OCCHIALINI, GIUSEPPE PAOLO STANISLAO

(b. Fossombrone, Italy, 5 December 1907; d. Paris, France, 30 December 1993)

cosmic ray physics, Geiger-Müller counters, counter-controlled cloud chamber, nuclear emulsions, particle physics, space physics.

Occhialini, one of the leading figures in twentieth-century physics, conducted cosmic ray studies and performed work in particle, and space physics. He was a consummate experimentalist and was involved in electron-positron pair production and pion discoveries.

Occhialini, nicknamed “Beppo,” was the son of Etra Grossi and Raffaele Augusto Occhialini. His father was a well-known physicist who was appointed to the University of Pisa in 1911. The young Giuseppe attended elementary school in Pisa. In 1917 he and his mother moved to Florence, where he frequented the Ginnasio and the Liceo Scientifico. In November 1925, under pressure from his father, he undertook scientific studies at the University of Florence. He did so without enthusiasm, for his romantic nature disposed him toward a liberal arts career.

Arcetri Day Occhialini graduated in physics from the University of Florence in 1929, with an (unpublished) dissertation on spectroscopy under his father’s supervision. He joined the Institute of Physics (of the University of Florence) in 1930 as a voluntary research assistant, which led to a permanent appointment the following year. Like the Institute of Physics of the University of Rome (Istituto di fisica di via Panisperna) where Enrico Fermi and his group were working, the Florence Institute of Physics enjoyed the revival of physics research that took place in Italy after World War I. Antonio Garbasso, both a scientist and a politician, directed the Florence institute, located on the Arcetri Hill where Galileo Galilei spent his last years. In Rome, Orso Mario Corbino played a similar double role as a senator and as the director of the Institute of Physics at the University of Rome. Both men were leading actors in the organization of scientific research in Italy.

The years in Florence were very important for Occhialini’s early formation. He belonged to a small, congenial group of young and brilliant physicists, including the charismatic Enrico Persico, Bruno Rossi, and Gilberto Bernardini. Persico’s seminar introduced Occhialini and the others to the mysteries of the new quantum physics and to the thriving atomic and nuclear physics. Bernardini and Rossi began research on the emission of slow electrons by radioactive sources, a relatively original topic at the institute. Occhialini’s earliest research paper (1930), supervised by Rossi, pertained to a related subject.

Occhialini’s paper presented a new magnetic spectrograph for beta rays, based on a principle by Rossi. Designed to work for weak radioactive sources such as rubidium and potassium, the apparatus involved a Geiger-Müller counter, a facing aluminum surface on which the source was spread, and a deflecting magnet. This was one of the Florence group’s first applications of the counter recently invented in 1928 by Hans Geiger and Walther Müller. Being in a precarious financial situation, the Florence laboratory favored this inexpensive device, which worked both for beta and gamma rays.

Occhialini’s scientific career and activities at the Arcetri laboratory underwent a radical change in 1929 when Walter Bothe and Werner Kolhörster published their famous experiment in which single cosmic ray events were detected by means of two Geiger-Müller counters used in coincidence. This setup was far superior to an ionization chamber, which could detect only global ionization caused by a large number of particles over a long duration. Rossi quickly understood the opportunity the new configuration offered for investigating the physical properties of local cosmic radiation, and he launched his own program in this field.

Cosmic radiation, discovered at the beginning of the century by the Austrian physicist Victor Hess, had already been the subject of important investigations by the American physicist Robert Millikan. Millikan’s program required extensive resources, including balloons equipped with electroscopes and ionization chambers; observations were made outdoors at sea level, at high altitude on California mountaintops, and in the upper atmosphere. In order to account for the wealth of results, Millikan proposed what he called the “birth cry theory,” a set of more or less scientific conjectures about the origin and nature of cosmic radiation. In particular, he believed that primary cosmic rays could be only hard gamma rays.

Occhialini and his Florentine colleagues had a different conception of cosmic rays. They were aware that Bothe and Kolhörster supported the interpretation that primary rays were charged particles. The Florentine group was to confirm this latter hypothesis by improving the counter technique. In their experiment, Bothe and Kolhörster placed two Geiger-Müller counters one above the other a small distance apart and recorded the simultaneous pulses (coincidences) caused by the passage of individual particles through both counters. They observed the coincidences by connecting the wires of the two counters to two separate electroscopes. They obtained only double coincidences, and the method was very cumbersome. In order to improve upon this technique, Rossi devised a clever valve circuit that easily recorded coincidences of any order. It was mainly from this work that Occhialini acquired the expertise that he later brought to Cambridge.

Cavendish Laboratory Occhialini joined the Cavendish Laboratory at Cambridge University in England in July 1931, at age twenty-four, thanks to a fellowship from Italy’s National Research Council (CNR). His mission was to learn the technique of cloud chambers from Patrick M. S. Blackett. The Cavendish Laboratory had long been one of the world’s premier centers for experimental physics. Under Ernest Rutherford’s direction, it currently housed many outstanding young physicists interested in radioactivity and atomic physics. James Chadwick was working on a program that led him to the neutron, and James Cockcroft and Ernest Walton were on their way to discovering the first artificial disintegration of an atomic nucleus. Occhialini enjoyed the friendly community that discussed science, politics, and philosophy in academic clubs at the Cavendish Laboratory and elsewhere in Cambridge. Among them was the Kapitsa Club, founded by the Russian physicist Peter Kapitsa in 1922. Membership was restricted and the main concern was experimental physics. Occhialini thus joined a group of prestigious figures including Cockcroft, Ralph Fowler, and Walton as well as Blackett, Paul Dirac, and Nevill Mott.

The Italian fellowship offered to Occhialini was to enable him to spend three months in Cambridge; as it turned out, Occhialini’s stay spanned three enthusiastic years, with very little further support from CNR or Rutherford. Occhialini’s mentor, Blackett, was a world authority in cloud chambers, acquired from a long series of studies by C. T. R. Wilson at the end of the nineteenth century on the condensation of moist air and the reproduction of atmospheric phenomena. Wilson’s method materialized the path of ionizing agents, being based upon a property of ions in which they serve as centers for the formation of droplets in a supersaturated vapor. When a charged particle passes through a vessel containing a gas saturated with vapor, ions are formed along the whole of the trajectory. Under sudden increase of the volume (sudden expansion), vapor condenses around whatever ions are present in the chamber. A cloud of minute droplets then shows the trajectory of the charged particle. This array of droplets may be recorded photographically for quantitative work.

When Occhialini arrived in Cambridge, Blackett was using an automatic cloud chamber that was very convenient for the study of nuclear disintegration processes. However, the device was not well adapted for the cosmic ray studies he wished to initiate with Occhialini. Since the expansions were done at random, cosmic ray tracks were found in only a very small fraction of the photographs. Blackett and Occhialini hit upon the idea of triggering the expansion of the chamber with two Geiger-Müller counters placed above and below it. The cosmic particles took their own photograph, so to speak.

In 1932 Blackett and Occhialini observed the massive production of what they came to identify as electron-positron pairs. Their photographs showed showerlike multiple tracks diverging from a region over the chamber (see Figure 1), half of which had the magnetic curvature, range, and ionization that corresponded to positively charged particles with mass comparable to that of an electron. In the ensuing publication, they identified the new particle as the antielectron predicted by Dirac’s relativistic theory of the electron. They also suggested a possible mechanism for the production of the showers, based on initial collision and nonionizing links. A few months earlier, Carl Anderson, Millikan’s assistant at the California Institute of Technology, had already reported the existence of new low-mass positive particles in an untriggered cloud chamber, without referring to Dirac’s theory. Anderson received the Nobel Prize in 1936 for this discovery. Blackett received the same honor in 1948 “for the development of the Wilson cloud chamber and his discoveries therewith in the fields of nuclear physics and cosmic rays.” Occhialini was not mentioned in the citation.

From Italy to Brazil Occhialini returned to Italy in 1934 and resumed work thanks to a permanent professorship

at the University of Florence. Unfortunately the Arcetri institute was no longer what it had been. Garbasso had passed away; Rossi had been appointed to Padua and Persico to Torino. Owing to the rise of Fascism, the political climate had become unbearable to Occhialini. He was a liberal and had become markedly anti-Fascist, although he had adhered to the Fascist Party in the 1920s. In 1936, during Italy’s invasion of Ethiopia and involvement in the Spanish civil war, he had to remain in the party in order to save his job, but it was a tormenting decision.

In 1937 Occhialini accepted a professorship in Brazil offered by the Italian government and perhaps mediated by his father. He joined the University of São Paulo, which had just been created in 1934. The physics department was under the direction of Gleb Wataghin, an Italian-naturalized physicist born in Ukraine. With Wataghin, whom he had already met during a visit to the Cavendish Laboratory, Occhialini worked to develop Brazilian scientific activity. Primarily as experimentalists and secondarily as theoreticians, they launched cosmic ray physics in Brazil. This was a suitable research topic for a country such as Brazil, as it had been for Italy, because it required only small laboratories.

The intellectual and scientific atmosphere in South America was not the same as in Europe; in Brazil, scientific research was still in the process of becoming institutionalized. However, Occhialini had a real opportunity there to extend his scientific career. He held several chairs at the University of São Paulo, in general and experimental physics. Not surprisingly, he built a triggered cloud chamber with his students and directed counter-based absorption experiments, both in the laboratory and outdoors. His leadership at an international symposium on cosmic rays held in Rio de Janeiro in 1941 testified to the excellence of this work. He extended his earlier work on the beta radioactivity of rubidium.

In August 1942 Brazil entered World War II. The Italian-run group at the University of São Paulo dissolved, and Wataghin resigned the direction of the physics institute. Occhialini remained in Brazil, although he could not take a position against Fascist Italy because his father was living there and there was a risk of reprisals. Information is scant concerning Occhialini’s life in Brazil during the war, probably because he tried to keep a low profile as a foreigner living in an enemy country. It is rumored that he went into hiding in the Itatiaia Mountains near São Paulo. After the Italian armistice in September 1943 he spent time at the biophysics laboratory headed by Carlos Chagas Jr. at the University of Brazil and lectured at the University of São Paulo. Occhialini’s interest focused on techniques of photography and plate processing.

The Bristol Period At the end of 1944 Occhialini left Brazil and returned to England, invited by Blackett to join the British team working with Americans on the atomic bomb. However, he was barred from participating in that military research owing to his Italian nationality. Instead he was assigned to collaborate with Cecil F. Powell, again thanks to Blackett’s help. In 1944 Powell still was a relatively unknown physicist, a pacifist who had strong left-wing views. He was engaged in experimental work, detecting and measuring the energy of nuclear particles through the utilization of photographic emulsions. Powell belonged to the H. H. Wills Physical Laboratory in Bristol, which was constructed in 1927 through generous funding by the Wills tobacco family. During the war period this laboratory was located in a bombarded neighborhood, but research continued to be carried on in a highly scientific atmosphere despite the insufferable conditions.

Photographic emulsions used by Powell in his n-p scattering studies (the detection and measure of nuclear particles) had major drawbacks: their thickness and sensitivity to ionizing particles needed to be increased. Through a fruitful collaboration with C. Waller of Ilford Ltd., Occhialini and Powell succeeded in preparing special emulsions with a greatly increased proportion of the heavy elements silver and bromine, which became a powerful tool for the study of elementary particles. Meanwhile the Bristol group had been enlarged by the arrival of Cesare Lattes and Ugo Camerini. These two young and brilliant former Occhialini students from the University of São

Paulo specialized in cloud chambers and cosmic ray studies. Very soon Occhialini and Lattes realized that the new concentrated emulsions could be effectively used for the detection and study of cosmic rays.

In the spring of 1946 Occhialini participated in a speleological expedition in France. He took a stack of the new Ilford C2 plates—which were only 2 inches by 1 inch and 50 μm thick—to the Pic du Midi, whose altitude was 2,867 meters, and exposed the plates for about one month to cosmic radiation. Occhialini, Lattes, Powell, and Hugh Muirhead (Powell’s young student) realized from plate processing and scanning in Bristol between autumn 1946 and spring 1947 that they were involved in the discovery of a new particle. As a matter of fact, the Bristol physicists—making use also of Lattes’s observations from plates exposed at the Chacaltaya Mountains in the Bolivian Andes—obtained evidence of the existence of a pi-meson, the particle associated with the force between protons and neutrons inside the nuclei. They also observed events showing a pi-meson coming to rest in the emulsion and decaying into a mu-meson, discovered ten years before, also in cosmic rays.

So with Powell, as with Blackett, Occhialini was involved in the development of a new experimental technique leading to a scientific discovery. In 1947 Powell and Occhialini prepared together an atlas titled Nuclear Physics in Photographs. In 1950 Powell received the Nobel Prize for “the development of the photographic method of studying the nuclear process and his discoveries regarding mesons with this method.” Occhialini was not mentioned in the citation.

In 1948 Occhialini moved to the Free University in Brussels. There he married Constance “Connie” Charlotte Dilworth, a physicist from Bristol. They formed a research group on cosmic rays detected by emulsions, which, like electric counters years before, were an inexpensive experimental technique. The Occhialinis increased the sensitivity of the emulsions, and they performed a method for processing emulsions as thick as 600 μm, with which very long particle tracks could be recorded.

Back to Italy In 1949 Occhialini moved to Genoa, where his father directed the physics institute at the university. He immediately formed a research group and trained students. There, as in Brussels, he continued developing nuclear emulsions for the study of cosmic rays. In 1951 his daughter Etra Mary Giovanna Occhialini was born. Augusto Occhialini died in the same year. The family moved to Milan in 1952, where Occhialini was offered a professorship in advanced physics at the university.

Occhialini played a major role in the revival of Italian physics in the postwar period. He and his wife stayed in touch with Powell in Bristol and developed collaborations with other countries. The G-stack project—a European collaboration to expose large numbers of plates of nuclear emulsions (37 x 27 x 15 cm) to cosmic rays with the expectation of detecting K-meson decay—was one of Occhialini’s great scientific and organizational contributions of the 1950s. With the efficient support of the rector of the University of Milan, Giovanni Polvani, part of the Genoa group moved to Milan, where Occhialini assembled another research group on cosmic rays and nuclear emulsions. In the same period, at the request of UNESCO, Occhialini went to Brazil to help to create a laboratory for studying cosmic rays in Chacaltaya. In Europe, the foundation of the European Organization for Nuclear Research (CERN) in 1954 led physicists to favor accelerators over cosmic rays as a source of high-energy particles. The Milan group soon began using this new source.

At the end of the 1950s Occhialini and his wife wanted to return to their work with cosmic rays. Invited by Rossi, they spent a year at the Massachusetts Institute of Technology in order to familiarize themselves with the American space research program and astrophysical studies of cosmic rays. Rossi had become an expert on the physics of the ionosphere and the magnetosphere as well as on cosmic x-rays and gamma rays. Back in Europe the Occhialinis began studying the properties of the cosmic ray components from the Sun and from other bodies in the galaxy. They created a research group at the University of Milan, applying the new space technology in cosmic ray studies and high-energy astrophysics. They also played a fundamental role in the establishment of European scientific collaborations and in the development of the European Space Research Organisation (ESRO), an institution created at the beginning of the 1960s, later to become the European Space Agency (ESA). The Milan group directed by Occhialini took a fundamental part in the COS-B project, a second-generation space instrument conceived exclusively for gamma-ray astronomy. It was launched in 1975 and supplied the first gamma map of the galaxy.

Occhialini retired in 1983. In 1993, the year of his death, he was elected an honorary member of the European Physical Society. It was the last among a long list of honors from learned societies and academies: he was awarded the Charles Vernon Boys Prize of the Council of the Physical Society (London) for distinguished work in experimental physics in 1950; became a member of the Accademia dei Lincei in 1958 (and recipient of the Sella and Einaldi Einaudi prizes of this academy in 1934 and 1949) and foreign member of the Royal Society in 1974; and was the recipient of the Wolf Prize, “for his contributions to the discoveries of electron pair production and of the charged pion,” in 1979.

BIBLIOGRAPHY

An extensive bibliography is in the Occhialini archive in the Istituto di Fisica Generale Applicata of the University of Milan, and in Leonardo Gariboldi’s “The Reconstruction of Giuseppe Occhialini’s Scientific Bibliography” (see below). See also the list of publications held in the Occhialini archive with Occhialini’s own indications of the publications he considered most important.

WORKS BY OCCHIALINI

With Patrick M. S. Blackett. “Photography of Penetrating Corpuscular Radiation.” Nature 130 (1932): 363.

_____. Some Photographs of the Tracks of Penetrating Radiation.” Proceedings of the Royal Society of London A 139 (1933): 699–720, 722, 724, 726.

With Patrick M. S. Blackett and James Chadwick. “New Evidence for the Positive Electron.” Nature 131 (1933): 473.

_____. Some Experiments on the Production of Positive Electrons.” Proceedings of the Royal Society of London A 144 (1934): 235–249.

With Cecil F. Powell, Derek L. Livesey, and L. V. Chilton. “A New Photographic Emulsion for the Detection of Fast Charged Particles.” Journal of Scientific Instruments 23, no. 5 (1946): 102–106.

With Cecil F. Powell. Nuclear Physics in Photographs. Oxford: Clarendon Press, 1947. Occhialini’s only book.

With Cecil F. Powell, Derek L. Livesey, and L. V. Chilton. “Processes Involving Charged Mesons.” Nature 159 (1947): 694–697.

With Cesare M. G. Lattes and Cecil F. Powell. “Observations on the Tracks of Slow Mesons in Photographic Emulsions, Part 1.” Nature 160 (1947): 453–456.

_____. Observations on the Tracks of Slow Mesons in Photographic Emulsions, Part 2.” Nature 160 (1947): 486–492.

With Constance C. Dilworth and Ron M. Payne. “Processing Thick Emulsions for Nuclear Research.” Nature 162 (1948): 102–103.

With B. Bhowmik et al. “The Interaction and Decay of KMesons in Photographic Emulsion, Part 1: General Characteristics of K-Interactions and Analysis of Events in Which a Charged μ-Meson Is Emitted.” Il Nuovo Cimento13, no. 4 (1959): 690–729.

“Occhialini, Giuseppe.” Scienziati e Tecnologi Contemporanei 2 (1974): 322–324; Milan: Mondadori, 1974. A short autobiography that is included in the Occhialini archive.

OTHER WORKS

Bignami, Giovanni F. “Giuseppe Paolo Stanislao Occhialini.” Biograpical Memoirs of the Fellows of the Royal Society of London 48 (2002): 331–340. A detailed account.

Gariboldi, Leonardo. Tesi di Dottorato Di Ricerca, “Giuseppe Paolo Stanislao Occhialini (1907–1993): A Cosmic Ray Hunter from Earth.” PhD diss., Universita Degli Studi di Milano, Facolta du Scienze Politiche, 2004–2005. Information on Occhialini’s scientific life.

_____. The Reconstruction of Giuseppe Occhialini’s Scientific Bibliography.” In Atti del XXIII Congresso Nazionale di Storia della Fisica e dell’Astronomia, edited by Pasquale Tucci, Augusto Garuccio, and Maria Nigro. Bari, Italy: Prima editione, 2004.

Redondo, Pietro, et al., eds. The Scientific Legacy of Beppo Occhialini. Berlin, Heidelberg, and New York: Societa Italiana di Fisica; Bologna-Springer-Verlag, 2006.

Russo, Arturo. “Vita di uno sperimentatore.” Sapere 4 (1996): 62–69.

Telegdi, Valentine L. “Giuseppe Occhialini.” Proceedings of the American Philosophical Society 146, no. 2 (2002): 218–222.

Martha Cecilia Bustamante