Who discovered positron

The new particles

7.2 First antimatter - the positron

At the end of the twenties of the 20th century it had finally been clarified that the secondary Cosmic rays, which we can measure at the earth's surface, is not ultra-high-energy γ-radiation, but particle radiation, the more precise properties of which were not yet completely clear, but it was assumed that electrons and protons in particular were caused by the primary Cosmic rays knocked out of the molecules of the air.

The American Robert Andrews Millikan, who was still convinced that the primary one, was not convinced Cosmic rays was γ-radiation that produced secondary particle radiation on its way through the atmosphere. To investigate this phenomenon more closely, he put one of his students on a new detector technology that was developed in Russia by a young physicist Dmitry Skobeltzyn from Leningrad. In 1924, Skobeltzyn set himself the goal of using Wilson’s cloud chamber to record the traces of electrons that the supposed cosmic ultragamma radiation emitted from the air atoms of his cloud chamber. He had the problem that electrons from the metallic wall of the cloud chamber were superimposed on the traces of electrons from the gas and made the results more difficult to analyze. Therefore, he decided to use a strong electromagnet to direct the electrons from the vessel wall out of the chamber volume (this is possible because moving electrically charged bodies in the magnetic field experience a force normal to the direction of movement, the so-called Lorentz force, and therefore be distracted on a curve). Despite the strong magnets Skobeltzyn used, there were still some particles left whose trajectories were barely curved. Skobeltzyn explained these strange traces with high-energy electrons that just came from the Cosmic Radiation knocked out of the atoms. In fact, and without knowing it, he was the first person to be the secondary Cosmic rays had proven himself. In any case, Skobeltzyn did not pursue these experiments any further, but Robert Andrews Millikan saw some potential in them.

So he commissioned his freshly graduated assistant Carl David Anderson to build a Wilson cloud chamber, to equip it with a strong electromagnet and the Cosmic rays to observe. The great achievement of Carl David Anderson was to actually build a cloud chamber in interwar America during the economic crisis, to equip it with a water-cooled electromagnet that was ten times as powerful as the one that Skobeltzyn had constructed, and thus up to 4,300 meters to drive high Pike's Peak.

The results of these measurements were surprising and in no way met the expectations of the experimenter or the client. It turned out that the secondary Cosmic rays seemed to consist of positive and negative particles in equal parts, since particle trajectories were found in roughly equal parts that were curved to the left and those curved to the right. Millikan was convinced that the positive particles had to be protons that must have been knocked out of the atomic nuclei of the atmosphere by the ultra-high-energy cosmic γ-radiation, because he could not imagine other positive particles. Anderson vehemently refused this explanation, however, as he could see that in no case could it be protons. The orbits of the positive particles were similarly strongly curved as those of the negative. So that the massive protons are deflected to the same extent as the light electrons, they have to travel much slower. But if they were slower, then the ionization density had to be much higher along their path; but there was no indication of that. Therefore, Anderson said that it could not be a question of protons, but had to be electrons that did not move from top to bottom through the cloud chamber, but from bottom to top. This explanation in turn refused Millikan, who did not see why Cosmic rays should suddenly come from below instead of from heaven.

To settle the dispute, Anderson inserted a lead plate in the middle of his cloud chamber, which should get in the way of the electrons in order to slow them down on their way. After the electrons passed through the lead plate, they must have lost energy due to interactions with the shell electrons of the lead atoms and thus move a lot more slowly. As a result, they would be more strongly deflected in the magnetic field after the lead plate and, based on the radii of curvature of the particle trajectory before and after the plate, it would be possible to clearly decide whether the strange particles were downward-moving protons or upward-moving electrons.

Again, the results of the experiment disappointed the expectations of the two American scientists. Anderson was able to show very clearly that the strange particle trajectories could neither be traces of protons nor of electrons. He got a very nice picture of a particle trajectory, which indicated a particle that had roughly the same mass as the electron, but had a positive electrical charge. This made Anderson the first person who succeeded in detecting the positive antielectron, the positron, which had been predicted a few years earlier by the theoretical physicist Paul Dirac. For this achievement, Carl David Anderson was awarded the Nobel Prize in Physics in 1936 at the age of 31, together with Viktor Franz Hess.

One question that was still unanswered was where this positron came from so suddenly. Electrons and protons, which had already been found, originated, as we knew, from the nuclei and the shells of the atoms in our environment, the atmosphere, etc. The positrons, however, were antimatter, which in this form did not exist in our environment occurs. So the question of where the positrons came from was extremely difficult to answer. Anderson and Millikan soon found an answer to this question, which again turned out to be wrong. Their explanation was that the positrons were exotic particles that were associated with the Cosmic Radiation came from the depths of the cosmos.

Patrick Blackett and Giuseppe Ochialini, who worked at the Cavendish Laboratory in Cambridge, were not satisfied with the Americans' explanation. They had succeeded in decisively improving Wilson’s cloud chamber. Your idea was brilliant because it was simple. They mounted a Geiger-Müller counter tube above and below the cloud chamber and took pictures of the particle tracks in their cloud chamber precisely when both counter tubes reported an event at the same time. Then the probability was high that a particle of the Cosmic Radiation also crossed the cloud chamber on its way through both counting tubes and triggered interesting events there. What they found were so-called showers of elementary particles that formed when the Cosmic rays underwent interactions in matter. This produced large quantities of electrons and positrons. With this, Blacket and Ochialini succeeded in proving that the positrons were by no means examples of exotic matter that came as guests from distant antimatter galaxies, but that this antimatter was generated on Earth in high-energy nuclear collisions.

Although the idea of ​​combining a cloud chamber with an electromagnet came from Dmitry Skobeltzyn, the idea to take up this technology again came from Robert Andrews Millikan, although the explanations of the peculiar and new phenomena around the positron came from Patrick Blackett and Giuseppe Ochialini, but the first person to photograph positron traces was Carl David Anderson, which is why he was also awarded the Nobel Prize. [33.34]