; in order to avoid this problem, we have developed a new trigger system. In this paper, we describe the use of this trigger system in the CANDLES experiment and its performance after installation in 2013.