Neutron detection with proportional counter

To a limited degree, the fill-gas will determine what type of radiation the proportional counter will be able to detect. Argon and helium are the most frequently used fill gases and allow for the detection of alpha, beta, and gamma radiation. When detection of neutrons is necessary, the detectors are usually filled with boron-triflouride gas. 

Neon is also used in scintillation counters, neutron fission counters, proportional counters, and ionization chambers for detection of charged particles. Its mixtures with bromine vapors or chlorine are used in Geiger tubes for counting nuclear particles. Helium-neon mixture is used in gas lasers. Some other applications of neon are in antifog devices, electrical current detectors, and lightning arrestors. The gas is also used in welding and preparative reactions. In preparative reactions it provides an inert atmosphere to shield the reaction from air contact. 

The fact that neutrons can be detected with reasonably high efficiency and with minimal interferences from other radiations permits the practical determination of fissionable species such as isotopes of uranium and thorium by delayed neutron counting. The known delayed neutron emitter precursors are all short lived and the irradiated samples are counted with 10BF3-filled proportional counters immediately after irradiation without any separation chemistry. 

C. Proportional Counters. Proportional counters are used to detect one type of radiation in the presence of other types of radiation or to obtain output signals greater than those obtainable with ionization chambers of equivalent size. Proportional counters may be used to either detect events or to measure absorbed energy (dose), because the output pulse is directly proportional to the energy released in the sensitive volume of the counter. Proportional counters are most widely used for the detection of alpha particles, beta, neutrons, and protons. 

Proportional counters can be used also for neutron detection by using a gas containing nuclei that capture neutrons and in the nuclear reaction produce charged particles, e.g. or He. In Bp3-counter the gas is BF3, usually enriched in B. With neutrons the reaction... 

Proportional counters filled with BF3 or He can be used to detect low-energy neutrons. The reaction °B(n,a) Li has a large cross section for thermal neutrons, and the charged reaction products produce a large, distinctive pulse. The cross section drops as energy increases, but it remains useful to keV neutron energies. Similarly, the He(n,p) H reaction has a large cross section, produces a distinctive pulse, and can be used for spectroscopy. 

The handheld neutron monitor (HHNM) is a portable ( 4 kg) neutron detection device with three He proportional neutron counters, a GM counter and integrated electronics, which provide a means of searching for and localizing neutron radiation sources. A measurement sequence consists of background and verification measurements. When a predetermined threshold is exceeded, the detector triggers an alarm and records the relevant information. 

In the case of neutrons, the detector is a proportional counter consisting of a container filled with He gas and an array of wires. When neutrons hit He atoms, electrons are released and their positions are detected by the array of wires, which gives both the position of the scattered neutrons and their number [16]. 

---