Bi-spectral extraction through elliptic neutron guides

In this paper we present the results of investigating a suggested guide extraction system utilizing both a thermal and a cold moderator at the same time, the so-called bi-spectral extraction. Here, the thermal moderator has line of sight to the sample position, and the neutrons from the cold source are reflected by a supermirror towards the sample. rk is motivated by the construction of the European Spallation Source (ESS) but the results are general and can be used at any neutron source. Due to the long pulse structure, most instruments at ESS will be long, often exceeding 50 m from moderator to detector. We therefore investigate the performance of bi-spectral extraction for instrument lengths of 30 m, 56 m, 81 m and 156 m. In all these cases, our results show that we can utilize both moderators (and thus high intensity in a wide wavelength band) in the same instrument at a cost of flux of 5–30% for neutrons with wavelength larger than 1 Å. In general, the divergence distribution is smooth at the sample position for all wavelengths.