Cosmic Rays 1922-1927

With Cosmic Rays we have some more back-tracking to do.  In some ways, Cosmic Rays are a tricker topic than I expected, though I maybe should have guessed that might be since (as Bruno Rossi points out early in his 1964 classic Cosmic Rays) not only was the exact nature of the “rays” not known, but not much was known about high energy interactions in general.  Moreover, since one thing I’m trying to do in this blog is to look at what people at the time thought was going on, all I can say is that what people thought was happening with Cosmic Rays in the period 1922 to 1927 was a long way from the whole story.  But anyway, let’s start with Millikan’s work since he was the person who decided to call whatever was happening “Cosmic Rays.”  Now, of course, ironically (as often happens with these things), at first, he was not convinced that the “rays” (actually just detectable ionization from an unknown source) were “cosmic” at all.

So…back to Millikan.  When Millikan started his work on the unknown source of ionization in 1922, he had finished measuring the photoelectric effect and was about to win the Nobel Prize for measuring the charge of the electron.  He produced small electroscopes and a device to record the ionization levels over time and sent those up in a set of balloons in March and April 1922 from Kelly Field in San Antonio, Texas.  Sure enough, the average “discharge rate” of the electroscopes at five kilometers above the Earth was three times what it was at the Earth’s surface.  Since that was only 25% of what earlier balloonists had found, Millikan naturally concluded that he was right and the rays came from somewhere else and not outer space ( as Qiaozhen Xu and Laurie M. Brown in M. j. Phys Jan 1987 point out on page 30 of their article on the history of Cosmic Ray studies).  But by 1925, Millikan was convinced that the powerful rays were coming from “above” – and could penetrate 22 meters of lake water.  Moreover, the rays were extremely powerful – fifty times (says Millikan based on absorption coefficients and the Compton effect) the most powerful gamma rays that had been measured up to that point or 30 MeV in our terms (or “32,000,000 volts” in Millikan’s terms).  That’s only, say, 15 times the energy of the gamma rays from the decay of Thorium — the “hard thorium C” line (2.65 times 106 e-volts)” – but that’s energies as known to L. H. Gray in 1930.  

Millikan was sure he was onto something big and the press went wild.  The “rays” (“Millikan Rays”) were described as “ultra-x-rays” a hundred times more penetrating than your basic x-rays (cited in the Qiaozhen Xu and Laurie M. Brown article).  Millikan managed the publicity well and got people to refer to the phenomena as “Cosmic Rays” and to attribute them to the formation of atomic nuclei all over the depths of space.  So that was a generally accepted picture of what was going on with Cosmic Rays up until 1927.  And here’s Millikan on the cover of Time for April 25, 1927 looking into a microscope or something for some reason: