## 5 Particle Detectors
### Neutron Detectors
A typical neutron imaging system is represented in Figure 14, where a scintillator screen is positioned perpendicular to the neutron beam and the light emitted is brought, via a 45\({}^{\circ}\) tilted mirror, to a lens, which focuses the scintillator image on a CCD camera. The mirror is necessary for easy positioning the camera out of the primary beam, so that a reduced amount of soft error or pixel damage occurs. Neutron detectors are based on the conversion of neutrons into other charged particles through nuclear reactions, followed by their direct detection or by their conversion into photons with wavelength in the visible range. The first category of detectors is well represented by gas detectors, as \({}^{3}\)He detectors. They are widely used in scattering experiments as neutron diffraction. \({}^{3}\)He has a very high absorption cross section for thermal and cold neutrons (5333 barn at 25 meV neutron energy). This nuclear reaction is characterized by the emission of charged particles:  
\[\mathrm{n}+{}^{3}\mathrm{He}\rightarrow{}^{3}\mathrm{H}+{}^{1}\mathrm{H}+0.7 64\ \mathrm{MeV}\]  
The assembly typically consists of a grounded steel chamber filled with pressurized \({}^{3}\)He gas with anode wire along the axis of the tube, at high gas pressure (10-50 bar). Between the wire and the chamber walls, a high voltage (few thousands of Volts) is applied. When a neutron enters the chamber and undergoes a nuclear reaction, a proton and a triton are produced and cause further ionization. The produced electrons are accelerated towards the anode by the potential difference between the anode and the external wall and generate an electron avalanche, which produces a high signal gain and enables the detection of single neutrons. \({}^{3}\)He detectors have the highest efficiency among neutron-to-charged particle conversion processes, but, due to the rarity of this isotope, many alternatives have been studied and used, such as boron-based detectors or GEM gas electron multiplier (GEM) detectors [245]. The other category of thermal- and cold-neutron detectors is based on the use of scintillators, being the elements which undergo nuclear reaction with neutrons in direct contact with the fluorescent material, so that light signals are generated. Beyond being used in scattering experiments, scintillator detectors are the preferred detectors in neutron imaging. Usually the double conversion, neutron to charged particle to light, is performed using a chemical vapor deposition (CVD) of a mixture of a neutron-absorbing component (e.g., LiF) and a fluorescent compound (e.g., ZnS). Although the use of self-supporting neutron detectors would be definitely preferable, the present detectors need a substrate. Figure 14: Typical neutron imaging system. The scintillator screen is perpendicular to the neutron beam. The emitted light is reflected, via a 45\({}^{\circ}\) tilted mirror, to a lens focusing the scintillator image on a CCD camera. Typically, a thin (order of 1 mm or less) Al sheet is used, which results quite transparent to thermal neutrons (Al attenuation coefficient 0.1 cm\({}^{-1}\)), minimizing the unavoidable spatial resolution worsening, due to neutron scattering in between sample and scintillator. The neutron absorption takes place thanks to the \({}^{6}\)Li nuclei present (isotopic abundance is approximately 7.5%), according to the reaction:  
\[\text{n}+{}^{6}\text{Li}\rightarrow{}^{3}\text{H}+{}^{4}\text{He}+4.79\text{ MeV}\]  
The thickness of this compound is a compromise between the need to increase the light output (which would suggest increasing the thickness of the fluorescent compound) and the requirement to improve the light transmission (for which it would be better to reduce the thickness of the compound). Optimizing the detection system is thus carried out by optimizing the thickness of the scintillator screen and tailoring the neutron-sensitive element composition and its relative concentration with respect to the fluorescent one.