An alpha particle is a particle consisting of two protons, two neutrons, and no electrons. Essentially, it is a helium-4 nucleus. Other names for alpha particles are alpha rays or alpha radiation. The symbol for an alpha particle is α, α2+, He2+, or 42He2+. Usually, alpha particles result from alpha radioactive decay, but they form via other processes, too.
Alpha Particle Properties
Alpha particles have a net spin of zero. Their kinetic energy varies, with higher energy from larger nuclei (~7 MeV) and lower energy from smaller nuclei (~3 MeV). The average kinetic energy is around 5 MeV, with a velocity approximately 4% the speed of light. Because it lacks electrons, an alpha particle has a net electrical charge of 2+. However, the particles readily accept electrons from other matter and quickly become regular electrically neutral helium atoms (42He).
Alpha particles are a form of ionizing radiation and can cause considerable damage to living tissue. However, alpha particles produced by radioactive decay have a lower penetration depth than, for example, beta or gamma radiation. Usually, all it takes is a few centimeters of air, a sheet of paper, or a few layers of skin to stop an alpha particle.
However, the alpha particles that form from ternary fission (as opposed to radioactive decay) are much more energetic and penetrate about three times further. Alpha particles that are cosmic rays are so energetic that they can pass through the human body or several meters of radiation shielding. Similarly, alpha particles produced in particles accelerators are highly energetic and display high penetration.
Sources of Alpha Particles
Most of the time, alpha particles come from radioactive decay of heavy atoms, such as uranium, thorium, actinium, radium, and transuranic elements. The smallest nucleus capable of alpha decay is beryllium-8. Alpha decay decreases the mass number of the parent nucleus by four nucleons (two protons and two neutrons). Since the atomic number decreases by two, alpha decay is a form of transmutation that results in the formation of a new element.
AZX → A-4Z-2Y + 42He
Note that even though an ionizing helium nucleus results (42He2+), the “2+” usually is omitted in the written reaction. This is because nuclear reactions only show what’s going on with the atomic nucleus.
Other sources of alpha particles are ternary fission (when three charged particle results from fission instead of two) particle accelerators, cosmic rays, and nuclear reactions within the cores of stars. In everyday life, products that contain alpha emitters include tobacco smoke (polonium), some smoke detectors (americium), heart pacemakers (plutonium), and static eliminators (polonium). Rocks, soils, and water contain varying amounts of uranium, thorium, and actinium and their decay products.
Biological Effects of Alpha Particles
Alpha particles resulting from alpha decay mainly pose a risk when inhaled, ingested, or injected. This is because they only penetrate the outer layers of skin or the cornea of the eye. They can also enter through an open wound.
If alpha particles enter the body, they are so massive and reactive that they are the most destructive form of radiation. Significant doses of alpha radiation cause chromosome damage and radiation poisoning that can cause cancer or death. On average, alpha particles are between 20 times (ingested) and 1000 times (inhaled) more dangerous than beta particles or gamma radiation.
Alpha emitters also find use in the treatment of tumors. For example, radium-223 finds use in treating bone cancer and actinium-225 is used in treating prostate cancer. Alpha emitters used in cancer treatment tend to be radioisotopes with short half-lives. They either rely on their short effective range or else they migrate to specific tissues in the body.
As with other particles, there is an antimatter version of an alpha particle, which is the anti-alpha. An international team of researchers detected the anti-alpha using the STAR detector at Brookhaven National Laboratory in 2011. The experiment involved colliding gold ions moving nearly at the speed of light. At the time, the anti-alpha or antimatter helium-4 nucleus was the heaviest antimatter particle.
- Agakishiev, H.; et al. (STAR collaboration) (2011). “Observation of the antimatter helium-4 nucleus”. Nature. 473 (7347): 353–6. doi:10.1038/nature10079
- Christensen, D. M.; Iddins, C. J.; Sugarman, S. L. (2014). “Ionizing radiation injuries and illnesses”. Emergency Medicine Clinics of North America. 32 (1): 245–65. doi:10.1016/j.emc.2013.10.002
- Firestone, Richard B. (1999). Table of Isotopes. Coral M. Baglin (8th ed.). New York: Wiley. ISBN 0-471-35633-6.
- Krane, Kenneth S. (1988). Introductory Nuclear Physics. John Wiley & Sons. ISBN 978-0-471-80553-3.
- Little, John B.; Kennedy, Ann R.; McGandy, Robert B. (1985). “Effect of Dose Rate on the Induction of Experimental Lung Cancer in Hamsters by α Radiation”. Radiation Research. 103 (2): 293–9. doi:10.2307/3576584