Spallation

In general, spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic weight. In the context of impact physics it describes ejection or vaporization of material from a target during impact by a projectile. In planetary physics, spallation describes meteoritic impacts on a planetary surface and the effects of a stellar wind on a planetary atmosphere. In the context of mining or geology, spallation can refer to pieces of rock breaking off a rock face due to the internal stresses in the rock; it commonly occurs on mine shaft walls. In the context of anthropology, spallation is a process used to make stone tools such as arrowheads by knapping.

Nuclear spallation

 * See also Cosmic ray spallation

Nuclear spallation occurs naturally in earth's atmosphere owing to the impacts of cosmic rays, and also on the surfaces of bodies in space such as meteorites 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 neutrons. 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. The concept of nuclear spallation was first coined by Nobelist Glenn T. Seaborg in his doctoral thesis on the inelastic scattering of neutrons in 1937.

Laser 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 Nd:YAG) is used to create a compressive stress pulse in the substrate wherein it propagates and reflects of as a tensile wave at the free boundary. This tensile pulse spalls/peels the thin film while propagating towards the substrate. Using theory of wave propagation in solids it is possible to extract the interface strength. The stress pulse created in this fashion is usually around 3-8 nanoseconds in duration while its magnitude varies as a function of laser fluence. Due to the non-contact application of load, this technique is very well suited to spall ultra-thin films (1 micrometre in thickness or less). It is also possible to mode convert a longitudinal stress wave into a shear stress using a pulse shaping prism and achieve shear spallation.

Production of neutrons at a spallation neutron source
Generally the production of neutrons at a spallation source begins with a high powered accelerator. This is more often than not a synchrotron. As an example, the ISIS neutron source is based on some components of the former Nimrod synchrotron. Nimrod was uncompetitive for high energy physics so it was replaced with a new synchrotron, initially using the original injectors, but which produces a highly intense pulsed beam of protons. Whereas Nimrod would produce around 2ųA at 7GeV, ISIS produces 200 ųA at 800 MeV. This is pulsed at the rate of 50 Hz, and this intense beam of protons is focused onto a target. Experiments have been done with depleted uranium targets but although these produce the most intense neutron beams, they also have the shortest lives. Generally, therefore, tantalum targets have been used. Spallation processes in the target produce the neutrons, initially at very high energies - a good fraction of the proton energy. These neutrons are then slowed in moderators filled with liquid hydrogen or liquid methane to the energies that are needed for the scattering instruments. Whilst protons can be focused since they have charge, chargeless neutrons cannot be, so in this arrangement the instruments are arranged around the moderators.

Inertial fusion energy has the potential to produce orders of magnitude more neutrons than spallation. Neutrons are capable of locating hydrogen atoms in structures, resolving atomic thermal motion and studying collective excitations of photons more effectively than X-rays.