Wide Range Detector Definition – Fission Chamber

Neutron ionization detectors, such as fission chambers, are ionization detectors. It is common practice in nuclear instrumentation systems to employ the fission chamber as a "wide range" channel due to its dual purpose usage.

In order to detect neutrons, fission chambers are employed as ionization detectors. Neutron flux (reactor power) may be monitored by using fission chambers as intermediate range detectors. In addition, they send out signals for reactor trip, indicator, warning, and other functions. Because of the way this instrument is constructed, the source range channels and the entire power range instruments can be used simultaneously.

Generally speaking, an ionization chamber, also known as an ion chamber, is an electrical device used to detect various forms of ionizing radiation (also known as charged particles). The voltage of the detector is adjusted to match the ionization zone. Gas amplification is not possible due to the low voltage (secondary ionization). The Geiger-Mueller tube's accuracy degrades with increasing dosage rate, hence ionization chambers are preferable for high radiation dose rates. In addition, an increase in voltage does not significantly increase the number of ion-pairs gathered in the ionization area. For each particle or ray in the radiation, the number of ion-pairs collected by the electrodes is directly proportional to how many ion-pairs are created.

A thin coating of highly enriched uranium-235 coats the chamber in fission chambers to detect neutrons. If you want to detect the presence of neutrons, you'll need to turn them into charged particles first. There are two fission fragments that are high in energy, and they cause the gas in the detector to be ionized by the thermal neutron that caused the fission. Uranium-235 is better for nuclear reactors because the fission fragments are far more powerful than the alpha particle from a boron reaction, which is lower in energy. There are a number of factors that contribute to this: First, the gamma radiation that hits the detector is much weaker than the neutrons that interact with the coating and produce fission pieces. As a result, the charge pulses created by neutrons are far greater than those generated by gamma rays. The undesired gamma pulses can then be blocked using pulse size discrimination circuitry. In contrast to an uncompensated ion chamber with a boron liner, fission chambers are particularly sensitive to neutron flux because of this.

Fission chambers can be employed as current indicators and pulse devices depending on the degree of neutron flux in the reactor. Because neutrons and gamma rays have highly different pulse lengths, fission chambers are particularly useful in pulse mode. Fission chambers can be operated in Campbelling mode (also known as "fluctuation mode" or "mean square voltage mode") to enable accurate neutron measurements when power is high in the intermediate or power range (i.e. in a high level mixed gamma and neutron flux). The gamma component is eliminated using the Campbelling procedure. There are a lot of "wide range" channels in nuclear instrumentation systems that make advantage of the fission chamber because of its dual purpose.