Alpha particle

Alpha particles, also called alpha ray or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+
or 4
2
He2+
indicating a helium ion with a +2 charge (missing its two electrons). If the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 4
2
He
.

Alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV, and a velocity in the vicinity of 5% the speed of light. (See discussion below for the limits of these figures in alpha decay.) They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) have low penetration depth. They can be stopped by a few centimeters of air, or by the skin.

However, so-called long range alpha particles from ternary fission are three times as energetic, and penetrate three times as far. As noted, the helium nuclei that form 10–12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and are thus capable of being highly penetrating and able to traverse the human body and also many meters of dense solid shielding, depending on their energy. To a lesser extent, this is also true of very high-energy helium nuclei produced by particle accelerators.

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage. Alpha radiation is an average of about 20 times more dangerous, and in experiments with inhaled alpha emitters, up to 1000 times more dangerous[3] than an equivalent activity of beta emitting or gamma emitting radioisotopes.

Alpha particle
Alpha Decay
Composition2 protons, 2 neutrons
StatisticsBosonic
Symbolα, α2+, He2+
Mass6.644657230(82)×10−27 kg[1]

4.001506179127(63) u

3.727379378(23) GeV/c2
Electric charge+2 e
Spin0[2]

Name

Some science authors use doubly ionized helium nuclei (He2+
) and alpha particles as interchangeable terms. The nomenclature is not well defined, and thus not all high-velocity helium nuclei are considered by all authors to be alpha particles. As with beta and gamma particles/rays, the name used for the particle carries some mild connotations about its production process and energy, but these are not rigorously applied.[4] Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions (for example the alpha processes), and even when they occur as components of cosmic rays. A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission. However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles".

Sources of alpha particles

Alpha decay

Physicist Studying Alpha Rays GPN-2000-000381
A physicist observes alpha particles from the decay of a polonium source in a cloud chamber
Alpha radiation in a cloud chamber
Alpha radiation detected in an isopropanol cloud chamber (after injection of an artificial source radon-220).

The best-known source of alpha particles is alpha decay of heavier (> 106 u atomic weight) atoms. When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due to alpha decay.

Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements. Unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it. The smallest nuclei that have to date been found to be capable of alpha emission are beryllium-8 and the lightest nuclides of tellurium (element 52), with mass numbers between 104 and 109. The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray then removes the excess energy.

Mechanism of production in alpha decay

In contrast to beta decay, the fundamental interactions responsible for alpha decay are a balance between the electromagnetic force and nuclear force. Alpha decay results from the Coulomb repulsion[2] between the alpha particle and the rest of the nucleus, which both have a positive electric charge, but which is kept in check by the nuclear force. In classical physics, alpha particles do not have enough energy to escape the potential well from the strong force inside the nucleus (this well involves escaping the strong force to go up one side of the well, which is followed by the electromagnetic force causing a repulsive push-off down the other side).

However, the quantum tunnelling effect allows alphas to escape even though they do not have enough energy to overcome the nuclear force. This is allowed by the wave nature of matter, which allows the alpha particle to spend some of its time in a region so far from the nucleus that the potential from the repulsive electromagnetic force has fully compensated for the attraction of the nuclear force. From this point, alpha particles can escape, and in quantum mechanics, after a certain time, they do so.

Ternary fission

Especially energetic alpha particles deriving from a nuclear process are produced in the relatively rare (one in a few hundred) nuclear fission process of ternary fission. In this process, three charged particles are produced from the event instead of the normal two, with the smallest of the charged particles most probably (90% probability) being an alpha particle. Such alpha particles are termed "long range alphas" since at their typical energy of 16 MeV, they are at far higher energy than is ever produced by alpha decay. Ternary fission happens in both neutron-induced fission (the nuclear reaction that happens in a nuclear reactor), and also when fissionable and fissile actinides nuclides (i.e., heavy atoms capable of fission) undergo spontaneous fission as a form of radioactive decay. In both induced and spontaneous fission, the higher energies available in heavy nuclei result in long range alphas of higher energy than those from alpha decay.

Accelerators

Energetic helium nuclei may be produced by cyclotrons, synchrotrons, and other particle accelerators, but they are not normally referred to as "alpha particles."

Solar core reactions

As noted, helium nuclei may participate in nuclear reactions in stars, and occasionally and historically these have been referred to as alpha reactions (see for example triple alpha process).

Cosmic rays

In addition, extremely high energy helium nuclei sometimes referred to as alpha particles make up about 10 to 12% of cosmic rays. The mechanisms of cosmic ray production continue to be debated.

Energy and absorption

The energy of the alpha emitted in alpha decay is mildly dependent on the half-life for the emission process, with many orders of magnitude differences in half-life being associated with energy changes of less than 50% (see alpha decay).

The energy of alpha particles emitted varies, with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV (mega-electron-volts), corresponding to extremely long and extremely short half-lives of alpha-emitting nuclides, respectively.

This energy is a substantial amount of energy for a single particle, but their high mass means alpha particles have a lower speed (with a typical kinetic energy of 5 MeV; the speed is 15,000 km/s, which is 5% of the speed of light) than any other common type of radiation (β particles, neutrons, etc.)[5] Because of their charge and large mass, alpha particles are easily absorbed by materials, and they can travel only a few centimetres in air. They can be absorbed by tissue paper or the outer layers of human skin (about 40 micrometres, equivalent to a few cells deep).

Biological effects

Due to the short range of absorption and inability to penetrate the outer layers of skin, alpha particles are not, in general, dangerous to life unless the source is ingested or inhaled.[6] Because of this high mass and strong absorption, if alpha-emitting radionuclides do enter the body (upon being inhaled, ingested, or injected, as with the use of Thorotrast for high-quality X-ray images prior to the 1950s), alpha radiation is the most destructive form of ionizing radiation. It is the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning. It is estimated that chromosome damage from alpha particles is anywhere from 10 to 1000 times greater than that caused by an equivalent amount of gamma or beta radiation, with the average being set at 20 times. A study of European nuclear workers exposed internally to alpha radiation from plutonium and uranium found that when relative biological effectiveness is considered to be 20, the carcinogenic potential (in terms of lung cancer) of alpha radiation appears to be consistent with that reported for doses of external gamma radiation i.e. a given dose of alpha-particles inhaled presents the same risk as a 20-times higher dose of gamma radiation.[7] The powerful alpha emitter polonium-210 (a milligram of 210Po emits as many alpha particles per second as 4.215 grams of 226Ra) is suspected of playing a role in lung cancer and bladder cancer related to tobacco smoking.[8] 210Po was used to kill Russian dissident and ex-FSB officer Alexander V. Litvinenko in 2006.[9]

History of discovery and use

Alfa beta gamma radiation
Alpha radiation consists of helium-4 nucleus and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminium plate. Gamma radiation is eventually absorbed as it penetrates a dense material. Lead is good at absorbing gamma radiation, due to its density.
Alphaparticlemagnetic
An alpha particle is deflected by a magnetic field
Растурање на алфа-честички на тенок метален лист
Dispersing of alpha particles on a thin metal sheet

In the years 1899 and 1900, physicists Ernest Rutherford (working in McGill University in Montreal, Canada) and Paul Villard (working in Paris) separated radiation into three types: eventually named alpha, beta, and gamma by Rutherford, based on penetration of objects and deflection by a magnetic field.[10] Alpha rays were defined by Rutherford as those having the lowest penetration of ordinary objects.

Rutherford's work also included measurements of the ratio of an alpha particle's mass to its charge, which led him to the hypothesis that alpha particles were doubly charged helium ions (later shown to be bare helium nuclei).[11] In 1907, Ernest Rutherford and Thomas Royds finally proved that alpha particles were indeed helium ions.[12] To do this they allowed alpha particles to penetrate a very thin glass wall of an evacuated tube, thus capturing a large number of the hypothesized helium ions inside the tube. They then caused an electric spark inside the tube, which provided a shower of electrons that were taken up by the ions to form neutral atoms of a gas. Subsequent study of the spectra of the resulting gas showed that it was helium and that the alpha particles were indeed the hypothesized helium ions.

Because alpha particles occur naturally, but can have energy high enough to participate in a nuclear reaction, study of them led to much early knowledge of nuclear physics. Rutherford used alpha particles emitted by radium bromide to infer that J. J. Thomson's Plum pudding model of the atom was fundamentally flawed. In Rutherford's gold foil experiment conducted by his students Hans Geiger and Ernest Marsden, a narrow beam of alpha particles was established, passing through very thin (a few hundred atoms thick) gold foil. The alpha particles were detected by a zinc sulfide screen, which emits a flash of light upon an alpha particle collision. Rutherford hypothesized that, assuming the "plum pudding" model of the atom was correct, the positively charged alpha particles would be only slightly deflected, if at all, by the dispersed positive charge predicted.

It was found that some of the alpha particles were deflected at much larger angles than expected (at a suggestion by Rutherford to check it) and some even bounced almost directly back. Although most of the alpha particles went straight through as expected, Rutherford commented that the few particles that were deflected was akin to shooting a fifteen-inch shell at tissue paper only to have it bounce off, again assuming the "plum pudding" theory was correct. It was determined that the atom's positive charge was concentrated in a small area in its center, making the positive charge dense enough to deflect any positively charged alpha particles that came close to what was later termed the nucleus.

Prior to this discovery, it was not known that alpha particles were themselves atomic nuclei, nor was the existence of protons or neutrons known. After this discovery, J.J. Thomson's "plum pudding" model was abandoned, and Rutherford's experiment led to the Bohr model (named for Niels Bohr) and later the modern wave-mechanical model of the atom.

Bragg Curve for Alphas in Air
Energy-loss (Bragg curve) in air for typical alpha particle emitted through radioactive decay.
WolfhartFig1a
The trace of a single alpha particle obtained by nuclear physicist Wolfhart Willimczik with his spark chamber specially made for alpha particles.

Rutherford went on to use alpha particles to accidentally produce what he later understood as a directed nuclear transmutation of one element to another, in 1917. Transmutation of elements from one to another had been understood since 1901 as a result of natural radioactive decay, but when Rutherford projected alpha particles from alpha decay into air, he discovered this produced a new type of radiation which proved to be hydrogen nuclei (Rutherford named these protons). Further experimentation showed the protons to be coming from the nitrogen component of air, and the reaction was deduced to be a transmutation of nitrogen into oxygen in the reaction

14N + α → 17O + p 

This was the first-discovered nuclear reaction.

To the adjacent pictures: According to the energy-loss curve by Bragg it is recognizable that the alpha particle indeed loses more energy on the end of the trace.[13]

Anti-alpha particle

In 2011, members of the international STAR collaboration using the Relativistic Heavy Ion Collider at the U.S. Department of Energy's Brookhaven National Laboratory detected the antimatter partner of the helium nucleus, also known as the anti-alpha.[14] The experiment used gold ions moving at nearly the speed of light and colliding head on to produce the antiparticle.[15]

Applications

  • Some smoke detectors contain a small amount of the alpha emitter americium-241. The alpha particles ionize air within a small gap. A small current is passed through that ionized air. Smoke particles from fire that enter the air gap reduce the current flow, sounding the alarm. The isotope is extremely dangerous if inhaled or ingested, but the danger is minimal if the source is kept sealed. Many municipalities have established programs to collect and dispose of old smoke detectors, to keep them out of the general waste stream.
  • Alpha decay can provide a safe power source for radioisotope thermoelectric generators used for space probes and artificial heart pacemakers. Alpha decay is much more easily shielded against than other forms of radioactive decay. Plutonium-238, a source of alpha particles, requires only 2.5 mm of lead shielding to protect against unwanted radiation.
  • Static eliminators typically use polonium-210, an alpha emitter, to ionize air, allowing the "static cling" to more rapidly dissipate.
  • Researchers are currently trying to use the damaging nature of alpha emitting radionuclides inside the body by directing small amounts towards a tumor. The alphas damage the tumor and stop its growth, while their small penetration depth prevents radiation damage of the surrounding healthy tissue. This type of cancer therapy is called unsealed source radiotherapy.

Alpha radiation and DRAM errors

In computer technology, dynamic random access memory (DRAM) "soft errors" were linked to alpha particles in 1978 in Intel's DRAM chips. The discovery led to strict control of radioactive elements in the packaging of semiconductor materials, and the problem is largely considered to be solved.[16]

See also

References

  1. ^ "CODATA Value: Alpha particle mass". NIST. Retrieved 15 September 2011.
  2. ^ a b Krane, Kenneth S. (1988). Introductory Nuclear Physics. John Wiley & Sons. pp. 246–269. ISBN 978-0-471-80553-3.
  3. ^ Little, John B.; Kennedy, Ann R.; McGandy, Robert B. (1985). "Effect of Dose Rate on the Induction of Experimental Lung Cancer in Hamsters by α Radiation". Radiation Research. 103 (2): 293–9. Bibcode:1985RadR..103..293L. doi:10.2307/3576584. JSTOR 3576584. PMID 4023181.
  4. ^ Darling, David. "Alpha particle". Encyclopedia of Science. Archived from the original on 14 December 2010. Retrieved 7 December 2010.
  5. ^ N.B. Gamma rays move at the speed of light (c). Beta particles often move at a large fraction of c, and exceed 0.5 c whenever their energy is > 64 keV, which it commonly is. Neutron velocity from nuclear reactions ranges from about 0.06 c for fission to as much as 0.17 c for fusion.
  6. ^ Christensen, D. M.; Iddins, C. J.; Sugarman, S. L. (2014). "Ionizing radiation injuries and illnesses". Emergency Medicine Clinics of North America. 32 (1): 245–65. doi:10.1016/j.emc.2013.10.002. PMID 24275177.
  7. ^ Grellier, James; et al. (2017). "Risk of lung cancer mortality in nuclear workers from internal exposure to alpha particle-emitting radionuclides". Epidemiology. 28 (5): 675–684. doi:10.1097/EDE.0000000000000684. PMC 5540354. PMID 28520643.
  8. ^ Radford, Edward P.; Hunt, Vilma R. (1964). "Polonium-210: A Volatile Radioelement in Cigarettes". Science. 143 (3603): 247–249. Bibcode:1964Sci...143..247R. doi:10.1126/science.143.3603.247. PMID 14078362.
  9. ^ Cowell, Alan (24 November 2006). "Radiation Poisoning Killed Ex-Russian Spy". The New York Times. Retrieved 15 September 2011.
  10. ^ Rutherford distinguished and named α and β rays on page 116 of: E. Rutherford (1899) "Uranium radiation and the electrical conduction produced by it," Philosophical Magazine, Series 5, vol. 47, no. 284, pages 109–163. Rutherford named γ rays on page 177 of: E. Rutherford (1903) "The magnetic and electric deviation of the easily absorbed rays from radium," Philosophical Magazine, Series 6, vol. 5, no. 26, pages 177–187.
  11. ^ Hellemans, Alexander; Bunch, Bryan (1988). The Timetables of Science. Simon & Schuster. p. 411. ISBN 0671621300.
  12. ^ E. Rutherford and T. Royds (1908) "Spectrum of the radium emanation," Philosophical Magazine, Series 6, vol. 16, pages 313–317.
  13. ^ Magazine "nuclear energy" (III/18 (203) special edition, Volume 10, Issue 2 /1967.
  14. ^ Agakishiev, H.; et al. (STAR collaboration) (2011). "Observation of the antimatter helium-4 nucleus". Nature. 473 (7347): 353–6. arXiv:1103.3312. Bibcode:2011Natur.473..353S. doi:10.1038/nature10079. PMID 21516103.. See also "Erratum". Nature. 475 (7356): 412. 2011. arXiv:1103.3312. doi:10.1038/nature10264.
  15. ^ "Antihelium-4: Physicists nab new record for heaviest antimatter". PhysOrg. 24 April 2011. Retrieved 15 November 2011.
  16. ^ May, T. C.; Woods, M. H. (1979). "Alpha-particle-induced soft errors in dynamic memories". IEEE Transactions on Electron Devices. 26 (1): 2–9. Bibcode:1979ITED...26....2M. doi:10.1109/T-ED.1979.19370.

Further reading

  • Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 978-0-7167-4345-3.
Alpha-particle spectroscopy

One method for testing of (and measuring) many alpha emitters is to use alpha-particle spectroscopy. For methods of gamma rays and beta particles, please see gamma spectroscopy and liquid scintillation counting respectively.

Alpha decay

Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. It has a charge of +2 e and a mass of 4 u. For example, uranium-238 decays to form thorium-234. Alpha particles have a charge +2 e, but as a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms – the charge is not usually shown.

Alpha decay typically occurs in the heaviest nuclides. Theoretically, it can occur only in nuclei somewhat heavier than nickel (element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitters being the lightest isotopes (mass numbers 104–109) of tellurium (element 52). Exceptionally, however, beryllium-8 decays to two alpha particles.

Alpha decay is by far the most common form of cluster decay, where the parent atom ejects a defined daughter collection of nucleons, leaving another defined product behind. It is the most common form because of the combined extremely high nuclear binding energy and relatively small mass of the alpha particle. Like other cluster decays, alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, it is governed by the interplay between both the nuclear force and the electromagnetic force.

Alpha particles have a typical kinetic energy of 5 MeV (or ≈ 0.13% of their total energy, 110 TJ/kg) and have a speed of about 15,000,000 m/s, or 5% of the speed of light. There is surprisingly small variation around this energy, due to the heavy dependence of the half-life of this process on the energy produced (see equations in the Geiger–Nuttall law). Because of their relatively large mass, electric charge of +2 e and relatively low velocity, alpha particles are very likely to interact with other atoms and lose their energy, and their forward motion can be stopped by a few centimeters of air. Approximately 99% of the helium produced on Earth is the result of the alpha decay of underground deposits of minerals containing uranium or thorium. The helium is brought to the surface as a by-product of natural gas production.

Alpha particle X-ray spectrometer

APXS is also an abbreviation for APache eXtenSion tool, an extension for Apache web servers.An alpha particle X-ray spectrometer (APXS) is a spectrometer that analyses the chemical element composition of a sample from the scattered alpha particles, and fluorescent X-rays after the sample is irradiated with alpha particles and X-rays from radioactive sources. This method of analysing the elemental composition of a sample is most often used on space missions, which require low weight, small size, and minimal power consumption. Other methods (e.g. mass spectrometry) are faster, and do not require the use of radioactive materials, but require larger equipment with less modest power requirements. A variation is the alpha proton X-ray spectrometer, such as on the Pathfinder mission, which also detects protons.

Over the years several modified versions of this type of instrument such as APS (without X-ray spectrometer) or APXS have been flown: Surveyor 5-7, Mars Pathfinder, Mars 96, Mars Exploration Rover, Phobos, Mars Science Laboratory and the Philae comet lander. APS/APXS devices will be included on several upcoming missions including the Chandrayaan-2 lunar rover.

Bathurst Inlet (rock)

Bathurst Inlet is a rock on the surface of Aeolis Palus, between Peace Vallis and Aeolis Mons ("Mount Sharp"), in Gale crater on the planet Mars. The rock was encountered by the Curiosity rover on the way from Bradbury Landing to Glenelg Intrique on September 30, 2012 and was named after Bathurst Inlet, a deep inlet located along the northern coast of the Canadian mainland. The "approximate" site coordinates are: 4.59°S 137.44°E / -4.59; 137.44.

The NASA rover team had assessed the rock to be a suitable target for one of the first uses of Curiosity's contact instruments, the Mars Hand Lens Imager (MAHLI) and the Alpha particle X-ray spectrometer (APXS). The rock is dark gray and seems to contain grains or crystals, if any at all, that are finer than Curiosity's cameras can resolve - less than 80 µm in size.

Borophosphosilicate glass

Borophosphosilicate glass, commonly known as BPSG, is a type of silicate glass that includes additives of both boron and phosphorus. Silicate glasses such as PSG and borophosphosilicate glass are commonly used in semiconductor device fabrication for intermetal layers, i.e., insulating layers deposited between succeedingly higher metal or conducting layers.

BPSG has been implicated in increasing a device's susceptibility to soft errors since the boron-10 isotope is good at capturing thermal neutrons from cosmic radiation. It then undergoes fission producing a gamma ray, an alpha particle, and a lithium ion. These products may then dump charge into nearby structures, causing data loss (bit flipping, or single event upset).

In critical designs, depleted boron consisting almost entirely of boron-11 is used to avoid this effect as a radiation hardening measure. Boron-11 is a by-product of the nuclear industry.

Charged particle

In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary particle, which are all believed to have the same charge (except antimatter). Another charged particle may be an atomic nucleus devoid of electrons, such as an alpha particle.

A plasma is a collection of charged particles, atomic nuclei and separated electrons, but can also be a gas containing a significant proportion of charged particles.

Cluster (physics)

In physics, the term clusters denotes small, multiatom particles. As a rule of thumb, any particle of somewhere between 3 and 3×107 atoms is considered a cluster. Two-atom particles are sometimes considered clusters as well. A two atom particle may also be a molecule.

The term can also refer to the organization of protons and neutrons within an atomic nucleus, e.g. the alpha particle (also known as "α-cluster"), consisting of two protons and two neutrons (as in a helium nucleus).

Geiger–Marsden experiment

The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a landmark series of experiment by which scientists discovered that every atom contains a nucleus where all of its positive charge and most of its mass are concentrated. They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil. The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

Geiger–Nuttall law

In nuclear physics, the Geiger–Nuttall law or Geiger–Nuttall rule relates the decay constant of a radioactive isotope with the energy of the alpha particles emitted. Roughly speaking, it states that short-lived isotopes emit more energetic alpha particles than long-lived ones.

The relationship also shows that half-lives are exponentially dependent on decay energy, so that very large changes in half-life make comparatively small differences in decay energy, and thus alpha particle energy. In practice, this means that alpha particles from all alpha-emitting isotopes across many orders of magnitude of difference in half-life, all nevertheless have about the same decay energy.

Formulated in 1911 by Hans Geiger and John Mitchell Nuttall, in its modern form the Geiger–Nuttall law is

where λ is the decay constant (λ = ln2/half-life), Z the atomic number, E the total kinetic energy (of the alpha particle and the daughter nucleus), and a1 and a2 are constants. The law works best for nuclei with even atomic number and even atomic mass. The trend is still there for even-odd, odd-even, and odd-odd nuclei but not as pronounced.

Heisenbug

In computer programming jargon, a heisenbug is a software bug that seems to disappear or alter its behavior when one attempts to study it. The term is a pun on the name of Werner Heisenberg, the physicist who first asserted the observer effect of quantum mechanics, which states that the act of observing a system inevitably alters its state. In electronics the traditional term is probe effect, where attaching a test probe to a device changes its behavior.

Similar terms, such as bohrbug, mandelbug, hindenbug, and schrödinbug (see the section on related terms) have been occasionally proposed for other kinds of unusual software bugs, sometimes in jest; however, unlike the term heisenbug, they are not widely known or used.

Iron star

In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.

The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.

Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.

Lunar Prospector

Lunar Prospector was the third mission selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition including polar ice deposits, measurements of magnetic and gravity fields, and study of lunar outgassing events. The mission ended July 31, 1999, when the orbiter was deliberately crashed into a crater near the lunar south pole after the presence of water ice was successfully detected.Data from the mission allowed the construction of a detailed map of the surface composition of the Moon, and helped to improve understanding of the origin, evolution, current state, and resources of the Moon. Several articles on the scientific results were published in the journal Science.Lunar Prospector was managed by NASA Ames Research Center with the prime contractor Lockheed Martin. The Principal Investigator for the mission was Alan Binder. His personal account of the mission, Lunar Prospector: Against all Odds, is highly critical of the bureaucracy of NASA overall, and of its contractors.In 2013 an unidentified object was discovered in an unstable orbit around the Earth, and assigned the provisional number WT1190F. After it crashed into the Indian Ocean it was identified as probably the translunar injector of Lunar Prospector.

Muon-catalyzed fusion

Muon-catalyzed fusion (μCF) is a process allowing nuclear fusion to take place at temperatures significantly lower than the temperatures required for thermonuclear fusion, even at room temperature or lower. It is one of the few known ways of catalyzing nuclear fusion reactions.

Muons are unstable subatomic particles. They are similar to electrons, but are about 207 times more massive. If a muon replaces one of the electrons in a hydrogen molecule, the nuclei are consequently drawn 196 times closer than in a normal molecule, due to the reduced mass being 196 times the mass of an electron. When the nuclei are this close together, the probability of nuclear fusion is greatly increased, to the point where a significant number of fusion events can happen at room temperature.

Current techniques for creating large numbers of muons require far more energy than would be produced by the resulting catalyzed nuclear fusion reactions. Moreover, each muon has about a 1% chance of "sticking" to the alpha particle produced by the nuclear fusion of a deuteron with a triton, removing the "stuck" muon from the catalytic cycle, meaning that each muon can only catalyze at most a few hundred deuterium tritium nuclear fusion reactions. These two factors prevent muon-catalyzed fusion from becoming a practical power source, limiting it to a laboratory curiosity. To create useful room-temperature muon-catalyzed fusion, reactors would need a cheaper, more efficient muon source and/or a way for each individual muon to catalyze many more fusion reactions.

Photodisintegration

Photodisintegration (also called phototransmutation) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The incoming gamma ray effectively knocks one or more neutrons, protons, or an alpha particle out of the nucleus. The reactions are called (γ,n), (γ,p), and (γ,α).

Photodisintegration is endothermic (energy absorbing) for atomic nuclei lighter than iron and sometimes exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for the nucleosynthesis of at least some heavy, proton-rich elements via the p-process in supernovae.

Rutherford scattering

Rutherford scattering is the elastic scattering of charged particles by the Coulomb interaction. It is a physical phenomenon explained by Ernest Rutherford in 1911 that led to the development of the planetary Rutherford model of the atom and eventually the Bohr model. Rutherford scattering was first referred to as Coulomb scattering because it relies only upon the static electric (Coulomb) potential, and the minimum distance between particles is set entirely by this potential. The classical Rutherford scattering process of alpha particles against gold nuclei is an example of "elastic scattering" because neither the alpha particles nor the gold nuclei are internally excited. The Rutherford formula (see below) further neglects the recoil kinetic energy of the massive target nucleus.

The initial discovery was made by Hans Geiger and Ernest Marsden in 1909 when they performed the gold foil experiment in collaboration with Rutherford, in which they fired a beam of alpha particles (helium nuclei) at foils of gold leaf only a few atoms thick. At the time of the experiment, the atom was thought to be analogous to a plum pudding (as proposed by J. J. Thomson), with the negatively-charged electrons (the plums) studded throughout a positive spherical matrix (the pudding). If the plum-pudding model were correct, the positive "pudding", being more spread out than in the correct model of a concentrated nucleus, would not be able to exert such large coulombic forces, and the alpha particles should only be deflected by small angles as they pass through.

However, the intriguing results showed that around 1 in 8000 alpha particles were deflected by very large angles (over 90°), while the rest passed through with little deflection. From this, Rutherford concluded that the majority of the mass was concentrated in a minute, positively-charged region (the nucleus) surrounded by electrons. When a (positive) alpha particle approached sufficiently close to the nucleus, it was repelled strongly enough to rebound at high angles. The small size of the nucleus explained the small number of alpha particles that were repelled in this way. Rutherford showed, using the method outlined below, that the size of the nucleus was less than about 10−14 m (how much less than this size, Rutherford could not tell from this experiment alone; see more below on this problem of lowest possible size). As a visual example, Figure 1 shows the deflection of an alpha particle by a nucleus in the gas of a cloud chamber.

Rutherford scattering is now exploited by the materials science community in an analytical technique called Rutherford backscattering.

Supernova nucleosynthesis

Supernova nucleosynthesis is a theory of the nucleosynthesis of the natural abundances of the chemical elements in supernova explosions, advanced as the nucleosynthesis of elements from carbon to nickel in massive stars by Fred Hoyle in 1954. In massive stars, the nucleosynthesis by fusion of lighter elements into heavier ones occurs during sequential hydrostatic burning processes called helium burning, carbon burning, oxygen burning, and silicon burning, in which the ashes of one nuclear fuel become, after compressional heating, the fuel for the subsequent burning stage. During hydrostatic burning these fuels synthesize overwhelmingly the alpha-nucleus (A = 2Z) products. A rapid final explosive burning is caused by the sudden temperature spike owing to passage of the radially moving shock wave that was launched by the gravitational collapse of the core. W. D. Arnett and his Rice University colleagues demonstrated that the final shock burning would synthesize the non-alpha-nucleus isotopes more effectively than hydrostatic burning was able to do, suggesting that the expected shock-wave nucleosynthesis is an essential component of supernova nucleosynthesis. Together, shock-wave nucleosynthesis and hydrostatic-burning processes create most of the isotopes of the elements carbon (Z = 6), oxygen (Z = 8), and elements with Z = 10–28 (from neon to nickel). As a result of the ejection of the newly synthesized isotopes of the chemical elements by supernova explosions their abundances steadily increased within interstellar gas. That increase became evident to astronomers from the initial abundances in newly born stars exceeding those in earlier-born stars. To explain that temporal increase of the natural abundances of the elements was the main goal of stellar nucleosynthesis. Hoyle's paper was the founding paper of that theory; however, ideas about nuclear reactions in stars providing power for the stars is often confused with stellar nucleosynthesis. Realize that nuclear fusion in stars can occur with negligible impact on the abundances of the chemical elements.

Elements heavier than nickel are comparatively rare owing to the decline with atomic weight of their nuclear binding energies per nucleon, but they too are created in part within supernovae. Of greatest interest historically has been their synthesis by rapid capture of neutrons during the r-process, reflecting the common belief that supernova cores are likely to provide the necessary conditions. But see the r-process below for a recently discovered alternative. The r-process isotopes are roughly a 100,000 times less abundant than the primary chemical elements fused in supernova shells above. Furthermore, other nucleosynthesis processes in supernovae are thought to also be responsible for some nucleosynthesis of other heavy elements, notably, the proton capture process known as the rp-process, the slow capture of neutrons (s-process) in the Helium-burning shells and in the carbon-burning shells of massive stars, and a photodisintegration process known as the γ-process (gamma-process). The latter synthesizes the lightest, most neutron-poor, isotopes of the elements heavier than iron from preexisting heavier isotopes.

Surveyor 6

Surveyor 6 was the sixth lunar lander of the American unmanned Surveyor program that reached the surface of the Moon. Surveyor 6 landed on the Sinus Medii. A total of 30,027 images were transmitted to Earth.

This spacecraft was the fourth of the Surveyor series to successfully achieve a soft landing on the Moon, obtain post landing television pictures, determine the abundance of the chemical elements in the lunar soil, obtain touchdown dynamics data, obtain thermal and radar reflectivity data, and conduct a Vernier engine erosion experiment. Virtually identical to Surveyor 5, this spacecraft carried a television camera, a small bar magnet attached to one footpad, and an alpha-scattering instrument as well as the necessary engineering equipment. It landed on November 10, 1967, in Sinus Medii, 0.49 deg in latitude and 1.40 deg w longitude (selenographic coordinates)–the center of the Moon's visible hemisphere. The spacecraft accomplished all planned objectives. The successful completion of this mission satisfied the Surveyor program's obligation to the Apollo project. On November 24, 1967, the spacecraft was shut down for the two-week lunar night. Contact was made on December 14, 1967, but no useful data was obtained.

Lunar soil surveys were completed using photographic and alpha particle backscattering methods. A similar instrument, the APXS, was used onboard several Mars missions.In a further test of space technology, Surveyor 6's engines were restarted and burned for 2.5 seconds in the first lunar liftoff on November 17 at 10:32 UTC. This created 150 lbf (700 N) of thrust and lifted the vehicle 12 feet (4 m) from the lunar surface. After moving west eight feet, (2.5 m) the spacecraft once again successfully soft landed and continued functioning as designed.

Targeted alpha-particle therapy

Targeted alpha-particle therapy (or TAT) is an in-development method of targeted radionuclide therapy of various cancers. It employs radioactive substances which undergo alpha decay to treat diseased tissue at close proximity. It has the potential to provide highly targeted treatment, especially to microscopic tumour cells. Targets include leukemias, lymphomas, gliomas, melanoma, and peritoneal carcinomatosis. As in diagnostic nuclear medicine, appropriate radionuclides can be chemically bound to a targeting biomolecule which carries the combined radiopharmaceutical to a specific treatment point.It has been said that "α-emitters are indispensable with regard to optimisation of strategies for tumour therapy".

Timothy C. May

Timothy C. May, better known as Tim May (December 21, 1951 – December 13, 2018) was an American technical, political writer, electronic engineer and senior scientist at Intel in the company's early history. He retired from Intel in 1986 at age 35 and died of natural causes at his home on December 13, 2018 at age 66.

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