The Science Book

A CENTURY OF PROGRESS 195

isotope has a half-life of 5,730 years.
Carbon-14 is constantly being
produced in the upper atmosphere
as nitrogen atoms are bombarded
with cosmic rays. This means
there is a relatively constant ratio
of carbon-12 to carbon-14 in the
atmosphere. Since photosynthesizing
plants take in carbon dioxide from
the atmosphere, and our food
consists of plants (or animals that
have eaten plants), there is also a
relatively constant proportion in
plants and animals while they are
alive, even though the carbon-14
is constantly decaying. When an
organism dies, no more carbon-14
is taken into its body, while the
carbon-14 already there continues
to decay. By measuring the ratio of
carbon-12 to carbon-14 in the body,
scientists can figure out how long
ago the organism died.
This radiometric method is used
to date wood, charcoal, bone, and
shells. There are natural variations
in the ratios of the carbon isotopes,
but dates can be cross-checked

with other dating methods such

as tree rings, and the corrections

applied to objects of similar age.

A wonder treatment

Curie realized that radioactivity

had medicinal uses. During World

War I, she used the small amount

of radium she had extracted to

The Curie laboratory...

was a cross between a stable

and a potato-cellar, and, if I

had not seen the worktable

with the chemical apparatus,

I would have thought it

a practical joke.

Wilhelm Ostwald

produce radon gas (a radioactive

gas produced when radium

decays). This was sealed into glass

tubes and inserted into patients’

bodies to kill diseased tissue.

It was seen as a wonder cure,

and even marketed in beauty

treatments to help firm up aging

skin. It was only later that the

importance of using materials with

a short half-life was recognized.

Radioactive isotopes are also

widely used in medical imaging to

diagnose disease, and in treatment

of cancer. Gamma rays are used to

sterilize surgical instruments, and

even food, to increase its shelf life.

Gamma ray emitters can be used

for the internal inspection of metal

objects, to detect cracks, or to

inspect the contents of cargo

containers to identify contraband. ■

The erection of Ale’s stones in

Sweden was dated to 600 CE by the

radiometric dating of wooden tools

found at the site. The actual stones are

hundreds of millions of years older.