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High-energy processes |
Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context ofimpact mechanics it describes ejection of material from a target during impact by aprojectile. Inplanetary physics, spallation describesmeteoritic impacts on a planetary surface and the effects ofstellar winds andcosmic rays onplanetary atmospheres andsurfaces. In the context ofmining orgeology, spallation can refer to pieces of rock breaking off arock face due to the internal stresses in the rock; it commonly occurs onmine shaft walls. In the context of metal oxidation, spallation refers to the breaking off of the oxide layer from a metal. For example, the flaking off ofrust from iron. In the context ofanthropology, spallation is a process used to make stone tools such asarrowheads byknapping. Innuclear physics, spallation is the process in which a heavy nucleus emits numerousnucleons as a result of being hit by a high-energyparticle, thus greatly reducing itsatomic weight. Inindustrial processes andbioprocessing the loss of tubing material due to the repeated flexing of the tubing within aperistaltic pump is termed spallation.
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 internalcavitation due to stresses, which are generated by the interaction of stress waves, exceeding the localtensile 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 (HESH) charges.
Laser induced spallation is a recent experimental technique developed to understand theadhesion ofthin films withsubstrates. A high energy pulsedlaser (typicallyNd:YAG) is used to create acompressive stress pulse in thesubstrate 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 ofwave propagation in solids it is possible to extract the interface strength. The stress pulse created in this example is usually around 3 to 8nanoseconds in duration while its magnitude varies as a function oflaser 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 ashear stress using a pulse shaping prism and achieve shear spallation.
Nuclear spallation from the impact of cosmic rays occurs naturally inEarth's atmosphere and on the surfaces of bodies in space such asmeteorites and theMoon. Evidence of cosmic ray spallation is seen on outer surfaces of bodies and gives a means of measuring the length of time of exposure. The composition of cosmic rays themselves may also indicate that they have suffered spallation before reaching Earth, because the proportion of light elements such as lithium, boron, and beryllium 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.Cosmogenicisotopes ofaluminium,beryllium,chlorine,iodine andneon, formed by spallation of terrestrial elements under cosmic ray bombardment, have been detected on Earth.
Nuclear spallation is one of the processes by which aparticle accelerator may be used to produce a beam ofneutrons. A particle beam consisting of protons at around 1 GeV is shot into a target consisting ofmercury,tantalum,lead[1] or another heavy metal. The target nuclei are excited and upon deexcitation, 20 to 30 neutrons are expelled per nucleus. Although this is a far more expensive way of producing neutron beams than by achain reaction ofnuclear fission in anuclear reactor, it has the advantage that the beam can be pulsed with relative ease. Furthermore, the energetic cost of one spallation neutron is six times lower than that of a neutron gained via nuclear fission. In contrast to nuclear fission, the spallation neutrons cannot trigger further spallation or fission processes to produce further neutrons. Therefore, there is no chain reaction, which makes the process non-critical. Observations of cosmic ray spallation had already been made in the 1930s,[2] but the first observations from a particle accelerator occurred in 1947, and the term "spallation" was coined byNobelistGlenn T. Seaborg that same year.[3] Spallation is a proposed neutron source insubcritical nuclear reactors like the upcoming research reactorMYRRHA, which is planned to investigate the feasibility ofnuclear transmutation ofhigh level waste into less harmful substances. Besides having a neutron multiplication factorjust belowcriticality, subcritical reactors can also produce net usable energy as the average energy expenditure per neutron produced ranges around 30 MeV (1GeV beam producing a bit over 30 neutrons in the most productive targets) while fission produces on the order of 200 MeV per actinide atom that is split. Even at relatively lowenergy efficiency of the processes involved, net usable energy could be generated while being able to use actinides unsuitable for use in conventional reactors as "fuel".
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Generally the production of neutrons at a spallation source begins with a high-powered protonaccelerator. The accelerator may consist of a linac only (as in theEuropean Spallation Source) or a combination of linac and synchrotron (e.g.ISIS neutron source) or a cyclotron (e.g.SINQ (PSI)) . As an example, theISIS neutron source is based on some components of the formerNimrod synchrotron. Nimrod was uncompetitive forparticle physics so it was replaced with a new synchrotron, initially using the originalinjectors, 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 withdepleted uranium targets but although these produce the most intense neutron beams, they also have the shortest lives. Generally, therefore,tantalum ortungsten targets have been used. Spallation processes in the target produce the neutrons, initially atvery high energies—a good fraction of the proton energy. These neutrons are then slowed inmoderators filled withliquid hydrogen or liquidmethane 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.[4] This could be useful forneutron radiography, which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion, and study collective excitations of phonons more effectively thanX-rays.
