Friday, May 30, 2014

Pierre Auger and Lise Meitner

Last week in this blog, I discussed Auger electrons and their role in determining the radiation dose to biological tissue. This week, I would like to examine a bit of history behind the discovery of Auger electrons.

Auger electrons are named for Pierre Auger (1899-1993), a French physicist. Lars Persson discusses Auger’s life and work in a short biographical article (Acta Oncologica, Volume 35, Pages 785-787, 1996)
“From the onset of his scientific work in 1922 Pierre Auger took an interest in the cloud chamber method discovered by Wilson and applied it to studying the photoelectric effect produced by x-rays on gas atoms. The Wilson method provided him with the most direct means of obtaining detailed information on the photoelectrons produced, since their trajectories could be followed when leaving the atom that had absorbed the quantum of radiation. He filled the chamber with hydrogen, which has a very low x-ray absorption coefficient, and a small proportion of highly absorbent and chemically neutral heavy gases, such as krypton and xenon. Auger observed some reabsorption in the gas, but most often found that the expected electron trajectory started from the positive ion itself. Numerous experiments enabled Auger to show that the phenomenon is frequent and amounts to non-radiactive transitions among the electrons of atoms ionized in depth. This phenomenon was named the auger effect, and the corresponding electrons auger electrons. His discovery was published in the French scientific journal Comptes Rendus as a note titled ‘On secondary beta-rays produced in a gas by x-rays’ (1925; 180: 65-8). He was awarded several scientific prizes and was also a nominee for the Nobel Prize in physics which however, he never received. He was a member of the French Academy of Science. Pierre Auger was certainly one of the great men who created the 20th century in science.”
What is most interesting to me about the discovery of Auger electrons is that Auger may have been scooped by one of my favorite physicists, Lise Meitner (1878-1968). I didn’t think I would have the opportunity to discuss Meitner in a blog about physics in medicine and biology, and her name never appears in the 4th edition of Intermediate Physics for Medicine and Biology. But the discovery of Auger electrons gives me an excuse to tell you about her. In the book Lise Meitner: A Life in Physics, Ruth Lewin Sime writes about Meitner’s research on UX1 (now known to be the isotope thorium-234)
“According to Meitner, the primary process was simply the emission of a decay electron from the nucleus. In UX1 she believed there was no nuclear gamma radiation at all. Instead the decay electron directly ejected a K shell electron, an L electron dropped into the vacancy, and the resultant Kα radiation was mostly reabsorbed to eject L, M, or N electrons from their orbits, all in the same atom. The possibility of multiple transitions without the emission of radiation had been discussed theoretically; Meitner was the first to observe and describe such radiationless transitions. Two years later, Pierre Auger detected the short heavy tracks of the ejected secondary electrons in a cloud chamber, and the effect was named for him. It has been suggested that the ‘Auger effect’ might well have been the ‘Meitner effect’ or at least the ‘Meitner-Auger effect’ had she described it with greater fanfare, but in 1923 it was only part of a thirteen-page article whose main thrust was the beta spectrum of UX1 and the mechanism of its decay.”
On the other hand, for an argument in support of Auger’s priority, see Duparc, O. H. (2009) Pierre Auger – Lise Meitner: Comparative contributions to the Auger effect. International Journal of Materials Research 100: 1162-1166.

Of course, Meitner is best know for her work on nuclear fission, described so eloquently by Richard Rhodes in his masterpiece The Making of the Atomic Bomb. Meitner was an Austrian physicist of jewish descent working in Germany with Otto Hahn. After the Anschluss in 1938, Hitler planned to expel jewish scientists from their academic positions, but also forbade their emigration. With the help of her Dutch colleague Dirk Coster (who is mentioned in IPMB because of Coster-Kronig transitions), she slipped out of Berlin in July 1938. Rhodes writes
“Meitner left with Coster by train on Saturday morning. Nine years later she remembered the grim passage as if she had traveled alone: ‘I took a train for Holland on the pretext that I wanted to spend a week’s vacation. At the Dutch border, I got the scare of my life when a Nazi military patrol of five men going through the coaches picked up my Austrian passport, which had expired long ago. I got so frightened, my heart almost stopped beating. I knew that the Nazis had just declared open season on Jews, that the hunt was on. For ten minutes I sat there and waited, ten minutes that seemed like so many hours. Then one of the Nazi officials returned and handed me back the passport without a word. Two minutes later I descended on Dutch territory, where I was met by some of my Holland colleagues.’”
Even better reading is Rhode’s description of Meitner’s fateful December 1938 walk in the woods with her nephew Otto Frisch, during which they sat down on a log, worked out the mechanism of nuclear fission, and correctly interpreted Hahn’s experimental data. Go buy Rhode’s book and enjoy the story (or listen to it here, the walk in the woods part starts at about 10 hours, 5 minutes and 42 seconds). Also, you can listen to Ruth Lwein Sime talk about Meitner's life and work here.

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