Geiger,
Hans Wilhelm
Geiger, Hans Wilhelm (1882-1945), German physicist and inventor of
the Geiger counter, a type of particle detector. Geiger researched the
detection of alpha particles (nuclear particles consisting of two neutrons and
two protons), ionization (the formation of electrically charged atoms or
molecules), and radioactivity (the emission of nuclear particles, such as alpha
particles, or energy by atoms). His work contributed significantly to the
development of nuclear physics.
Born in Neustadt, Geiger studied physics
at the Universities of Munich and Erlangen. After
receiving his doctorate from Erlangen in 1906, he
accepted a fellowship at the University of Manchester in England, where he
became an assistant to British physicist Ernest Rutherford in 1907. The
following year, Geiger built his first device to detect radioactive
particles (particles spontaneously emitted by atoms). His device became
known as the Geiger counter.
The detection component of the Geiger counter is a metal tube filled
with gas under low pressure. The tube and a copper cathode (see Electrode)
are under a high voltage. When an alpha particle emitted from the nucleus of a
radioactive atom passes through the tube, it ionizes a gas molecule, leaving
the molecule with an electric charge. The electrically charged molecule, or
ion, is then attracted to the cathode. As it moves toward the cathode, the ion
collides with other gas molecules and produces more ions. This brief cascade of
ions creates sufficient energy to produce a momentary electrical current. The
Geiger counter records each cascade electronically and indicates a cascade with
a click. Rapidly repeating clicks indicate the presence of a large number of
alpha particles and, therefore, an increased level of radioactivity. Rutherford
subsequently identified the alpha particle as the nucleus of a helium atom.
In 1909 Geiger and British physicist Ernest Marsden
studied the influence of metal reflectors on alpha particles emitted by the
element radium. Although most alpha particles passed straight through the
metal, some were deflected at wide angles. After further research, Geiger and Marsden demonstrated that deflection of alpha particles
depended on the metal used, and that the degree of deflection was reduced as
the atomic weights of the metals decreased. They determined that the positively
charged alpha particles were being deflected by the positively charged nuclei
of the metal in the reflectors similar to the way that electrical charges repel
each other. Their research helped Rutherford develop his theory that an atom
consists of a nucleus with a positive charge and an electron shell with a
negative charge.
In 1912 Geiger was appointed head of the radioactivity
laboratories at the Physikalische Technische
Reichsanstalt in Berlin, where he organized a team of
researchers and continued his study of atomic structure. His work was
interrupted when he served in the German artillery during World War I
(1914-1918). When Geiger returned to research following the war, he worked with
German physicist Walther Bothe to improve the Geiger
counter. They used it in 1924 to research the increase in the wavelength of
high-energy electromagnetic radiation when it collides with electrons, called
the Compton effect after its discoverer, American
physicist Arthur Compton.
Geiger received an appointment as professor of physics at the
University of Kiel in 1925. While there he worked with German physicist Walther
Müller to improve the sensitivity and durability of
the Geiger counter, which became known as the Geiger-Müller
counter. In addition to alpha particles, the Geiger-Müller
counter detected beta particles (high-speed electrons, or positrons) and
ionizing electromagnetic photons. In 1929 Geiger was appointed professor of
physics at the University of Tübingen, where he began
his study of cosmic radiation. He became head of the physics department at the
Technical University of Charlottenberg-Berlin in 1936
and was named editor of Zeitschrift für Physik, a technical
publication. Illness slowed the pace of Geiger’s work from 1940 until his
death.
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1993-2002 Microsoft Corporation. All rights reserved.
Geiger
Counter
Geiger Counter, device that detects and records
information about subatomic particles emitted by radioactive substances. These
particles are invisible to the eye, and Geiger counters are used to guard
against human exposure to radiation, which can cause illness or death. See
also Particle Detectors; Elementary Particles.
A Geiger counter is a sealed metal tube filled with an inert gas,
such as argon. One fine wire, called an electrode, runs down the center of the tube. When the Geiger counter is on, the
difference in electric voltage between the positively charged electrode and the
negatively charged walls of the tube is about 1000 volts. Although the tube
carries such high voltage, no electric current passes between the electrode and
the walls because the inert gas does not normally conduct electricity.
When a charged particle, such as an electron, enters the tube, it
interacts with the gas to create ions, atoms or molecules with an
electric charge. The positive ions move toward the negatively charged walls of
the tube and the negative ions move toward the positively charged electrode,
creating a momentary electric current between the electrode and the tube wall.
The Geiger counter relays the current to the user with an audible click or a
visual signal to indicate the presence of a radioactive particle. Each signal
represents that one particle has entered the detector.
In 1908 German physicist Hans Wilhelm Geiger created the first
version of this counter. Later, Geiger collaborated with German physicist
Walther Müller to develop the Geiger-Müller counter, which counts subatomic particles with no
charge, such as neutrons.
Microsoft ® Encarta ® Reference Library 2003. ©
1993-2002 Microsoft Corporation. All rights reserved.
Geiger,
Hans
b. Sept. 30, 1882, Neustadt
an der Haardt, Ger.
d. Sept. 24, 1945, Potsdam
byname of JOHANNES WILHELM GEIGER, German
physicist who introduced the first successful detector (the Geiger counter) of individual alpha particles and other ionizing
radiations.
Geiger was awarded the Ph.D. by the University of Erlangen
in 1906 and shortly thereafter joined the staff of the University of Manchester,
where he became one of the most valuable collaborators of Ernest Rutherford. At
Manchester, Geiger built the first version of his particle counter and
used it and other radiation detectors in experiments that led to the
identification of the alpha particle as the nucleus of the helium atom and to
Rutherford's correct proposal (1912) that, in any atom, the nucleus occupies a
very small volume at the centre.
Moving in 1912 to
the Physikalisch-Technische Reichsanstalt
("German National Institute for Science and Technology") in Berlin, Geiger
continued his studies of atomic structure. During World War I he served as an
artillery officer in the German army. With Walther Bothe,
Geiger devised the technique of coincidence counting and used it in 1924
to clarify the details of the Compton effect. In 1925 Geiger
accepted his first teaching position, at the University of Kiel. There, he and
Walther Müller improved the sensitivity, performance,
and durability of the particle counter; the Geiger-Müller
counter detects not only alpha particles but beta particles (electrons) and
ionizing electromagnetic photons. In 1929 Geiger took up a post at the
University of Tübingen, where he made his first
observation of a cosmic-ray shower. He continued to investigate cosmic rays,
artificial radioactivity, and nuclear fission after accepting a position in
1936 at the Technische Hochschule
in Berlin, which he held until he died.
The concept of
the atom is an ancient one; the Greek philosopher Democritus (c. 460-c.
370 BC) proposed a form of "atomism"
that contained the essential features of the chemical atom later introduced by
the British chemist John Dalton in 1810. The British
physicist Ernest Rutherford spoke of
counting the atoms and in 1908, with the German physicist Hans Geiger, disclosed the first
electrical detector for ionizing radiations. The development of wavelength-tunable lasers has made it possible to carry out
Rutherford's concept of counting atoms. As stated above, RIS can be used to
remove one electron from each of the atoms of a selected type, and the modern
version of the electrical detector, known as the proportional counter, can even be
made to count a single electron. Thus, all that is required for the most
elementary form of atom counting is to pulse the proper laser beam through a
proportional counter.
Experimental
demonstrations of atom counting can be performed by introducing low
concentrations of cesium
vapour into proportional counters, commonly used for nuclear radiation
detection, that contain a "counting" gas composed of a mixture of
argon and methane. Pulsed laser beams used to implement the RIS scheme of Figure 14A
Figure 14A: Figure 14:
Resonance-ionization schemes. Photons from lasers are tuned so that their...
can be directed through a
proportional counter to detect individual atoms of cesium
without interference from the much larger number of argon atoms and methane
molecules.
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