Investigation of post-irradiation molecular changes in LiPCDA-based radiochromic film (Gafchromic EBT4) using Raman spectroscopy

Although radiochromic films are widely used for two-dimensional radiation measurements, the radiation-induced polymerization that produces their radiochromic response remains incompletely characterized. In this study, we investigated the radiation-induced chemical and structural changes in the active layer of a currently available radiochromic film (GafchromicTM EBT4) using Raman spectroscopy in relation to dose-dependent radiochromic reactions. By examining the Raman shift spectra across an absorbed dose range of up to 20 Gy, the analysis revealed consistent and systematic modifications in the vibrational bands associated with the polymeric active layer. After normalization using the 1530 cm-1 peak that was specifically observed in EBT4, a key Raman peak around 1455 cm-1 was identified as a highly sensitive marker of dose response, showing distinct, monotonic intensity increases with increasing radiation dose. The clear separation between dose levels confirms that this principal vibrational feature directly reflects the underlying diacetylene polymerization and radical formation mechanisms triggered by ionizing radiation. The dose-response behavior was well described by a simple increasing curve that showed a shape similar to that of the green color intensity. These new findings, including the successful normalization based on the 1530 cm-1 reference peak, provide an important basis for elucidating the molecular-level mechanisms underlying the radiochromic reactions in EBT-series films.