Reactions of neutrons with light nuclei

The present section excludes discussion of the two- and three-body problems, and reduces to a minimum the treatment of resonant processes, since these topics are dealt with in other articles in this Encyclopedia (Vol. XXXIX and XL). General information on neutron induced reactions is contained in references [d] and [7] and total cross sections are given by Adair and by Hughes and Harvey [3d]. The reactions observed are listed in Table 7. 

Differential cross sections for the scattering of neutrons by deuterons have been measured at several energies up to 14 MeV. Results are summarised by Allred et al. and by Massey [36] who compare them with those for the similar [pd) reaction and with theory. There is evidence for the deuteron disintegration E. [n,p) 2n. Total cross sections are also known as a function of energy. 

The He ( ) reaction has a cross section of about 5000 bams for thermal neutrons and the reaction cross section follows a 1/v-law up to neutron energies of about 3 MeV . The observed value agrees with that calculated from the cross section of the inverse reaction (Table 4). 

Excitation of bound states by radiative capture of slow neutrons. Radiative capture is mainly important for thermal neutrons for which there is the technical advantage that very large intensities are available from reactors. The energy spectmm of thermal capture radiations has been observed by Kinsey and his collaborators [37] for a large number of nuclei. In this work the neutron capturing sample was placed in a high flux near the core of a nuclear reactor, and collimated 

Burcham Nuclear Reactions, Levels, and Spectra of Light Nuclei. Sect. 40. 

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Table 7. Reactions of neutrons with light nuclei fA 3j.

Resonant reactions of neutrons with light nuclei. Most of the resonances observed in light nuclei are due to scattering, but when reactions are energetically possible, these may also show resonance. Some of the reactions studied are listed in Table 8 total cross sections have also been measured as a function of energy by the transmission method for the normal isotopic mixture of all the elements listed. 

Reactions of high energy neutrons with light nuclei. 

Reactions of high energy neutrons with light nuclei. In many experiments made with the or neutron source emphasis has been placed on... 

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. 

A few elements, among them fluorine and phosphoras, occur naturally with just one isotope, but most elements are isotopic mixtures. For example, element number 22 is titanium (Ti), a light and strong metal used in Jet engines and in artificial human Joints. There are five naturally occurring isotopes of Ti. Each one has 22 protons in its nuclei, but the number of neutrons varies from 24 to 28. In a chemical reaction, all isotopes of an element behave nearly identically. This means that the isotopic composition of an element remains essentially constant. The isotopic composition of Ti (number percentages) is... 

In the case of silicon fusion, which begins at around 2 billion K, the reactions proceed in a slightly different manner and we return to a fusion scheme similar to that of neon. At this temperature, silicon nuclei are gradually gnawed down by thermal photons which detach helium nuclei, protons and neutrons from them. These light nuclei combine with intact silicon to give nuclei in the region of iron. Schematically,... 

In conclusion thre first half-life measurements of light neutron rich nuclei using the MSU Reaction Product Mass Separator has resulted in the measurement of eight half-lives,two of which represent first time measurements and three of which are second measurements.The RPMS coupled with fast beam switching has proven to provide a very clean environment in which to study the decays of neutron rich nuclei. 

The debris from a type II supernova explosion includes ejecta rich in heavy elements which come from reaction of light elements with the mantle of the iron core and a residual star which is made of neutron-rich material (Fig. 30). A neutron star is believed to have a dense crust made of normal matter, but including many heavy elements. At deeper layers, more neutron-rich (and normally unstable) nuclei are expected and below that, layers containing free electrons and free neutrons. In the core itself, neutrons and possibly more... 

The n, 2n) reaction is expected to show an increased cross section for high neutron energies at the expense of the [n, n ) process. This is probably so for heavier elements, but the small number of nucleons involved in nuclei even as heavy as calcium makes it rather doubtful that thermal equilibrium is actually obtained for light nuclei, particularly with 14 MeV neutrons and particularly when it is known that direct interaction processes can contribute appreciably to the total cross section. 

In light nuclei it is often possible to follow the decay of compound nuclei in resonance reactions for the spacing of levels can be wide compared with the resolving power of existing apparatus. For heavy nuclei, discrete levels in the compound nucleus usually can be detected at the present time only by the interaction of slow neutrons with nuclei. This subject is discussed elsewhere in this series. We are concerned here with compound nuclei in which a great many levels are excited. The theoretical discussion therefore necessitates a. statistical approach. Thus if the probability of decay of the compound nucleus C in the direction (7.1) is represented hy F B,b), and if Fq is the sum of the probabilities of all possible reactions, then the cross section for the reaction aA->bB is ... 

As discussed previously, targets for use in heavy-ion irradiations are restricted to nuclides with Z < 98. While shallow transfer reactions with light heavy ions have been used with some success to produce new neutron-rich nuclei in the heavy actinides [477, 478], transfers of less than six protons have no application to the production of superheavy nuclei. Since we are interested primarily in neutron-rich exchanges, only projectiles much heavier than Ne should be considered. With... 

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