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NUCLEAR SPALLATION Nuclear spallation occurs naturally in Earth's Atmosphere owing to the impacts of Cosmic Ray s, and also on the surfaces of bodies in space such as Meteorite s and the Moon . Evidence of cosmic ray spallation is evidence that the material in question has been exposed on the surface of the body of which it is part, and gives a means of measuring the length of time of exposure. The composition of the cosmic rays themselves also indicates that they have suffered spallation before reaching Earth, because the proportion of light elements such as Li, B,and Be in them exceeds average cosmic abundances; these elements in the cosmic rays were evidently formed from spallation of Oxygen, Nitrogen, Carbon and perhaps Silicon in the cosmic ray sources or during their lengthy travel here. ''Cosmogenic'' Isotopes of Aluminium , Beryllium , Chlorine , Iodine and Neon , formed by spallation of terrestrial elements under cosmic ray bombardment, have been detected on earth. Nuclear spallation is one of the processes by which a Particle Accelerator may be used to produce a beam of Neutron s. A Mercury , Tantalum or other heavy metal target is used, and 20 to 30 neutrons are expelled after each impact. Although this is a far more expensive way of producing neutron beams than by a Chain Reaction of Nuclear Fission in a Nuclear Reactor , it has the advantage that the beam can be pulsed with relative ease. LASER INDUCED SPALLATION Laser induced spallation is a recent experimental technique developed to understand the adhesion of thin films with substrates. A high energy pulsed laser (typically ) is used to create a compressive pulse in the substrate where in it propagates and reflects of as a tensile wave at the free boundary. This tensile pulse spalls/peels the thin film on the substrate. Using theory of wave propagation in solids it is possible to extract the interface strength. SEE ALSO EXTERNAL LINKS
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