Section 7.3.2. Neutron capture

Neutrons' lack of charge and the fact that they are only weakly absorbed by most materials require specific nuclear reactions to capture them and convert them into detectable secondary particles. Table 7.3.2.1 lists the neutron-capture reactions that are commonly used in thermal neutron detection. The incoming thermal neutron brings a negligible energy to the nuclear reaction, and the secondary charged particles or fission fragments are emitted in random directions following the conservation-of-momentum law . The capture or absorption cross sections for a number of nuclei of interest are plotted as a function of neutron energy in Fig. 7.3.2.1. These cross sections are commonly expressed in barns (1 barn = 10⁻²⁸ m²). At low energies, they are inversely proportional to neutron velocity, except in the case of Gd, which has a nuclear resonance at 0.031 eV. The total efficiency of neutron detection can be expressed by the equation where N is the number of absorbing nuclei per unit volume, is their energy-dependent absorption cross section, and t is the thickness of the absorbing material. The factor gives the neutron-capture efficiency, while ξ is a factor that depends on the detector geometry and materials (absorption and scattering in the front window) and on the efficiency of the secondary particles.

Table 7.3.2.1| top | pdf | Neutron capture reactions used in neutron detection n = neutron, p = H+ = proton, t = 3H− = triton, α = 4H+ = α, alpha, e− = electron. Capture reaction Cross section at 1 Å (barns) Secondary-particle energies (MeV) 3He + n → t + p 3000 t  0.20 p  0.57   6Li + n → t + α 520 t  3.74 α  2.05   2100 α  1.47 α  1.78 7Li  0.83 7Li  1.01 γ  0.48 74000 (natGd: 17000) e− spectrum γ spectrum 0.07 to 0.182 up to 8   235U + n → fission fragments 320 Fission fragments up to 80

Figure 7.3.2.1| top | pdf | The capture cross sections for a number of nuclei used in neutron detection. [Adapted from Convert & Forsyth (1983).]