Characterization of the graphite pile as a source of thermal neutrons

Abstract

The absorption cross sections of low energy neutrons obey 1/v law for most materials. Therefore, the detection efficiency of a particular detector increase with decreasing neutron energy. Consequently, most of neutron measuring instruments consist of the detector of thermal neutrons which is located inside of the polyethylene moderator. Development of such instruments requires testing and response calibrations in the field of thermal neutrons. Availability of thermal neutron beams on nuclear reactors is limited and access to them is rather complicated, so it is more convenient to moderate neutrons from the radionuclide neutron sources. Since radiunuclide neutron sources are producing fast neutrons it is necessary to use an appropriate moderator material like heavy water or graphite to thermalize neutrons from the source and to avoid thermal neutron capture in the same time.Thermal neutron fluence rate is commonly measured by means of gold activation detectors. The quality of a moderated thermal neutron source spectra is then characterized by so called 'cadmium ratio' parameter. This parameter is based on properties of Cd-113 isotope (12.2% natural abundance of Cd) with very high neutron cross-section as depicted in Fig. 1. Gold foil is activated in the thermal neutron field both bare and encapsulated in a Cd housing and the correspondig activity ratio characterize the thermal neutron field. Au-197 has strong resonance at 4.9 eV which allows measurable activation of the gold inside the cadmium capsule. Basic moderated thermal neutron field characteristics are: Cadmium ratio: =  Cadmium coefficient:where A tot is activity of openly irradiated Au foil and A epi is activity of a Au foil irradiated in the Cd capsule. The effective energy cut-off E Cd of used cadmium covers is 0.5 eV.

A two-layer Timepix3 stack for improved charged particle tracking and radiation field decomposition

“…A first measurement was done within an isotropic thermal neutron field at the Czech Metrology Institute (CMI) created by moderating fast neutrons from Pu-Be source in a graphite pile [16,17]. The fast neutron response was studied at the Weapons Neutron Research (WNR) facility of the Los Alamos Neutron Science Center (LANSCE) [18].…”