Neutron flux

The physical size of the neutron flux or equivalently, the neutron flux density is the number of times per unit time passing through a unit area ( free ) neutrons. It can also be interpreted as the product of the neutron flux (e.g., in neutron per cm3 ) and the (average) speed of the neutrons (e.g., in cm / s). The direction of motion of the neutron does not matter; there are also counted all neutrons that pass through an imaginary ball at the location in the cross section one unit area. The neutron flux density is therefore a scalar quantity.

The customary unit is cm -2s - 1, ie neutrons per square centimeter and second. In a nuclear reactor, the neutron flux density generally depends on place and time, in consistently held reactor critical only of the place. They can be measured by neutron detectors.

Total and energy-dependent neutron flux density

The existing in a space free neutrons have different kinetic energies in general. The above defined neutron flux density, usually denoted by (big phi), refers to neutrons of all energies together and is therefore more accurately known as total or integral neutron flux density. Your derivative with respect to the neutron energy is called energy-dependent neutron flux density or neutron spectrum (small phi):

Your unit is accordingly eg cm -2s - 1MeV -1. is the number of neutrons in the energy interval for traversing said ball per second.

By integrating a neutron flux density over time, e.g. the duration of irradiation to give the corresponding ( or total energy dependent ) neutron.

Importance in reactor technology

The neutron flux density is relatively easy to measure, such as with built-in into the reactor core fission chambers. It is a suitable measure for the (volume ) power density at a location in the reactor. Since the energy spectrum is approximately equal in the whole reactor, it is sufficient to measure the total flux in practice. In contrast, a temperature measurement would not be suitable as a performance, as the temperature at a location is a result of the performance of the body itself, the power in adjacent areas in the younger and middle past as well as the cooling capacity.

The flow measurement signal is therefore generally used for control and monitoring of the reactor. Even when the ( sub-critical ) reactor, the flow measurement is kept in operation. A built for this purpose radioactive neutron source always provides a low neutron flux; thus the function of the measurement instrumentation is continuously monitored.