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 Ei = 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 Ei (e.g. 106 n/cm2/s at Ei = 20meV with ΔE/Ei = 3%). The newly developed fast diskchopper enables a fine and tunable energy resolution (ΔE/Ei ≥ 1% at Ei = 20 meV). A large area detector bank (±1358 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/ki, 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.
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