Neutrons interaction with matter

Greater detail in the treatment of neutron interaction with matter is required in modem reactor design. The neutron energy distribution is divided into groups governed by coupled space-dependent differential equations. 

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. 

According to the wave theory of elementary particles, a particle of mass m moving with a velocity v has a wavelength A = h/mr, where h is Planck s constant. Thermal (neither hot nor cold ) neutrons have a wavelength of about 1 A, Neutrons interact with matter in two different ways. On the one hand, they Interact with atomic nuclei, thus the image produced by a neutron microscope ... 

The neutron spectroscopy method consists of the measurement of changes in energy and momentum of neutrons interacting with matter in order to obtain information about the dynamics and the geometry of constituent atoms. Let us denote by indexes i andf the initial and final states of the physical quantities attached to the neutron. In the scattering process, the sample transfers momentum TiQ and energy hot to the neutron according to... 

The initial obvious statement is that, overall, neutrons interact with matter even less strongly than do X-rays. Table 10.8 summarizes the differences between the two probes. For both neutrons and X-rays, it is not as easy to direct the beam as it is with electrons or ions. In both cases, the experimental method involves measuring the intensity of the scattered beam as a function of scattering angle. 

As mentioned earlier, neutron radiation is rarely encountered in most laboratories that use radioisotopes in research programs. However, it is useful to understand the difference in the mechanisms by which a neutron interacts with matter compared to those involving other types of radiation since neutron radiation may make the matter with which it interacts radioactive. The neutron has no charge, but it does have about one fourth of the mass of an alpha particle, so that it does have an appreciable mass compared to the atoms with which it interacts. 

The fundamental aspects of neutron diffraetion and its use in the study of materials has been covered in detail in several excellent books whieh should be eonsulted for details. Neutrons interact with matter in a variety of ways which make neutron diffraetion both similar to and yet different from X-ray diffraction. 

Neutrons interact with matter via the short-range nuclear interactions and hence see the nuclei in a sample rather than the diffuse electrons cloud observed by X-rays. In magnetic samples neutrons are scattered by the magnetic moments associated with unpaired electron spins (dipoles). Unlike the X-rays, the neutrons are able to "see" light atoms in the presence... 

The future for electron diffraction is very bright for two reasons. First, electron diffraction pattern can be reeorded seleetively from individual nanostrueture at sizes as small as a nanometer using the electron probe forming lenses and apertures, while eleetron imaging provides the selectivity. Second, electrons interact with matter mueh more strongly than X-ray and Neutron diffraction. These advantages, eoupled with quantitative analysis, enable the structure determination of small, nonperiodic, structures that was not possible before. 

There are three significant possible effects when radiation interacts with matter (5,6). First, the radiation can interact with the nucleus and induce radioactivity as in the case of neutrons. Second, displacement of atoms can occur. This has happened in a number of uranium- and thorium-containing minerals over geological periods. The outstanding example is zircon, which can contain over 10% Th and 2% U. The internal bombardment from these materials and their decay products over geological periods produces low or metamict zircon, where the disorder gives an amorphous state having a low density. 

Neutrons can interact with matter via a number of different reactions, depending on their energy. The following are among the most important of these reactions ... 

X-rays interact with matter because their electromagnetic oscillations are affected by the electrons of the material. Neutrons take no notice whatsoever of electrons when they pass through matter. They interact with the nuclei. Neutron diffraction is sensitive to the atomic number and atomic weight of the atoms constituting the substance. For example, it can distinguish easily between Fe and Co in alloys and between isotopes such as and Cl. 

The sum of the kinetic energy of the fission products and the energy of decay can be determined calorimetrically. The energy of the neutrons and the y rays is usable only inasmuch as neutrons and y rays are absorbed in the medium considered. The energy of the neutrinos is lost, because of their small interaction with matter. 

Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles interact with matter. 

Since neutrons are uncharged, their detection must depend on an interaction with matter which produces energetic charged particles. There are several nuclear reactions initiated by neutrons which result in charged particles. One of the most useful for slow neutrons is the reaction in which a neutron is incident on a boron nucleus. This reaction produces a lithium nucleus and an alpha particle, both of which are rapidly moving. Note that it is the boron isotope of mass 10, with a natural abundance of about 20%, that is required for this reaction and that the alpha particle is simply the nucleus of the helium atom. The boron is usually incorporated in... 

Indirectly ionizing radiations include some types of electromagnetic radiations and neutrons. These radiations interact with matter by giving rise to secondary radiation which is ionizing. Indirectly ionizing radiations lose energy by collisions with electrons, or atomic nuclei, and the charged particles thus set in motion interact in turn with the orbital electrons or nuclei. 

Since neutrons are uncharged their main interaction with matter (in contrast to electromagnetic waves) is not with the electrons in atoms, but with the nuclei through the nuclear interaction. This has a number of consequences. 

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