title: A spectroscopic study of the chromatic properties of GafChromicEBT3 films.
abstract: OBJECTIVE
  This work provides an interpretation of the chromatic properties of GafChromicEBT3 films based on the chemical nature of the polydiacetylene (PDA) molecules formed upon interaction with ionizing radiation. The EBT3 films become optically less transparent with increasing radiation dose as a result of the radiation-induced polymerization of diacetylene monomers. In contrast to empirical quantification of the chromatic properties, less attention has been given to the underlying molecular mechanism that induces the strong decrease in transparency.
  
  
  METHODS
  Unlaminated GafChromicEBT3 films were irradiated with a 6 MV photon beam to dose levels up to 20 Gy. The optical absorption properties of the films were investigated using visible (vis) spectroscopy. The presence of PDA molecules in the active layer of the EBT3 films was investigated using Raman spectroscopy, which probes the vibrational modes of the molecules in the layer. The vibrational modes assigned to PDA's were used in a theoretical vis-absorption model to fit our experimental vis-absorption spectra. From the fit parameters, one can assess the relative contribution of different PDA conformations and the length distribution of PDA's in the film.
  
  
  RESULTS
  Vis-spectroscopy shows that the optical density increases with dose in the full region of the visible spectrum. The Raman spectrum is dominated by two vibrational modes, most notably by the ν(C≡C) and the ν(C=C) stretching modes of the PDA backbone. By fitting the vis-absorption model to experimental spectra, it is found that the active layer contains two distinct PDA conformations with different absorption properties and reaction kinetics. Furthermore, the mean PDA conjugation length is found to be 2-3 orders of magnitude smaller than the crystals PDA's are embedded in.
  
  
  CONCLUSIONS
  Vis- and Raman spectroscopy provided more insight into the molecular nature of the radiochromic properties of EBT3 films through the identification of the excited states of PDA and the presence of two PDA conformations. The improved knowledge on the molecular composition of EBT3's active layer provides a framework for future fundamental modeling of the dose-response.
authors.affiliation: Wave Propagation and Signal Processing, KU Leuven-KULAK, Kortrijk 8500, Belgium.
authors.forename: M
authors.initials: M
authors.lastname: Callens
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven 3000, Belgium.
authors.forename: W
authors.initials: W
authors.lastname: Crijns
authors.affiliation: imec, Kapeldreef 75, Leuven 3001, Belgium.
authors.forename: V
authors.initials: V
authors.lastname: Simons
authors.affiliation: imec, Kapeldreef 75, Leuven 3001, Belgium and Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium.
authors.forename: I
authors.initials: I
authors.lastname: De Wolf
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven 3000, Belgium.
authors.forename: T
authors.initials: T
authors.lastname: Depuydt
authors.affiliation: Department of Electrical Engineering, ESAT/PSI, KU Leuven, Leuven 3001, Belgium.
authors.forename: F
authors.initials: F
authors.lastname: Maes
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven 3000, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: Haustermans
authors.affiliation: Department of Cardiovascular Sciences, KU Leuven, Leuven 3000, Belgium.
authors.forename: J
authors.initials: J
authors.lastname: D'hooge
authors.affiliation: DoseVue NV, Hasselt 3500, Belgium.
authors.forename: E
authors.initials: E
authors.lastname: D'Agostino
authors.affiliation: Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium.
authors.forename: M
authors.initials: M
authors.lastname: Wevers
authors.affiliation: Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium.
authors.forename: H
authors.initials: H
authors.lastname: Pfeiffer
authors.affiliation: Wave Propagation and Signal Processing, KU Leuven-KULAK, Kortrijk 8500, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: Van Den Abeele
mesh_terms: D003116:Color; D005373:Film Dosimetry; D013059:Spectrum Analysis, Raman