About the Nobel Prize in Physics 1992
Georges Charpak
14 October 1992
The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 1992 to Professor Georges Charpak, France, École Supérieure de Physique et Chimie, Paris and CERN, Geneva, Switzerland, for his invention and development of particle detectors, in particular the multiwire proportional chamber.
This year's Nobel Prize in physics is
awarded to Georges Charpak, France, for his invention and
development of detectors in high energy physics. Since 1959
Charpak is working at CERN, the European laboratory for particle
physics situated in the canton of Geneva in Switzerland. Charpak
invented the multiwire proportional chamber at CERN. The
pioneering work was published in 1968. Largely due to his work
particle physicists have been able to focus their interest on
very rare particle interactions, which often reveal the secrets
of the inner parts of matter. Sometimes only one particle
interaction in a billion is the one searched for. The
experimental difficulty lies in choosing the very few but
exceptionally interesting particle interactions out of the many
observed. Photographic methods, once so very successful in
exploring particle processes, are not good enough for this. In
the new wire chamber Charpak used modern electronics and realised
the importance of connecting the detector directly to a computer.
The invention made it possible to increase the data collection
speed with a factor of a thousand compared to previous methods
for registering charged particle trajectories. At the same time
the high spatial resolution was very often considerably improved.
His fundamental idea has since been developed and for more than
two decades Charpak has been at the forefront of this
development.
The development of detectors very often goes hand in hand with
progress in fundamental research. Various types of particle
detectors based on Charpak's original invention have been of
decisive importance for many discoveries particle physics during
the last two decades. Several of these have been awarded the
Nobel Prize in physics. Charpak has actively contributed to the
use of is new type of detector in various applications in for
example medicine and biology.
Background information
The study of reactions between elementary particles provides
knowledge of their properties and of the forces that act between
them. The reactions are often very complex, sometimes several
hundred particles can be created in a single reaction and to
interpret them the scientists very often need to register every
single particle trajectory. Up to about 1970 this registration
was often done with photographic methods. The pictures were
analysed with the help of special measuring devices, a slow and
laborious process.
Charpak's invention consists of using an earlier development, the
proportional counter, in a particularly unconventional way. The
classical proportional counter, like the Geiger Müller tube,
consists of a thin wire in the middle of a tube with a diameter
of about a centimetre. Between the wire and the wall of the tube
a high voltage of a few kilovolts is applied. A charged particle
passing through the gas-filled tube will ionise the gas. In this
process electrons, which have negative electric charge, are
liberated from the neutral atoms of the gas, which then become
positively charged. In the electric field the electrons move
towards the central wire, the anode. Near the wire the electric
field is very strong and results in a rapid acceleration of the
electrons. They then have enough energy to ionise the gas and
more electrons are liberated, which in their turn are accelerated
and so on. This results in an avalanche of electrons and positive
ions and it is the movement of the electrons and the ions that
gives rise to an electric signal on the wire. The position of the
charged particle that started the ionisation in the gas can
however only be determined with a precision of about a
centimetre, the size of the tube.
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The principle of the multiwire
proportional chamber. The distance between the anode wires is
about 2 mm and the distance between the cathode planes is about 2
cm. A charged particle ionises the gas between the cathode planes
and the charges - the electrons and ions - move towards the anode
and the cathodes respectively. Several chambers are placed at
different distances from each other to make it possible to
determine the particle trajectory precisely.
To cover large surfaces with layers of these classical
proportional tubes is impractical and the desired spatial
precision cannot be reached. The break-through occurred with
Charpak's invention of the multiwire proportional chamber. It
consists of a large number of thin, parallel wires arranged in a
plane between two cathode planes a few centimetres away. The thin
anode wires have a diameter of about a tenth of a millimetre and
are placed about one or a few millimetres apart. In 1968 Charpak,
contrary to the general belief, realised that each wire would
behave as a proportional counter and result in a spatial
precision of about a millimetre or less. Each wire could stand a
very high rate of particles, several hundred thousand per second,
at that time an exceptionally high rate.
Each wire has an amplifier. The use of such a large number of
amplifiers is feasible thanks to the developments in electronics
which make it possible to construct compact amplifiers with very
small power requirements. An additional very important advantage
is the ability to register the signals with computers and handle
large amounts of data.
In this pioneering work from 1968 Charpak also points to possible
developments of the multiwire proportional chamber. One such
application makes use of the time it takes for the primary
ionisation to drift to the anode wire. A measurement of the drift
time results in an improved spatial precision. This application
is called a drift chamber and a spatial resolution better than a
tenth of a millimetre has been obtained.
History
Very often discoveries in physics are related to detector
development. For the development of the cloud chamber, which
registers tracks of charged particles in a gas, the 1927 Nobel
Prize was awarded to C.T.R.
Wilson. The cloud chamber was used in the discovery of the
first antiparticle, the positron, for which C.D. Anderson (of
Swedish descent) was awarded the 1936 Nobel Prize. The 1948 Nobel Prize
in physics went to P.M.S.
Blackett for his development of the cloud chamber technique
and its use in the study of the atom nucleus and the cosmic
radiation. In studies of the cosmic radiation during the 1940's
and 1950's special photographic emulsions were used to register
the tracks of charged particles. C.F. Powell was awarded the 1950 Nobel
Prize in physics for the development of the emulsion technique
and the discovery of the pi meson.
The invention of the bubble chamber, for which D.A. Glaser received the 1960 Nobel
Prize in physics, was of great importance for the evolution of
particle physics in the 1960's. In the bubble chamber, which is
filled with an overheated liquid, charged particles give rise to
small bubbles where the liquid is boiling along the track. These
strings of bubbles are photographed. However, pictures can only
be taken about once per second. During the 1960's a large number
of new elementary particles were discovered thanks to the bubble
chamber technique and L.W.
Alvarez was awarded the 1968 Nobel Prize in physics for the
development of this technique.
Charpak's discovery in 1968 started a massive development of
different types of wire chambers. Today practically every
experiment in particle physics uses some type of track detector
that has been developed from Charpak's original invention.
Charpak himself has been in the centre of this development from
which thousands of scientists, both at CERN and elsewhere, have
profited. When the charm quark was discovered in 1974, resulting
in the award of the 1976 Nobel Prize in physics to B. Richter and S.C.C. Ting, several
multiwire proportional chambers were used. The wire chamber was
also used in the discovery of the intermediate bosons at CERN in
1983. For this discovery the 1984 Nobel Prize in physics was
awarded to C. Rubbia and S. Van der
Meer. Detectors developed by Charpak are being used more and
more outside physics, e.g. in medicine for the detection of
X-rays.
