Development of the proton beam monitor based on the thin diamond crystal for the COMET Experiment

In the COMET experiment, it is essential to establish the online monitor that enables to detect few residual protons between consecutive main proton bunches to search for the muon-to-electron([Formula presented]) conversion with a sensitivity below [Formula presented]. A thin diamond based detector is proposed because of its ideal characteristics. To realise the diamond based residual proton monitor, a noise suppression is essential since the number of electron–hole pairs generated by a single proton is relatively small in a thin diamond ([Formula presented]5000 [Formula presented]/[Formula presented]). The prototype detector based on a 0.5 mm thick diamond was developed to evaluate the proton counting capability. The beam test using a 30 GeV proton beam was performed with this prototype and the result is reported in this paper.