LET, a cold neutron multi-disk chopper spectrometer at ISIS

LET is a new multi-chopper direct geometry cold neutron spectrometer, recently installed on target station 2 (TS2) at the ISIS spallation neutron source. The characteristics of the primary spectrometer are a 25 m straight super-mirror guide viewing the new highly efficient coupled solid methane moderator system. This combination yields a high incident flux of neutrons with a wide dynamic range of 0.6-80 meV. LET employs a novel flux compression guide design which in combination with two 300 Hz counter rotating choppers produces fine energy resolutions of >= 0.8% delta E/Ei, to be realised with very little flux sacrifice compared to conventional methods. The multi-chopper system is designed to make full use of the long 100 ms time frames of TS2, allowing the user to make multiple measurements within a single frame. The secondary spectrometer is characterised by a wide area position sensitive He-3 multidetector with pi steradians of nearly gapless coverage over the angular range from -40 degrees to +/- 140 degrees in the horizontal plane and +/- 30 degrees in the vertical plane. The multidetector utilises the worlds first 4 m position sensitive He-3 neutron detector tubes. LET has been designed specifically to allow the use of polarised neutrons allowing the user to exploit full XYZ neutron polarisation analysis. In addition a new 9 T magnet has been designed for LET allowing the use of large samples of 25 mm x 25 mm with a 30 degrees vertical opening to make use of the large multidetector. (C) 2011 Elsevier B.V. All rights reserved.