Characterization with X-rays of a Large-Area GEMPix Detector with Optical Readout for QA in Hadron Therapy

Abstract

Quality Assurance (QA) in hadron therapy is crucial to ensure safe and accurate dose delivery to patients. This can be achieved with fast, reliable and high-resolution detectors. In this paper, we present a novel solution that combines a triple Gas Electron Multiplier (GEM) and a highly pixelated readout based on a matrix of organic photodiodes fabricated on top of an oxide-based thin-film transistor backplane. The first LaGEMPix prototype with an active area of 60 × 80 mm2 was developed and characterized using low energy X-rays. The detector comprises a drift gap of 3.5 mm, a triple-GEM stack for electron amplification, and a readout featuring 480 × 640 pixels at a 126 µm pitch. Here, we describe the measurements and results in terms of spatial resolution for various experimental configurations. A comparison with GAFCHROMIC® films and the GEMPix detector used in the charge readout mode was performed to better understand the contribution to the spatial resolution from both the electron diffusion and the isotropic emission of photons. The measurements were compared to Monte Carlo simulations, using the FLUKA code. The simulation predictions are in good agreement with the GEMPix results. Future plans with respect to applications in hadron therapy are discussed.

A Large Area GEMPix detector for treatment plan verification in hadron therapy

“…The experimental conditions were identical as in [11]: 30 kV (N-5 series filter) or 40 kV X-rays (N-6 series filter), minimum aperture of 1 cm, 230 cm source-detector distance and same electric fields in the GEM structure: transfer field 1 = 2.0 kV/cm; transfer field 2 = 1.75 kV/cm; induction field = 5.0 kV/cm; drift field = 1 kV/cm. The electric fields between the top and bottom copper layers in each GEM foil were kept constant corresponding to a total voltage of 940 V.…”

“…Measurements similar to those reported in [11] were carried out with the new detector. For the ESF, we placed a lead block of 10 x 20 x 2.5 cm 3 dimensions in front of the detector, covering part of its active area.…”

“…Scintillation photons generated in the GEM holes after electron avalanche multiplication are detected by the matrix of OPDs, placed at 3 mm from the production point. A more detailed description of the LaGEMPix and the results summarized in the following can be found in reference [11].…”

“…A region of interest (ROI) was set perpendicularly to the edge, and the edge response profile was calculated from the average of 200 background corrected images. The spatial resolution was estimated by fitting the edge profile by a logistic function [11,21].…”

“…As expected, the upgraded LaGEMPix with reduced gap exhibits the best spatial resolution. A more detailed discussion of the different methods presented here can be found in [11].…”