The Most Emergetic Photons Known (1931)

A small detour into radioactive photon sources before going into Cosmic Rays.  In 1929, the most energetic photons known (ie reliable sources with the highest measured energies) were the gamma rays from the decay of Thorium — the “hard thorium C” line (2.65 times 106 e-volts)” to quote L. H. Gray’s 1930 paper on what those gamma rays do when you scatter them from various elements as targets.  So that’s gamma rays at what we would call 2.65 MeV, though Schweber on page 82 of QED and the Men Who Made it, says 2.62 MeV which is presumably the modern (1990s) value of those photons.  Of course, these days we have seen and measured photons in Cosmic Rays with about 500 million times the energy of gamma rays from the hard thorium C” line.  Still, if you want to look at how electrons scatter photons, 2 or 3 MeV is a good place to notice how things are happening.

L. H. Gray, who was looking into what was going on with energetic gamma rays in 1929-1931, later went on to work on the effects of radiation on biological systems.  In his 1931 paper he summarized the earlier work on photon scattering (the Meitner is the Lise Meitner who was the first observer of nuclear fission a few years later):

 Tarrant, ‘ Proc. Roy. Soc.,’ A, vol. 128, p. 345 (1930);

 Meitner and Hupfeld, Natur wiss.,’ vol. 22, p. 534 (1930); and in ‘ Phys. Z.,’  vol. 31, p. 947,(1930)

Chao, ‘ Proc. Nat. Acad. Sci.,’ vol. 16, p. 431 (1930)

And his own earlier: (part i): ‘ Proc. Roy. Soc.,’ A, vol. 128, p. 361 (1930).

What everyone was noticing was that, at energies over about 2 MeV, the expected scattering was not quite happening as the Klein-Nishina equation based on Dirac’s work suggested that it would.  The energy was going somewhere – being absorbed and re-radiated or at least not going anywhere as the high energy photons that were expected.  Gray describes the disappearance of predicted scattering as an “increase in ionization,” ie generalized energy in the experimental space.  Which is to say, the energy is still there but not in the form of photons with their frequencies altered according to how they scattered.  Still, the Klein-Nishina formula worked very well for photon energies up to around 2 MeV.

As Pais points out in Inward Bound (pages 348-9), this correct functioning of the formula in relatively low energies happens partly because Klein and Nishina treated the radiation field semi-classically.  The purely quantum treatments of the radiation field in scattering situations done by Ivar Waller and Evgenievich Tamm in 1930 showed that electron momenta had to be summed over positive and negative values for the low energies in the purely classical Thomson Scattering range well below 2 MeV.

So, what was going on in even higher energies as we move into the 1930s?   

So again: next time: Cosmic Rays.

Compton looking for Cosmic Rays in the 1930s