Cold-neutron disk-chopper spectrometer at J-PARC

Abstract

We are constructing a cold-neutron double-chopper spectrometer for the new spallation neutron source at the Materials and Life Science Facility (MLF), J-PARC. The spectrometer is dedicated to inelastic and quasi-elastic measurements in the cold and thermal neutron incident energy region of E_i =1-80 meV. Looking at a H₂-coupled moderator and employing a combination of two fast disk-choppers, one as the pulse-shaping and another as the monochromating chopper, the spectrometer realizes both high intensity and fine resolution. The expected neutron flux at the sample position is of the order of 10⁴-10⁷ n/cm²/s depending on E_i (e.g. 10⁶ n/cm²/s at E_i = 20 meV with ΔE/E_i = 3%). The newly developed fast disk- chopper enables a fine and tunable energy resolution (ΔE/E_i ≥ 1% at E_i = 20 meV). A large area detector bank (± 135° in horizontal, −22-+30° in vertical and 1.3π in solid angle) covers wide momentum transfer range with the resolution of a few percent of ΔQ/k_i, where ΔQ is the uncertainty in the longitudinal component of the scattering vector. The spectrometer is planned to be ready in 2008, when the first neutron beam at MLF, J-PARC is expected.