how antimatter was first photographed

The Mechanism

*Carl David Anderson* (born New York City, *3 September 1905*; died San Marino, California, *11 January 1991*, aged 85) was an American experimental physicist who, on the night of *2 August 1932* at the *California Institute of Technology* in Pasadena, photographed the first cosmic-ray cloud-chamber track ever recorded of a positively-charged particle of electron mass — the *positron*, the *antielectron*, the first directly-detected piece of antimatter. The experiment was the centrepiece of his Caltech PhD thesis under the cosmic-ray physicist *Robert A. Millikan*. The apparatus was a *vertical magnetic cloud chamber* — a glass-walled cylinder filled with saturated water vapour at slightly above atmospheric pressure, mounted between the poles of a powerful electromagnet that produced a *magnetic field of up to 2.4 tesla* (over fifty thousand times the strength of the Earth's field). When the chamber was suddenly expanded by withdrawing a piston at its base, the gas inside cooled adiabatically and the vapour became supersaturated; any charged particle passing through the chamber at that moment ionised the gas along its path, and the supersaturated vapour condensed onto the ions in a thin trail of microscopic droplets. The path was photographed and the curvature of the track in the magnetic field gave both the *sign of the charge* (left or right curvature) and the *momentum* of the particle (the radius of curvature multiplied by the field strength). The chamber was housed in a small wooden hut on the *roof of the Guggenheim Aeronautical Laboratory* at Caltech, eight floors above ground; the *600-kilowatt DC generator* needed to power the electromagnet was in the basement of the same building, fed by an underground cable. Anderson's key technical innovation, introduced earlier in 1932, was to install a *6-mm-thick lead plate* across the middle of the chamber. A charged particle passing through the chamber would have to traverse the lead, lose energy in the process, and have its curvature increase on the far side. The lead plate solved a difficult problem in cosmic-ray cloud-chamber analysis: from a single bent track, you could not tell whether the particle was moving up or down — the same curved line could be a negative particle moving in one direction or a positive particle moving the opposite. With the lead plate in place, the side of the plate with the *tighter curvature* was unambiguously the *exit* side; the direction of motion was solved; and the sign of the charge could be read off without ambiguity. On the evening of *2 August 1932*, the chamber recorded what Anderson logged as *"Track 75 — through Pb, showing energy change or double ejection."* The track entered the chamber from below, curved gently as it passed through the lower half, traversed the 6-mm lead plate at the centre, and emerged into the upper half with the curvature in *the same rotational direction* — but tighter, by a measurable factor, indicating that the particle had lost about 38 MeV of energy in the lead. The mass-to-charge ratio computed from the track's curvature radius before and after the lead was *consistent with the electron mass to better than 20%*, and inconsistent with the proton mass (which is about 1,836 times the electron mass) by a factor of more than ten. The lower-half curvature direction, given the direction of motion fixed by the lead-plate energy loss, identified the particle as *positively charged*. A positively-charged particle with the mass of the electron had no place in any theory of matter as written in the summer of 1932: the only known positive particle was the proton. Anderson photographed the track at *9:30 in the evening*. He spent the next several hours examining the photograph under a magnifying glass with his thesis advisor Millikan; the two argued for most of the night about whether the track could be explained as a proton, or a deflected electron entering from a wrong angle, or any of three other artefacts. By the next morning Anderson was certain. He wrote up the result as a short note titled *"The Apparent Existence of Easily Deflectable Positives,"* submitted it to *Science* on *1 September*, and published it on *9 September 1932* (*Science* vol. 76, no. 1967, pp. 238-239). A longer paper with the full analysis of multiple tracks followed in *Physical Review* in March 1933 ("The Positive Electron," vol. 43, pp. 491-494). The implications were immediate and enormous. In 1928, the British theoretical physicist *Paul Dirac* had written down a relativistic wave equation for the electron — the *Dirac equation* — that, mathematically, had two solutions for every value of the momentum: one with positive energy (the ordinary electron) and one with negative energy. Dirac, finding the negative-energy solutions paradoxical, had reinterpreted them as describing *a positively-charged particle of the electron's mass*, originally identifying this hypothetical particle with the proton (1929) and then, in 1931, after Hermann Weyl pointed out that the new particle had to have *exactly* the electron's mass and could not be the proton, predicting it as a then-unobserved *new* particle that he called the *antielectron*. Anderson, working in Pasadena, had never read Dirac's 1931 paper and made the positron discovery *independently of the Dirac prediction*; his Nobel Lecture in 1936 notes wryly that "the discovery of the positron was wholly accidental." The connection between the experimental detection and the Dirac prediction was made over the next year by *Patrick Blackett* and *Giuseppe Occhialini* at the Cavendish Laboratory in Cambridge, who in early 1933 published a series of cloud-chamber photographs showing *electron-positron pairs* produced together by cosmic rays — the first direct observation of *pair production*, the process by which a high-energy photon converts into a particle and its antiparticle, the empirical realisation of the Dirac equation's central prediction. The *1936 Nobel Prize in Physics* was awarded to Anderson, *"for his discovery of the positron"*; he shared the prize with the Austrian *Victor Hess*, who had discovered cosmic rays in 1912. Anderson was 31, the *second-youngest physicist ever to receive the Nobel* at the time. He went on, in 1936, to discover a second new particle in the same cloud chamber — the *muon* (originally the *mesotron*), a heavier cousin of the electron that turned out to have no role in atomic structure and prompted I.I. Rabi's famous reaction, *"Who ordered that?"* The positron is now the working principle of *positron emission tomography (PET)* in clinical medicine: a patient is injected with a short-lived positron-emitting radioisotope (typically *fluorine-18 fluorodeoxyglucose*), the emitted positrons annihilate with tissue electrons within 1-2 mm of the emission point, producing back-to-back 511-keV gamma rays that are detected in coincidence by a ring of scintillators around the patient. Approximately *eight million PET scans are performed worldwide per year*. The positron is also the working principle of the world's largest particle physics experiments — the electron-positron colliders at SLAC, KEK, and the planned *Future Circular Collider* at CERN — and is the simplest example of the broader principle, first written down by Dirac and first photographed by Anderson on a roof in Pasadena on the evening of 2 August 1932, that every kind of matter has a mirror-image antimatter counterpart.