LET will be a versatile direct geometry spectrometer operating over a wide 0 – 80meV energy range, with an energy resolution ranging from 5μeV at an incident energy of 1 meV, or 260 μeV at 20 meV. It will have a large detector solid angle populated with MAPS type position sensitive detectors (PSD) that will make it possible to map a vast swathe of Q-E space in a single measurement.
The design of the instrument is similar to IN5 at the ILL or NEAT at HMI. It will have a very large flux at the sample position, similar to the new improved IN5 at low energies but much larger at higher energies, greater than 20 meV. This is because such a spectrometer ideally requires a high ‘peak’ flux such as that from the coupled moderator of TSII, and not a large time averaged flux produced by reactor sources. It will be both: an excellent quasi-elastic spectrometer with a resolution that exceeds the 17μeV available on IRIS, and a low energy chopper spectrometer with count rates and resolution superior to that of any other existing machine. As of yet, such an instrument has never been built on a pulsed spallation source although one is planned for the SNS.
LET will use two sets of fast counter-rotating disk choppers to monochromate the incident beam rather than the single Fermi-chopper used on the existing ISIS chopper instruments. This will make the instrument much more versatile, with the first chopper controlling the
pulse width of the moderator while the second chopper monochromates the beam: This has several key advantages:
- It will give control over the pulse width and shape; producing a symmetric triangular resolution function.
- It makes it possible to match the pulse width from the moderator and the chopper to maximise the flux for any given resolution.
- Its resolution will not be limited by the intrinsic width of the moderator, only by the opening time of the choppers; thus making it possible to get better percentage ( ΔE / Ei ) energy resolution than the existing instruments.
- It will have the ability to trade flux for resolution by altering the speed of the choppers.
This chopper configuration will make it possible to take advantage of the high peak flux from the coupled moderator on TSII. Together with a converging supermirror guide, LET will deliver a very high flux to the sample over a very wide range of energies. Indeed, it is only above around 80meV that the sample flux on MAPS will begin to exceed LET, see figure 2, and its flux will be competitive with the IN5 at energies as low as 1meV. On reactor based spectrometers such as IN5 or IN6, the flux begins to drop away rapidly above 20meV.