ALPHA DECAY

'Alpha decay' is a type of radioactive decay in which an atomic nucleus emits an alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less.
For example:
:$$
{}^2{}^{38}_{92}hbox{U}; o;{}^2{}^{34}_{90}hbox{Th};+;{}^4_2hbox{He}^{2+},

although this is typically written as:
:$$
{}^{238}hbox{U}; o;^{234}hbox{Th};+;lpha.

(The second form is preferred because the first form appears electrically unbalanced. Fundamentally, the recoiling nucleus is very quickly stripped of two electrons to neutralize the ionized helium cation.)
An alpha particle is identical to a helium nucleus, and both mass number and atomic number are the same.
Alpha decay is a form of nuclear fission where the parent atom splits into two daughter products. Alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the strong nuclear force.
Alpha particles have a typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy, i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05c. Because of their relatively large mass, +2 charge and relatively low velocity, they are very likely to interact with other atoms and lose their energy, so they are effectively absorbed within a few centimeters of air.

Most of the helium produced on Earth comes from the alpha decay of underground deposits of minerals containing uranium or thorium. The helium is brought to the surface as a by-product of natural gas production.

Contents

History

Uses

Toxicity

References

History

By 1928, George Gamow had solved the theory of the alpha decay via tunneling. The alpha particle is trapped in a potential well by the nucleus. Classically, it is forbidden to escape, but according to the then newly discovered principles of Quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus.

Uses

Americium-241 is used in smoke detectors. The alpha particles ionize air between a small gap, leading to a small current that can be easily interrupted by smoke particles.
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.

Toxicity

Because they are heavy and charged, alpha particles tend to have a very short mean free path, and therefore lose their kinetic energy within a short distance of their source. This can result in several MeV being deposited in a relatively small area. If they penetrate live tissue, this can cause significant cellular damage. Generally, external alpha radiation is not harmful because alpha particles are completely absorbed by a few centimeters of air. Even touching an alpha source is usually not harmful; the thin layer of dead skin cells in the outermost layer of the skin will absorb them. However, if a substance radiating alpha particles is ingested, inhaled by, injected into, or introduced through the skin (shrapnel, corrosive chemicals) into an organism it may result in a high dose to that area.
Radon is a naturally occurring, radioactive gas found in soil, rock, and sometimes groundwater. When radon gas is inhaled, some of the radon particles stick to the inner lining of the lung. The particles that remain continue to decay over time, emitting alpha particles which may damage cells in the lung tissue.^[1]. The death of Marie Curie at age 66 from leukemia was likely caused by prolonged exposure to high doses of ionizing radiation. Curie worked extensively with Radium, which decays into Radon^[2], along with other radioactive materials that emit beta and gamma rays. Shrapnel deposited in the body from depleted uranium poses another such internal risk of alpha particle radiation dose.
The 2006 assassination of Russian dissident Alexander Litvinenko is thought to have been caused by poisoning with Polonium-210, an alpha emitter.

References

1. EPA Radiation Information: Radon. October 6 2006, [1], Accessed Dec. 6 2006
2. Health Physics Society, "Did Marie Curie die of a radiation overexposure?" [2]