Thermal neutron standards

Abstract

Over the 50-year history of Section III (Neutron Measurements) of the Consultative Committee for Ionizing Radiation of the International Committee for Weights and Measures, only a single interlaboratory comparison of thermal neutron measurements has been completed, although a new comparison of this kind is in progress at the time of this writing. The initial comparison was perhaps considered to have demonstrated a sufficient degree of agreement that its repetition was not of particularly urgent priority. Thermal neutron measurements are indeed an old and well-developed field, as is seen in the many references cited in this paper. This paper describes briefly the production of thermal neutron fields from fast neutron sources, the moderation of neutron spectra towards thermal equilibrium, various conventions for describing thermal neutron fields, corrections for self-absorption and scattering in detector materials and the selection of detectors based on established thermal neutron cross-section standards.

Thermal neutron reference radiation facility with high thermalization and large uniformity area

“…Thermal neutron field can be achieved by embedding isotopic neutron sources in moderator assemblies consisting of graphite or polyethylene [7][8][9], which are the most popular materials used to attenuate the neutron energy by elastic collisions [10]. It is necessary to increase the thermal neutron fraction in the reference radiation field as much as possible since some epithermal and fast neutrons will penetrate the moderator directly and contribute significantly to radiation-protection dose measurements [11]. The thermal neutron fields, depending on the design of the moderation framework, show different thermal neutron fluence rates and fractions.…”

Precision determination of absolute neutron flux

“…The two recoil peaks of the 7 Li are also clearly resolved. In principle, the 7 Li peaks could be counted as well, but they fall on the noise tail. Additionally, the α signal has the highest rate of all the signals in the thin target mode, so increasing its rate by accepting an additional peak has little statistical benefit.…”

“…Prompt gamma lines from Si activation can be seen, as well as small amounts of background gammas from ambient environmental radioactivity (e.g., Ra, K, etc.). The 478 keV gamma from capture on 10 B is broadened by the distribution of Doppler shifts caused by the relativistic energy of the ejected 7 Li nucleus. The signal peak is still clearly resolved from the electron-positron annihilation peak present at 511 keV and is only an order of magnitude resolved from background gammas.…”

“…where σ is the 10 B absorption cross section and ρ N is the areal number density of the deposit. Neutron absorption in 10 B produces 7 Li and an alpha particle. The 7 Li nucleus is in an excited state 93.70 % of the time [33,34] The emitted alpha particles and gamma rays are detected in the silicon detector and HPGe detectors, respectively.…”

“…There exist many techniques for the detection of slow neu trons [5][6][7]. In the majority of these techniques, a neutron is incident upon an isotope that has a large reaction cross sec tion and produces energetic charged particles that can be detected through several methods.…”