Alpha Beta Gamma rays

Radioactive nuclei emit three types of radiations

Physicists have called he three types of radiations emitted by nuclei, alpha, beta and gamma.

Map of decay modes
This map of the various nuclei is coloured with regards to the types of decay they undergo. Stable nuclei, self-evidently found along the stability line, are in black. The beta-emitters can be found on either side of this curve – beta negatives (in blue) occur in neutron-heavy nuclei, whereas beta-positives (in orange) occur in proton-heavy nuclei. At the right-hand side of the graph, the line of stability is replaced by a zone where alpha-emitters (in yellow) predominate. One can also see some exceedingly heavy particles (very far from the line of stability) which undergo spontaneous fission (in green), and a few nuclei which emit either protons (in red) or neutrons (in purple)

Naming the three types of radiation alpha, beta and gamma is a convention that has been in use since their discovery, and still applies today. Alpha radiation is the name for the emission of an alpha particle in fact an helium nuclei, beta radiation is the emission of electrons or positrons, and gamma radiation is the term used for the emission of energetic photons.

When uranium salts were found in 1896 to produce unknown emissions, two additional types of radiation, X-rays and cathode rays, have been just discovered. At that time, nuclei, electrons and photons were unknown. It would take decades before the origins of all these rays of were properly understood, but a few years to identify their nature. Incidentally cathode rays and X-rays were found to be electrons and photons like beta and gamma radiations.

Disintegration diagram
This nucleus of radium-226, the one discovered by Marie Curie, decays directly in 93 % of the cases in a nucleus of radon by emitting an alpha particle. However in 7 % of the cases, the nucleus is left in an excited state. Very rapidly, the excited radon nucleus get rid of its excess of energy, by emitting a gamma ray. Physicists represents the various decays modes of a nucleus by decay charts similar to the one represented on the right. The emission of gamma very often accompany the transformations of nuclei.
Alpha, beta and gamma decay are a result of the three fundamental forces working in the nucleus – the ‘strong’ force, the ‘weak’ force and the electromagnetic force. In all three cases, the emission of radiation increases the nucleus’s stability, by adjusting its proton : neutron ratio.

In the case of alpha radiation, the nucleus attempts to find stability by emitting an ‘alpha particle’ – identical to a helium nucleus (two protons and two neutrons).

Beta radiation involves the transformation of a proton into a neutron through the emission of an electron, or the reverse process through the emission of a positron (identical to an electron, but with positive charge).

Gamma radiation is simply a loss of energy by the nucleus; much like an emission of light or X-rays by energetic atoms. This type of radiation is usually instantaneous, and brings the nucleus down to a more stable energetic state. As the previous two types of decay involve an increase in nucleus energy, gamma rays almost always follow the emission of an alpha- or beta- particle.

Alpha and beta decay are comparatively infrequent. As a result, the lifetimes of such radioactive particles as uranium or thorium can extend to several billions of years.

All three types of radiation change the structure of the nucleus, and hence the nature of the atom. While no type of decay can transform one element into another – such as changing lead into gold – any reaction occurring in the nucleus does indeed cause a transmutation of matter.

Access to page in french