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Group of History and Theory of Science
Lattes' description of the
discovery
of the pi meson
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The description of the discovery of the pi meson,
found below, was written by prof. César Lattes in 1984. It was first
published in: BELLANDI FILHO, José & PEMMARAJU, Ammiraju (eds.).
Topics
in cosmic rays. 2 vols. Campinas: Editora da UNICAMP, 1984, vol. 1,
pp. 1-5. You can obtain a copy of the whole text
in Word for Windows format.
MY WORK IN MESON PHYSICS WITH NUCLEAR EMULSIONS
Cesare Mansueto Giulio Lattes
At the end of World War II, I was working at the
University of São Paulo, Brazil, with a slow-meson-triggered cloud
chamber that I had built in collaboration with Ugo Camerini and A. Wataghin.
I sent pictures obtained with this cloud chamber to Giuseppe P. S. Occhialini
who had recently left Brazil and had joined Cecil F. Powell at Bristol.
On receiving from Occhialini positive prints of photomicrographs of tracks
of protons and a particles, obtained in a new
concentrated emulsion just produced experimentally by Ilford Ltd., I immediately
wrote to him asking to work with, the new plates, which obviously opened
great possibilities. Occhialini and Powell arranged for a grant from the
University of Bristol; I somehow managed to get to Bristol during the winter
of 1946.
I was given the task of obtaining the shrinkage
factor of the new emulsion (which was much more concentrated than the old
ones); Occhialini and Powell were still at work on n-p scattering at around
10 MeV, using the old emulsions. I decided that the time allotted to me
at the Cambridge Cockroft-Walton accelerator, which provided artificial
disintegration particles as probes for the shrinkage factor, was sufficient
for a study of the following reactions:
D (d, p) H31
Li63 (d, p) Li73
Li73 (d, p) Li83
Be94 (d, p 2n) Be84
B105 (d, p) B105
B115 (d, p) B125
Through analysis of the tracks, we obtained a range-energy
relation for protons up to about 10 MeV that was used for several years
in research where single charged particles were detected (e.g., pions and
muons) (1).
In the same experiment I placed borax-loaded plates,
which Ilford had prepared at my request, in the direction of the beam of
neutrons from the reaction
which gives a peak of neutrons at about 13 MeV. The idea, which worked
well, was to obtain the energy and momentum if neutrons, irrespective of
their direction of arrival (which was not known), through the reaction
n0 + B105 ®
He42 + He42 + H31
Occhialini and I decided that he should take some plates
to the Pic-du-Midi in the Pyrenees for an exposure of about one month;
some were loaded with borax, and some were normal plates (without borax).
All were made of the new concentrated B1-type emulsion for which a range-energy
relation already existed. The normal plates were to be used for the study
of low-energy cosmic rays and as a control, to see if we were detecting
cosmic-ray neutrons.
When Occhialini processed the emulsion after their
recovery, on the same night on which they were received in Bristol it became
clear that borax-loaded emulsions had many more events than the unloaded
ones; borax somehow kept the latent image from fading; normal plates had
a great amount of fading. The variety of events in the borax plates, and
the richness in detail, made it obvious that the neutron energy detection
was but a side result. The normal events seen in the plate were such as
to justify putting the full force of the laboratory into the study of normal
low-energy cosmic-ray events. After a few days of scanning, a young lady,
Marietta Kurz, found an unusual event: one stopping meson and, emerging
from its end, a new meson of about 600 m range,
all contained in the emulsion. I should add that mesons are easily distinguished
from protons in the emulsion we used because of their much larger scattering
and their variation of grain density with range. A few days later, a second
"double" meson was found; unfortunately , in this case the secondary did
not stop in the emulsion, but one could guess, by studying its ionization
(grain counting), that its extrapolated range was also about 600 m.
The first result on the double mesons were published in Nature(2).
By the way, the cosmic-ray neutrons (direction, energy) were also obtained
in the same plates, and the results were published in the same volume of
Nature (3).
Having one and a half double mesons that seemed
to correspond to a fundamental process (although it could have been an
exothermal reaction of the type m–
+ Xba ® Xba-2
+ m+), the Bristol group realized
that one should quickly get more events. I went to the Department of Geography
of Bristol University and found that there was a meteorological station
at about 18,600 ft some 20 Km by road from the capital of Bolivia, La Paz.
I therefore proposed to Powell and Occhialini that if they could get funds
for me to fly to South America, I could take care of exposing borax-loaded
plates at Chacaltaya Mountain for one month. That was done, and I left
Bristol with several borax-loaded plates plus a pile of pound notes sufficient
to carry me to Rio de Janeiro and back. Contrary to the recommendation
of Professor Tyndall, director of the H. H. Wills Physical Laboratory,
I took a Brazilian airplane, which was wise, because the British plane
crashed in Dakar and killed all its passengers.
After the agreed time, I developed one plate in
La Paz. The water was not appropriate, and the emulsion turned out stained.
Even so, it was possible to find a complete double meson in this plate;
the range of the secondary was also around 600 m.
Back in Bristol, the plates were duly processed
and scanned; about 30 double mesons were found. It was decided that I should
try to get the mass ratio of the first and second mesons by doing repeated
counting on the tracks. The result convinced us that we were dealing with
a fundamental process (4). We
identified the heavier meson with the Yukawa particle and its secondary
with Carl Anderson's mesotron. A neutral particle of small mass was needed
to balance the momenta.
At the end of 1947, I left Bristol with a Rockefeller
scholarship with the intention of trying to detect artificially produced
pions at the 184-in. cyclotron that had started operation at Berkeley,
California. The beam of a particles was only
380 MeV (95 MeV per nucleon), an energy insufficient for producing pions.
I took my chance on the "favourable" collisions in which the internal momentum
of a nucleon in the a and the momentum of the
bean provided sufficient energy in the center-of-mass system. The results
showed that mesons were indeed being produced. Two papers describe the
method of detection and the results, the first referring to negative pions,
the second to positive (5). By making
use of the range of pions and their curvature in a magnetic field, it was
possible to estimate the masses to be about 300 electron masses.
Around February 1949, I was preparing to leave Berkeley
to return to Brazil. At that time, Edwin McMillan, who had his 300 MeV
electron synchrotron in operation, asked me to look at some plates that
had been exposed to g rays from his machine.
In one night I found about a dozen pions, both positive and negative, and
the next morning I delivered to McMillan the plates and maps that 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.
Notes
1. C.M.G. Lattes, R.H.Fowler, and R.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, R.H.
Fowler, and R.Cuer, "A Study of the Nuclear Transmutations of Light Elements
by the Photographic Method", Proc. Phys. Soc. (London) 59
(1947), 883-900. BACK
2. C.M.G.Lattes, H.Muirhead, G.P.S.Occhialini, and
C.F.Powell, "Processes Involving Charged Mesons", Nature 159
(1947), 694-7. BACK
3. C.M.G.Lattes and G.P.S.Occhialini, "Determination
of the Energy and Momentum of Fast Neutrons in Cosmic Rays", Nature159
(1947), 331-2. BACK
4. C.M.G.Lattes, G.P.S.Occhialini, and C.F.Powell,
"Observation on the Tracks of Slow Mesons in Photographic Emulsions”, Nature160
(1947) 453-6 and 486-92; C.M.G.Lattes, G.P.S.Occhialini, and C.F.Powell,
"A Determination of the Ratio of the Masses of m–
and p– Mesons by the Method of Grain-Counting",
Proc.
Phys. Soc. (London) 61 (1948), 173-83. BACK
5. Eugene Gardner and C.M.G. Lattes, "Production of
Mesons by the 184-Inch Berkeley Cyclotron," Science 107 (1948),
270-1; John Burfening, Eugene Gardner, and C.M.G.Lattes, "Positive Mesons
Produced by the184-Inch Berkeley Cyclotron", Phys. Rev. 75
(1949), 382-7. BACK
César
Lattes' paper on the discovery of the pi meson
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here, in Word for Windows 6.0. The file clattes.doc is 17.0
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