LET science
22 Jul 2009




The LET spectrometer showing the large detecord bank and multiple high speed disk choppers.

LET is a versatile direct geometry spectrometer operating over a wide 0 – 30meV energy range, with an energy resolution ranging from 20μeV at an incident energy of 1 meV, or 500 μeV at 20 meV. It has a large detector solid angle populated with 4m long position sensitive detectors (PSDs) that 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, CNCS at SNS, Amateras at J-Parc, or NEAT at HZB. It has a very large flux at the sample position, comparable with the aforementioned instruments. 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 is an excellent quasi-elastic spectrometer rep-rate multiplication enabling a single measurement to span multiple time and length scales. For the measurement of excitations (both magnetic and structural) the high flux, extremely low background and versatile sample environment make LET the perfect instrument for energies below 25 meV.

LET uses two sets of fast counter-rotating disk choppers to monochromate the incident beam rather than the single Fermi-chopper used on other ISIS chopper instruments. This makes 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:

  1. It allows control over the pulse width and shape; producing a symmetric triangular resolution function.
  2. It makes it possible to match the pulse width from the moderator and the chopper to maximise the flux for any given resolution.
  3. The resolution is not limited by the intrinsic width of the moderator, only by the opening time of the choppers.
  4. It has the ability to trade flux for resolution by altering the speed and opening size of the choppers.

Two additional choppers enable the instrument to be run in Rep-Rate Multiplication mode, where several (4 to 10) incident energies are able to pass through the chopper system. This exploits the full 100 ms time window of the second target station and limits the need to repeat measurements with different incident energy.​