IMAGING WITH X-RAYS

Illuminating the Archimedes Palimpsest with X-rays

The Stanford Synchrotron Radiation Laboratory (SSRL) at SLAC makes intense x-ray beams that can reveal the nature of molecules and atoms within a material. The x-ray beam can be used for research like mapping out the three-dimensional shape of proteins, or showing how atoms are arranged in chemical pollutants. X-rays can also be used to illuminate secrets hidden in a 1,000-year old parchment without damaging the delicate pages.

Synchrotron radiation is created when electrons traveling near the speed of light are made to follow a curved path around a storage ring. Bending the path of an electron causes it to emit electromagnetic radiation—light—that ranges up the spectrum from infrared to x-rays.

Synchrotron light is a powerful tool. X-ray florescence is typically used to detect small concentrations of metals and other elements. For the Archimedes Palimpsest, the 13th century monk failed to scrape all of the ink from the parchment fibers when he scrubbed off the original text. Scientists use x-ray fluorescence to detect traces of iron pigment in the remnants of that ancient ink.

Other techniques—notably multi-spectral imaging—have revealed most of the pages, certain pages are covered in gold paint and stains, which only x-rays can penetrate. The bright synchrotron beam passes through the document, makingthe iron in the ink fluoresce.

A special frame holds one page of the palimpsest at a time and slowly moves it in front of the x-ray beam like a book being read by a stationary eye. The hair-thin x-ray beam rests on any one spot of parchment for only a few milliseconds to prevent damaging the delicate fibers.

The x-ray beam is tuned to a specific energy to trigger fluorescence. An iron atom has 26 electrons in different orbits around its nucleus. An x-ray tuned to 7.1 kilo-electron volts (keV) will knock an electron out of the innermost orbit of an iron atom. Missing an electron, the unstable atom immediately fills the hole by grabbing an electron from an outer orbital. The replacement electron emits x-ray fluorescence as it falls into its new place. The fluorescent signal has 6.4 keV — the detectors are set to pick up signals of this energy, which can come only from iron atoms.

The detector builds an image dot by dot, mapping out each speck of iron-containing ink. Researchers watch the images form on their computer screens, and character recognition software helps decipher the ancient Greek letters. Scholars start reading the letters immediately, comparing the x-ray data with other information to help pick apart the two sets of text and decipher Archimedes’ thoughts. In this way, during a two-day period, Reviel Netz, Stanford University Professor of Classics and Philosophy, transcribed a previously unknown page of On Floating Bodies in the original Greek for the first time.

The palimpsest is actually a tapestry of many texts. The monk needed more pages for his prayer book than the Archimedes parchment provided, so he also palimpsested other texts and added them to the book. Using multispectral images, scholars recently discovered another gem hidden beneath the monk’s script: the first texts by ancient Athenian orator Hyperides ever to be found in a codex, the modern form of the book. SSRL’s x-ray beam will be used to retrieve more of this text as well.

The Walters Art Museum hopes to finish imaging and conservation of the palimpsest by 2007. Transcriptions of the complete text and images of several pages are already available for scholars and the public online.