ABSTRACT
Attenuation of neutron beam, as shown in Figure 9.1, by the specimen depends on thickness and the attenuation coecient similar to gamma ray as follows:
It = I0 exp (−Σt) where
It is the transmitted neutron intensity (n/cm2/s) I0 is the incident neutron intensity (n/cm2/s) t is the specimen thickness (cm) Σ is the macroscopic cross section (cm−1)
Σ is equivalent to the linear attenuation coecient of gamma ray (μ) and is the characteristic of elements in the specimen. Σ and μ are the products of atom density of elements contained in the specimen (in atoms/cm3) and their eective microscopic cross sections (σ) to the reactions of interest (in cm2). σ is the eective cross section, not the actual physical cross section of the nucleus. It indicates probability of occurrence for each neutron interaction. For example, σ(n,γ) indicates the probability of (n, γ) reaction and σs indicates the probability of scattering reaction that combines elastic (n, n) and inelastic (n, n′) scattering cross sections. Σ and σ of pure elements and common compounds or mixtures (such as water, heavy water, and concrete) can be found in literatures.
