The ionization chamber, also known as the ion chamber, is electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the ionization region. The voltage is not high enough to produce gas amplification (secondary ionization). Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon which affects the accuracy of the Geiger-Mueller tube at high dose rates.
The compensated ion chamber is utilized in the intermediate range because the current output is proportional to the relatively stable neutron flux, and it compensates for signals from gamma flux. The compensated ion chamber consist of two detectors in one case. The outer chamber is coated on the inside with boron-10, while the inner chamber is uncoated. The coated chamber is sensitive to both gamma rays and neutrons, while the uncoated chamber is sensitive only to gamma rays. By connecting the two chambers in a proper way, the net electrical output from the detector will be the current due to neutrons only.
To achieve the proper amount of gamma compensation, the voltages between these two sets of electrodes must be balanced. The consequences of operating with an overcompensated or undercompensated chamber are important. If the voltage in the compensation chamber is too high, the detector is overcompensated and some neutron current, as well as all of the gamma current, is blocked, and indicated power is lower than actual core power. If the compensating voltage is too low, undercompensation will occur. At high power, gamma flux is relatively small compared to neutron flux, and the effects of improper compensation may not be noticed. It is extremely important, however, that the chamber be properly compensated during reactor startup and shutdown.
See also: U.S. Department of Energy, Instrumentation and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.
Since the neutrons are electrically neutral particles, they are mainly subject to strong nuclear forces but not to electric forces. Therefore neutrons are not directly ionizing and they have usually to be converted into charged particles before they can be detected. Generally every type of neutron detector must be equipped with converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).
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 take place. 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 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 nuclear reactor. It must be noted, BF3 counters are usually operated in the proportional region.
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