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Alpha particle
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Alpha particles (named after and denoted by the first letter in the Greek alphabet, a) 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 are a highly ionizing form of particle radiation, and have low penetration.
The alpha decay process
Alpha particles are emitted by radioactive nuclei such as uranium, thorium, actinium, or radium in a process known as alpha decay.
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Alpha particles (named after and denoted by the first letter in the Greek alphabet, a) 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 are a highly ionizing form of particle radiation, and have low penetration.
The alpha decay process
Alpha particles are emitted by radioactive nuclei such as uranium, thorium, actinium, or radium in a process known as alpha decay. This 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 interaction responsible for alpha decay is the electromagnetic force—alpha decay results from the Coulomb repulsion between the alpha particle and the rest of the nucleus, which both have a positive electric charge. In classical physics, alpha particles do not have enough energy to escape the potential well inside the nucleus. However, the quantum tunnelling effect allows them to escape.
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.
The energy of alpha particles varies, with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV (million electron-volts). This 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) than any other common type of radiation (ß particles, ? rays, neutrons, etc). 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) and so are not generally dangerous to life unless the source is ingested or inhaled. Because of this high mass and strong absorption, however, if alpha radiation does enter the body (most often because radioactive material has been inhaled or ingested), it 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.
Applications
Most smoke detectors contain a small amount of the alpha emitter americium-241. This 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.
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 hypothesisized 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, and some bounced 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.
Rutherford's work also improved on previous measurements of the ratio of an alpha particle's mass to charge, allowing him to deduce that alpha particles were helium nuclei.
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".
See also
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