Alpha particle

From Wikipedia, the free encyclopedia

An alpha particle is deflected by a magnetic field

Alpha radiation consists of helium-4 nuclei 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.

Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) are a highly ionizing form of particle radiation that 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⁺. The alpha particle mass is 6.644656×10^-27 kg, which is equivalent to the energy of 3.72738 G eV. The charge of an alpha particle is equal to +2e, where e is the magnitude of charge on an electron, e=1.602176462x10^-19C.

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. Classically, alpha particles do not have enough energy to escape the potential of the nucleus. However, the quantum tunnelling effect allows them to escape.

When an alpha particle is emitted, the atomic mass of an element goes down by roughly 4.0015 u, due to the loss of 2 neutrons and 2 protons. The atomic number of the atom goes down by 2, as a result of the loss of 2 protons; the atom becomes a new element. An example of this is when 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. 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 (β particles, γ rays, neutrons, etc). Because of their charge and large mass, alpha particles 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 J. J. Thomson'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 was 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 positive 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'.