FIRES: a novel neutron backscattering spectrometer

The principal design parameters are presented for the proposed new high resolution time-of-flight (tof) backscattering instrument FIRES at ISIS. The primary spectrometer consists of a pulse shaping chopper combined with a coupled hydrogen moderator and a flight path about 80 m long. The use of a pulse shaping chopper will enable changing the resolution by choosing different pulse widths. Trading resolution against intensity provides a powerful and flexible tool. The secondary spectrometer is built up of large area silicon analysers which reflect the neutrons in near backscattering geometry to position-sensitive detectors. We assess the performance of the instrument by analytical calculations of resolution, intensity and dynamic range. The resulting numbers indicate a performance comparable with the most modern instruments available now and in the near future.