Delbruck scattering

Delbruck scattering, the deflection of high-energy photons in the Coulomb field of nuclei as a consequence of vacuum polarization has been observed. However, the process of scattering of light by light, has not been observed. ^[1] In both cases, it is a process described by Quantum Electrodynamics (QED).

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The Feynmann diagram of Delbruck scattering is shown in Fig 1. The wavy line represents a photon and the double line an electron in the external field of a nucleus.

The lowest order diagram has four vertices and consists of two incoming photons, which annihilate into a virtual electron-positron pair, which then annihilates into two real photons again.

Discovery

From 1932 to 1937, Max Delbrück worked in Berlin as an assistant to Lise Meitner, who was collaborating with Otto Hahn on the results of irradiating uranium with neutrons. During this period he wrote a few papers, one of which turned out to be an important contribution on the scattering of gamma rays by a Coulomb field due to polarization of the vacuum produced by that field (1933). His conclusion proved to be theoretically sound but inapplicable to the case in point, but 20 years later Hans Bethe confirmed the phenomenon and named it "Delbrück scattering".^[2]

In 1953, Robert Wilson observed Delbruck scattering of 1.33 MeV gamma-rays by the electric fields of lead nuclei.

References

1. ^ http://puhep1.princeton.edu/~mcdonald/e144/pic98talk.ps
2. ^ Biographical Memoirs: Volume 62 pp66-117 "MAX LUDWIG HENNING DELBRÜCK 4 September 1906 - 10 March 1981" BY WILLIAM HAYES http://books.nap.edu/openbook.php?record_id=2201&page=66