The Orph�e reactor

Laboratoire Léon Brillouin

UMR12 CEA-CNRS, Bât. 563 CEA Saclay

91191 Gif sur Yvette Cedex, France

+33-169085241 llb-sec@cea.fr

The Orph�e reactor

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).

The variety 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 and g -rays and to optimize the signal-to-background ratio.