The reactor ORPHEE
The 14 MW reactor ORPHEE consists of a core of very small size, highly enriched in 235 Uranium which provides a high neutron density, surrounded by a heavy water reflector tank to give a good thermal flux (3 1014n/cm2s). The neutron flux has a nearly Maxwellian distribution peaking at a wavelength of 1.7 Å (corresponding to a neutron kinetic energy of 30 meV).
Thevariety of problems studied by neutron scattering requires an as broad as possible neutron wavelength (and energy) distribution : short wavelengths to explore a large volume of reciprocal space for atomic structure resolution, long wavelengths (and therefore low energy) to study large scale structures up to a fraction of a ”m and to perform high resolution energy spectroscopy (neV scale to meV scale). This is achieved by local thermalisation of 3 neutron beams by a hot source (heated graphite, 1400 K), of 8 others by two cold sources (liquid hydrogen, 20 K), the 6 remaining ("thermal" beams) being thermalised at the temperature of the D2O moderator (300 K). This makes available incident neutrons of any wavelength ranging from 0.7 to 15 Å.
The design of the 9 horizontal neutron beam tubes (which point tangentially towards the core) and of the 6 curved neutron guides, has allowed to considerably reduce the background due to fast neutrons andg -rays and to optimize the signal-to-background ratio.