# Medical Applications of the GEMPix
## 6 Microdosimetry  
The most recent application of the GEMPix to medical physics lies in the field of microdosimetry. Microdosimetry is the'systematic study of the spatial and temporal distributions of absorbed energy in irradiated matter' [51]. The golden standard for detectors in microdosimetry is the Tissue Equivalent Proportional Counter (TEPC). A TEPC is a device consisting of an active volume filled with tissue equivalent (TE) gas, with TE walls and a thin anode wire. The gas pressure is only a few percent of the atmospheric pressure and therefore the ionization of a particle in the active volume is the same as in a small piece of tissue. Both the lineal energy and the absorbed dose to tissue can be measured [52]. It is a well-established technology that can measure the microdosimetric distributions accurately and reliably, but it has important limitations that the GEMPix could in principle overcome. For example, the equivalent mean path length in a TEPC can be selected by adjusting the operational pressure, but it is typically limited to a few micrometres, with a lower limit of operation of 0.3 \(\upmu\)m in case of special design [53], which is not enough to obtain details on the particle track structure. Detectors with the best spatial resolutions tend to be complex and bulky [54]. Furthermore, conventional TEPCs feature only a single readout channel in a detection volume of fixed dimensions. While GEMs had been used before in microdosimetry [55], the combination with a highly pixelated readout is unique to the GEMPix. In case of the GEMPix detector, it is possible to achieve a resolution down to the scale of tens of nanometres because the inherently good spatial resolution (pixel pitch of 55 \(\upmu\)m) is scaled down using a gas instead  
Figure 12: On the left, the portion of the phantom used in the test, on the right the final reconstructed image. The total exposure time was 100 s at a beam intensity of 3 kHz [25]. of a solid material by a factor of approximately 1000. The particle track imaging capability of the GEMPix is completely new to microdosimetry. The version of GEMPix for use as microdosimeter is called GEMTEQ ('GEMPix detector for microdosimetry with tissue equivalent gas'). For this application, the GEMTEQ is operated with a so-called propane-based TE gas (C\({}_{3}\)H\({}_{8}\):CO\({}_{2}\):N\({}_{2}\), 55:39.6:5.4). It was calibrated using an \({}^{55}\)Fe source and its performance was tested in various radiation fields (photons from an X-ray generator and from radioactive sources, neutrons from a neutron generator and a mixed photon/neutron field from an AmBe radioactive source). One of the GEMTEQ prototypes is equipped with a tissue equivalent cathode replacing the Mylar foil by an A150 conductive plastic. Figure 13 shows the GEMTEQ detector setup for measurements in a neutron field. Standard dose spectra as with a TEPC were obtained. These results were achieved by grouping together several pixels in a'superpixel': the superpixels can be defined offline to adjust the equivalent mean path length. The energy deposition of all pixels in a superpixel is summed and these values are used to populate a histogram similar to the pulse height histograms obtained in standard microdosimetry. Figure 14 shows the results of measurements with an AmBe source (solid black line). In order to show one of the potential advantages of the pixelated readout of the GEMPix, an analysis to discriminate photons and neutrons in the mixed radiation field provided by the AmBe source was set up: a particle track in the GEMPix often stretches over several superpixels. While for the standard dose distributions, the energy deposition in each superpixel is used to populate the histogram (as shown for the complete data set in Figure 14), the total energy deposition of the entire track is used to discriminate neutrons and photons. The value of the discrimination parameter--the total energy deposition of a particle in the GEMPix--was chosen to be the maximum energy deposition observed in the measurements with an Am-241 source.