A radionuclide has an unstable combination of nucleons and emits radiation in the process of regaining stability.Reaching stability involves the process of radioactive decay.Each one of these decay types may also involve the release of one or more photons of gamma radiation.

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In alpha decay, the nucleus ejects a helium nucleus (alpha particle) composed of two neutrons and two protons, dropping the mass of the original nucleus by four mass units.

A decay, also known as a disintegration of a radioactive nuclide, entails a change from an unstable combination of neutrons and protons in the nucleus to a stable (or more stable) combination. Radioactive atoms decay principally by alpha decay, negative beta emission, positron emission, and electron capture.

The type of decay determines whether the ratio of neutrons to protons will increase or decrease to reach a more stable configuration. How does the neutron-to-proton number change for each of these decay types?

In the process of this conversion, a beta particle with a negative charge is then ejected from the nucleus. In positron decay, the opposite situation occurs: the proton to neutron ratio is greater than desired.

Accordingly, a proton is converted into a neutron and a beta particle (but with a positive charge! Again, the nucleus remains the same size, but the number of protons decreases by one. Electron capture results in the same outcome as positron decay in that, in this process, the nucleus stays the same size and the number of protons decreases by one.

In this type of decay, however, the nucleus captures an electron and combines it with a proton to create a neutron.

X-rays are given off as other electrons surrounding the nucleus move around to account for the one that was lost.

This smaller nucleus is easier to keep in a stable form. In negative beta decay, the nucleus contains an excess of neutrons.

To correct this unstable condition, a neutron is converted into a proton, which keeps the nucleus the same size (i.e., the same atomic mass) but increases the number of protons (and therefore the atomic number) by one.

The purpose of this chapter is to explain the process of radioactive decay and its relationship to the concept of half-life.

The primary intent is to demonstrate how the half-life of a radionuclide can be used in practical ways to “fingerprint” radioactive materials, to “date” organic materials, to estimate the age of the earth, and to optimize the medical benefits of radionuclide usage. Remember that a radionuclide represents an element with a particular combination of protons and neutrons (nucleons) in the nucleus of the atom.