Spallation
1220790
220995741
2008-06-22T15:57:03Z
DOI bot
6652755
Citation maintenance. Initiated by [[User:Fconaway|Fconaway]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]].
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 [[nucleon]]s as a result of being hit by a high-energy [[Elementary_particle|particle]], thus greatly reducing its [[atomic weight]]. In the context of [[Impact force|impact physics]] it describes ejection or vaporization of material from a target during impact by a [[projectile]]. In [[Planetary science|planetary physics]], spallation describes [[meteor|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 [[arrowhead]]s by [[flintknapping|knapping]].
==Nuclear spallation==
:''See also [[Cosmic ray spallation]]''
Nuclear spallation occurs naturally in [[earth's atmosphere]] owing to the impacts of [[cosmic ray]]s, and also on the surfaces of bodies in space such as [[meteorite]]s 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 [[neutron]]s. A [[Mercury (element)|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.<ref>http://www.khwarzimic.org/takveen/seaborg.pdf , "A Man Beyond Elements: Glenn T. Seaborg," website, accessed July 30, 2006</ref>
==Laser spallation==
Laser induced spallation is a recent experimental technique developed to understand the [[adhesion]] of [[thin films]] with [[Wafer (electronics)|substrate]]s. A high energy pulsed [[laser]] (typically [[Nd:YAG_laser|Nd:YAG]]) is used to create a compressive [[Compressive stress|stress]] pulse in the [[Wafer (electronics)|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 [[Particle accelerator|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]] synchrotron. Nimrod was uncompetitive for [[high energy physics]] so it was replaced with a new synchrotron, initially using the original [[injector]]s, 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 [[fast neutron|very high energies]] - a good fraction of the proton energy. These neutrons are then [[neutron moderation|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.
{{seealso|Spallation Neutron Source}}
[[Inertial confinement fusion|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]].<ref>{{cite journal | author=Taylor, Andrew | title=A Route to the Brightest Possible Neutron Source? | journal=[[Science (journal)|Science]] | volume=315 | month=February | year=2007 | pages=1092–1095 | pmid=17322053 | doi=10.1126/science.1127185 <!--Retrieved from Yahoo! by DOI bot-->}}</ref>
== References ==
{{reflist}}
*[http://www.isis.rl.ac.uk/accelerator/index.htm Description of ISIS accelerator etc.]
==See also==
*[[Spallation Neutron Source]]
*[[ISIS neutron source]]
==External links==
*[http://www.sns.gov/aboutsns/what-why.htm Spallation Neutron Source] technical background.
*[http://www.isis.rl.ac.uk/aboutIsis/index.htm How spallation works] at the ISIS neutron and muon source
[[Category:nuclear technology]]
[[Category:Particle physics|Spallation]]
[[de:Spallation]]
[[es:Espalación]]
[[fr:Spallation]]
[[it:Spallazione nucleare]]
[[pl:Spalacja]]
[[sr:Спалација]]
[[ur:تَشظیہ]]