International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 7.3, p. 644

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 neutroncapture 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 conservationofmomentum 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} m^{2}). 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 energydependent absorption cross section, and t is the thickness of the absorbing material. The factor gives the neutroncapture 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.

References
Convert, P. & Forsyth, J. B. (1983). Positionsensitive detection of thermal neutrons: Part 1, Introduction. Positionsensitive detection of thermal neutrons, pp. 1–90. London: Academic Press.