Decay Modes in Table Form
Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with mass number A and atomic number Z is represented as (A, Z). The column "Daughter nucleus" indicates the difference between the new nucleus and the original nucleus. Thus, (A − 1, Z) means that the mass number is one less than before, but the atomic number is the same as before.
| Mode of decay | Participating particles | Daughter nucleus |
|---|---|---|
| Decays with emission of nucleons: | ||
| Alpha decay | An alpha particle (A = 4, Z = 2) emitted from nucleus | (A − 4, Z − 2) |
| Proton emission | A proton ejected from nucleus | (A − 1, Z − 1) |
| Neutron emission | A neutron ejected from nucleus | (A − 1, Z) |
| Double proton emission | Two protons ejected from nucleus simultaneously | (A − 2, Z − 2) |
| Spontaneous fission | Nucleus disintegrates into two or more smaller nuclei and other particles | — |
| Cluster decay | Nucleus emits a specific type of smaller nucleus (A1, Z1) smaller than, or larger than, an alpha particle | (A − A1, Z − Z1) + (A1, Z1) |
| Different modes of beta decay: | ||
| β− decay | A nucleus emits an electron and an electron antineutrino | (A, Z + 1) |
| Positron emission (β+ decay) | A nucleus emits a positron and an electron neutrino | (A, Z − 1) |
| Electron capture | A nucleus captures an orbiting electron and emits a neutrino; the daughter nucleus is left in an excited unstable state | (A, Z − 1) |
| Bound state beta decay | A nucleus beta decays to electron and antineutrino, but the electron is not emitted, as it is captured into an empty K-shell; the daughter nucleus is left in an excited and unstable state. This process is suppressed except in ionized atoms that have K-shell vacancies. | (A, Z + 1) |
| Double beta decay | A nucleus emits two electrons and two antineutrinos | (A, Z + 2) |
| Double electron capture | A nucleus absorbs two orbital electrons and emits two neutrinos – the daughter nucleus is left in an excited and unstable state | (A, Z − 2) |
| Electron capture with positron emission | A nucleus absorbs one orbital electron, emits one positron and two neutrinos | (A, Z − 2) |
| Double positron emission | A nucleus emits two positrons and two neutrinos | (A, Z − 2) |
| Transitions between states of the same nucleus: | ||
| Isomeric transition | Excited nucleus releases a high-energy photon (gamma ray) | (A, Z) |
| Internal conversion | Excited nucleus transfers energy to an orbital electron, which is subsequently ejected from the atom | (A, Z) |
Radioactive decay results in a reduction of summed rest mass, once the released energy (the disintegration energy) has escaped in some way (for example, the products might be captured and cooled, and the heat allowed to escape). Although decay energy is sometimes defined as associated with the difference between the mass of the parent nuclide products and the mass of the decay products, this is true only of rest mass measurements, where some energy has been removed from the product system. This is true because the decay energy must always carry mass with it, wherever it appears (see mass in special relativity) according to the formula E = mc2. The decay energy is initially released as the energy of emitted photons plus the kinetic energy of massive emitted particles (that is, particles that have rest mass). If these particles come to thermal equilibrium with their surroundings and photons are absorbed, then the decay energy is transformed to thermal energy, which retains its mass.
Decay energy therefore remains associated with a certain measure of mass of the decay system invariant mass. The energy of photons, kinetic energy of emitted particles, and, later, the thermal energy of the surrounding matter, all contribute to calculations of invariant mass of systems. Thus, while the sum of rest masses of particles is not conserved in radioactive decay, the system mass and system invariant mass (and also the system total energy) is conserved throughout any decay process.
Read more about this topic: Radioactive Source
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