The Flash of Genius, 14
Collected by Alfred B. Garrett Ohio State University Columbus
The Positron:
T h e duality of particles in n a t u r e a n d hence the theory of anti-matter-was redly established with the discovery of the positron in 1832. The discovery had been recognized but the significance not realized after the identification of the proton and electron in the latter part of the 19th century and the early part of the 20th. This new duality of particles is termed "particle twins," i.e., they are oppositely charged particles which have the same properties. The verv idea of the existence of anv chareed narticle. other than the p k & n (+) and the electron (I) was ,inconceivable t o most of these earlier scientists. Diracl in 1928, had produced a brilliant treatment of the application of the quantum theory to the electron for the equation which he derived successfully collated and unified a variety of diverse ideas and calculations. Hans Bethe called i t "the greatest paper of our century." Bul W. Paulia concluded that even though the Dirao equation was a wonderful contribution to physics, it must be erroneous beeituse i t entailed the existence of positive electrons. Experimental evidence for the positron was obtained from "fog tracks" produced in a cloud chamber. Thelength of these tracks, their width, branching, and direction of curvature are all clues to the nature of the particle that produces them. Over a period of several years nncleer scientists had been observing some "fog
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' DIEAC,P. A. M., Proc. Royal. Soc.,117,610 (1928). PAULI,W., Handbueh der Physik,242 (1933).
C. D. Anderson
tracks" that seemed to bc the correct type to have heen caused by electrons but their curvature was in the wrong direction. These tracks were commonly referred t o as "curving the wrung way," "moving backward,'' "falling back into the sourrc," e t c s Such tracks were dso often referrod to as "dirt tracks" or just spurious. I n this hostile atmosphere, C. D. Anderson proposed his hypothesis "I muat assume s particle the mass of an electron but of positive charge." He published his conrlusion first in a note4 rn follows:
"Up to the present a positive electron has always been found with an associated mass 1850 times that associated with the negative electron. In measuring the energies of charged particles produced by cosmic rays some tracks have recently been found which seem to be produced by positive particles, but if so the masses of these particles must be small compared to the mass of the proton. The evidence for this statement is found in several photographs, three of which are discussed below. HANBON, N. R., Bdish Joumol for the Philosophy of Science, 47, 194 (1961); 48, 299 (1962). 'ANDERSON,C. D., Sdace,76,238-9 (1932).
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"ln one instance, in which a lead plate of 6 mm thickness was inserted in the cloud-chamber, tracks of a particle were observed above and below the lead. The curvature due to the magnetic field was measurable both above and below the lead. There are the following alternative interpretations: A pmitive particle of small mass penetrates the lesd plate and loses shout two thirds of its energy; or Two particles are simultaneously ejected from the lead, in one direction a positive particle of small mass, in the opposite direction an electron; or An electron of about 20 million volts energy penetrates the lead plate and emerges with an energy of 60 million volts, having gained 40 million volts energy in traversing the lead; or The chance occurrence of two independent electron tracks in the chamber, so placed as to give the appearance of one particle traversing the lead plate.
"In another instance two tracks of opposite curvature appear below the lead. The alternate interpretations are: A positive particle of small mess and an electron emerging from the same point in the lead; or A positive particle of smdl mass strikes the lead and rebounds with a loss in energy; or An electron of sbout 20 million volts energy strikes the lead and rebounds with 30 million volts energv; or The chance occurrence of two independent electron tracks.
"In the third instance two tracks appear below the lead plate. The alternative interpretations are: A pmitive particle of small mass and another positive particle emerge from the same point in the lead; or A 4 million volt electron rebound8 from the lead producing the second track; but here a difficulty is met with, since a change in the sign of the charge would have to he assumed to take place in the rebound of the electron; or The chance occurrence of two independent tracks.
"For the interpretation of these effects it seems necessary to call upon a positively charged particle having a mass comparable with that of an electron, or else admit the chance occurrence of independent tracks on the same photagraph so placed as to indicate a common point of origin of two particles. The latter possibility on a probability basis is exceedingly unlikely. ' "The interpretation of these tracks as due to protons, or other heavier nuclei, is mled out on the basis of range and curvature. Protons or heavier nuclei of the observed curvatures could not have ranges as great as those observed. The specific-ionization is close to that for an electron of the same curvature, hence indicating a positively-charged particle comparable in mass and magnitude of charge with an electron." The next year he published a more detailed report5:
"On August 2,1932, during the course of photographing cosmic-ray tracks produced in a vertical Wilson chamber (magnetic field of 15,000gauss) designed in the summer of 1930 by Professor R. A. MiUikan and the writer, tracks were obt~ined which seemed to be interpretable only on the basis of the existence in this case of a particle carrying a positive charge but having a mass of the same order of magnitude as that normally possessed by a free negative electron. Later study of the photograph by a whole group of men of the Norman Bridge Laboratory only tended to strengthen this view. The reason that this interpretation seemed so inevitable is that the track appearing on the upper half of the figure cannot possibly have a mass as large as that of a proton for as soon as the mass is fixed the energy is a t once fixed by the curvature. The energy of a proton of t h ~ curvature t comes out 300,000 v, but it proton of that energy according to well established and universally accepted determinations has a total range of about 5 mm in air while that portion of the range actually visible in this case exceeds 5 cm without a noticeable change in curvature. The only escape from this conclusion would he to assume that a t exactly the same instant (and the sharpness of the tracks determines that instant to within about a fiftieth of a second) two independent electrons happened to produce two tracks so placed as to give the impression of a single particle shooting through the lead plate. This assumption was dismissed on a probability basis, since a sharp track of this order of curvature under the experimental condition prevailing occurred in the chamber only once in some 500 exposures, and since there was practically no chance a t all that two such tracks should line up in this way. We also discarded as completely untenable the assumption of an electron of 20 million v entering the lead on one side and coming out with an energy of 60 million v on the other side. A fourth possibility is that a photon, entering the lead from above, knocked out of the nucleus of a lead atom two particles, one of which shot upward and the other downward. But in this case the upward moving one would be a positive of small mass so that either of the two possibilities leads to the existence of the positive electron. "In the course of the next few weeks other photographs were obtained which could be interpreted logically only on the positive-electron basis, and a brief report was then published with due reserve in interpretation in view of the importance and striking nature of the announcement." 6
ANDERSON, C. D., Phys Rev., 43, 4 9 2 4 (1933).
forces. With such an undemtanding, an intelligent citizen can make intelligent judwents between good and bad policy on sciatzfie and technical matters. W i t b u t it, he can hardly participate fully i n a scientific demoemey.
GLENN T . SEABOBG Chairman, AEC
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