How X-rays Read Medieval Ink
By Heather Rock Woods

Synchrotron radiation is a powerful tool for studying the Archimedes palimpsest. In contrast to an x-ray tube, the synchrotron beam is more intense, is collimated (parallel), does not need to be focused, can be polarized, and is very easy to tune.

Iron has 26 electrons in different orbits around the nucleus. An X-ray tuned to an energy of 7.1 kilo-electron volts (keV) can knock out an electron from the innermost orbit of an iron atom.

Missing an electron, the unstable atom immediately fills the hole by grabbing an electron from a farther out orbit. Since the replacement electron has less energy (it was less tightly bound to the nucleus), as it falls into its new place it emits x-ray fluorescence, a photon with 6.4 keV, exactly the difference between the two electrons.

This creates a fluorescent signal at an energy specific to iron. The detector window is set to 6.4 keV to capture the iron signals. Like an old dot-matrix printer, the detector builds an image dot by dot, mapping out each spec of iron-containing ink.

Extra ink in one spot causes a more intense signal. Generally, the ink from the Archimedes text is no more than a faint stain in the fibers of the parchment, while the thicker, unerased ink of religious text sits on its surface. Where the two overlap (the texts are written perpendicular to each other) the iron signal is stronger, which may allow researchers to separate the two texts.

(Reprinted with the permission of Stanford Linear Accelerator Center)