|Ioannis Giomataris (2011), Scholarpedia, 6(7):11431.||doi:10.4249/scholarpedia.11431||revision #91312 [link to/cite this article]|
Georges Charpak the man, his work
Georges Charpak died on October 28 in Paris at the age of 86. As inventor of the first operating Multiwire Proportional Wire Counter (MPWC) he was awarded the Nobel Prize in Physics in 1992.
Georges was born in eastern Poland on 8 March 1924, and in 1932 he and his family moved to Paris. During World War II, he took part in the French Resistance and was later imprisoned by the Nazis in the Dachau concentration camp in Germany. Georges became a French citizen after the war, and in 1948 he entered the CNRS. He studied under Nobel laureate Frédéric Joliot-Curie at the Collège de France in Paris and earned his PhD in nuclear physics in 1954. He received the CNRS silver medal in 1960.
In 1959 Georges went to work at CERN in Geneva where his scientific interest switched from nuclear physics to high-energy particle physics and he spent his entire career there. He was part of a team that in 1961 determined that a particle known as the muon was not a separate particle of the nucleus but just a heavy electron. That was a first successful attempt to precisely measure the magnetic-moment anomaly for muons. Very high precision of that value is of great importance in particle physics; a small deviation from the theoretical value could predict the energy scale of new physics. Georges became a CERN employee in 1963. There he invented a new type of particle detector, the Multiwire Proportional Chamber, which revolutionized particle physics. Fabio Sauli, who joined the group at CERN played an important role in developing this detector, its descendant the drift chamber and a novel concept, the Multistep Avalanche Chamber, aiming to reach higher counting rates. These detectors have been further developed and adapted to detect UV light and to tackle new applications ranging from fundamental research to medicine, biology, and industry. The experimental effort was shared with other distinguished physicists, among them Amos Breskin, Stan Majewski, and Wotjek Dominic. In collaboration with Tom Ypsilantis and his group, a particular effort was devoted to improve the RICH, a counter which was serving to identify elementary particles. New UV photocathodes have been developed in collaboration with Vladimir Peskov and Jacques Seguinot.
The Multiwire Proportional Chamber (MWPC)
Working to improve readout methods for spark chambers, at the end of 1967 Georges Charpak had the idea to put together his technological expertise to reliably assembly the wire planes used for digital spark chambers, and his former experience with classic proportional counters. The first Multiwire Proportional Chamber was born: a gas-filled box with a large number of parallel detector wires, each connected to individual amplifiers. The wire plane is placed in the middle between two cathode planes. Typical distances between the anode wires are 1 to 4 mm, while the two cathode planes are from 5 to 15 mm apart. The figure shows the electric field lines: the electrons that are produced in the constant field region will drift towards the closest anode wire, where under the force of the higher field they will be accelerated and produce an avalanche.
Linked to a computer, it could achieve counting rates thousand times better than existing detectors, providing an excellent spatial resolution. This was an experimental technique that many others had attempted without much success. Indeed wire proportional counters have been in use since the beginning of the 20th century, and examples of counter arrays exist. What was missing certainly was an understanding of the formation of pulses in a proportional multiwire chamber. Working with similar detectors in the College de France, Charpak realized that signals were not produced by the drifting electrons but rather by the movement of positive ions, which induced pulses of opposite polarity near the wires.
Unlike earlier detectors, such as the bubble chamber, which records a few photographs per second, the multiwire chamber records up to one million tracks per second and sends the data directly to a computer for analysis. The demanding experimental needs emerging with the new generation of accelerators and the rapid development of integrated electronics in the late sixties led to a massive use of large MWPC arrays in particle physics experimentats. The speed and precision of the multiwire chamber and its descendants, the drift chamber and the time projection chamber, revolutionized high-energy physics. It allowed physicists to operate experiments at much higher particle collision rates and test theories predicting production of rare events and new massive particles.
With his collaborators Charpak found that pulses induced in a multiwire proportional chamber on orthogonal cathode strips are permitting a bidimensional read-out. Moreover determination of the charge centroid provides a better accuracy along the wires which is better than 200 μm. The new read-out was used by many experiments and today is still in use at LHC.
Drift chamber and TPC
A drift chamber is an apparatus for measuring the space coordinates of the trajectory of a charged particle. This is achieved by detecting the ionization electrons produced by the charged particle in the gas of the chamber and by measuring their drift times and arrival positions on sensitive electrodes.
In order to cover large areas with MWPCs very many wires, and therefore many channels of amplification and readout, are required. With drift chambers, the wires are much more widely spaced. Electrons from ionisation drift in regions of low electric field before reaching the high field or avalanche region near a wire, where amplification and detection occur. The drift time is then a measure of the position of the original particle. (Note that to measure this drift time, the transit time of the original particle must be known, e.g. from independent scintillation counters as illustrated in the figure above).
Historically, when the multiwire proportional chamber was introduced in 1968, its authors had already noted that the time of a signal could be useful for a coordinate determination, and first studies with a drift chamber were made by Bressani, Charpak, Rahm and Zupancic in 1969. The first operational drift-chamber system with electronics and readout was developed by Walenta, Heintze and Schürlein in 1971 and a new real instrument for particle experiments had appeared. Drift chambers have been widely used by many experiments because of economic read-out, high accuracy and the possibility to build large area devices.
In 1974 David Nygren has introduced a descendant of the drift chamber, the Time Projection Chamber (TPC) that has been widely used by many experiments and especially at LEP. This contains a large cylindrical drift volume (several meters in the case of LEP experiments) creating a uniform electric field along the axis. A field of the order of 100 kV per meter drifts charge onto MWPCs at the end of the drift volume. The MWPC, equipped with pad induction elements, provides the two-dimensional spatial coordinates x and y in the plane perpendicular to the TPC axis, while the third coordinate z along the axis is given by measuring the drift time of the electrons. To minimize diffusion, a magnetic field parallel to the electric one forces the electrons to spiral about the field direction. The spatial resolution achieved goes down to 150 μm in the x-y plane and approximately 1mm on the z axis.
Parallel Plate and Multistep chambers
Parallel-Plate Avalanche Counters (PPAC) have found wide-spread use predominantly as timing detectors for heavily ionizing radiation. Their simple design and operation, their excellent timing resolution (better than 100 ps) is obtained with a small size detector. Very early on PPAC were made position-sensitive by inserting a grid of wires between the anode and cathode and by dividing the cathode into strips.
The idea of developing the multistep avalanche chamber, developed in the period 1979-1989, was to reach even higher counting rates. The basic idea was to split the amplification into two stages and overcome space charge-effects that otherwise counter-acted the gain. In fact a MultiStep Avalanche Chamber (MSAC) is a stack of two or three PPACs. One MSAC has several electrodes which are called meshes, as schematically shown in the figure. A mesh consists of crossed stainless steel wires, equally spaced at a distance of 500 μm, each having a 50 μm diameter. The frames are made of a very light composite of vetronite. The frames span the meshes with a tension of about 15-50 kg/m. Such a tension is needed because the parallelism of the electrodes is a critical factor for the operation of the PPAC. It should be better than 20 μm for the 4 mm amplification gaps. Special care is taken to prevent sparking at the edges of the chambers where the electromagnetic field is not homogeneous.
Imaging and light read-out
The multiplication in gases of ionization electrons, in the electric field between parallel electrodes, leads to the emission of light from the molecules excited in the avalanche process. The optical imaging of this light, with intensifiers, on charge-coupled devices permits the localization, in the gaseous volume, of the entrance points of converted radiation. The discovery that adding appropriate vapors to noble gases yields a copious intensity of light opened an active research and induced new developments. It also offered prospects of promising applications in all the domains where autoradiography techniques are used for quantitative measurement of the spatial distribution of radioactive compounds carried by biological samples or gels.
Georges Charpak has spent time and effort to push possible applications of these detector in medical radiology where the trend is the digital read-out technology in order to replace the photographic film, with improved sensitivity and a spatial resolution comparable to the film. In comparison with photographic emulsion, there is a significant gain in time for data taking, with the advantage of linearity, wider dynamic range in the intensity measurement and a greatly improved signal-to-noise ratio. Beta radiography is employed in medical and biological investigations to image human or animal tissues labeled with beta-emitting radionuclides.
A considerable effort was invested in the detection and imaging of Ultraviolet (UV) light targeting many applications in Cherenkov imaging and astronomy. In these detectors compounds of low photoionization potential were used to convert UV photons to photoelectrons.
Cherenkov counters and new detector developments
In the 1980s, Charpak started a close collaboration with Ioannis Giomataris elaborating new detector concepts adapted to solve specific problems in particle physics, including a high energy gamma telescope with good energy resolution. In 1990, they began working with Leon Lederman on a new device called Optical Trigger to select in real time particles carrying the fifth quark (bottom) in high intensity proton collisions. These particles fly a short distance before decaying and this enhances the Cherenkov light produced by relativistic particles inside a thin crystal cell. This is a way to tag these particles and enrich the collected sample with good events.
In 1991 they proposed the Hadron Blind Detector, which was then developed by an international collaboration. In this concept, most of the particles produced in proton collisions, called hadrons, are not seen by the detector, while electrons and high momentum muons are efficiently reconstructed. This selection criterion rejects unwanted events and highly improves the signal to background ratio. This detector concept was demonstrated in October 1992 at CERN just before the Nobel prize announcement, when Charpak and other team members were conducting experiments at night. For these investigations a gaseous parallel plate detector was used and during optimization process the advantage of a narrow amplification gap was experimentally shown. This triggered the idea of building an even narrower amplification gap and from that a new detector concept was born, the MicroMeGAS (Micro Mesh Gaseous Structure).
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Georges Charpak was spending many weekends and summer holidays at his house in Cargese, a village established by Greeks at the end of the 18th century in Corsica. His house was located two steps from the Institut d’Etudes Scientifiques de Cargèse, a facility developed by a physicist Maurice Levy in the sixties. It became an important summer school of theoretical physics, gathering some distinguished physicists and many actors of the standard model of particle physics. Georges used to join lectures during summer and there was constant feedback between his house and the Institut.
He also used to invite to his house other physicists to discuss new ideas in physics.
Georges Charpak was a concerned citizen involved in many social and educational activities. In 1978 he canceled his participation to a congress in DUBNA to protest against the imprisonment of Jury Orlov, a Russian physicist. After the Nobel prize in 1992 Georges moved to Paris and a new life with diversified activities started.
Keen to popularise science, he also became widely known among the general public through several books that made physics accessible to as many people as possible. In 2004, with Henri Broch, he wrote a book deriding pseudoscience, astrology and other misconceptions. He wrote an other book with Richard Garwin in which they evaluate the benefits of nuclear energy and show how it can provide an assured, economically feasible, and environmentally responsible supply of energy that avoids the hazards of weapon proliferation. They make a strong statement in favor of arms control and outline specific strategies for achieving this goal worldwide.
Education and research lead the life and work of Georges Charpak. He is also the creator of La Main à la Pâte, an association to introduce hands-on science education in primary schools in France, an idea that had been first initiated in Chicago by his friend Leon Lederman. Since 1996 with the support of the French Academy of Sciences and some colleagues he propagated the new idea of teaching science in primary schools. A motivated partner was the École Nationale Supérieure des Mines (ENSM) at Saint-Étienne where a laboratory was created in his honor, gathering teachers and scientists and a special prize ’the PuRkwa Prize’ has been created to reward pedagogical initiatives which help children to acquire a scientific spirit. The new education approach has spread to several countries around the world.
Every two years since 2002, we started organizing a conference in Paris ‘Large TPCs For Low Energy Detection’. The purpose of the meeting is an extensive discussion of present and future projects using a large TPC for low energy, low background detection of rare events (low-energy neutrinos, double beta decay, dark matter, solar axions). Georges was actively participating in this Conference giving introductory talks pointing out links between science, education and technology.
Georges has been active until the end. Recently he published, together with Francois Vannucci, a new book to celebrate physics that he loved. I met him at his home a day before his death and I was impressed by the clarity of his mind. He was excited to hear the new progress on physics and detector developments conducted by my group: the Micromegas and a novel spherical detector.
Georges liked music and especially classic songs. He was often inviting musicians at his home in Paris or enjoying the company of artists of the opera and friends at a typical ‘parisian bistrot’ where they sang around a piano player. For his last residence, as he wished, several musicians were playing classic music during the ceremony. I will keep in my memory a kind man, a humanist, enthusiastic, optimistic and always open to new ideas. I have the feeling, as many other collaborators do, that our second ‘father’ has passed away.