III.30.
Ionization chambers were first developed to measure exposure to X rays and gamma radiation. If hydrogen is introduced into the walls and the gas, ionization chambers can be made more sensitive to neutron radiation. However, they are also sensitive to photons, and so it is necessary to provide a second chamber that is relatively insensitive to neutrons (e.g. with graphite walls and a carbon dioxide gas mixture, or aluminium walls and argon gas) to correct for the gamma radiation that is always associated with neutron radiation. Such ionization chambers measure the neutron absorbed dose, not the dose equivalent rate.
III.31.
Since the response of an ionization chamber to gamma radiation per unit dose is similar to the response to neutron radiation, it is not possible to discriminate efficiently between the two types of radiation with a single chamber. However, ionization chambers can be used where it is not necessary to discriminate between the contributions from gamma radiation and from neutron radiation in a radiation field. It is possible to make the wall of the ionization chamber in near tissue equivalent material, and to fill the detector with near tissue equivalent gas. The energy deposition in the detector then mimics the energy deposition in tissue, regardless of the type of radiation. These ionization chambers are operated in proportional counter mode. Such tissue equivalent proportional counters are mostly used at low gas pressure, and can thus be used for microdosimetric purposes, but they are also useful as ambient monitors.