A GEM-based dose imaging detector with optical readout for proton radiotherapy

New techniques in proton radiation therapy and advances in beam delivery systems design such as beam scanning require accurate 2D dosimetry systems to verify the delivered dose distribution. Dose imaging detectors based on gas electron multipliers (GEMs) are capable of providing high sensitivity, improved dose measurement linearity, position resolution, fast response and accurate characterization of depth-dose distributions. In this work, we report on the development of a GEM-based dose imaging detector with optical readout using a CCD camera. A 10×10 cm2 detector has been tested in a 205 MeV proton beam in single- and double-GEM configurations. The detector demonstrates linearity in dose rate up to 100 Gy/min and position resolution (σ) of 0.42 mm. Transverse non-uniformity of the detector response is ≤10% before correction and the stability of the detector output throughout the day is within ±1%, with day-to-day reproducibility of about 10%. The depth-dose response of the detector is close to that of a wide-aperture air-filled ionization chamber and is in good agreement with Monte Carlo simulations.