Beta Decay
Introduction
Beta decay is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus. This process allows the atom to move closer to the optimal ratio of protons and neutrons. Beta decay is a common mode of radioactive decay for isotopes with a high neutron-to-proton ratio.
Types of Beta Decay
There are two types of beta decay: beta-minus decay and beta-plus decay.
Beta-Minus Decay
In beta-minus decay, a neutron in an atomic nucleus is transformed into a proton, an electron, and an electron antineutrino. The electron and the antineutrino are emitted from the nucleus, while the proton remains. This type of decay is observed in neutron-rich nuclei.
Beta-Plus Decay
In beta-plus decay, also known as positron emission, a proton in the nucleus is transformed into a neutron, a positron, and an electron neutrino. The positron and the neutrino are emitted from the nucleus, while the neutron remains. This type of decay is observed in proton-rich nuclei.
Conservation Laws in Beta Decay
Beta decay, like all other forms of decay, must obey a number of conservation laws. These include the conservation of electric charge, the conservation of linear and angular momentum, and the conservation of energy. The conservation of lepton number is also crucial in beta decay, as it differentiates between the emission of an electron and a positron.
The Weak Interaction
Beta decay is mediated by the weak interaction, one of the four fundamental forces of nature. The weak interaction is responsible for the transformation of one type of quark into another, which is the underlying process in beta decay. The weak interaction is unique among the fundamental forces in its ability to change the flavor of particles, a property that is crucial for the occurrence of beta decay.
The Neutrino in Beta Decay
The neutrino plays a crucial role in beta decay. In beta-minus decay, an electron antineutrino is emitted, while in beta-plus decay, an electron neutrino is emitted. The neutrino carries away a variable amount of the energy released in the decay, which results in a continuous energy spectrum for the beta particles. The discovery of the neutrino was a direct result of studies of beta decay.
Fermi's Theory of Beta Decay
Enrico Fermi proposed the first successful theory of beta decay in 1933. Fermi's theory, which is now known as the theory of the weak interaction, describes beta decay as a four-fermion interaction, involving a neutron, a proton, an electron (or positron), and a neutrino (or antineutrino). Fermi's theory was later incorporated into the electroweak theory, which unifies the weak interaction with electromagnetism.
Beta Decay and the Neutrino Mass
Studies of beta decay have provided important insights into the properties of neutrinos, including their mass. The observation of neutrino oscillations, which implies that neutrinos have mass, has important implications for beta decay. In particular, it opens up the possibility of neutrinoless double beta decay, a hypothetical process that would violate the conservation of lepton number.
Double Beta Decay
Double beta decay is a radioactive decay process where a nucleus decays by emitting two beta particles simultaneously. This process can occur in two ways: with or without the emission of neutrinos. The neutrinoless double beta decay, if observed, would be a clear indication of new physics beyond the Standard Model.
Applications of Beta Decay
Beta decay has a number of practical applications. It is used in carbon dating, a method for determining the age of organic materials. Beta decay is also used in medical imaging and therapy, in particular in the form of beta-emitting isotopes. In addition, beta decay provides a source of neutrinos, which are used in neutrino detectors for studying the properties of these elusive particles.