Neutron Detectors

Mechanisms for detecting neutrons in matter are based on indirect methods. Neutrons, as their name suggests, are neutral. Also, they do not interact directly with the electrons in matter, as gamma ray$ do. The process of neutron detection begins when neutrons, interacting with various nuclei, initiate the release of one or more charged particles. The electrical signrdsproduced by the charged particles can then be processed by the detection system. Two basic types of neutron interactions with matter are available. First, the neutron can be scattered by a nucleus, transferring some of its kinetic energy to the nucleus. If enough energy is transferred the recoiling nucleus ionizes the material surrounding the point of interaction. This mechanism is only efficient for neutrons interacting with light nuclei. In fact, only hydrogen and helium nuclei are light enough for practical detectors. Seem@ the neutron can cause a nuclear reaction. The products from these reactions, such as protons, alpha particles, gamma rays, and fission flrqpnents, can initiate the detection process. Some reactions require a:minimum neutron energy (threshold), but most take place at thermal energies. Detectors exploiting thermal reactions are usually surrounded by moderating material to take ,maximum advantage of this feature. Detectors employing either the reco~ or rekction m~hanism can use solid, liquid, or gas-filled detection media. Although the choice of reactions is limited, the detecting media can be quite variedj leading to many options. ~This chapter describes gas-filled proportional counte~ scintillators, fission chambers, ‘%I-lined chambew and other types of neutron detectors. Gas detectors are discussed in the order of their frequency of use in Sections 13.4.1through 13.4.* plastic&d liquid scintillators, in Section 13.5;and other types of detectors, in Section 13.6. The energy information obtained in neutron detection systems is usually poor because of the limitations of the available neutron-induced reactions. Recoil-type counters measure only the fir$ inte~tion event. we fill neutron energy is usually not deposited in the detector, and the oj$y energy ififomadon obtined is whether a hi@or low-energy neutron initiated tie iriteractioq. Reactio~type counttake advantage of the increased reaction probability at low netiyon eneq$es by moderating the incoming neutrons. But knowledge of the i~tial rieutron eneti before moderation is’lost. The ene~ recorded by the detector Ii the reaction energy (plus, perhaps, some of the remaining initial neutron energy). Thus, in gerieral, neutron ‘detectorsprovide information only on the number of neutrons detecte$land not on their energy. Information on