Absorption spectrum of gafchromic® EBT2 film with angular rotation

Abstract

Study of the absorption spectrum in film dosimetry is important because the spectral absorbance of the film is related to the film’s total absorption dose. We investigated the absorption spectra of Gafchromic EBT2 film at various rotational angles in a visible wavelength band. The film was irradiated with 6- MV photon beams for a total dose of 300 cGy. The absorption spectra for different angles of rotation were obtained 24 h after the irradiation and we fitted the spectra by using Lorentzian functions. Two dominant absorption peaks, one at approximately 586 nm (green) and the other at 634 nm (red) were observed. The measured spectrum was decomposed into peaks at 542 nm, 558 nm, 578 nm, 586 nm, 626 nm, 634 nm, and 641 nm. The maximum total area of the red absorption band occurred at 45° (225°) and the minimum at 90° (270°). As the angle of rotation changed, the intensities and integrated areas of the blue and the green peaks also changed with a 180° period, with minima at 90° and 270°, and maxima at 0° and 180°, although the overall absorbance was very low. The spectral peak wavelengths remained constant within ±1.2 nm for all angles. The absorption spectrum of the film should no hysteresis of; spectra taken at 0° and 360° were substantially the same and showed similar behaviors for all rotational angles. The change in the absorbance with the rotational angle of the film affected the dosimetric properties, resulting in rotational variations of the film’s dosimetry in each red-green-blue channel.