Whole body counters are sensitive radiation detecting and measuring
instruments that provide information not easily obtainable otherwise
about that most important of all chemical systems, the human body. They
can do this because, strange as it may seem, every person who ever lived
is slightly radioactive.
Quickly, accurately, and painlessly, whole body counters reveal the
kinds and amounts of radioactive substances that have accumulated in the
body from natural sources, from man-made fallout, or from tracer
isotopes given for medical purposes. They count emissions from these
radioactive materials, as do other kinds of instruments known as
“counters”.
In contrast to devices that disclose concentrations of radioactivity in
a small area or a particular organ, whole body counters usually are used
to total up the burden of radioactivity in all parts of a human body.
They also are distinguished from many radiation detecting instruments
with the same general purpose by their large size, their heavy
shielding, and their sensitivity to low levels of radioactivity.
Whole body counters are useful in many studies of physiological activity
in living persons and animals. They have proved valuable in calculating
the radiation absorbed by victims of overexposure to radioactive
materials. They can show a doctor how much of his patient’s body is fat
and how much lean. Whole body counters also gave medical scientists
clues to the relation of potassium deficiency to muscular dystrophy and
other diseases. And new medical and scientific uses are being found
regularly.
The need for an instrument that would measure whole body radioactivity
was first felt in the 1920s when the hazardous nature of radium was
recognized. Other sorts of instruments had to be used to estimate the
amount of radium that some factory workers inadvertently had absorbed
while painting luminous watch dials with a radium-containing coating.
(See pages 15 and 16.) But instruments then available were without
adequate shielding to eliminate background radiation, and so the
measurement efforts were of limited value.
It was not until the 1950s that new types of radiation-detecting
instruments were designed, making use of the discovery that some
crystals, liquids, and plastics give off light when struck by gamma rays
(one form of radioactive emission). Two principal types of instruments
have been developed to detect these emissions in human tissues.
The most common whole body counter employs a sodium iodide crystal as
the radiation detector. The person being examined usually sits in a
tilted chair in a room that has thick steel walls to absorb background
radiation. During the counting period, the crystal is centered a few
inches above the subject. This type is useful for examination involving
low levels of radiation or emissions from more than one kind of
radioactive atom.
In the other type the subject is surrounded by a tank of a liquid that
detects gamma rays. This type is faster, but less sensitive, than the
crystal type.
This booklet is intended to enable you to make imaginary visits to
several whole body counters, to understand the scientific principles
that are applied in their design, to learn the interesting ways they are
used, and to appreciate the promise they hold for increasing our
knowledge of ourselves and the world we live in.