Shifting Standards: Experiments in Particle Physics in the Twentieth Century

Chapter 7 “Electron Polarization”

• 65 Shull, Chase, and Myers’s (1943) experiment was designed not only to demonstrate the existence of free electron polarization (electrons with more spins pointing in one direction than in the opposite direction) but also to compare the result with theoretical predictions on electron scattering from heavy nuclei. In addition, previous experimental results were in disagreement with theoretical predictions: “In a number of experiments, starting with that of Cox, McIlwraith, and Kurrelmeyer (1928), the attempt has been made to find evidence of polarization in a beam of free electrons. The early experiments were made without the guidance of any clear theory , their methods being suggested by analogy with phenomena of light or X-rays or with the Stern-Gerlach experiment. The first scatter was expected to polarize the electrons and the second scatter would detect the polarization. The results were either negative or were rendered questionable by the results of later experiments or by theoretical developments” (Shull, Chase, and Myers 1943, 29). The lack of a theory of the phenomenon was remedied when Mott (1929, 1932), on the basis of Dirac’s electron theory, calculated that there should be a forward-backward (0º–180º) asymmetry in the double scattering of high-energy electrons (E > 30 keV), from high-Z nuclei at large angles (~90º) with targets thin enough to guarantee single scattering (figure 7.1). Shull and his collaborators discussed only three previous experiments in any detail: those of Dymond (1934) and Richter (1937), both of which gave negative results and did not support Mott’s predictions, and an experiment by Kikuchi (1940), which gave a positive result but was regarded as questionable because of the thick scattering targets used, which did not satisfy the conditions for Mott scattering. The brief history gave some of the flavor of the controversy but not the details. Although the discussion of only a few recent results is typical of modCHAPTER 7 “Electron Polarization” 66 • “Electron Polarization” ern papers,1 the authors omitted a rather interesting and complex history of previous experiments, which I will summarize briefly (for details see Franklin 1986, chapter 2). Beginning in the late 1920s, before Mott’s calculation , and continuing through the 1930s, the double-scattering experiments had been performed numerous times and the general consensus was that the results showed no evidence of electron polarization and were in disagreement with Mott’s theoretical calculation. The situation was even more complex. Dymond (1932), for example, had initially reported a positive result that was five times smaller than the theoretical prediction. He later found a considerable experimental asymmetry in his apparatus and repudiated that result in a 1934 paper (Dymond 1934). By the late 1930s a consensus had been reached that Mott’s predictions had not been confirmed . As Richter (1937) remarked, “despite all the favorable conditions of the experiment, however, no sign of the Mott effect could be observed. With this experimental finding, Mott’s theory of the double scattering of electrons from the atomic nucleus can no longer be maintained” (554). This led to several attempts by theoreticians to check Mott’s calculations and to see if other effects might explain the negative results. None were successful . During this period the only positive results reported were a series of experiments by Rupp (1929, 1930a, 1930b, 1931, 1932a, 1932b, 1932c, 1934), and it turns out that Rupp’s results were fraudulent. In 1935 he published a note (Rupp 1935) in which he withdrew the results of several, but not all, of his experiments on electron polarization. This paper included a note from a doctor stating that Rupp had suffered from a mental illness that did not allow him to distinguish between fantasy and reality. Rupp’s work merely complicated an already complex situation. Figure 7.1. Schematic diagram of the electron polarization experiment. From Shull, Chase, and Myers (1943). “Electron Polarization” • 67 In the early 1940s it was found that the number of electrons scattered at 90º from two sides of an inclined foil differed (Chase and Cox 1940). It was later shown by Goertzel and Cox (1943) that this was due to plural scattering , in which a single large-angle scatter was made up of several smallangle scatters. This effect was significant only in reflection-type experiments and not in transmission experiments (figure 7.2). Because plural scattering does not result in polarized electrons in the first scatter, the effect predicted by Mott could not be...