The thermal neutron facility HOTNES: theoretical design.

HOTNES (HOmogeneous Thermal NEutron Source) is a thermal neutron irradiation facility with extended and very uniform irradiation area. A 241Am-B radionuclide neutron source with nominal strenght 3.5×106 s-1 is located on bottom of a large cylindrical cavity (30cm diameter, 70cm in height) delimited by polyethylene walls. The upper part of this volume (30cm diameter, 40cm in height) is used to irradiate samples. A polyethylene cylinder, acting as shadowing object, prevents fast neutrons to directly reach the irradiation volume. Indeed neutrons can only reach the irradiation volume after multiple scattering with the cavity walls. The facility was designed trough extensive calculations with MCNPX. Irradiation planes are disks with 30cm diameter, centred on the cavity axis, and parallel to the cavity bottom. The value of thermal fluence in a given irradiation plane is as uniform as 1-2%. The value of thermal fluence rate simply depends on the height from the cavity bottom. Values of thermal fluence rate in the range 700-1000cm-2s-1 are available, depending on the irradiation plane chosen. The fraction of thermal neutrons is in the order of 90%, also depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic. Taking advantage of the HOTNES design, even large devices can be uniformly irradiated. This work presents HOTNES's design and describes the neutron field in the irradiation volume in terms of spatial, energy and direction distributions.