Radioactivity is the spontaneous emission of energy from atoms in the form of particles or waves. Emission comes from the nucleus through the three different types of nuclear decay. Unstable atoms try to reach a stable form and will decay until that is accomplished. A familiar phenomenon of nuclear decay is half-life. An isotope’s half-life is the measure of the time it takes for half of its mass to decay. When an isotope undergoes nuclear decay (alpha or beta), it turns into another element. This is transmutation and is useful for nuclear power, among other things.
Alpha decay occurs when a nucleus emits 2 protons and 2 neutrons. This is the equivalent of a helium nucleus, which in itself acts as a quantum particle. Every element has an equal amount of protons as they do electrons. Since elements are defined by the number of protons and electrons they have, the original element transmutes into another one. In the image above, Uranium-238 undergoes alpha decay, losing two protons and two neutrons. The original atom is now Thorium-234.
Nuclear reactions focus on what is going on in the nucleus, i.e., it ignores the electrons. However, taking a look at this reaction, the thorium would actually be an anion with a -2 charge. The alpha particle, or helium nucleus, is a cation with a +2 charge. Eventually, the electrons on the thorium either react chemically or attach to the alpha particles. This process is much slower than the alpha decay.
During beta decay, a neutron will split into two particles: either an electron and an antineutrino, or a positron and a neutrino. Positrons are the antiparticles of electrons and thus have the same mass but opposite charge of electrons. Neutrons have more mass than protons, so the remaining mass after the particle emission is roughly equal to that of a proton.
- Beta-minus decay involves the emission of an electron and an antineutrino. The neutron emits an electron and now has a positive charge. Since it is now a proton, the element changes.
- Beta-plus decay involves the emission of a positron and a neutrino. The proton emits a positron and now has a neutral charge. Since the atom now has one less proton, the element changes.
Gamma decay is almost always seen with alpha and beta decay. This occurs when a nucleus emits a high-energy gamma photon (or gamma ray, depending on how you’d like to look at it). If only energy is released and the nucleus remains with the same number of particles, how is this possible? Atoms like to arrange their particles in a way that reduces the energy as much as possible. If a proton needs to shift a little to reduce the overall energy, that energy must go somewhere. Nuclear forces are very strong and have high energies associated with them, so it makes sense for such a high-energy particle (or wave) to be emitted. To give some perspective, the strong force, which allows protons and neutrons to be close together in the nucleus, is 1038 times stronger than gravity. The high energy of this decay is dangerous, but has been used for radiation therapy to fight cancer.