Angular distributions of beam loss protons at J-PARC linac

In J-PARC linac, at ACS (Annular-Coupled Structure linac) section, highest beam loss was observed. This beam loss is considered to be caused mainly by H⁰ generated by ionization of H^- by residual gas in the beam duct. The H⁰ is further changed to H⁺ when penetrating the beam duct. We have developed a detector system consisting of 8 planes of scintillating fiber hodoscopes. Each hodoscope consists of 16 fibers of 4x4 mm² square or 4 mm diameter circle cross sections with 64 mm length. The upstream detector (4 planes) is separated from the downstream detector (4 planes) by about 1.5 m to measure time-of flight of charged particles. Each detector can move in horizontal and vertical directions with stepping motors. We have measured angular and energy distributions of the protons from Oct. 2012 to May 2013. (author)