Georges Charpak: Hero of the Resistance, Particle Hunter

I met him in the main restaurant of the European High Energy Research Laboratory (CERN), which I will call the Skeptical City. CERN, as well as Fermilab, located outside of Chicago; DESY, in the Altona district, Hamburg; KEK, in the city of Tsukuba, Japan, are places where the bowels of matter are observed and nothing is taken for granted. “We are skeptical by nature,” he assured me. Georges Charpack, while we had an espresso. It is a small city with avenues bearing the names of Marie Curie, Ernst Rutherford, Albert Einsteinn and other paladins of the atom, which lead to labyrinthine, Cortazarian buildings, in whose subsoil there is a tunnel 23 kilometers in diameter where the powerful machine, the Large Hadron Collider (LHC), rests. As in the novel Julio Cortazar, Hopscotchbuilding 1 does not necessarily lead to 2. There is a hotel with four buildings, three restaurants, cafes, post office, bank, souvenir shop.

Charpak won the Nobel Prize in Physics in 1992 for realizing the dreams of the particle hunters of his day. And it is that several decades earlier he had conceived and directed the construction of detector instruments, without which it would have been impossible to delve further into this extravagant universe of subatomic particles, some of them from the cosmos. Another morning he told me about those romantic times. In 1968, within the same underground ring where the LHC rests, the SPS (Super Proton Synchrotron) was found, a machine that was modified to also generate antiprotons. Thus, two colossal experiments were launched: UA1 and UA2. I remember Charpak emphasizing that a single collision between a proton and an antiproton generated hundreds of electrical signals as the particles passed through the various layers of UA1.

Computing became increasingly sophisticated, so that both experiments incorporated a series of microprocessors and computers connected to a main computer; then he recorded on magnetic tape all the information contained in these signals. Each event took up the equivalent of about 70,000 bytes of space, and it took the computer a quarter of a second to write it onto the tape, an eternity! The spinning proton and antiproton packets in the SPS bumped into each other every 7.6 microseconds (that is, 7.6 millionths of a second). So while the computer was busy writing the entire record of a single collision or event, there was already a row of three thousand other collisions that might be interesting to record.

For this, the subsidiary system of microprocessors and computers available at the time in the Skeptical City was used. Programmed with the purpose of making evaluations in four millionths of a second, they discarded the useless signals and saved the relevant ones. How to know what to record and what not? Charpak enlightened me. “Through a program written by physicists and experts in binary languages, they told the machines what to keep and what to discard, depending on their intrinsic values.” A new era of computing was beginning. The ingenuity of the programmers had to allow the protocols to be rewritten and adjusted to different types of events. A crucial step in the search for speed and precision was taken by the man in front of me, about to stop us and go in search of another espresso. Hero of the resistance during World War II, imprisoned in Dachau for a year, a moment that coincided with the end of the conflict. He later became a prolific inventor of detectors, “a way to entertain the demons of captivity,” he told me. Among his inventions, the proportional wire camera stands out. In the 1960s, multiwire scintillation cameras proved valuable because they were faster than bubble cameras, although they could not provide the same detailed information.

The camera that Charpak devised in the following years was faster than the scintillation camera and, at the same time, as precise as the bubble camera. When a charged particle passes through a gas, it leaves behind a trail of ionized atoms. All detectors, from the cloud chamber to the scintillation and wires, were only good if they could reveal such an ionized signature. In 1968 Charpak’s group discovered new ways of making such ionization show the passage of different particles and, to do so, devised two basic types of detectors, the proportional wire chamber and the flux chamber. This innovation in subatomic traps gave a renewed impetus to the hunt for the smallest entities. Today, a street in the French town on the border with Geneva, Saint-Genis Pouilly, whose population lives largely thanks to the economic benefits of those who visit and work at CERN, bears his name.

AQ