If you’ve heard of substances emitting radiation (also known as radioactivity), you may have wondered what causes it. The reason goes down to the nuclear level, and it comes down to how the strong nuclear forces fare against repulsion forces in an atomic nucleus.
The Forces Within A Nucleus
In an atomic nucleus, which is a very tight bundle of protons and neutrons, there are a few forces at play. The first of them is the strong nuclear force. This force uses particles known as gluons to keep the protons and neutrons together in the nucleus. Then there are the repulsion forces. The nucleus contains many protons very close together — and as these protons are of the same charge (positive), they repel each other.
In the nuclei of most of the atoms we see today, the attraction forces can keep the repulsions at bay. However, as atomic numbers get larger and there are more protons repelling each other, the repulsion forces can at times overpower the attractions — and the nucleus is in an unstable state, causing a decay to occur which pushes it back to a more stable state eventually. An excess of protons or neutrons could also increase the internal energy of the nucleus and make it unstable.
And sometimes things can happen inside the individual particles in the nucleus too. The weak force allows the quarks inside the nucleons to change their flavors — basically, for example, a down quark can change to an up quark, turning a neutron into a proton. An up quark can also change into a down quark, turning a proton into a neutron. The energy released in this conversion is converted into matter and split into an electron or positron, and a neutrino. This is the basis of the beta decay, which we’ll talk about later.
Types of Radioactive Decay
There are a diverse range of of radioactive decays that can happen to a nucleus. Common examples include:
- alpha decay (emits a helium-4 nucleus with two protons and two neutrons; example: uranium-238)
- beta decay (splits a neutron into a proton, an electron, and a neutrino, occurs when there is an excess of neutrons (example: carbon-14); or splits a proton into a neutron, a positron, and a neutrino, occurs when there is an excess of protons)
- electron capture (combines a proton and electron into a neutron, occurs when there is an excess of protons; for example: potassium-40 (11% chance))
Other types of decays include:
- proton / neutron emission (for nuclei with very high proton-to-neutron or neutron-to-proton ratios, to balance the numbers of protons and neutrons)
- spontaneous fission (some nuclei split itself spontaneously into two lighter nuclei of comparable mass)
How Fast Does Radioactive Decay Occur?
The speed of radioactive decay depends on the radioactive isotope that is being decayed. And since the decay is a random event, it is best measured by its half-life — the time it takes for half of a particular substance to decay. The longer the half-life, the more stable the isotope.
The most stable radioactive isotope ever found is bismuth-209 (with 83 protons and 126 neutrons), with a half-life of about 2 * 10^19 years. The least stable ones are often those with very low or very high proton-to-neutron ratio (i.e., with too many protons or neutrons), such as hydrogen-7 (1 proton to 7 neutrons). These isotopes are often synthesized in the lab instead of being found in nature because of their extreme instability.
Speaking of the randomness of the radioactive decay event, there could also be multiple paths that an isotope can take on in its decay chain (the sequence of decay products). For example, the isotope potassium-40 has an approximately 89% chance that it will go through beta decay to become calcium-40, turning a neutron into a proton and increasing its atomic number by 1 (while keeping its mass number unchanged). However, 11% of the time, it goes through an electron capture and becomes argon-40 (where a proton encounters an electron and is converted into a neutron).
Conclusion
In this article, we’ve talked about why some isotopes are radioactive, and it’s because in some atomic nuclei, the combination of the attraction (strong force), the repulsion forces (e.g., electrostatic repulsion), and the weak force are unstable. This leads to a radioactive decay, which is a random event whose rate is measured by the half-life of that isotope. If you have any questions about this article, please leave them in the comments below.