Terahertz imaging and spectroscopy of papyrus phantoms

Terahertz (THz) radiation is known to penetrate layers of materials opaque to the visible light, discriminating among materials with similar elemental composition and low atomic number such as organic molecules. THz imaging is useful to investigate layered structures such as medieval manuscripts (e.g., pages stuck to each other and materials re-used in bookbinding structures), painting layers, or multi-layered papyrus fragments. The Herculaneum papyri are a collection of more than 1,800 manuscripts that were charred during the 79 AD Vesuvius eruption. Some of these papyri are rolled and others are unrolled. A portion of the latter is written on both sides but permanently sticked to kraft paper, so the “verso” side is unreadable. Any attempt to open the rolled scrolls or to remove the extremely fragile papyrus fragments from their kraft paper, results in irreparable damage. Therefore, an imaging technique with high penetration through papyrus is needed. X-ray phase-contrast tomography (XPCT) technique revealed many portions of text hidden inside scrolls without opening them. However, THz offers advantages such as using non-ionising radiation and portable instruments. We employed THz frequency domain spectroscopy and imaging to analyse carbonised papyrus phantom sample, written using a black-carbon based ink. We observed different values of transmittance and reflectance in the 0.1 - 0.4 THz range, according to the degree of carbonisation of the papyrus fiber. We also verified, by imaging a handwritten letter on a carbonised papyrus phantom, that carbon-based ink has a measurable contrast at 0.1 THz.