VESUVIO spectrometer, has been designed and built by British and Italian scientists, for inelastic neutron scattering at high energy and momentum transfers operational at the ISIS pulsed neutron source. The instrument is operational at the world most intense pulsed neutron source ISIS (UK) since September 2001. This instrument has upgraded the original eVS instrument operating at the ISIS pulsed source since the year 1985, which pioneered the inelastic neutron scattering at the eV energy in the last decades.
A reactor neutron source produces a Maxwell Boltzmann distribution of neutrons from the large moderator which falls exponentially with energy above the thermal peak (F ≈ exp -(E/23 meV)). At a pulsed spallation source, it is important to have a neutron pulse width small enough to maintain good time-of-flight resolution. To this aim moderators are kept small with the result that neutrons are undermoderated, with a spectrum which is distinctly not a Maxwell Boltzmann distribution for energies above 100 meV (F ≈ 1/E). This results in an order of magnitude higher flux of epithermal neutrons than existing research reactors.
The experimental technique employed is known as Deep Inelastic Neutron Scattering (DINS) or Neutron Compton Scattering in analogy with Compton scattering of photons from electrons. It is the only experimental technique presently available which directly allows the study of mean kinetic energies, and of the atomic momentum distribution, n(p) in condensed matter.
The nuclear resonance absorption foil is a basic element for eV neutron spectroscopy, in that it selects the final energy of the scattered neutrons. The experimental method on the eVS instrument consists of cycling the foil in and out of the scattered neutron beam. Two measurements were taken, one with the foil between sample and detector and one with the foil removed. The difference between these two data sets (i.e the number absorbed by the foil) was the experimental signal, which provided a measurement of the intensity of neutrons scattered from the sample with final energy E1. The width of the nuclear resonances used for energy analysis contributes to the finite resolution of the spectrometer and it has already experimentally shown that it can be effectively reduced by filter cooling. The present VESUVIO spectrometer employs routinely Uranium and Gold foils.
The widths of the resonances of the Uranium are quite narrow compared to the resonance energy (DEr/Er ~0.5%), however the broadening due to the thermal motion of the lattice in which the absorbing nuclei are embedded, is an important contribution to the spectrometer resolution function. On VESUVIO a cooling device keeps the U filter chamber in backscattering at ~30K, thereby reducing the Doppler width, and improving by a factor ~2 the energy resolution of the instrument.
- Momentum distributions in quantum and molecular fluids and solids
- Hydrogen momentum distributions
- Single particle dynamics in amorphous materials, polymers, catalysts and metal hydrides
Biology and Biomedical Research