Beta decay is one of the forms of radioactive decay, where the nucleus of an atom transforms into a different nuclear species while emitting an electron or positron(beta particle). Unlike alpha decay, where part of the nucleus is simply ejected, in beta decay one of the protons or neutrons that make up the nucleus is transformed into a neutron or proton, respectively. This transformation changes the nucleus into that of a new element but leaves the total number of constituents, called the mass number (A), unchanged.
Discovery and Significance
Shortly after Becquerel discovered the existence of radioactivity, it was shown that there were three types of radioactive emission: positively-charged alpha particles, negatively-charged beta particles, and neutral gamma rays. It was soon discovered that the beta particle was an electron, but properties of beta decay remained a strong driver for theoretical physics through the first half of the 20th century.
The primary mystery in beta decay dealt with the energy of the emitted electron. The energy difference between the initial nucleus and the product nucleus is a fixed value, so if the electron were emitted on its own, it would always have the same energy for any given decay. What Lise Meitner and Otto Hahn discovered in 1911, though, was that the beta particle had a smoothly varying spectrum of energies ranging from zero to the full amount of the nuclear energy difference, seemingly violating conservation of energy.
This result led to significant controversy in the then-emerging field of particle physics, with some even ready to abandon conservation of energy itself to explain beta decay. The final solution to the problem was first published by Wolfgang Pauli in 1930, positing that a second, invisible particle was emitted along with the electron, carrying away a variable fraction of the energy. This particle, which Pauli called the 'neutron', was included in Enrico Fermi's successful theory of beta decay, where it was renamed the neutrino due to the pre-existing use of the word 'neutron'.
Fermi's theory was the basis for a more comprehensive theory of what became known as the weak nuclear force. An important feature of weak theory that was discovered in beta decay by Chien-Shiung Wu is parity violation; beta particles are not emitted equally in all directions but have a preferred direction relative to the spin axis of the nucleus. As beta decay is the classic weak nuclear process, and the only one that occurs at low energy, precision measurements of beta decay continue to be done to refine the limits of the theory.
Beta decay has several technical applications. The range of beta particles in matter is short, so small samples of beta-emitting isotopes can be implanted in a cancerous tumor to wreak havoc with the cancer cells while avoiding damage to the healthy surrounding tissue. β+ decay is the best-known source of positrons, and is used in this capacity as a source for particle physics experiments and for medical imaging through Positron Emission Tomography (PET). As beta particles are high-energy electrons, it is possible to capture them and use them to power electrical circuitry; this technology is known as betavoltaics.
Properties of Beta Decay
The simplest form of beta decay is that of a free neutron. Since the neutron is slightly heavier than the proton (by about .1% of its mass), it will tend to decay into a proton thus minimizing its energy. To do this, one of the down quarks in the neutron must transform into an up quark, increasing its electric charge by one unit. Thus, to satisfy conservation of charge, it must emit a negatively-charged electron, and then to satisfy conservation of lepton number it must also emit an electron antineutrino. This process is mediated by the weak nuclear force and is usually referred to as β- decay due to the negative charge of the emitted beta particle.
Beta decay of a nucleus is similar, connecting nuclear species with the same mass number but different proportions of neutrons to protons, referred to as isobars. (The free proton and neutron can be thought of as the two A=1 isobars.) The equilibrium proportion of neutrons and protons is where the total energy of the nucleus (and thus its mass by E=mc2) is a minimum, so other isobars will tend to decay towards this (most tightly bound) nucleus. For isobars with too many neutrons, the beta decay process proceeds through the same β- process as the free neutron, but the decay of the proton-rich nuclei proceeds through an analogous process where a positron and electron neutrino are emitted, called β+ decay.
The usual picture of beta decay in the context of nuclear physics is that each set of isobars gradually decays into the most tightly-bound species, and thus only these energy minima constitute stable states of the system. However, there are several ways this can be prevented. The simplest case is when two neighbouring isobars have masses that differ by less than the electron mass. In this case, it is impossible to emit a beta particle as there is insufficient energy to create one. For proton-rich isobars, it is still possible to reach the lower-energy state through the related process of electron capture. Since this requires that there be an electron bound to the nucleus for it to capture, there is no analogous process for neutron-rich isobars due to the lack of available positrons.
A more interesting situation is where the binding energy does not change smoothly with proton number, but rather has species that are separated from the next-lowest energy species by a species with higher energy. This happens quite frequently with even-A nuclei, as 'paired' neutrons or protons are more tightly bound than unpaired neutrons or protons. Thus two even-even nuclei can easily be separated by an odd-odd nucleus with higher energy than either of the even-even nuclei. In this case normal beta decay of the higher-energy even-even nucleus is impossible, but there is a very small probability of two beta decays occurring simultaneously in a process called double beta decay, which is under active study due to its interesting implications for neutrino physics.