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

Performance of converging neutron guides was studied for the cold neutron double-chopper spectrometer proposed for the spallation neutron source at J-PARC. Intensity, spatial distribution, and angular distribution of a neutron beam at a sample are compared for linearly tapered and elliptic geometries by Monte Carlo simulation. For low-energy neutrons, the flux on the sample is much increased by an elliptic geometry whose focal points are on the moderator and on the sample in compensation for the angular resolution. To keep a substantial gain for a wide energy range, an elliptic guide with one of the focal points far behind the moderator is better. It also gives good spatial and angular intensity profiles at the sample.