Observations of beam loss proton trajectories in J-PARC linac

In J-PARC linac, highest beam loss has been observed in the ACS section at the H- beam energy of 181 MeV. The primary source of the loss is considered to be H0's produced by the electron stripping of H-'s by the remnant gas inside the beam duct. The H0 almost conserves the momentum of the H- at the point where it has been produced, and almost 100% of the H0's are converted into H+'s (protons) through the beam duct. The H+'s are expected to have small angles around 5 degrees with respect to the beam, with energies 100-150 MeV, according to a simulation. To detect H+'s, we developed six-plane scintillating fiber detectors. The 2 planes are located at an upstream position and the other 4 planes are located about 1.1 m downstream of the beam. We measure each PMT signal by defining a trigger signal by coincidence of dynode signals of the most upstream and the most downstream PMT's. We present preliminary results of time-of-flight measurements of charged particle tracks consistent with protons, and distributions of tracks along the beam axis. (author)