Interaction of Neutrons with Matter

Neutrons, with protons, are the constituents of nuclei (see Sec. 3.4). Since a neutron has no charge, it interacts with nuclei only through nuclear forces. When it approaches a nucleus, it does not have to go through a Coulomb barrier, as a charged particle does. As a result, the probability (cross section) for nuclear interactions is higher for neutrons than for charged particles. This section discusses the important characteristics of neutron interactions, with emphasis given to neutron cross sections and calculation of interaction rates. 

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One of the technologically most important interactions of neutrons with matter is their loss of energy ( slowing down ) by a series of elastic collisions. Let us consider the case where particle 1 of mass mx, speed Vi, collides with particle 2, mass m2, at rest. After the collision, the particles will have speeds v and in the lab system. 

Because neutrons are electrically neutral, their interaction with electrons is very small and primary ionization by neutrons is negligible. The interaction of neutrons with matter is practically confined to the nuclei and comprises elastic and inelastic scattering and nuclear reactions. In elastic collisions the total kinetic energy remains constant, whereas in inelastic collisions part of the kinetic energy is given off as excitation energy. 

The interaction of neutrons with matter is quite different from that of either charged particles or gamma rays. Depending on their energy, neutrons interact with matter by various processes. 

The interaction of neutrons with matter is weaker than for X-rays and electrons. Therefore higher neutron fluxes are needed to measure appreciable scattered intensities. 

A model of structure has been developped from electron microscopy and X-ray studies in Nafions (6). No basic difference exists between X-ray and neutron techniques. Nuclear interactions of neutrons with matter are characterized by the coherent scattering length and the corresponding values for H and D are very different Because of the different origins of the contrast, X-ray and neutron small angle scattering techniques are complementary. 

Much like X-rays, the interactions of neutrons with matter are atomic in nature. The difference is that neutrons are sensitive to nuclei directly, whereas X-rays interact with electrons. Hence, while X-rays are unsuitable to detect light elements because of the low atomic electron count, neutron scattering factors depend on the properties of the nucleus [206]. The most relevant consequence in the context of this discussion is that neutron-based tools are better suited for the detection of H and Li than X-rays, as H and Li are among the most highly neutron-absorbing atoms, and that they offer isotope resolution capability. In principle, they are also nondestructive. 

As a consequence of the above interactions of neutrons with matter, neutron diffraction has both advantages and disadvantages as compared to X-ray diffraction. 

From 50 years to 100 years after Dalton proposed his theory, various discoveries showed that the atom is not indivisible, but really is composed of parts. Natural radioactivity and the interaction of electricity with matter are two different types of evidence for this subatomic structure. The most important subatomic particles are listed in Table 3-2, along with their most important properties. The protons and neutrons occur in a very tiny nucleus (plural, nuclei). The electrons occur outside the nucleus. 

Two different types of information can be obtained by bombarding soil with neutrons. Fast neutrons are slowed when they interact with water and thus can be used to measure the amount of water present. This type of analysis is most often conducted in the field rather than in the laboratory. Figure 8.2 illustrates the use of a fast neutron source and a slow neutron detector to measure the moisture content of soil. This method depends on the interaction of neutrons with hydrogen and so it is not as useful in soils with significant or highly variable organic matter contents. 

The extremely weak interaction of neutrons and matter is dominated by spin-spin interaction with nuclei, whilst interactions with electron-spins are negligible. Nuclear cross-sections for neutron scattering are strictly independent of the electronic structure (ionic or neutral, chemical bonding, etc). There-... 

The interaction of neutrons and matter is extremely weak, for the neutron has no electric charge. It is essentially spin-spin interaction with nuclei, whilst interaction with electron spins is negligible. Nuclei can be treated as dimensionless scattering centres (Fermi potential). The nuclear cross-sections are strictly independent of the electronic surrounding (ionic or neutral, chemical bonding, etc.). Therefore, the scattering cross-section of any sample can be calculated exactly, from the known cross-section of each constituent. Compared to optical techniques, INS intensities can be fully exploited and the spectra can be interpreted with more confidence. They are related to nuclear displacements involved in each vibrational eigenstate. 

Chen, S.-H. and Kotlarchyk, M. (1997). Interactions of Photons and Neutrons with Matter - An Introduction. World Scientific, New Jersey. 

The lion s share of fluorine is produced by the intense burst of neutrinos that occurs when the Type II supernova core collapses. Although neutrinos interact only infrequently with matter, a tiny fraction of their intense flux during a 10-second burst drives a proton or neutron from the 20Ne nucleus, in either case resulting in 19F. This occurs where both 20Ne and the neutrino flux are most abundant, near the core of the exploding massive star. Much of this 19F is subsequently destroyed by nuclear reactions in the heated gas when the shock wave passes, but enough survives to account for the 19F/2°Ne abundance ratio in the Sun. 

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