Beta decay is a radioactive process in which a nucleus transforms by emitting an electron (beta-minus) or a positron (beta-plus). This process occurs to adjust the neutron-to-proton ratio for better stability.
In beta-minus decay, a neutron transforms into a proton, an electron, and an antineutrino. The energy released (Q-value) is shared among these three particles.
Unlike alpha particles, which have discrete energies, beta particles are emitted with a continuous range of energies. The energy varies from zero up to a maximum value (the endpoint energy), which corresponds to the total Q-value of the reaction.
These are alternative modes of beta decay primarily observed in proton-rich nuclei.
The continuous nature of the beta spectrum and the apparent violation of energy and angular momentum conservation led Wolfgang Pauli to propose the Neutrino Hypothesis in 1930.
Definition: A neutrino is an elementary particle with extremely small (or zero) mass, no electric charge, and spin 1/2. It carries away the "missing" energy and momentum in beta decay.
Due to their weak interaction with matter, neutrinos are incredibly difficult to detect. The Rein & Cowans experiment (1956) provided the first experimental evidence of neutrinos using a large tank of water and detecting the results of inverse beta decay.
After alpha or beta decay, the daughter nucleus is often left in an excited state. It reaches the ground state by emitting high-energy electromagnetic radiation called Gamma Rays.
Gamma rays are emitted with discrete energies corresponding to the difference between nuclear energy levels.
Instead of emitting a gamma ray, the excited nucleus may transfer its energy directly to an inner orbital electron (K or L shell), which is then ejected from the atom. These are called Conversion Electrons.
A nuclear reaction occurs when a target nucleus is bombarded by a projectile particle, resulting in a change in the nucleus.
In every nuclear reaction, the following must be conserved:
The Q-value is the net energy released or absorbed in a nuclear reaction.
The Scattering Cross-Section (σ) is a measure of the probability that a specific nuclear reaction will occur when a target is hit by a beam of particles.
Concept: It is imagined as an "effective area" presented by the target nucleus to the incoming projectile. Its unit is the barn (1 barn = 10^-28 m^2).
Note on Threshold Energy: For endoergic reactions, the threshold energy is always slightly higher than the absolute value of Q because some energy must go into the kinetic energy of the products to conserve momentum.