Maurice de Broglie and the First Observation of an X-ray Absorption Edge

A slightly modified version of Figure 15.2 from Intermediate Physics for Medicine and Biology.

If you shine x-rays through a material and measure the number absorbed by it, you create an x-ray absorption spectrum. The absorption is related to the cross section; the bigger the cross section, the more the x-rays are absorbed. Figure 15.2 from Intermediate Physics for Medicine and Biology is shown above, where the cross section for carbon is plotted as a function of the x-ray energy. I’ve drawn an oval around what’s the most interesting feature of the plot, the jump in the cross section at an energy of about 0.28 keV. This abrupt rise is known as the K edge, and is an example of an absorption edge.

The cross section jumps up when the photon’s energy rises above the binding energy of a K-shell electron [an electron in the innermost energy level]. It’s not a small effect; the cross section increases by more than a factor of ten at the K edge (note that this is a log-log plot).

When I see such a dramatic effect, I imagine how surprising it must have been for the person who observed it first. Who was the person who discovered the K edge? Maurice de Broglie.

Maurice de Broglie in 1932.

Maurice was the elder brother of the more-famous Louis de Broglie, who Russ Hobbie and I mention when talking about electron waves and the electron microscope. Maurice was born in Paris in 1875. After more than a decade in the French navy, he left the military to study physics. He was interested in x-rays, which were discovered by Wilhelm Röntgen in 1895. In 1913, Maurice published the first observation of an absorption edge (Comptes Rendus, Volume 157, Pages 924–926). When World War I began, he went back to the navy to do research on detecting U-boats (German submarines). After a long career in science, including being awarded the Hughes Medal by the Royal Society of London, he died in 1960 at the age of 85.

Farrel Lytle, an x-ray spectroscopy pioneer, tells Maurice’s story in his review article (Journal of Synchrotron Radiation, Volume 6, Pages 123–134, 1999).

Apparently it took a while to figure out that the absorption edges belonged to materials in the photographic film and not the x-ray tube or the crystal, but eventually it was all sorted out. A German scientist, Julius Hedwig (1879–1936), independently studied x-ray spectroscopy, and may have observed an x-ray absorption edge before Maurice, but he soon abandoned the work while Maurice pursued it further, becoming the father of x-ray spectroscopy.

Originally published at http://hobbieroth.blogspot.com.

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Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.

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Brad Roth

Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.