Nuclear reactions spallation

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

In addition to all these fusion and neutron capture processes, there is a further type of nuclear reaction, called spallation. Rather than fusing together, nuclei are smashed up or chipped to produce smaller species. This process is thought to be the origin of most of the lithium, beryllium and boron in the Universe. 

Energetic particles react with solid matter in a variety of ways. Low-energy particles in the solar wind ( 1 KeV/nucleon) are implanted into solids to depths of 50 nm. Energetic heavy particles penetrate more deeply and disrupt the crystal lattice, leaving behind tracks that can be imaged by or chemically etched and observed in an optical microscope. Particles with energies of several MeV or more may induce a nuclear reaction. The two main modes of production of cosmogenic nuclides are spallation reactions and neutron capture. 

There are three principal types of nuclear reactions due to the interactions of terrestrial materials with cosmic rays (i) by high-energy spallation of nucleons (E > 40MeV), principally by neutrons, (ii) by thermal neutron capture, and (iii) muon-induced nuclear disintegrations. Muon reactions become important only at depths below sea level. The estimation of the production ratio is difficult because of lack of knowledge of the probabilities of formation of nuclides in the different reactions. 

Cosmic rays The 6Li/7Li ratio near 0.6 observed in the cosmic rays is much larger than in the solar system because in cosmic rays both isotopes are spallation fragments of carbon and oxygen created by nuclear reactions between the cosmic rays and the interstellar atoms with which they collide. (See Nucleosynthesis origin, Cosmic rays above, and also see 7Li, below). Thus the Li isotope anomaly in the cosmic rays is understood in terms of the nuclear physics that alters the cosmic-ray composition from what it was originally when the cosmic rays began their high-speed journey. The 6Li/ 7Li ratio observed in the cosmic rays does not, therefore, represent the isotopic composition of Li in any bulk sample of stellar or planetary matter. 

Marti (1967) introduced the Kr/Kr method for determining CRE ages. The Al/ Ne method sketched above rests only on analyses of meteorites in contrast, the Kr/Kr method also demands knowledge of the relative cross-sections for certain nuclear reactions. To see how the method works, we write Equation (1) for Kr, a stable isotope produced mainly by spallation, and Equation (2) for Kr, which has a half-life of 0.229 Myr, and then divide one by the other, obtaining... 

The spallation reaction is another example of special nuclear reactions, by which many nuclides relatively small mass number (about 10 to 20, the smaller the number, the higher the yield) in comparison with the target nuclide is produced simultaneously. The example is ... 

Accelerators provide a variety of nuclear reactions for production of neutrons. Cockcroft-Walton accelerators can generate 14.8 MeV neutrons by accelerating deuterons ( H) onto a tritium target to produce 10 -10 neutrons s Cyclotrons and linear accelerators can produce high-energy neutrons with a broad spectrum of energies in spallation reactions that result from the bombardment of heavy elements by charged particles. 

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