Radioactivity is the spontaneous emission of ionizing radiation from nuclear decay and reactions. The three main types of radioactive decay are alpha, beta, and gamma decay, but there are other nuclear reactions responsible for radioactivity. Here is a look at the definition of radioactivity, its units, the types of radioactive decay, and how radioactivity penetrates matter.
Radioactivity Definition
Radioactivity is defined as the emission of particles and radiation from nuclear reactions. These nuclear reactions include radioactive decay by unstable atomic nuclei, fission, and fusion.
It’s important to note that not all radiation comes from radioactivity. For example, a fire emits heat (infrared radiation) and light (visible radiation) from a chemical reaction and not a nuclear reaction. Infrared and visible light are types of non-ionizing radiation. Radiation from radioactivity is ionizing radiation. Ionizing radiation is sufficiently energetic to change the electrical charge of an atom. Usually, this is from removing an electron from an atom, but sometimes ionizing radiation affects the atomic nucleus. A substance that emits ionizing radiation is radioactive.
In a radioactive material, the emission of radioactivity occurs at the atomic level. An unstable atomic nucleus eventually decays, but it’s not possible to predict exactly when this will occur. But, in a sample of material, the half-life is the time it takes for half of the atoms to decay. The half-life of a radioactive element ranges from a fraction of a second to a time longer than the age of the universe.
Difference Between Stable and Unstable
A radioactive isotope or radioisotope undergoes radioactive decay. A stable isotope is one that never breaks apart. Examples of stable isotopes include protium and carbon-12. A stable radioisotope has a half-life so long that it’s stable for all practical purposes. An example of a stable radioisotope is tellurium-128, which has a half-life of 7.7 x 1024 years. An unstable isotope is a radioisotope with a relatively short half-life. An example of an unstable isotope is carbon-14, which has a half-life of 5730 years. But, many unstable isotopes have half-life values that are much, much shorter.
Radioactivity Units
The becquerel (Bq) is the International System of Units (SI) unit of radioactivity. Its name honors French scientist Henri Becquerel, the discoverer of radioactivity. A bequerel is one disintegration or decay per second.
Another common unit of radioactivity is the curie (Ci). One curie is 3.7 x 1010 disintegrations per second or 3.7 x 1010 bequerels.
While the becquerel and curie reflect the rate of radioactive decay, they don’t address the interaction between radiation and human tissue. The gray (Gy) is the absorption of one joule of radiation energy per kilogram of body mass. The sievert (Sv) is the amount of radiation resulting in a 5.5% chance of cancer eventually resulting from exposure.
Types of Radioactive Decay
Radioactive decay occurs when an unstable isotope (the parent isotope or parent nuclide) undergoes a reaction, producing at least one daughter nuclide. The daughter(s) may be either stable or unstable isotopes. Some types of decay involve transmutation, where the parent isotope decays and yields a daughter isotope of a different element. In other types of decay, the atomic number and element identity of the parent and daughter are the same.
Alpha (α), beta (β), and gamma (γ) decay were the first three types of radioactivity that were discovered, but there are other nuclear reactions. When discussing types of decay, remember A is the mass number of an atom or the number of protons plus neutrons, while Z is the atomic number or number of protons. A identifies the isotope of an atom, while Z identifies which element it is.
| Decay Mode | Symbol | Reaction | Daughter Nucleus |
| Alpha decay | α | The parent nucleus emits an alpha particle or helium nucleus (A=4, Z=2) | (A − 4, Z − 2) |
| Proton emission | p | The parent nucleus ejects a proton | (A − 1, Z − 1) |
| Double proton emission | 2p | The nucleus ejects two protons simultaneously | (A − 2, Z − 2) |
| Neutron emission | n | The nucleus ejects a neutron | (A − 1, Z) |
| Double neutron emission | 2n | The nucleus ejects two neutrons simultaneously | (A − 2, Z) |
| Spontaneous fission | SF | The nucleus disintegrates into two or more smaller nuclei and other particles | varies |
| Cluster decay | CD | The nucleus emits a specific smaller nucleus that is larger than an alpha particle | (A − A1, Z − Z1) + (A1, Z1) |
| Beta minus decay | β− | The nucleus emits an electron and electron antineutrino | (A, Z + 1) |
| Beta plus decay | β+ | The nucleus emits a positron and an electron neutrino | (A, Z − 1) |
| Electron capture | ε (EC) | The nucleus captures an orbiting electron and emits a neutrino, leaving an excited unstable daughter | (A, Z − 1) |
| Bound-state beta decay | A nucleus or free neutron decays into an electron and antineutrino, but retains the electron in a vacant K-shell | (A, Z + 1) | |
| Double beta decay | β−β− | A nucleus emits to electrons and two antineutrinos | (A, Z + 2) |
| Double electron capture | εε | A nucleus absorbs two orbital electrons and emits two neutrinos, yielding an excited unstable daughter | (A, Z − 2) |
| Electron capture with positron emission | A nucleus absorbs one orbital electron and emits one positron and two neutrinos | (A, Z − 2) | |
| Double positron decay | β+β+ | A nucleus emits two positrons and two neutrinos | (A, Z − 2) |
| Isomeric transition | IT | An excited nucleus releases a high-energy gamma ray photon (after >10−12 s) | (A, Z) |
| Internal conversion | – | An excited nucleus transfers energy to an orbital electron and the electron is ejected | (A, Z) |
| Gamma decay | γ | An excited nucleus (often after alpha or beta decay) emits a gamma ray photon (~10−12 s) | (A, Z) |
Example Decay Schemes
The alpha decay of uranium-238 is:
23892U → 42He +23490Th
The beta decay of thorium-234 is:
23490Th → 0-1e + 23491Pa
Gamma decay accompanies more nuclear reactions, including alpha or beta decay. The gamma decay of uranium-238 is:
23892U → 42He + 23490Th + 200γ
But, gamma decay is not usually shown when writing nuclear reactions.
Penetration of Matter
Alpha, beta, and gamma decay are named for the first three letters of the Greek alphabet in order of their matter penetration ability.
- Alpha particles are essentially helium nuclei. They have the greatest mass, the highest ionization ability, and the shortest penetration distance. Skin, a thick sheet of paper, or a layer of clothing are enough to stop alpha particles. Alpha radiation mainly poses a threat when inhaled, injected, or ingested.
- Beta particles are electrons or positrons. They have much less mass than alpha particles, so they penetrate further into tissue than alpha particles, but they are less likely to ionize atoms. A thick sheet of aluminum foil stops beta particles. Again, the main health threat occurs when they are ingested, injected, or inhaled.
- Gamma rays are a form of electromagnetic radiation. Gamma rays are so energetic that they penetrate deeply into matter. While gamma rays may pass through a human body without interacting, they are stopped by lead shielding. When gamma rays do interact with living tissue, they cause considerable damage.
References
- L’Annunziata, Michael F. (2007). Radioactivity: Introduction and History. Amsterdam, Netherlands: Elsevier Science. ISBN 9780080548883.
- Loveland, W.; Morrissey, D.; Seaborg, G.T. (2006). Modern Nuclear Chemistry. Wiley-Interscience. ISBN 978-0-471-11532-8.
- Martin, B.R. (2011). Nuclear and Particle Physics: An Introduction (2nd ed.). John Wiley & Sons. ISBN 978-1-1199-6511-4.
- Soddy, Frederick (1913). “The Radio Elements and the Periodic Law.” Chem. News. Nr. 107, pp. 97–99.
- Stabin, Michael G. (2007). Radiation Protection and Dosimetry: An Introduction to Health Physics. Springer. doi:10.1007/978-0-387-49983-3 ISBN 978-0-387-49982-6.