Development and production of the ionization chamber for the T2K muon monitor

Abstract

We report on the development of an ionization chamber for the muon monitor in the T2K (Tokai-to-Kamioka) long baseline neutrino oscillation experiment.In the T2K experiment, we generate a muon neutrino beam using the 50-GeV proton synchrotron in J-PARC and detect it in the Super-Kamiokande detector, which is 295km away from J-PARC. We aim the neutrino beam off-axis by 2.5° from Super-Kamiokande in order to maximize the sensitivity to the neutrino oscillation measurement. Since the beam direction is required to be aimed within 1mrad, we need to monitor it with a precision better than 1mrad.The muon monitor is the only instrument that can monitor the neutrino beam direction in real time by measuring a profile of muons which are generated along with neutrinos. It is required to measure the intense muon beam precisely and stably in a high radiation area. Therefore, we use two independent systems for the muon monitor: an array of ionization chambers and another array of solid-state detectors. We developed the ionization chamber and performed beam tests with an electron beam at the Laboratory of Particle Beam Science in Kyoto University. We also had a long-term test in the NuMI beamline at Fermilab. Then we finalized the design of the ionization chamber and built it. We expect the ionization chambers as the muon monitor to monitor the beam direction with a precision of 0.2mrad.