Section 4.4.6. Absorption coefficients for neutrons

The cross sections σ discussed in Section 4.4.4 represent the area of each nucleus as seen by the neutron. To calculate the beam attenuation arising from absorption it is more convenient to use the macroscopic cross section , which is the cross section per unit volume in units of cm⁻¹. is derived by multiplying σ for the element by the number of atoms per unit volume. Thus, for element j, with density and atomic weight , where is Avogadro's number.

Table 4.4.6.1 gives the macroscopic absorption cross sections of the elements. They are tabulated for a neutron velocity v = 2200 m s⁻¹, corresponding to a wavelength of 1.80 Å. The cross sections are larger at longer wavelengths (Section 4.4.4). Apart from a few exceptions, such as boron and cadmium, the absorption cross section is vastly smaller than for X-rays. The 1/e penetration depth (l) is listed separately – most metals, for example, have a penetration depth of several cm. The data in Table 4.4.6.1 have been derived from the review article by Hutchings & Windsor (1987).