Spallation

Spallation (from English to spall, " chipping ") is a non-elastic interaction of a projectile with its target object.

Spallation in solids

In fracture mechanics, spallation may occur when a pressure pulse traveling through a material and reflected on the free surface as a pulse of tension. The ensuing voltage can exceed the maximum tensile strength of the material and thereby create voids which can lead to one or more or even Zugbrüchen spalling of material.

This type of spallation occurs, for example at the edge of impact craters by the at the free surface (eg planet's surface ) incident shock waves.

Nuclear spallation

In the nuclear spallation of the atomic nucleus with a projectile ( neutron, proton, another nucleus or elementary particle ) kinetic energy is bombarded (> 100 MeV). The nucleus is thereby shattered several neutrons into smaller fragments and generally.

The spallation reaction proceeds in two stages. The projectile interacts initially with the individual nucleons of the target nucleus, which in turn interact with other nucleons, so that a Teilchenkaskade within the target nucleus is formed. Some of these protons and neutrons leave it the target nucleus with energies up to the initial energy of the projectile. The main direction of the emitted particles coincides with the direction of the projectile. Projectile with energies greater than 400 MeV pions can be produced.

What remains is a highly excited residual nucleus in which the kinetic energy is statistically distributed evenly over all nucleons. The second stage of the spallation reaction then consists of the " evaporation " of neutrons and protons - less frequently also of α - particles and other nuclear fragments - from the target nucleus. These nucleons have energies up to 10 MeV and an isotropic angular distribution. The remaining core is usually still unstable and therefore radioactivity.

The first reaction stage lasts approximately 10-22 seconds. This is the time it takes for the projectile to pass through the core. The second stage takes about 10-16 seconds.

In nature spallation reactions occur in the interaction of cosmic rays with interstellar or interplanetary matter, for example. Thus the irradiation age may be about by measuring the frequency of caused by spallation reaction products, such as certain noble gas isotopes are determined in meteorites. Technically, the spallation is used in spallation neutron sources for the production of neutrons, which can be an alternative to their usual production in research reactors. Also for the disposal of nuclear waste by transmutation spallation neutron sources are considered. In Europe, there are the short-pulse source ISIS at Oxford, which works with neutron pulse lengths in the microsecond range, as well as continuous source SINQ in Switzerland. The European Spallation Source ESS major to go into operation in 2019.