a device used to detect radioactive
radiations, such as those given off in the
release of nuclear energy
The German physicist Hans
Geiger and the British scientist Sir
developed the first radiation counter in 1908.
Geiger counters in use today are based on designs
made by Geiger and E.W. Müller in the 1920's.
A Geiger counter usually
consists of a glass tube about 3/4 inch in
diameter enclosing a metal cylinder, often of
copper, about 4 inches long along the axis of
which runs a thin metal wire, which is often of
tungsten. The cylinder and wire are connected
through the end wall of the glass tube to a
source of electrical voltage. The tube is filled
with a gas, usually argon, at a low pressure,
equivalent to a pressure of a few centimeters of
mercury. A voltage is set up between the cylinder
(the negative electrode or cathode) and the wire
(the positive electrode or anode) which is just a
little less than that needed to create an
electrical discharge between the two electrodes.
How It Works
When a charged particle with
high energy flies through the glass tube it
knocks electrons out of the atoms of the gas.
These electrons, being negatively charged, make
for the wire anode and the damaged atoms, which
are positively charged argon ions, make for the
cathode. The electrons pick up enough energy to
knock more electrons out of atoms, which in turn
pick up further energy and liberate even more
electrons. This is known as electron
avalanche. At the same time, the positive
ions hit the cylinder with enough energy to
release still more electrons. An avalanche of
electrons therefore descends on the wire, which
can be detected as a pulse of electric current,
indicating that a charged particle has passed
through the tube. This pulse is revealed by light
signals, by clicks picked up by earphones, or by
readings on a meter.
The Illustrated Science and Invention
Encyclopedia New York: H.S.
Stuttman Co., Inc., 1977
Sir Ernest Rutherford
Questions or comments about