My Work in Meson Physics with Nuclear Emulsions Cesar Lattes
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'Sgaa»_ &>***- &Lj d~^tZL, f F. Caruso, A. Marques & A. Troper (Eds.), Cesar Latias, a descoberta do méson ne outras histórias, pp. 9-11. My Work in Meson Physics I decided that the time allotted to me at the Cambridge Cockroft-Walton accelerator, which with Nuclear Emulsions provided artificial disintegration particles as probes for the shrinkage factor, was sufficient for a study of the reactions: Cesare Mansueto Giulio Lattes 7 D(d,p)H\ Belt'd, pin)Be\ Li\(d,p)Li] B\()(d, p)B \1 Li\ (d, p)Li\ B\}(d,p)B \2 At the end of the Second World War, I was working at the University of S. Paulo, Brazil, with a slow meson triggered cloud chamber, which I had Through analysis of the tracks, we obtained a built in collaboration with Ugo Camerini and A. range-energy relation for protons up to about 10 Wataghin. I sent pictures obtained with this cloud MeV, which was used for several years in research chamber to Giuseppe P.S. Occhialini, who had where single charged particles were detected, e.g., recently left Brazil and had joined Cecil F. Powell pions and muons.1 at Bristol. Upon receiving from Occhialini positive In the same experiment, I placed borax- prints of photomicrographs of tracks of protons and loaded plates, which Ilford had prepared, at my a-particles, obtained in a new concentrated request, in the direction of the beam of neutrons emulsion just produced experimentally by Ilford from the reaction Ltd., I immediately wrote to him asking to work with the new plates, which obviously opened great nQ, possibilities. Occhialini and Powell arranged for a Sj1 + H }^ Cl1 + grant from the University of Bristok I somehow managed to get to Bristol during the wmter of 1946. which gives a peak of neutrons at about 13 MeV. I was given the task o y obtaining the The idea, which worked well, was to obtain the shrinkage factor of the new en^ifision (which was energy and momentum of neutrons, irrespective of much more concentrated than the old ones); their direction of arrival (which was not known), Occhialini and Powell w^rfe still at work on n-p through the reaction scattering at around 10 MeV, using the old n \, + Bl° => He* + He* + H :; I asked Occhialini, who had decided to * Reprinted from Proceedings of the Symposium on the take a vacation in the Pyrenees (Pic-du-Midi and History of Particle Physics - The Birth of Particle environs), to take with him for about a one month Physics, Fermi lab, May I960. 9 CL c* o ' My work in Meson Physics with Nuclear Emulsions Cesar Lattes exposure, boxes of emulsions; some were loaded were also obtained in the same plates and the with borax, some were normal plates (without results published in the same volume of Nature'^. borax). All were made of the new concentrated B1 Having one and a half ‘double mesons’ type emulsion for which a range-energy relation which seemed to correspond to a fundamental already existed. The normal plates were to be used process (although it could have been an exothermal for the study of low energy cosmic rays and as a reaction of the type => + ju+A, the control, to see if we were detecting cosmic ray Bristol group realized that one should quipkly get neutrons. more events. I went to the Department of When Occhialini pi/oc^ssedjthe emulsions,„ . Geography rf Bristol University.aijd^found that on the same night in whiclyne arrived back Tram 2 r 'l \ there was a meteorological station at about 18600- vacation», it became clear that borax loaded - \ x / . ft, about 20 km by road from the capital of Bolivia, emulsions had many more events than the unloaded s p <■ La Paz. I therefore proposed to Powell and ones; borax somehow kept the latent image from ^ *** Occhialini that if they could get funds for me to fly fading; normal plates had a great amount of fading. to South America, I could take care of exposing The variety of events in the borax plates, and the /lu i, borax loaded plates at Chacaltaya Mountain for one richness in detail, made it obvious that the neutron month. That was done and I left Bristol with several energy detection was but a side result. The normal borax-loaded plates plus a pile of pound notes events seen in the plates were such as to justify sufficient to carry me to Rio de Janeiro and back. putting the full force of the Laboratory into the Contrary to the recommendation of Professor study of normal low-energy cosmic ray events. Tyndall, Director of the H.H. Wills Laboratory, I After a few days of scanning, a young lady — took a Brazilian airplane, which was wise, since the M arietta Kurz — found an u n c i a l event*1: one British plane crashed in Dakar and killfed/jall its j stopping meson and, emeramsyi&n its end, a new p a s s e n g e r s C ^ \ [ j a ( ^ U # $ S ) meson of about 600 f.i range^N contained in the After the agreed ^ime I deveroped/ one V emulsion. I should add that mesons are easily plate in La Paz. The water was not appropriate and I distinguished from protons in the emulsion we the emulsion turned out stained. Even so it was used, due to^their much larger scattering and their possible to find a complete ‘double meson’ in this variation of grain density with range. A few days plate; the .range of the secondary was also around later a secorid ‘double’ meson was found; 60°i-i! j\ji 1 I nr ^fortunately in, this case the secondary did n^t stop BatK in Bristol She plated (wbe *duly in the emulsion|b«t-©rrc'cauldy^iKi<,,<(t^ slud> iirglts processed and scanned; about 30 ‘double mesons’ icuHzaiion” (grain 'Cntrrrt^ng), that its exprSffiffiffe'd were found. It was decided that I should try to get range was also ~600 |i. The first results on the the mass ratio of the first and second mesons by ‘double’ meson were published in Nature" By the doing repeated grain counting on the tracks. The way, the cosmic ray neutrons (direction, energy) results convinced us that we were dealing with a fundamental process4. We identified the heavier # N.E.: The sequence in which the two first mesons were meson with the Yukawa particle and its secondary observed was mistaken in the text; the one with a with Carl Anderson’s mesotron. A neutral particle complete secondary within the emulsion’s thickness was of small mass was needed to balance the momenta. actually the second to be found. 10 • » Cesar Lattes My work in Meson Physics with Nudeur Emulsions J . a . J c tllL ß / y\.^ At the pid of 1947 I left Bristol with a Rockefeller Scholarship with the ipfention of trying ' C.M.G. Lattes, H. Muirhead, G.P.S. Occhialini, to detect artificially produced pions at the C.F. Powell, “Processes Involving Charged 184”cyclotron which h^started operation at Mesons”, Nature 159 (1947), 694-7. Berkeley, CaliforniarTne beam of a-particles was C.M.G. Lattes and G.P.S. Occhialini, only 380 MeV (95 MeV/nucleon), an energy “Determination of the Energy and Momentum of insufficient to produce pions. I took my chance on Fast Neutrons in Cosmic Rays”, Nature 159 (1947), the ‘favourable’ collisions in which tne internal 331-2. momentum of a nucleon in the a and the 4 C.M.G. Lattes, G.P.S. Occhialini and C.F. Powell, momentum of the beam provided sufficient energy “Observations on the Tracks of Slow Mesons in in the centre o f mass s^sT2i?TbQsu1tssfiowed that Photographic Emulsions”, Nature 160 (1947), 453- mesons were indeed being produced. Two papers 6 and 486-92; C.M.G. Lattes, G.P.S. Occhialini and describe the method of detection and the results, the C.F. Powell, “A Determination of the Ratio of the first referring to negative pions, the second to Masses o f n~ and |i~ M esons by the Method of positive5. By making use of the range of pions and Grain-Counting”, Proc. Phys. Soc. (London) 61 their curvature in a magnetic field, it was possible (1948), 173-83. to estimate the masses to be about 300 electron 5 Eugene Gardner and C.M.G. Lattes, “Production masses. of Mesons by the 184-lnch Berkeley Cyclotron”, Around February 1949, 1 was preparing to Science 107 (1948), 270-1; John Burfening, Eugene leave Berkeley to return to Brazil. At that time Gardner and C.M.G. Lattes, "Positive Mesons Edwin McMillan, who had his 300 MeV electron Produced by the 184-Inch Berkeley Cyclotron”, synchrotron in operation, asked me to look at some Phys. Rev. 75 (1949), 382-7. plates which had been exposed to y - rays from his machine. In one night 1 found about a dozen pions, both positive and negative and the next morning delivered to McMillan the plates and maps which allowed the finding of the events. I do not know what use McMillan made of the information, but there is no doubt that they were the first artificially photoproduced pions detected. 1 C.M.G.Lattes, P.H.Fowler, and P. Cuer, “Range- Energy Relation for Protons and a-Particles in the New Ilford ‘Nuclear Research’ Emulsions”, Nature 159 (1947), 301-2; C.M.G. Lattes, P.H. Fowler and P. Cuer, “A Study of Nuclear Transmutations of Light Elements by the Photographic Method”, Proc.