Kind of Neutron Detectors
Neutrons are detected by nuclear reactions which result in prompt charged particles such as protons, alpha, and so on. Virtually, all types of the neutron detectors include the combination of a target material to carry out neutron to ionizing particle conversion together with one of the charged particle detectors. Because the neutron energy is the important parameter in use of the cross section for neutron interactions in most materials, several techniques have been introduced for neutron detection in various energy ranges.
In selecting for nuclear reactions that could be used in neutron detection, several elements must be considered. For example, the cross section of the reaction must be as large as possible so that efficient detectors with small dimensions can be used. In many applications, fields of gamma rays are also presented with neutrons, so the choice of reaction is involved on the ability of discriminate gamma rays from the neutrons in the detection process (Fazzi, Agosteo, Pola, Varoli, & Zotto, 2003).
The discovery of the neutron is based on the measurements of neutron energy due to scattering neutrons on hydrogen (Chadwick, 1932) or nitrogen (Feather, 1932), and measuring the energy of the recoiled nuclei. Neutron detection has adopted to the development of nuclear industry since 1932 and also in many research fields such as nuclear physics, nuclear technology, fusion plasma diagnostics, radiotherapy and radiation protection, it is very significant that measure fast neutron spectra.
Methods of neutron spectrometry can be categorized into seven groups (Brooks & Klein, 2002):
- 1.
Methods based on measurement energy of the recoiled nuclei, as in the discovery of the neutron.
- 2.
Methods in which the neutron is induced in nuclear reaction and results the charge particles such as alpha particles.
- 3.
Methods based on the measurements of neutron velocity.
- 4.
Threshold methods, so that minimum neutron energy is indicated by the presence of a neutron irradiation effect such as neutron activation, radioactivity, specific gamma-ray energy or a phase transition.
- 5.
Methods based on the determination of neutron energy distribution by unfolding a set of detectors (or detector geometries) which their response to neutrons are differed.
- 6.
Methods in which neutron diffraction effects are observed and
- 7.
Methods based on the measurement of time-distribution of the high-energy neutron slowing downs in a suitable medium.
In this survey we particularly discuss methods belonging to group 1 and we introduce detectors that working phenomena lies in this class.
The most useful method of fast neutron spectrometry is based on elastic scattering of neutrons by hydrogenous materials. The recoil nuclei that result from (n,p) reaction are called recoil protons and consequently, spectrometry method is known as proton recoil method.
One of the proton recoil detector is the ‘SP2’ counter, investigated by Benjamin et al. (Benjamin, Kemshall, & Redfearn, 1968). A thin thickness spherical stainless steel chamber, 40mm in diameter, filled with hydrogen gas at a pressure range from 105 to 106 Pa (1–10 atm), which neutrons interact by scattering from a proton in the hydrogen atoms and the obtained proton deposits its energy in the gas medium, and generates primary ionization along its track until it stopped by the gas (or hits the wall). In the nearby to the anode wire, that the electric field is relatively strong, multiplication of the primary charge is started. The main disadvantage of the SP2 counter is to gamma ray sensitivity, which primary photoelectrons from the gamma rays generate secondary electrons near the anode wire.