Biology Reference
In-Depth Information
The key phrase here is “relative age,” because so far the methods dis-
cussed reveal the kind, sequence, and arrangement of the rocks and the bio-
logical information they contain; that is, they reveal ways of putting things
in the right chronological order. Assigning an age in years, however, had to
await the discovery of radioactivity and the development of absolute dating
techniques.
Radiometric Dating
This broad category of techniques is based on the property of certain un-
stable variants (isotopes) of elements to decay (decompose) by emitting
packets of energy. One form is a particle consisting of two protons and two
neutrons. This emission is called alpha radiation. It was discovered by the
nuclear physicist Ernest Rutherford (1871-1937), winner of the 1908 No-
bel Prize in Chemistry. Among elements emitting alpha particles are plu-
tonium, uranium, thorium, radium, and radon. Another form of radiation
consists of electrons (beta radiation), also discovered by Rutherford, and
emitted, for example, by isotopes of potassium. When these particles are
lost, the nucleus is often in an “excited state.” That is, there is an imbalance
of energy between the nucleus and the electron shells, and this energy is
given off, for example, in cobalt, in the form of electromagnetic radiation,
or gamma rays. In 1900 gamma radiation was discovered by physicist Paul
Villard (1860-1934), working in France at the time of Marie and Pierre
Curie.
Many isotopes are familiar because they have practical applications
in medicine (e.g., radium). Others are less well known, but among alpha
emitters are polonium, prominent recently in the Russian spy news and
used to remove static charges that develop during the manufacture of pa-
per, and americium, which generates the current in some smoke detectors.
This form of radiation travels relatively slowly (one-twentieth the speed of
light), dissipates in a few centimeters, and has little penetrating power. It
is stopped, for example, by the outer layer of dead skin and by materials
the thickness of a sheet of paper. If inhaled or taken internally, however, it
causes severe damage—hence the monitoring of radon levels in homes and
in the workplace.
Beta radiation from strontium is used to treat bone and eye cancers, and
other beta particles track thickness during the manufacture of materials
such as paper. Gamma rays have great penetrating power. They can pass, for
example, through a centimeter of aluminum, are used in the sterilization of
