COSMIC RAYS AND PARTICLE PHYSICS AT BERKELEY

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1 COSMIC RAYS AND PARTICLE PHYSICS AT BERKELEY W. Fretter To cite this version: W. Fretter. COSMIC RAYS AND PARTICLE PHYSICS AT BERKELEY. Journal de Physique Colloques, 1982, 43 (C8), pp.c8-191-c <1.151/jphyscol:198281>. <jpa > HAL Id: jpa Submitted on 1 Jan 1982 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE CoZZoque C8, suppze'ment au no 12, Tome 43, de'cernbre 1982 page C8-191 COSMIC RAYS AND PARTICLE PHYSICS AT BERKELEY W.B. Fretter University of Cazifomia, Berkeley, CA 9472, U. S. A. The study of cosmic rays and particle physics was initiated by Professor Robert B. Brode at the University of California, Berkeley, in Brode, who had formerly worked in atomic physics at Berkeley, started to work on cosmic rays after having spent a year working with Blackett in London. There he learned the technique of the counter-controlled Wiison cloud chamber, and on his return to Berkeley constructed two chambers, one of 18 centimeters in diameter and 8 centimeters deep and another 3 centimeters in diameter and 1 centimeters deep. The first was used for a study of the specific ionization of the cosmic ray particles, and the second, which was fitted with lead plates, was used to study the creation and development of cosmic ray showers. Brode and his student Merle Starr took thousand of photographs, and noticed that less than ten percent of the "electrons" passing through a six millimeter lead plate produced showers, and less than 5 percent for thicker plates. Calculations by Furry had shown that forty percent of electrons should have produced showers. Thus Brode and Starr's observations agreed with Neddermeyer and Anderson, and Street and Stevenson regarding these "penetrating" particles. Brode and his student Dale Corson then set about determining the mass of these penetrating particles. They placed the 18 centimeter chamber in a magnetic field of 8 gauss, with Geiger counters above and below. The specific ionization of 12 cosmic-ray tracks was measured by counting droplets in photographs of delayed expansion tracks, and the combination of curvature in the magnetic field, which gives the momentum of the particle, and measurement of the ionization, which gives the velocity of the particle, yielded values of the masses of the particles. They were able to measure a track in this way with mass about 25 me. Making use of other tracks reported in the literature, they concluded that all were consistent with a mass of (2+5) me. The next step was taken by Wayne Hazen, who measured the relativistic increase in ionization of high energy electrons and mesotrons, as these heavy electrons were called. Hazen remained at Berkeley during World War II, while both Brode and I, who had joined the group in 1938, were away at war work. Article published online by EDP Sciences and available at

3 C8-192 JOURIVAL DE PHYSIQUE Hazen placed the 3 centimeter cylindrical cloud chamber in the gap of a large electro-magnet built by Brode with the aid of a grant from the Carnegie Foundation. Hazen made accurate droplet counts on diffuse tracks in the cloud chamber, and was able to show that there was a substantial component of "penetrating" particles whose ionization was at the minimum, even though their momenta were sufficiently large to have given a substantial rise above the minimum had they been electrons. Hazen's work was published in 1945, the year I returned to Berkeley to complete my graduate work for the Ph.D. Hazen had realized that to obtain a larger sample of penetrating particles for which the mass could be determined, an additional cloud chamber containing a series of lead plates placed below the chamber in the magnetic field would be useful to determine the range of the particle in lead. It was with this apparatus that I was able to determine the mass of mesotrons to a considerably greater precision than had been possible before. In 1946 I published results on 26 particles - 14 positive and 12 negative, whose measured masses were all consistent with a value of 22 me, with a probable error of 5 me. Until then, the total number of reasonably reliable determinations was about 25, all consistent with 2 me, but with much larger individual errors. During the course of this experiment, Hazen and I noticed a large numbers of "penetrating showers" in the lower chamber produced in the lead plates. Several of the particles in these showers produced additional showers in successive plates, and we were able to show that the successive showers in some cases were produced by non-electronic particles. Some of these must have been what we now know as pions, nuclear active particles, in contrast to the penetrating mesotrons (now called muons), which were nuclear-inactive particles. Protons, which are of course nuclear - active at high energies, appeared in these pictures as heavily ionizing tracks with short ranges in the lead. Following this work in Hazen's 16 inch diameter chamber, 9 inch deep with 8% inch thick lead plates, I decided to make a larger cloud chamber, rectangular, with more lead plates. The resulting rectangular chamber had 16 lead plates, each K inch thick, separated at the center by.8 inch. The chamber was triggered by counters designed to detect showers and was used in Berkeley and at 1, feet in Tioga Pass, near Yosemite in California. Some spectacular photographs of penetrating showers were made and inferences made about penetrating particles, mesons, and electrons, in a paper published in 1949.

4 But V-particles had been discovered in 1947 by Rochester and Butler, and I did not observe any in this experiment. It was simple to correct this by reducing the number of plates from 16 to 7, spaced 2 inches apart instead of.8 inches. We immediately started seeing V-particles in the chamber, and together with Mike May and Paul Nakada, I published a paper in 1953 that described our results. We observed 6 V-particles, mostly from our high altitude station, and were able to publish data on 39 coplanar measurable V decays : 23 p + IT decays and 16 IT + a decays. Kinematic analysis made it possible to determine life times and masses consistent with those being found in other experiments done in those years. In preparation for my talk, Salmeron asked me to say something about the work of Thompson on V-particles, and the contribution of Leighton to these studies. Thompson, Cohn and Flum reported in 1951 observing 9 examples of V decay at sea level, with a 12 inch in a magnetic field. Three of these Vds were identified as Vo-p + an assumed v:-;, + IT + Q, the Q values found were 31 I - +5 and Mev. a V2-IT + IT was quoted with Q value of " 49 Mev, possibly the first observation of this particle (now called K ). Thompson then proceeded to build a much larger cloud chamber and a large electromagnet. Results were published first in 1953 and in a comprehensive review article in Nuovo Cimento in with a magnetic field of 7, gauss. The apparatus was carefully designed for accuracy of measurement The cloud chamber was 13 inches wide, 8 inches deep, and 24 inches high, thermally insulated from the magnetic field. Thompson found 23 VO decays, 38 V+ events and 1 T event V-3 n. He analyzed his data in terms of a Q - surface representation, which was useful in separating V2 - IT + IT from anomalous events. The review article of 1956 (Supp Nuvo Cim 4, 286, 1956) is an excellent example of careful analysis of the data he obtained. a mass of M = me. He found Q (n,a) = ( ) Mev, giving Much of Thompson's work on V-particles was reported at the Congres International sur le Rayonnement Cosmique - at Bagneres de Bigorre in July Others reporting on this subject included Armenteros, Bridge and Rossi, Gregory, Barker and Butler, Reynolds, Deutschmann, Astbury, Page, Newth, and Leighton. Since Leighton is also not present, Salmeron asked me to say something about the Cal Tech work. Leighton had been working on V-particles for several years with Anderson at Cal Tech. I remember well the second Rochester Conference, which I attended in 1951, reporting my own results on V-particles, and talking at length with Leighton on

5 C8-194 JOURNAL DE PIiYSIQUE the telephone to get his results. At Bagneres, Leighton reported on charged V-particles and heavy mesons, of which they had 36 V+ and 37 V-. They found masses for the primaries somewhat less than 1, me, a T - meson with Q value of 7$3 Mev and 3 cases of cascade decay - a neutral V coming from the decay point of a charged V. The group from the Ecole Polytechnique reported similar results at Bagneres. Now is the time to recall that I attended the Bagneres conference following a year ( ) spent at the Ecole Polytechnique in Paris within the Laboratoire LePrince Ringuet. There I worked with Gregory, Lagarrigue, Meyer, Muller, and Peyrou, together with Johnston, from Dublin, also visiting. I spent several weeks working with the group at the Pic du Midi, and worked on scanning and measurement of the film in Paris. It was my first visit to Paris, and the beginning of a life-long relationship with M. LePrince-Ringuet and his group. The Conference at Bagneres was by far the most exciting scientific meeting I ever attended. Results bubbled to the surface from all laboratories; the papers were interesting and the discussion excellent. The proceedings make difficult reading now because of the terminology used - different names for the same particle. But the title page stated " Les particules dgcrites au cours de ce congres ne sont pas entisrement fictives, et toute analogie avec des particules existant dans la nature n'est pas une pure coincidence." Returning to Berkeley in 1953, I constructed a cloud chamber for precision measurements of momentum and ionization. The chamber, 2 inches long by 16 inches wide by 5 inches deep, was placed in a magnetic field of 7,5 gauss provided by an air-cooled magnet. The conditions of the cloud chamber were strictly controlled and it was possible to make accurate measurements of ionization of the particles traversing the chamber. helium as the gas and droplets were readily counted. analyzing tracks of a negative and a positive.r meson. We used In 1955 Lagarrique joined us in In the case of the positive, we were able to make accurate measurements of both ionization and momentum of the primary and all three secondary particles, yielding a mass of M For the e' negative, the primary was too short to measure, but measurements on the secondaries gave a mass of Me. We continued to use this chamber for several years, until I made the decision to abandon cosmic rays for accelerators and bubble chambers, which I did in 196.