High 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. 

The high depth resolution, nondestructive nature of thermal neutrons, and availability of deuterium substituted materials has brought about a proliferation in the use of neutron reflectivity in material, polymer, and biological sciences. In response to this high demand, reflectivity equipment is now available at all major neutron facilities throughout the country, be they reactor or spallation sources. 

The discovery of cosmic ray produced 81Kr in meteorites [l]1 introduced a new method of high sensitivity measurements of 81Kr concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive 81Kr atoms (Ty2 = 2.13 x 105y, [2] and of stable spallation Kr atoms by a... 

Well, not quite everything. The hght elements lithium, beryllium, and boron are formed mostly by the break-up of heavier nuclei when hit by cosmic rays and other high-energy particles in interstellar space. This process, which whittles the nuclei down to hghter elements, is called spallation. Nucleosynthesis in stars produces very httle of these three elements. 

The Mn- Cr system can be studied by TIMS, ICPMS, and SIMS techniques. For TIMS and ICPMS work, bulk samples or mineral separates are dissolved and the solutions are passed through ion-exchange columns to produce clean solutions of manganese and chromium. For minerals with high Mn/Cr ratios SIMS can obtain isotopic data while retaining the petrographic context of the measurements. The chromium isotopic compositions may have to be corrected for small additions of chromium from spallation reactions induced by cosmic rays. This is particularly important in iron-rich meteorites. 

Beryllium-10 P-decays to 10B with a half-life of 1.5 Myr. Beryllium and boron (along with lithium) are several orders of magnitude less abundant than the other light elements in the solar system because, except for 7Li, they are not produced in stars. They are produced when high-energy cosmic rays, mostly protons, fragment atomic nuclei into small pieces in a process called spallation. Beryllium-10 is constantly being produced at low levels by spallation in the solar system, and its abundance in bulk meteorites is used to estimate the amount of time that they were exposed to cosmic rays as small bodies (their cosmic-ray... 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an... 

Noble gas isotopes are also produced through irradiation by cosmic rays. These rays are mostly high-energy protons that produce a cascade of secondary particles when they bombard other target nuclei, in a process called spallation. Neon produced by spallation reactions has similar abundances of all three isotopes (Fig. 10.8). Cosmic-ray irradiation occurs on the surfaces of airless bodies like the Moon and asteroids, as well as on small chunks of rock orbiting in space. Using these isotopes, it is possible to calculate cosmic-ray exposure ages, as described in Chapter 9. 

Generator Preparation. Strontium-82 is obtained from Los Alamos National Laboratory (LASL). The isotope is produced by a high energy spallation reaction on molybdenum and the purified mixture contains other strontium isotopes, notably Sr-83/Rb-83 and Sr-85. 

The high energy primary cosmic rays produce many secondary neutrons and protons which in turn are responsible for most of the spallation reactions resulting in radionuclide production in the atmosphere. The formation of these radionuclides occurs at reaction thresholds of 10-40 M.e.v. Because of local ionization losses the secondary protons lose their... 

If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

---