Simulation of a multiple-wavelength time-of-flight neutron spectrometer for a long-pulsed spallation source

We have performed design studies and ray-tracing simulations of a direct-geometry spectrometer at the cold moderator of a long-pulsed 5 MW neutron spallation source. A 100 m long guide path makes it possible to achieve 2% elastic resolution at 5 angstrom incoming neutron wavelength, without a need to shorten the 2 ms long source pulse. Assuming a 16 2/3 Hz repetition rate, the 60 ms time between pulses allows us to extract from each source pulse several monochromatic pulses at the sample with a spacing in the range 5-10 ms. These pulses are equidistantly spaced in wavelength with delta lambda approximate to 0.2-0.4 angstrom depending on the spectrometer setting. If all these bands contain useful information, the spectrometer gains up to an order of magnitude in incident flux from the repetition scheme. The incident flux will then reach values of roughly 100 times the present IN5 spectrometer at the ILL at similar resolution and frame length. There are possibilities of optimizing the guide design further, to achieve even higher flux values. Virtual experiments provide us with the final energy resolution of the spectrometer, including the line shapes. The performance of the instrument is found to be excellent, judged by the virtual data. Such an instrument positioned at a long-pulsed neutron spallation source would be very powerful for cold neutron spectroscopy for energy transfers in the range 10 mu eV to 10 meV.