# Medical Applications of the GEMPix
## 2 The GEMPix  
The GEMPix couples two CERN technologies, the Gas Electron Multiplier [9; 10] as amplifier for electric charges with four highly pixelated Timepix ASICs (the so-called quad or quadboard) as readout [12]. The concept for gas pixel detectors is not new [26]; it was first developed by [PERSON] et al. [27] as an X-ray polarimeter using a custom ASIC [28] for X-ray astronomy [29; 30]. Other examples include the GRIDPIX designed for high-energy physics applications [31], but sparks and discharges have proven to be persistent problems with these devices. Thanks to a specially designed High Voltage power supply (HVGEM) [32] and a carefully designed GEM electrode layout, the GEMPix demonstrates good reliability and discharge resistance. Figure 1 shows the GEMPix. It consists of (1) a drift gap of about 1 cm, (2) a triple-GEM setup for charge amplification and (3) an array of 2 \(\times\) 2 'naked' Timepix ASICs, i.e., Timepix ASICs without the usual silicon sensor layer. The GEMPix is thoroughly described in [33; 34]. Its design and operating principles are recalled here, first illustrating those of its two main components, and then their combination in the GEMPix. The GEM [9] is a thin (50 \(\upmu\)m) kapton foil, copper cladded on each side and chemically perforated by a high density of holes having bi-conical structure with an external (internal) diameter of 70 \(\upmu\)m (50 \(\upmu\)m) and a pitch of 140 \(\upmu\)m, as shown in Figure 2. By applying a suitable voltage difference (typically 400 \(\div\) 500 V) between the two metallic sides, an electric field with a field strength as large as 100 kV/cm is produced inside the holes. The holes act as spatially well-confined electron multiplication channels for the electrons released by ionizing radiation in the drift gap. The maximum gain reachable with a single GEM foil detector is of the order of 10\({}^{3}\). In safe operating conditions, effective gas gains (i.e., the product between the electron multiplication and the transparency, which is the overall efficiency of transporting the electrons through the GEM holes [11]) of up to 10\({}^{4}\)-10\({}^{5}\) are reachable by assembling several GEM foils close to each other. The distance between the three GEM foils of the GEMPix is the same as that used by the LHCb muon chambers in order to optimize the time resolution (the first gap is 1 mm) and to reduce the probability of  
Figure 1: The open GEMPix, showing the four Timepix ASICs and the FITPix readout module. The position of the drift gap and the GEMs is labelled (source: CERN). discharge as much as possible (the second gap is 2 mm, which results in the electron clouds being shared by more holes of the third GEM foil). Typically, the electric field applied between two GEMs to transfer the electrons to the next GEM range between 0.5 and 3 kV/cm depending on the gas and the performance required by the detector application. Figure 3 shows a scheme of the Timepix chip. The movement of the electric charge produced by the last GEM induces a signal on a group of pads of the chip underlying the charge cluster. The Timepix is able to collect and measure the information of this charge in three different modes, always referring to the time window defined a priori. The first mode ('Medipix mode') allows measuring the number of hits per pixel in the time window. The second mode is the Time-over-Threshold ('ToT'): the Timepix is able to measure with a precision on the order of 10 ns the time spent above the threshold by the analog signal induced on the single pixel; this time is proportional to the collected charge and, therefore, if the detector is calibrated, to the energy deposited by the radiation. This mode is typically used for spectroscopy of X-rays, dosimetry, microdosimetry and for dE/dX measurements. The third mode is the Time-of-Arrival ('ToA'): the Timepix measures the time of arrival of the electron cloud with respect to an external trigger for a maximum of a 2 ms time window.