Spallation

Last updated December 30, 2023

Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection 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 stellar winds and cosmic rays on planetary atmospheres and surfaces. 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 numerous nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic weight. In industrial processes and bioprocessing the loss of tubing material due to the repeated flexing of the tubing within a peristaltic pump is termed 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 (HESH) 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 (typically Nd:YAG) 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 example is usually around 3 to 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.

Nuclear spallation

Nuclear spallation from the impact of cosmic rays occurs naturally in Earth's atmosphere and on the surfaces of bodies in space such as meteorites and the Moon. Evidence of cosmic ray spallation (also known as "spoliation") 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. 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 particle beam consisting of protons at around 1 GeV is shot into a target consisting of mercury, 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 a chain reaction of nuclear fission in a nuclear 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 by Nobelist Glenn T. Seaborg that same year.^[3] Spallation is a proposed neutron source in subcritical nuclear reactors like the upcoming research reactor MYRRHA, which is planned to investigate the feasibility of nuclear transmutation of high level waste into less harmful substances. Besides having a neutron multiplication factor just below criticality, 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 low energy 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".

Production of neutrons at a spallation neutron source

Generally the production of neutrons at a spallation source begins with a high-powered proton accelerator. The accelerator may consist of a linac only (as in the European Spallation Source) or a combination of linac and synchrotron (e.g. ISIS neutron source) or a cyclotron (e.g. SINQ (PSI)) . 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 or tungsten 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.^[4] This could be useful for neutron radiography, which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion, and study collective excitations of photons more effectively than X-rays.

Related Research Articles

The neutron is a subatomic particle, symbol n or n⁰
, which has a neutral charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one dalton, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.

Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.

A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power.

Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino. Free neutrons have a mean lifetime of 887 seconds.

In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.

In nuclear physics, an energy amplifier is a novel type of nuclear power reactor, a subcritical reactor, in which an energetic particle beam is used to stimulate a reaction, which in turn releases enough energy to power the particle accelerator and leave an energy profit for power generation. The concept has more recently been referred to as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.

The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River Aare, and covers an area over 35 hectares in size. Like ETH Zurich and EPFL, PSI belongs to the Swiss Federal Institutes of Technology Domain of the Swiss Confederation. The PSI employs around 2,100 people. It conducts basic and applied research in the fields of matter and materials, human health, and energy and the environment. About 37% of PSI's research activities focus on material sciences, 24% on life sciences, 19% on general energy, 11% on nuclear energy and safety, and 9% on particle physics.

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion–fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.

The Spallation Neutron Source (SNS) is an accelerator-based neutron source facility in the U.S. that provides the most intense pulsed neutron beams in the world for scientific research and industrial development. Each year, this facility hosts hundreds of researchers from universities, national laboratories, and industry, who conduct basic and applied research and technology development using neutrons. SNS is part of Oak Ridge National Laboratory, which is managed by UT-Battelle for the United States Department of Energy (DOE). SNS is a DOE Office of Science user facility, and it is open to scientists and researchers from all over the world.

Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from beyond Earth, ranging from protons, alpha particles, and nuclei of many heavier elements. About 1% of cosmic rays also consist of free electrons.

The ISIS Neutron and Muon Source is a pulsed neutron and muon source, established 1984 at the Rutherford Appleton Laboratory of the Science and Technology Facilities Council, on the Harwell Science and Innovation Campus in Oxfordshire, United Kingdom. It uses the techniques of muon spectroscopy and neutron scattering to probe the structure and dynamics of condensed matter on a microscopic scale ranging from the subatomic to the macromolecular.

The European Spallation Source ERIC (ESS) is a multi-disciplinary research facility currently under construction. The ESS is currently under construction in Lund, Sweden, while its Data Management and Software Centre (DMSC) is situated in Copenhagen, Denmark. The 13 European member countries are partners in the construction and operation of ESS. ESS is scheduled to begin its scientific user program in 2023, with the construction phase set to be completed by 2025. ESS will enable scientists to observe and understand basic atomic structures and forces, which is not achievable with other neutron sources in terms of lengths and time scales. The research facility is located close to the Max IV Laboratory, which conducts synchotron radiation research. The construction of the facility began in the summer of 2014 and the first science results are planned for 2023.

Hybrid nuclear fusion–fission is a proposed means of generating power by use of a combination of nuclear fusion and fission processes.

A neutron research facility is most commonly a big laboratory operating a large-scale neutron source that provides thermal neutrons to a suite of research instruments. The neutron source usually is a research reactor or a spallation source. In some cases, a smaller facility will provide high energy neutrons using existing neutron generator technologies.

The Los Alamos Neutron Science Center (LANSCE), formerly known as the Los Alamos Meson Physics Facility (LAMPF), is one of the world's most powerful linear accelerators. It is located in Los Alamos National Laboratory in New Mexico in Technical Area 53. It was the most powerful linear accelerator in the world when it was opened in June 1972. The technology used in the accelerator was developed under the direction of nuclear physicist Louis Rosen. The facility is capable of accelerating protons up to 800 MeV. Multiple beamlines allow for a variety of experiments to be run at once, and the facility is used for many types of research in materials testing and neutron science. It is also used for medical radioisotope production.

A Fixed-Field alternating gradient Accelerator is a circular particle accelerator concept that can be characterized by its time-independent magnetic fields and the use of alternating gradient strong focusing.

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

The Neutron Time Of Flight (n_TOF) facility is a neutron spectrometer at CERN, with the aim of studying neutron-nucleus interactions over a range of kinetic energies, using the time of flight method. The research conducted at the facility has applications in nuclear technology and nuclear astrophysics. The facility has been in operation at CERN since 2001, following a proposal from the former Director General, Carlo Rubbia, for a high-intensity neutron source.

An accelerator-driven subcritical reactor (ADSR) is a nuclear reactor design formed by coupling a substantially subcritical nuclear reactor core with a high-energy proton or electron accelerator. It could use thorium as a fuel, which is more abundant than uranium.

The MYRRHA is a design project of a nuclear reactor coupled to a proton accelerator. MYRRHA will be a lead-bismuth cooled fast reactor with two possible configurations: sub-critical or critical.

References

↑ "Spallation Target | Paul Scherrer Institut (PSI)". Psi.ch. Retrieved 2015-12-12.
↑ Rossi, Bruno (1933). "Über die Eigenschaften der durchdringenden Korpuskularstrahlung im Meeresniveau" [About properties of penetrating, corpuscular radiation at sea level]. Zeitschrift für Physik. 82 (3–4): 151–178. Bibcode:1933ZPhy...82..151R. doi:10.1007/BF01341486. S2CID 121427439.
↑ Krása, Antonín (May 2010). "Neutron Sources for ADS" (PDF). Faculty of Nuclear Sciences and Physical Engineering. Czech Technical University in Prague. S2CID 28796927. Archived from the original (PDF) on 2019-03-03. Retrieved October 20, 2019.
↑ Taylor, Andrew; Dunne, M; Bennington, S; Ansell, S; Gardner, I; Norreys, P; Broome, T; Findlay, D; Nelmes, R (February 2007). "A Route to the Brightest Possible Neutron Source?". Science . 315 (5815): 1092–1095. Bibcode:2007Sci...315.1092T. doi:10.1126/science.1127185. PMID 17322053. S2CID 42506679.

External links

IAEA database of spallation neutron sources in the Accelerator Knowledge Portal
Description of ISIS accelerator etc.
Spallation Neutron Source technical background.
How spallation works at the ISIS neutron and muon source

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[1] "Spallation Target | Paul Scherrer Institut (PSI)". Psi.ch. Retrieved 2015-12-12.

[2] Rossi, Bruno (1933). "Über die Eigenschaften der durchdringenden Korpuskularstrahlung im Meeresniveau" [About properties of penetrating, corpuscular radiation at sea level]. Zeitschrift für Physik. 82 (3–4): 151–178. Bibcode:1933ZPhy...82..151R. doi:10.1007/BF01341486. S2CID 121427439.

[3] Krása, Antonín (May 2010). "Neutron Sources for ADS" (PDF). Faculty of Nuclear Sciences and Physical Engineering. Czech Technical University in Prague. S2CID 28796927. Archived from the original (PDF) on 2019-03-03. Retrieved October 20, 2019.

[taylor2007-4] Taylor, Andrew; Dunne, M; Bennington, S; Ansell, S; Gardner, I; Norreys, P; Broome, T; Findlay, D; Nelmes, R (February 2007). "A Route to the Brightest Possible Neutron Source?". Science . 315 (5815): 1092–1095. Bibcode:2007Sci...315.1092T. doi:10.1126/science.1127185. PMID 17322053. S2CID 42506679.

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