KOTO is the first experiment in the world specialized in observing the decay KL → π0VV̄. The decay is sensitive to new physics scenarios beyond the Standard Model. The most important detector of KOTO is the CsI calorimeter, which measures positions and energies of gammas from KL, in order to reconstruct the vertexes of them. There are mainly three requirements for the calorimeter. First, dynamic range for each channel of CsI is more than 3 order of magnitude. Second, single counting rate is about 120 kHz comes from beam intensity. And linearity is required less than ±5% in order to discriminate the signals and backgrounds. Finally, to suppress the backgrounds, we will put the calorimeter at the vacuum level of 0.1Pa. So we must reduce its power consumption. That is why we plan to use the Cockcroft-Walton circuit as a high voltage supply for PMTs. It can reduce power down to about 150mW.
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