Neutron interaction with atoms

SANS is a valuable research technique, since it provides information about the size and shape of the molecular scale structures in the size range 1-10 nm found in polymer or dendrimer systems. Neutrons interact with atomic nuclei, hence are sensitive not only to the details of the way such nuclei are arranged but also to the presence of different isotopes. 

Neutron interaction with atomic nuclei is directly linked to the production of several noble gas isotope species in the crust. Before considering the production of crustal noble gases it is essential to first understand the factors controlling the subsurface neutron flux. There are three main types of reaction that produce neutrons in the crust Cosmic ray interactions spontaneous fission and alpha particle interaction with light nuclei. Cosmic ray interaction is only important within the top few meters of the crust (Niedermann 2002, this volume) and we neglect this source of neutrons here. 

Because the neutron has a magnetic moment, it has a similar interaction with the clouds of impaired d or f electrons in magnetic ions and this interaction is important in studies of magnetic materials. The magnetic analogue of the atomic scattering factor is also tabulated in the International Tables [3]. Neutrons also have direct interactions with atomic nuclei, whose mass is concentrated in a volume whose radius is of the order of... 

The real component of the neutron refractive index 8 is related to the wavelength X of the incident neutrons, the neutron scatterir length (a measme of the extent to which neutrons interact with different nuclei), the mass density and the atomic... 

Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays. 

Figure 17.8 Schematic of neutron activation. When a neutron interacts with the target nucleus an isotope forms that can be unstable. If so, it will almost instantaneously de-excite into a more stable configuration through emission of y-rays. Then, this new radioactive nucleus decays by emission of an electron and one or more characteristic y-rays, at a rate according to the half-life of this nucleus. Illustration with Ag atom.

Since neutrons do not directly ionize atoms, they are detected indirectly upon producing a charged particle or a photon, which is then recorded with the help of an appropriate detector. The charged particle or the photon is the result of a neutron interaction with a nucleus. If the mechanism of the interaction is known, information about the neutron can be extracted by studying the products of the reaction. Many types of interactions are used, divided into absorptive and scattering reactions. 

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... 

Atoms in a crystal are not at rest. They execute small displacements about their equilibrium positions. The theory of crystal dynamics describes the crystal as a set of coupled harmonic oscillators. Atomic motions are considered a superposition of the normal modes of the crystal, each of which has a characteristic frequency a(q) related to the wave vector of the propagating mode, q, through dispersion relationships. Neutron interaction with crystals proceeds via two possible processes phonon creation or phonon annihilation with, respectively, a simultaneous loss or gain of neutron energy. The scattering function S Q,ai) involves the product of two delta functions. The first guarantees the energy conservation of the neutron phonon system and the other that of the wave vector. Because of the translational symmetry, these processes can occur only if the neutron momentum transfer, Q, is such that... 

Neutron scattering involves interactions of a neutron beam with atomic nuclei (Figure 4.3). The wave like nature of a beam of neutron particles each with mass M is incorporated in the deBroglie equation which states that the wave length of a neutron beam, A, is inversely proportional to the particle velocity, v, or... 

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. 

In order to date a potassium-bearing mineral by the 40Ar/3 >Ar method the sample is first irradiated by energetic neutrons in the core of a nuclear reactor (Faure and Mensing 2005). The neutrons interact with the nuclei of atoms and transform them into nuclei of Ar by a nuclear reaction of the form ... 

Finally, one must consider the differences in scattering mechanism. Electromagnetic waves excite oscillations in the electrons in the structure that emit the scattered light. Neutrons interact with nuclei. Electrons interact with the charges of electrons and nuclei of atoms. 

In neutron diffraction, Bragg scattering of neutrons occurs when neutrons interact with the nuclei of the atoms in a single crystal both light and heavy atoms can be directly detected. 

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... 

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