Alpha decay

Visual representation of alpha decay

Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into an atom with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. For example, uranium-238 decays to form thorium-234:^[1]

238
92U → 234
90Th + α

or

238
92U → 234
90Th + 4
2He

Both mass number and atomic number are conserved: the mass number is 238 on the left side and (234 + 4) on the right side and the atomic number is 92 on the left side and (90 + 2) on the right side. Alpha particles have a charge +2, but as a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms – the charge is not usually shown.

Alpha decay typically occurs in the heaviest nuclides. Theoretically, it can occur only in nuclei somewhat heavier than nickel (element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitter being the lightest isotopes (mass numbers 106–110) of tellurium (element 52).

Alpha decay is by far the most common form of cluster decay, where the parent atom ejects a defined daughter collection of nucleons, leaving another defined product behind. It is the most common form because of the combined extremely high binding energy and relatively small mass of the alpha particle. Like other cluster decays, alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, it is governed by the interplay between both the nuclear force and the electromagnetic force.

Alpha particles have a typical kinetic energy of 5 MeV (or ≈ 0.13% of their total energy, 110 TJ/kg) and have a speed of about 15,000,000 m/s, or 5% of the speed of light. There is surprisingly small variation around this energy, due to the heavy dependence of the half-life of this process on the energy produced (see equations in the Geiger–Nuttall law). Because of their relatively large mass, +2 electric charge and relatively low velocity, alpha particles are very likely to interact with other atoms and lose their energy, and their forward motion can be stopped by a few centimeters of air. Approximately 99% of the helium produced on Earth is the result of 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.

History

Alpha source beneath a radiation detector

Alpha particles were first described in the investigations of radioactivity by Ernest Rutherford in 1899, and by 1907 they were identified as He²⁺ ions.

By 1928, George Gamow had solved the theory of 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. Gamow solved a model potential for the nucleus and derived, from first principles, a relationship between the half-life of the decay, and the energy of the emission, which had been previously discovered empirically, and was known as the Geiger–Nuttall law.^[2]

Uses

Americium-241, an alpha emitter, is used in smoke detectors. The alpha particles ionize air in an open ion chamber and a small current flows through the ionized air. Smoke particles from fire that enter the chamber reduce the current, triggering the smoke detector's alarm. See Smoke Detector Ionization for details.

Alpha decay can provide a safe power source for radioisotope thermoelectric generators used for space probes^[3] and were used for artificial heart pacemakers.^[4] Alpha decay is much more easily shielded against than other forms of radioactive decay. Plutonium-238, for example, requires only 2.5 millimetres 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 dissipate more rapidly.

Toxicity

Highly charged and heavy, alpha particles lose their several MeV of energy within a small volume of material, along a very short mean free path. This increases the chance of double-strand breaks to the DNA in cases of internal contamination, when ingested, inhaled, injected or introduced through the skin. Otherwise, touching an alpha source is typically not harmful, as alpha particles are effectively shielded by a few centimeters of air, a piece of paper, or the thin layer of dead skin cells that make up the epidermis; however, many alpha sources are also accompanied by beta-emitting radio daughters, and both are often accompanied by gamma photon emission.

RBE relative biological effectiveness quantifies the ability of radiation to cause certain biological effects, notably either cancer or cell-death, for equivalent radiation exposure. Alpha radiation has high linear energy transfer (LET) coefficient, which is about one ionization of a molecule/atom for every angstrom of travel by the alpha particle. The RBE has been set at the value of 20 for alpha radiation by various government regulations. The RBE is set at 10 for neutron irradiation, and at 1 for beta radiation and ionizing photons.

However, the recoil of the parent nucleus (alpha recoil) gives it a significant amount of energy, which also causes ionization damage (see ionizing radiation). This energy is roughly the weight of the alpha (4 u) divided by the weight of the parent (typically about 200 u) times the total energy of the alpha. By some estimates, this might account for most of the internal radiation damage, as the recoil nucleus is part of an atom that is much larger than an alpha particle, and causes a very dense trail of ionization; the atom is typically a heavy metal, which preferentially collect on the chromosomes. In some studies,^[5] this has resulted in an RBE approaching 1,000 instead of the value used in governmental regulations.

The largest natural contributor to public radiation dose is radon, a naturally occurring, radioactive gas found in soil and rock.^[6] If the gas is inhaled, some of the radon particles may attach to the inner lining of the lung. These particles continue to decay, emitting alpha particles, which can damage cells in the lung tissue.^[7] The death of Marie Curie at age 66 from aplastic anemia was probably caused by prolonged exposure to high doses of ionizing radiation, but it is not clear if this was due to alpha radiation or X-rays. Curie worked extensively with radium, which decays into radon,^[8] along with other radioactive materials that emit beta and gamma rays. However, Curie also worked with unshielded X-ray tubes during World War I, and analysis of her skeleton during a reburial showed a relatively low level of radioisotope burden.

The russian dissident Alexander Litvinenko's 2006 murder by radiation poisoning is thought to have been carried out with polonium-210, an alpha emitter.

See also

• Alpha particle
• Beta decay

References

1. Suchocki, John. Conceptual Chemistry, 2007. Page 119.
2. For Gamow's derivation of this law, see
3. "Radioisotope Thermoelectric Generator". Solar System Exploration. NASA. Retrieved 25 March 2013.
4. "Nuclear-Powered Cardiac Pacemakers". Off-Site Source Recovery Project. LANL. Retrieved 25 March 2013.
5. Winters TH, Franza JR (1982). "Radioactivity in Cigarette Smoke". New England Journal of Medicine 306 (6): 364–365. doi:10.1056/NEJM198202113060613.
6. ANS : Public Information : Resources : Radiation Dose Chart
7. EPA Radiation Information: Radon. October 6, 2006, , Accessed December 6, 2006
8. Health Physics Society, "Did Marie Curie die of a radiation overexposure?"

• Alpha emitters by increasing energy (Appendix 1)

External links

• The LIVEChart of Nuclides - IAEA with filter on alpha decay
• Alpha decay with 3 animated examples showing the recoil of daughter