Study of converging neutron guides for the cold neutron double-chopper spectrometer at J-PARC

Kajimoto, Ryoichi  ; Nakamura, Mitsutaka   ; Osakabe, Toyotaka  ; Sato, Taku*; Nakajima, Kenji  ; Arai, Masatoshi

Performance of a neutron guide has been studied for the Cold Neutron Double-Chopper Spectrometer (CNDCS) proposed for the spallation neutron source at J-PARC. This spectrometer is dedicated to inelastic neutron scattering studies in vast research fields in an energy range of 80 meV. In order to detect weak inelastic signals, increasing neutron flux on sample with suppressing background at detector is very important. Installing a neutron guide is a well-known solution to these problems, because it can deliver much more neutrons to sample, and it can also cut off unwanted fast neutrons when installed in a curved layout. The performance of a neutron guide is much affected by its geometry. We have studied efficiency of the beam transport by a supermirror-coated guide designed for the CNDCS with conventional geometries such as straight, curved and tapered, as well as with advanced geometries such as ballistic, parabolic and elliptical. Energy dependence of gain in intensity, and beam distributions in space and angle obtained by Monte Carlo simulation will be discussed.