Spallation Neutron Source at J-PARC and Its Cryogenic Hydrogen System

The Japan Atomic Energy Agency and the High-Energy Accelerator Research Organization are collaborating in the construction of the Japan Proton Accelerator Research Complex, a high-intensity proton accelerator complex with MW beam power. The use of various secondary particle beams (neutrons, mesons, antiprotons, etc.) that are produced in proton-nucleus reactions is the prime purpose of the project. Accordingly, four science experimental facilities are being constructed, including a materials and life science experimental facility, a nuclear and particle experimental facility, a neutrino experimental facility, and a nuclear transmutation facility (planned for the future). At the materials and life science experimental facility, where materials or biological structures are analyzed by neutron beam scattering experiments, a spallation neutron source has been constructed to provide experiment users with neutron beams that have the world's highest pulse intensity. Neutrons produced using spallation reaction should possess a high energy of MeV order, but neutrons used for experiments should have energy of a low meV order. Therefore, an effective material that is capable of moderating neutron energy by approximately nine orders is required. That material is supercritical hydrogen; and the spallation neutron source should therefore be equipped with a cryogenic hydrogen system that provides supercritical hydrogen to the neutron energy moderating system. This paper discusses the spallation neutron source and introduces the cryogenic hydrogen system that is used for constructing the neutron source.