Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U²³⁵. The neutrons interact with the U²³⁵, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall.

One advantage of using U²³⁵ coating rather than boron is that the fission fragment has a much higher energy level than the alpha particle from a boron reaction. Neutron-induced fission fragments produce many more ionizations in the chamber per interaction than do the neutroninduced alpha particles. This allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining. Fission chambers are often used as current indicating devices and pulse devices simultaneously. They are especially useful as pulse chambers, due to the very large pulse size difference between neutrons and gamma rays. Because of the fission chamber's dual use, it is often used in "wide range" channels in nuclear instrumentation systems. Fission chambers are also capable of operating over the source and intermediate ranges of neutron levels.

Activation Foils and Flux Wires

Whenever it is necessary to measure reactor neutron flux profiles, a section of wire or foil is inserted directly into the reactor core. The wire or foil remains in the core for the length of time required for activation to the desired level. The cross-section of the flux wire or foil must be known to obtain an accurate flux profile. After activation, the flux wire or foil is rapidly removed from the reactor core and the activity counted.

Activated foils can also discriminate energy levels by placing a cover over the foil to filter out (absorb) certain energy level neutrons. Cadmium covers are typically used for this purpose. The cadmium cover effectively filters out all of the thermal neutrons.

Photographic Film

Photographic film may be utilized in x-ray work and dosimetry. The film tends to darken when exposed to radiation. This general darkening of the film is used to determine overall radiation exposure. Neutron scattering produces individual proton recoil tracks. Counting the tracks yields the film's exposure to fast neutrons. Filters are used to determine the energy and type of radiation. Some typical filters used are aluminum, copper, cadmium, or lead. These filters provide varying amounts of shielding for the attenuation of different energies. By comparing the exposure under the different filters, an approximate spectrum is determined.

Summary

A description of how self-powered neutron detectors, wide range fission chambers, flux wires, and photographic film detect radiation is summarized below.

Miscellaneous Detector Summary

Self-powered neutron detector

The central wire, made of a neutron-absorbing material, absorbs a neutron and undergoes beta decay.

As more beta decays occur, the remaining atoms cause the wire to become more positively charged.

The voltage potential set up causes a current flow in a resistor, which is measured by either a millivoltmeter or electrometer.

Wide range fission chamber

Neutrons interact with the U²³⁵ coated chamber causing fission of the U²³⁵.

A highly positive charged fission fragment interacts with the detector gas and causes ionizations.

The electrons produced are collected as pulses on the electrode.

Flux wire

The wire is inserted directly into the core and becomes activated by the neutron flux.

When the desired activation time is reached, the wire is removed from the core and counted.

Photographic film

Detects total radiation dose by darkening; film darkness determines overall exposure.

Fast neutron exposure determined by counting individual proton recoil tracks.