Alpha particle

Alpha particle
	Alpha decay
Composition	2 protons, 2 neutrons
Statistics	Bosonic
Symbol	α, α2+, He2+
Mass	6.64465620(33)×10−27 kg; 4.001506179127(62) u; 3.727379109(93) GeV/c2
Electric charge	2 e
Spin	0

Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is produced in the process of alpha decay. The alpha particle can be written as
He²⁺
, ⁴
₂He²⁺
or ⁴
₂He
(as it is possible that the ion gains electrons from the environment. Also, electrons are not important in nuclear chemistry). Not all helium nuclei are always considered by all authors as alpha particles. As with beta particles and gamma rays, the name used for the particle carries some mild connotations about its production process and energy.^[3]

Some science authors may use doubly-ionized helium nuclei (He²⁺) and alpha particles as interchangeable terms. 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. However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles" because the high energies produced by these sources highlights the striking difference in behavior of their particles from the classical alpha particles produced (and originally defined by) the process of radioactive alpha decay.

Alpha particles, like helium nuclei, have a net spin of zero, and (due to the mechanism of their production in nuclear decay) classically a total energy of about 5 MeV. They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) have low penetration depth. They are able to be stopped by a few centimeters of air, or by the skin. However, as noted, the helium nuclei which form 10-12% of cosmic rays are usually of much higher energy than those produced by radioactive decay, and are thus capable of being highly penetrating, able to traverse the human body and also many meters of dense solid shielding, depending on their energy.

The alpha decay process

Sources

When an atom emits an alpha particle, 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 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 which can support it. The smallest nuclei which have to date been found to be capable of alpha emission are the lightest nuclides of tellurium (element 52), with mass numbers between 106 and 110. The process of emitting an alpha sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy.

Energetic helium nuclei may be produced by cyclotrons, synchrotrons, and other particle accelerators, but they are not normally referred to as alpha particles. As noted, helium nuclei may participate in nuclear reactions in stars, and occasionally and historically these have been referred to, as alpha reactons (see triple alpha process.)^{[citation needed]} Very high energy helium nuclei sometimes referred to as alpha particles make up about 10 to 12% of cosmic rays.

Mechanism of production

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.

Energy and absorption

The energy of the alpha emitted 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 to 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.). γ rays, being an electromagnetic radiation, move at the speed of light. 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

Because of the short range of absorption, alphas are not generally dangerous to life unless the source is ingested or inhaled, in which case, they become extremely dangerous. Because of this high mass and strong absorption, if alpha emitting radionuclides do enter the body (if radioactive material has been 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 about 100 times greater than that caused by an equivalent amount of other radiation. 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.^[4] ²¹⁰Po was used to kill Russian dissident and ex-FSB officer Alexander V. Litvinenko in 2006.^[5]

Not only do alphas themselves cause damage, but approximately equal ionization is caused by the recoiling nucleus after alpha emission, and this energy may in turn be especially damaging to genetic material, since the positive cations of many soluble transuranic elements which emit alphas, are chemically attracted to the net negative charge of DNA, causing the recoiling atomic nucleus to be in close proximation to the DNA.

History of discovery and use

An alpha particle is deflected by a magnetic field

In the years 1899 and 1900 physicists Ernest Rutherford and Paul Villard separated radiation into three types: alpha, beta, and gamma, based on penetration of objects and ability to cause ionization. Alpha rays were defined by Rutherford by their lowest penetration of ordinary objects.

Rutherford's work also included measurements of the ratio of an alpha particle's mass to its charge, allowing him to hypothesize that alpha particles were helium nuclei (deuterium nuclei, which have the same mass to charge ratio, were not then known).^[6] In 1907, Ernest Rutherford and Thomas Royds finally proved that alpha particles were indeed helium nuclei. 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 nuclei inside the tube. They then caused an electric spark inside the tube, which provided a shower of electrons which were taken up by the nuclei to form neutral atoms of a gas. Subsequent study of the spectra of the resulting gas showed that alpha particles were indeed the hypothesized helium nuclei.

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.

Note: 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.

Applications

Most smoke detectors contain a small amount of the alpha emitter americium-241. The alpha particles ionize air between 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 RAM 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 was largely considered to be solved.^[7]

References

^ Template:CODATA2006
^ ^a ^b Krane, Kenneth S. (1988). Introductory Nuclear Physics. John Wiley & Sons. pp. 246–269. ISBN 047180553X.
^ Definition from dictionary of science Accessed Dec 7, 2010.
^ Radford, Edward P.; Hunt, Vilma R. (1964). "Polonium-210: A Volatile Radioelement in Cigarettes". Science. 143 (3603): 247–249. doi:10.1126/science.143.3603.247. PMID 14078362.
^ Radiation Poisoning Killed Ex-Russian Spy
^ Hellemans, Alexander; Bunch, Bryan (1988). The Timetables of Science. Simon & Schuster. p. 411. ISBN 0671621300.
^ May, T. C.; Woods, M. H. (1979). "Alpha-particle-induced soft errors in dynamic memories". IEEE Transactions on Electron Devices. 26 (1): 2–9. doi:10.1109/T-ED.1979.19370. ISSN 0018-9383.

Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 0-7167-4345-0.