The Timepix detectors are hybrid active pixel detectors, which were developed within the Medipix collaboration at CERN (http://medipix.web.cern.ch/medipix) as the successors of the Medipix detectors.
The main part of a Timepix detector is the active sensor layer, which is segmented into a square matrix of 256 x 256 pixels with a pixel pitch of 55 um, thus covering an area of 1.4 x 1.4 cm2, which is bump-bonded to the readout ASIC (Application specific integrated circuit) (see Figure 1).
Figure 1: Picture of the Timepixes sensor layer.
Each pixel is connected to its own readout chain. Timepix detector assemblies with different sensor materials (CdTe, GaAs and silicon) of different thicknesses (300 um to 1500 um) are available. Whereas the higher Z-materials are mainly used for X-ray imaging, Timepix detectors with silicon sensors have made their way into particle physics experiments, such as the ATLAS detector in the LHC, Space applications, and dosimetry in hadron therapy or in accelerator facilities.
Ionizing radiation creates free charge carriers (electrons and holes) in the sensor layer. These drift towards the corresponding electrode due to the applied electrical field, where they are finally collected. During their drift and collection, a current is induced at the electrodes of the pixel cells. The induced analog signal is shaped, amplified, and compared to a threshold level, which can be adjusted globally after a so-called threshold equalization. The voltage output signal is then used in different ways, depending on the selected mode:
The detector segmentation allows for particle type differentiation by evaluating the characteristic imprints in the pixelscreens. A basic pattern recognition procedure can be used, e.g. to separate gammas, electrons or minimal ionizing particles (MIPs) from charged hadrons such as low energy protons, alphas or even heavier ions and neutrons. Figure 2 gives an overview of the basic cluster categories.
Figure 2: Overview of basic cluster categories.
In the MoEDAL experiment 5 Timepix detectors with silicon sensor layers of thicknesses 300 um and 1 mm were installed. They are mainly used for the characterization of the radiation field at their positions, shown in Figure 3. However, due to their tracking capability, they are also able to identify highly ionizing events.
Figure 3: Position of the Timepix detectors in the MoEDAL experiment.