Medical Applications of the GEMPix

Abstract

The GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region, three Gas Electron Multipliers (GEMs) for signal amplification, and four Timepix ASICs with 55 µm pixel pitch and a total of 262,144 pixels. A continuous flow of a gas mixture such as Ar:CO2:CF4, Ar:CO2 or propane-based tissue equivalent gas is supplied externally at a rate of 5 L/h. This article reviews the medical applications of the GEMPix. These include relative dose measurements in conventional photon radiation therapy and in carbon ion beams, by which on-line 2D dose images provided a similar or better performance compared to gafchromic films. Depth scans in a water phantom with 12C ions allowed measuring the 3D energy deposition and reconstructing the Bragg curve of a pencil beam. Microdosimetric measurements performed in neutron and photon fields allowed comparing dose spectra with those from Tissue Equivalent Proportional Counters and, additionally, to obtain particle track images. Some preliminary measurements performed to check the capabilities as the detector in proton tomography are also illustrated. The most important on-going developments are: (1) a new, larger area readout to cover the typical maximum field size in radiation therapy of 20 × 20 cm2; (2) a sealed and low-pressure version to facilitate measurements and to increase the equivalent spatial resolution for microdosimetry; (3) 3D particle track reconstruction when operating the GEMPix as a Time Projection Chamber.

ICRU Report 98, Stochastic Nature of Radiation Interactions: Microdosimetry

“…The application of GEMs in the design of microdosimetric detectors could simplify the construction of miniature counters and also of multielement counter configurations by avoiding the need to incorporate one or more anode wires. Furthermore, the possibility to pixelate the collector electrode provides the means for particle tracking and imaging (Leidner et al, 2021;2022).…”

Applications of Medical Physics

“…It couples two technologies, the Gas Electron Multiplier as an amplifier for electric charges and four highly pixelated Timepix ASICs as readout. Leidner et al [17] propose a comprehensive review about the medical applications of GEMPix. The authors describe examples of the use of this device for relative dose measurements in X-ray radiation therapy and hadron therapy.…”

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

“…In this work, we used an OPD based on a bulk heterojunction (BHJ) structure. It consists of a 280 nm thick blend of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, a p-type (donor) polymer, and [6,6]-phenyl-C61-butyric acid methyl ester, PCBM, (acceptor) fullerene [22]. The EQE of the OPD is dictated by the absorption of the heterojunction components, potentially modified by interference effects derived from the electrodes and encapsulation.…”

“…In order to evaluate the contribution of the isotropic emission of the scintillation light to the spatial resolution in the LaGEMPix, we carried out comparative measurements with the GEMPix, a 2.8 × 2.8 cm 2 triple-GEM detector that uses a Timepix quad ASIC to readout charges directly [6,54]. The experimental conditions were identical: 40 kV X-rays (N-6 series filter), minimum aperture 1 cm, 230 cm source-detector distance and same electric fields in the GEM structure.…”

“…It consists of a triple-GEM (Gas Electron Multiplier) [4] coupled to a highly pixelated readout based on a matrix of organic photodiodes (OPDs). The potential of a triple-GEM detector combined with high pixel granularity for QA in hadron therapy was initially demonstrated with the GEMPix [5,6]. The GEMPix has shown very promising results but its application is limited by the relatively small active area of 28 × 28 mm 2 .…”