Radioactivity

Radioactivity

Natural radioactive decay of atoms has been happening on the earth

ever since it was formed, over 4000 million years ago. Radioactivity

is a property exhibited by certain types of matter of emitting energy

and subatomic particles spontaneously. It is, in essence, an attribute

of individual atomic nuclei.

An unstable nucleus will decompose spontaneously, or decay, into a

more stable configuration but will do so only in a few specific ways

by emitting certain particles or certain forms of electromagnetic

energy. Radioactive decay is a property of several naturally occurring

elements as well as of artificially produced isotopes of the elements.

The rate at which a radioactive element decays is expressed in terms

of its half-life; i.e., the time required for one-half of any given

quantity of the isotope to decay. Half-lives range from more than

1,000,000,000 years for some nuclei to less than 10-9 second. The

product of a radioactive decay process--called the daughter of the

parent isotope--may itself be unstable, in which case it, too, will

decay. The process continues until a stable nuclide has been formed.

The discovery of radioactivity in uranium by the French physicist

Antoine Henri Becquerel (1852 - 1908) in 1896 forced scientists to

radically change their ideas about atomic structure. Radioactivity

demonstrated that the atom was neither indivisible nor immutable.

Instead of serving merely as an inert matrix for electrons, the atom

could change form and emit an enormous amount of energy. Furthermore,

radioactivity itself became an important tool for revealing the

interior of the atom.

The German physicist Wilhelm Conrad Röntgen (1845 - 1923) had

discovered X rays in 1895, and Becquerel thought they might be related

to fluorescence and phosphorescence, processes in which substances

absorb and emit energy as light. In the course of his investigations,

Becquerel stored some photographic plates and uranium salts in a desk

drawer. Expecting to find the plates only lightly fogged, he developed

them and was surprised to find sharp images of the salts. He then

began experiments that showed that uranium salts emit a penetrating