alpha particle

Alpha particle
Alpha decay
Composition:	2 protons, 2 neutrons
Statistical behavior:	Boson
Symbol(s):	α, α2+, He2+
Mass:	6.64465620(33)×10−27 kg 4.001506179127(62) u 3.727379109(93) GeV/c2[1]
Electric charge:	2 e
Spin:	0[2]

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; hence, it can be written as He²⁺ or 42He²⁺. They have a net spin of zero, and normally a total energy of about 5 MeV. They are a highly ionizing form of particle radiation, and have low penetration.

The alpha decay process

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.

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.

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).

Because of the short range of absorption, alphas are not generally dangerous to life unless the source is ingested or inhaled, but then they become extremely dangerous. Because of this high mass and strong absorption, if alpha emitting radionuclides do enter the body (if the radioactive material has been inhaled or ingested), 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 alpha emitter polonium-210 is suspected of playing a role in lung cancer and bladder cancer related to tobacco smoking.^[3]

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

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.

Rutherford originally separated out radiation into alpha, beta, and gamma components, based on its penetration of objects and its ability to cause ionization. Alpha rays were defined by their lowest penetration of ordinary objects.

An alpha particle is deflected by a magnetic field

Rutherford's work also included measurements of the ratio of an alpha particle's mass to charge, allowing him to hypothesize that that alpha particles were helium nuclei (deuterium nuclei, which have the same mass to charge, were not then known).^[4] That alphas were helium nuclei was finally proven by allowing alpha particles to penetrate the thin glass into a vacuum tube, which was then energized to provide a gas discharge tube glow. The spectrum produced turned out to be that of helium.

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. Physicist Ernest 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) it was found that 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: it was not known at the time that alpha particles were themselves nuclei nor was the existence of protons or neutrons known. 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, for example, 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 memory 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".^{[citation needed]}

See also

Wikimedia Commons has media related to: Alpha particle

• beta particle
• cosmic rays
• list of alpha emitting materials
• nuclear physics
• particle physics
• radioactivity
• radioactive isotope
• radioactive decay
• rays:
  • γ gamma ray
  • δ delta ray
  • ε epsilon radiation

External links

• BIGS-animation - Alpha particles, determination of speed in a virtual experiment
• Alpha Particles Tutorial on YouTube

References

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