Types of Decays

Alpha Decay

The emission of an alpha particle, or 4He nucleus, is a process called alpha decay. Since alpha particles contain protons and neutrons, they must come from the nucleus of an atom. The nucleus that results from alpha decay will have a mass and charge different from those of the original nucleus. A change in nuclear charge means that the element has been changed into a different element. Only through such radioactive decays or nuclear reactions can transmutation, the age-old dream of the alchemists, actually occur. The mass number, A, of an a particle is four, so the mass number, A, of the decaying nucleus is reduced by four. The atomic number, Z, of 4He is two, and therefore the atomic number of the nucleus, the number of protons, is reduced by two. This can be written as an equation analogous to a chemical reaction. For example, for the decay of an isotope of the element seaborgium, 263Sg:

263Sg ----> 259Rf + 4He

The atomic number of the nucleus changes from 106 to 104, giving rutherfordium an atomic mass of 263-4=259a decay typically occurs in heavy nuclei where the electrostatic repulsion between the protons in the nucleus is large. Energy is released in the process of a decay. Careful measurements show that the sum of the masses of the daughter nucleus and the a particle is a bit less than the mass of the parent isotope. Einstein's famous equation, E=mc2, which says that mass is proportional to energy, explains this fact by saying that the mass that is lost in such decay is converted into the kinetic energy carried away by the decay products.

Beta Decay

Beta particles are negatively charged electrons emitted by the nucleus. Since the mass of an electron is a tiny fraction of an atomic mass unit, the mass of a nucleus that undergoes beta decay is changed by only a tiny amount. The mass number is unchanged. The nucleus contains no electrons. Rather, beta decay occurs when a neutron is changed into a proton within the nucleus. An unseen neutrino,v, accompanies each beta decay. The number of protons, and thus the atomic number, is increased by one. For example, the isotope 14C is unstable and emits β particles, becoming the stable isotope 14N:

14C ----> 14N + e- + v

In a stable nucleus, the neutron does not decay. A free neutron, or one bound in a nucleus that has an excess of neutrons, can decay by emitting a beta particle. Sharing the energy with the beta particle is a neutrino. The neutrino has little or no mass and is uncharged, but, like the photon, it carries momentum and energy. The source of the energy released in beta decay is explained by the fact that the mass of the parent isotope is larger than the sum of the masses of the decay products. Mass is converted into energy just as Einstein predicted.

Gamma Decay

Gamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus. A gamma ray is a high energy photon. The only thing which distinguishes a gamma ray from the visible photons emitted by a light bulb is its wavelength; the gamma ray's wavelength is much shorter. For complex nuclei there are many different possible ways in which the neutrons and protons can be arranged within the nucleus. Gamma rays can be emitted when a nucleus undergoes a transition from one such configuration to another. For example, this can occur when the shape of the nucleus undergoes a change. Neither the mass number nor the atomic number is changed when a nucleus emits a gamma ray in the reaction.

152Dy* ----> 152Dy + γ

This whole page is provided by Lawrence Berkeley National Laboratory

The Nuclear Equation