ABSTRACT
In this work, we calibrate the newly developed EBT-XD radiochromic films (RCFs) manufactured by Gafchromictm using protons in the energy range of 4-10 MeV. Irradiation was performed on the 2 × 6 MV tandem linear accelerator located at the Université de Montréal. The RCFs were digitized using an Epson Perfection V700 flatbed scanner using both the red-green-blue and grayscale channels. The proton fluences were measured with Faraday cups calibrated in absolute terms. The linear energy transfer function within the active layer of the films was calculated using the mass stopping power tables coming from the PSTAR database from the National Institute of Standards and Technology (NIST) to allow retrieval of the deposited dose. We find that the calibration curves for 7 and 10 MeV protons are nearly equivalent. The 4 MeV calibration curves exhibit a quenching effect due to the Bragg peak that falls close to the active layer. A linearization of this energy dependence was developed using a semiempirical parametric model to allow the generation of calibration curves for any incident proton energy within the present range. Excellent correspondence (<5% dose difference for the same netOD) of the 10 MeV calibration curves was noted when compared to existing high-energy proton (148.2 MeV) calibration curves reported in the literature. Our calibration extends the range of operation of EBT-XD films to low-energy proton beam dosimetry.
ABSTRACT
We present a Target Positioning Interferometer (TPI), a system that uses variations of the wavefront curvature to position solid reflective surfaces with submicrometric precision. The TPI is a Michelson interferometer into which a lens is inserted in the target arm and the mirror of the reference arm is slightly tilted. The TPI configuration presented in this work allows us to position the surface of a reflective target on a beam focus within an uncertainty of 350 nm (2σ) in a subsecond timeframe, using a lens with a numerical aperture of NA = 0.20. We support our experimental findings with numerical simulations of the interference pattern using the ABCD matrices' method, allowing us to define scaling laws for using the TPI with different optics and environments, as well as suggestions to improve the TPI accuracy and adapt the system to different applications. This system is very well suited for accurate and repeatable target positioning used in laser-driven ion acceleration, where a precise alignment is key to optimize the proton acceleration mechanism.
