# Medical Applications of the GEMPix
## 7 LaGEMPix, a Large Area GEMPix  
The superior imaging and particle discrimination capabilities of the GEMPix have been shown in the previous sections, but the current sensitive area of the detector (2.8 \(\times\) 2.8 cm) is too small for a practical application as QA tool in hadron therapy. This section focuses on the application in hadron therapy, but a larger area detector is also needed in other fields such as conventional radiation therapy with photon beams. A large area GEMPix (LaGEMPix) with a 20 \(\times\) 20 cm\({}^{2}\) size (the typical maximum clinical field size with scanned hadron therapy beams) cannot be easily (and economically) achieved by simply scaling up the current design by tiling more Timepix ASICs. First, the current Timepix chip has wire bonds located on one side, so the quad configuration (2 \(\times\) 2 chips) used in the GEMPix cannot be enlarged to an arbitrary n x n chip configuration. For this, one would need to use the latest Timepix version that is about four times larger than the Timepix and in which the connection for data transfer is from below the chip by Through-silicon Via (vertical interconnect access). However, such a solution would be very expensive just for the cost of the Timepix4 ASICs. In addition, the 55 \(\upmu\)m pixel size would require complex electronics to handle the large number of signals and each acquisition would generate a huge amount of data, which are actually not needed as spatial resolution of the order of 1 mm is sufficient for application in QA.  
We have considered a few alternative readouts for a large area detector. In the present section, we describe the one currently being explored; alternative solutions are briefly illustrated in the next section. The on-going development uses a novel readout based on the detection of the scintillation photons generated in the GEM holes. A first LaGEMPix prototype with a six time larger active area than the GEMPix (6 \(\times\) 8 cm\({}^{2}\)) has been developed in a collaboration between CERN and the Holst Centre/TNO (Eindhoven, The Netherlands) within an EU ATTRACT funded project [58]. The LaGEMPix combines a triple-GEM detector and an optical readout based on three main building blocks: a Thin Film Transistor (TFT) backplane, a light sensitive OPD (optical photo detectors) frontplane and a transparent thin-film encapsulation, serving as a protection against ambient conditions [59]. The triple-GEM detector includes a 20 \(\upmu\)m thick Mylar window used as the cathode at 3.5 mm from the first GEM. On the bottom of the third GEM, a 1.1 mm thick ITO (Indium Tin Oxide) coated glass anode is placed at 1.9 mm distance to collect the electrons produced during the amplification process while allowing the optical photons to pass through [60, 61].  
A LaGEMPix prototype (Figure 16) has been built. A first account of the results obtained are given in [62], the full results can be found in [63]. A number of tests were performed to characterize the detector, placed inside a custom-made black Polymethyl-methacrylate (PMMA) box to shield the ambient light and provide a well aligned set-up. A gain scan was performed while exposing the device to photons from a 3 TBq \({}^{137}\)Cs source, varying the sum of the GEM voltages from 0 to 1030 V, to define the optimal operating conditions. The spatial resolution was determined with 40 kV X-rays from an X-Ray irradiator employing a 3 mm thick copper plate placed on the inner wall of the box at 7 mm from the Mylar window, with a set of holes of different spacing (from 3 to 20 mm) drilled in it. An image obtained with this copper plate is shown in Figure 17. The FWHM of a single hole of 5 mm diameter is 6.7 \(\pm\) 0.1 mm. Two holes at a distance of 3 mm (edge to edge) can be resolved in the sense that two peaks with a dip in the intensity between the peaks are visible. The current results are promising but also show that the required sub-millimeter spatial resolution is not yet achieved.