Timepix is an ASIC (Application-specific integrated circuit) pixelated readout chip that was designed by CERN within the Medipix collaboration. Timepix is a versatile device that can be combined with variety of radiation sensors (Silicon, CdTe, Micro-Channel-Plates, …) and used for a large number of applications. ASI focuses on Timepix devices with Silicon sensors for X-ray imaging and charge particle tracking. Our devices are also used together with Micro-Channel-Plates (MCP) for ion imaging.

Fig.1: Photo of the readout chip and sensor with indication of bump-bonds. The bump-bonds connect the sensor pixels directly to the pixel electronics in the readout chip

The X-ray imaging and particle-tracking detector consists of a silicon sensor attached onto the Timepix readout chip (Fig. 1). The silicon sensor is a diode with common electrode on the top. The bottom electrode is divided into pixels 55×55µm² and each pixel is connected via bump-bond to its own electronics in the readout chip. A simplified schema of the pixel electronics is in the image Fig. 2.

Each pixel contains amplifier, comparator and counter. Pixel electronics can be operated in three different modes:

• counting mode,
• time-over-threshold mode,
• time-of-arrival mode.

Number of detected radiation quanta is recorded by the counter in the counting mode (Fig. 2a). The counter depth is 11810 counts and the pixel can handle count-rates up to 10⁵ counts/s. Different energies of incoming radiation can be discriminated in this mode thanks to the adjustable energy threshold.

A pixel configured in the Time-Over-Threshold mode (TOT) works differently. Signals that are above the set threshold in TOT mode start clock and the counter counts number of clocks until the signal is again under the threshold (Fig. 2b). This number is proportional to the detected charge and therefore also to the energy deposited by the radiation in the sensor. Thus, an energy spectrum of incoming radiation can be recorded for each single pixel. The TOT mode together with advanced data analysis techniques allows also achieving even sub-pixel spatial resolution for detection of heavy charged particles.

The last possible configuration is the Time-Of-Arrival mode where the pixel works as a single stop TDC (Time-to-Digital Converter). The maximum time interval that could be measured depends on the clock used. It is 118 µs for clock of 100 MHz.

The Timepix readout chip is interfaced with computer using electronics providing 1 Gb/s Ethernet connection. The electronics was designed by NIKHEF and it is shown in Fig. 3.

The Ethernet interface allows frame rates up to 120 frames/s with both versions of the detector, i.e. STPX-65k and QTPX-262k. The software to operate the detector provides a simple to use user interface. However, libraries to integrate the detector into custom DAQ systems are provided as well. All three major operating systems are supported (Windows, Linux, Mac OS X).