Thermal neutrons are neutrons in thermal equilibrium with a surrounding medium of the temperature of 290K (17 °C or 62 °F). Most probable energy at 17°C (62°F) for Maxwellian distribution is 0.025 eV (~2 km/s). This part of the neutron’s energy spectrum constitutes the most important part of the spectrum in thermal reactors.
Thermal neutrons have a different and often much larger effective neutron absorption cross-section (fission or radiative capture) for a given nuclide than fast neutrons.
In general, there are many detection principles and many types of detectors. In nuclear reactors, gaseous ionization detectors are the most common since they are very efficient, reliable, and cover a wide range of neutron flux. Various types of gaseous ionization detectors constitute the so-called excore nuclear instrumentation system (NIS). The excore nuclear instrumentation system monitors the reactor’s power level by detecting neutron leakage from the reactor core.
Detection of Neutrons using Ionization Chamber
Ionization chambers are often used as the charged particle detection device. For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n, alpha) reaction can occur. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.
Moreover, isotope boron-10 has a high (n, alpha) reaction cross-section along the entire neutron energy spectrum. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations.
Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in a nuclear reactor. It must be noted that BF3 counters are usually operated in the proportional region.
Fission Chamber – Wide Range Detectors
Fission chambers are ionization detectors used to detect neutrons. Fission chambers may be used as the intermediate range detectors to monitor neutron flux (reactor power) at the intermediate flux level. They also provide indications, alarms, and reactor trip signals. The design of this instrument is chosen to provide overlap between the source range channels and the full span of the power range instruments.
In the case of fission chambers, the chamber is coated with a thin layer of highly enriched uranium-235 to detect neutrons. Neutrons are not directly ionizing and usually have to be converted into charged particles before they can be detected. A thermal neutron will cause an atom of uranium-235 to fission, with the two fission fragments produced having high kinetic energy and causing ionization of the argon gas within the detector. One advantage of using uranium-235 coating rather than boron-10 is that the fission fragments have much higher energy than the alpha particle from a boron reaction. Therefore fission chambers are very sensitive to neutron flux, and this allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining.
Activation Foils and Flux Wires
Neutrons may be detected using activation foils and flux wires. This method is based on neutron activation, where an analyzed sample is first irradiated with neutrons to produce specific radionuclides. The radioactive decay of these produced radionuclides is specific for each element (nuclide). Each nuclide emits the characteristic gamma rays, which are measured using gamma spectroscopy, where gamma rays detected at particular energy are indicative of a specific radionuclide and determine concentrations of the elements.
Selected materials for activation foils are, for example:
These elements have large cross sections for the radiative capture of neutrons. The use of multiple absorber samples allows characterization of the neutron energy spectrum. Activation also allows the recreation of a historic neutron exposure. Commercially available criticality accident dosimeters often utilize this method. By measuring the radioactivity of thin foils, we can determine the number of neutrons to which the foils were exposed.
Flux wires may be used in nuclear reactors to measure reactor neutron flux profiles. The principles are the same. Wire or foil is inserted directly into the reactor core and remains in the core for the length of time required for activation to the desired level. After activation, the flux wire or foil is rapidly removed from the reactor core, and the activity is counted. Activated foils can also discriminate energy levels by placing a cover over the foil to filter out (absorb) certain energy level neutrons. For example, cadmium is widely used to absorb thermal neutrons in thermal neutron filters.