Callens M, Crijns W, Simons V, De Wolf I, Depuydt T, Maes F, Haustermans K, D'hooge J, D'Agostino E, Wevers M, Pfeiffer H, Van Den Abeele K
Wave Propagation and Signal Processing, KU Leuven-KULAK, Kortrijk 8500, Belgium.
Department of Radiation Oncology, University Hospitals Leuven, Leuven 3000, Belgium.
Med Phys. 2016 Mar;43(3):1156-66. doi: 10.1118/1.4941312.
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.
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.
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.
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.
本研究基于与电离辐射相互作用时形成的聚二乙炔（PDA）分子的化学性质，对GafChromic EBT3薄膜的颜色特性进行了解释。由于二乙炔单体的辐射诱导聚合，EBT3薄膜随着辐射剂量的增加，光学透明度降低。与颜色特性的经验量化相比，对导致透明度大幅下降的潜在分子机制关注较少。
将未层压的GafChromic EBT3薄膜用6 MV光子束辐照至剂量水平高达20 Gy。使用可见光谱研究薄膜的光吸收特性。使用拉曼光谱研究EBT3薄膜活性层中PDA分子的存在，拉曼光谱可探测该层中分子的振动模式。将分配给PDA的振动模式用于理论可见吸收模型，以拟合我们的实验可见吸收光谱。从拟合参数中，可以评估不同PDA构象的相对贡献以及薄膜中PDA的长度分布。
可见光谱表明，在整个可见光谱区域，光密度随剂量增加。拉曼光谱主要由两种振动模式主导，最显著的是PDA主链的ν（C≡C）和ν（C = C）拉伸模式。通过将可见吸收模型拟合到实验光谱中，发现活性层包含两种具有不同吸收特性和反应动力学的不同PDA构象。此外，发现平均PDA共轭长度比嵌入其中的晶体PDA小2 - 3个数量级。
可见光谱和拉曼光谱通过识别PDA的激发态和两种PDA构象的存在，对EBT3薄膜的放射变色特性的分子本质有了更深入的了解。对EBT3活性层分子组成的进一步了解为未来剂量响应的基础建模提供了框架。
