Alpha particle

An alpha particle is deflected by a magnetic field

Alpha particles or alpha rays (named after the first letter in the greek alphabet, α) are a highly ionizing form of particle radiation which have low penetration. They consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He²⁺.

Alpha particles are emitted by radioactive nuclei such as uranium 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, alpha decay is mediated by the strong nuclear force.

When an alpha particle is emitted, the atomic mass of an element goes down by roughly 4 amu, due to the loss of 4 nucleons. The atomic number of the atom goes down by 2, as the atom loses 2 protons, essentially becoming a new element. An example of this is when radium becomes radon gas due to alpha decay.

Because of their charge and large mass, alpha rays are easily absorbed by materials and can travel only a few centimeters 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 and bladder cancer related to tobacco smoking.

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, rather than let them go into 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. The physicist Ernest Rutherford famously used alpha particles to infer that Lord Kelvin's "plum pudding" model of the atom was fundamentally flawed. He did this by coating a screen which flashed wherever it was struck by an alpha particle then surrounding a thin piece of gold foil with this screen. He then aimed alpha particles at the foil, hypothesizing 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, with some even bouncing 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 cannonball at tissue paper only to have it bounce off, again assuming the "plum pudding" theory were correct. It was soon determined that the positive charge of the atom was concentrated in a small area in the center of the atom, hence making the postive charge dense enough to deflect any positively charged alpha particles that happened to come close to what was later termed the nucleus. (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 subsequently led to the Bohr model and later the modern wave-mechanical model of the atom.

In computer technology, 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 'solved'.

See also

• radioactivity
• beta particle
• gamma ray
• cosmic rays
• nuclear physics
• radioactive isotope
• radioactive decay
• rays: α — β — γ — δ — ε

References

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