Spallation is a process in which fragments of material are ejected from a
body due to impact or stress. In the context of impact mechanics 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. In
nuclear physics, spallation 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. Spallation in solid mechanics
Spallation can occur when a tensile stress wave propagates through a
material and can be observed in flat plate impact tests. It is caused by an
internal cavitation due to stresses, which are generated by the interaction
of stress waves, exceeding the local tensile strength of materials. A
fragment or multiple fragments will be created on the free end of the plate.
This fragment known as "spall" acts as a secondary projectile with velocities
that can be as high as one third of the stress wave speed on the material. This
type of failure is typically an effect of high explosive squash head charges.
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 is used to create a compressive
stress pulse in the substrate wherein it propagates and reflects 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. It is also possible to mode convert a longitudinal stress wave into
a shear stress using a pulse shaping prism and achieve shear spallation.
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,
lead 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.
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 particle 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 2 µA at 7 GeV, ISIS produces 200 µA at 0.8 GeV. 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 confinement fusion has the potential to produce orders of magnitude
more neutrons than spallation. Neutron radiography can be used to locate
hydrogen atoms in structures, resolve atomic thermal motion and study
collective excitations of photons more effectively than X-rays.
See also Spall, or spalling
Cosmic ray spallation Energy amplifier
Subcritical reactor = Spallation facilities =
European Spallation Source ISIS neutron source
J-PARC LANSCE Los Alamos
PSI Spallation Neutron Source Spallation Neutron Source Oak Ridge
References Description of ISIS accelerator etc.
External links Spallation Neutron Source technical
background. How spallation works at the ISIS neutron
and muon source