Geiger counter

The invention: the first electronic device able to detect and measure
radioactivity in atomic particles.
The people behind the invention:
Hans Geiger (1882-1945), a German physicist
Ernest Rutherford (1871-1937), a British physicist
Sir John Sealy Edward Townsend (1868-1957), an Irish physicist
Sir William Crookes (1832-1919), an English physicist
Wilhelm Conrad Röntgen (1845-1923), a German physicist
Antoine-Henri Becquerel (1852-1908), a French physicist
Discovering Natural Radiation
When radioactivity was discovered and first studied, the work
was done with rather simple devices. In the 1870’s, Sir William
Crookes learned how to create a very good vacuum in a glass tube.
He placed electrodes in each end of the tube and studied the passage
of electricity through the tube. This simple device became known as
the “Crookes tube.” In 1895, Wilhelm Conrad Röntgen was experimenting
with a Crookes tube. It was known that when electricity
went through a Crookes tube, one end of the glass tube might glow.
Certain mineral salts placed near the tube would also glow. In order
to observe carefully the glowing salts, Röntgen had darkened the
room and covered most of the Crookes tube with dark paper. Suddenly,
a flash of light caught his eye. It came from a mineral sample
placed some distance from the tube and shielded by the dark paper;
yet when the tube was switched off, the mineral sample went dark.
Experimenting further, Röntgen became convinced that some ray
from the Crookes tube had penetrated the mineral and caused it to
glow. Since light rays were blocked by the black paper, he called the
mystery ray an “X ray,” with “X” standing for unknown.
Antoine-Henri Becquerel heard of the discovery of X rays and, in
February, 1886, set out to discover if glowing minerals themselves
emitted X rays. Some minerals, called “phosphorescent,” begin to
glow when activated by sunlight. Becquerel’s experiment involved wrapping photographic film in black paper and setting various
phosphorescent minerals on top and leaving them in the sun. He
soon learned that phosphorescent minerals containing uranium
would expose the film.
Aseries of cloudy days, however, brought a great surprise. Anxious
to continue his experiments, Becquerel decided to develop film
that had not been exposed to sunlight. He was astonished to discover
that the film was deeply exposed. Some emanations must be
coming from the uranium, he realized, and they had nothing to do
with sunlight. Thus, natural radioactivity was discovered by accident
with a simple piece of photographic film.
Rutherford and Geiger
Ernest Rutherford joined the world of international physics at
about the same time that radioactivity was discovered. Studying the
“Becquerel rays” emitted by uranium, Rutherford eventually distinguished
three different types of radiation, which he named “alpha,”
“beta,” and “gamma” after the first three letters of the Greek alphabet.
He showed that alpha particles, the least penetrating of the three, are
the nuclei of helium atoms (a group of two neutrons and a proton
tightly bound together). It was later shown that beta particles are electrons.
Gamma rays, which are far more penetrating than either alpha
or beta particles, were shown to be similar to X rays, but with higher
energies.
Rutherford became director of the associated research laboratory
at Manchester University in 1907. Hans Geiger became an assistant.
At this time, Rutherford was trying to prove that alpha particles
carry a double positive charge. The best way to do this was to measure
the electric charge that a stream of alpha particles would bring
to a target. By dividing that charge by the total number of alpha particles
that fell on the target, one could calculate the charge of a single
alpha particle. The problem lay in counting the particles and in
proving that every particle had been counted.
Basing their design upon work done by Sir John Sealy Edward
Townsend, a former colleague of Rutherford, Geiger and Rutherford
constructed an electronic counter. It consisted of a long brass
tube sealed at both ends from which most of the air had been pumped. A thin wire, insulated from the brass, was suspended
down the middle of the tube. This wire was connected to batteries
producing about thirteen hundred volts and to an electrometer, a
device that could measure the voltage of the wire. This voltage
could be increased until a spark jumped between the wire and the
tube. If the voltage was turned down a little, the tube was ready to
operate. An alpha particle entering the tube would ionize (knock
some electrons away from) at least a few atoms. These electrons
would be accelerated by the high voltage and, in turn, would ionize
more atoms, freeing more electrons. This process would continue
until an avalanche of electrons struck the central wire and the electrometer
registered the voltage change. Since the tube was nearly
ready to arc because of the high voltage, every alpha particle, even if
it had very little energy, would initiate a discharge. The most complex
of the early radiation detection devices—the forerunner of the
Geiger counter—had just been developed. The two physicists reported
their findings in February, 1908.
Impact
Their first measurements showed that one gram of radium
emitted 34 thousand million alpha particles per second. Soon, the
number was refined to 32.8 thousand million per second. Next,
Geiger and Rutherford measured the amount of charge emitted
by radium each second. Dividing this number by the previous
number gave them the charge on a single alpha particle. Just as
Rutherford had anticipated, the charge was double that of a hydrogen
ion (a proton). This proved to be the most accurate determination
of the fundamental charge until the American physicist
Robert Andrews Millikan conducted his classic oil-drop experiment
in 1911.
Another fundamental result came froma careful measurement of
the volume of helium emitted by radium each second. Using that
value, other properties of gases, and the number of helium nuclei
emitted each second, they were able to calculate Avogadro’s number
more directly and accurately than had previously been possible.
(Avogadro’s number enables one to calculate the number of atoms
in a given amount of material.)The true Geiger counter evolved when Geiger replaced the central
wire of the tube with a needle whose point lay just inside a thin
entrance window. This counter was much more sensitive to alpha
and beta particles and also to gamma rays. By 1928, with the assistance
of Walther Müller, Geiger made his counter much more efficient,
responsive, durable, and portable. There are probably few radiation
facilities in the world that do not have at least one Geiger
counter or one of its compact modern relatives.