Neutron attenuation coefficient

The method of neutron radiography is simple to implement. Since water has a large neutron attenuation coefficient, the technique of radiography has a high sensitivity for small water mass changes... 

Figure 5. The measured attenuation coefficient for die NIST MCP detector at die BT2 thermal neutron beam and the NG1 cold neutron beam.

The macroscopic cross section is analogous to the linear attenuation coefficient of -y rays (Sec. 4.8.4). If a parallel beam of monoenergetic neutrons with intensity 1(0) impinges upon a material of thickness t, the number of neutrons that emerges without having interacted in the material is (see Fig. 4.24)... 

Neutrons, being uncharged, do not interact electromagnetically with electrons or nuclei in matter. Instead, the nuclear interaction with nuclei is the most common interaction, but this can occur only if the neutron comes within 1 fm of the nucleus. Hence, the attenuation coefficient for neutrons is small and neutrons can penetrate large amounts of matter. The main interaction processes are elastic scattering A n,ri)A, inelastic scattering A(n,n )A, radioactive capture [A(n,y)A+1, and other nuclear captures A(n,2n)A - 1, A(n,p)A(Z - 1), A(n,np)A - 1(Z - 1), A(n,a), A(n,f). ... 

The moment method can be formulated without reference to the Fourier transform of the flux. It can be regarded simply as a technique of constructing flux distribution functions from their (numerically calculated) moments. When information about the singularities of the flux transform is not utilized, the method becomes less well-founded theoretically, but it gains in flexibility and can be applied even when the singularities of the transform are not well understood. This situation arises in the case of the plane oblique source, as well as in connection with the penetration of fast neutrons whose attenuation coefficient may be a rapidly varying function of the energy with many maxima and minima. 

X-ray and neutron imaging are complementary techniques for materials research. X-rays interact mainly with the electronic shell of atoms whereas neutrons as charge-neutral particles interact with the nuclei (Figure 18.1a,b). The different interaction mechanisms yield different beam attenuation properties. Figure 18.1c shows the values for the attenuation coefficients of X-rays and neutrons for different element numbers. In the case of X-rays, the attenuation increases with the number of electrons in the atom and, therefore, with the element number. In case of neutrons, no clear dependence on the amount of nuclei within the atomic core can be found. In contrast to X-rays, some light elements such as H and Li have a very... 

Figure 18.1 Interaction of matter with (a) X-rays and (b) neutrons, and (c) mass attenuation coefficient as a function of atomic number for all elements. Adapted from [12, 13] by permission of lOP Publishing and DCZfP - Deutsche Gesellschaft fur Zerstorungsfreie Prufung.

Beam hardening artifacts (for polychromatic X-rays or neutrons) prevent exact quantification of the attenuation coefficients. 

Transport coefficients like the diffusion constant, the viscosity or the sound velocity, and dynamic light scattering or inelastic neutron scattering intensities are all dynamical properties of a system in thermal equilibrium. We concentrate here on the viscosity and on sound velocity and attenuation. 

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