Timepix

Operation modes

These are the Time over Threshold (ToT), Time of Arrival (ToA) and single photon counting modes. At XCS the Timepix chip is used in ToT mode to be able to image X-ray photons coming from a single LCLS pulse. The collected signal is integrated by a capacitor then slowly discharged via a constant current hence the time a signal is over a predefined threshold is a measure of its magnitude. Each pixel records this ToT value by counting clock pulses in a 14 bit counter which is proportional to the deposited energy in a pixel. The energy discrimination threshold of the Timepix pixel allows noise free detection, so no dark frame subtraction is needed.

Each pixel cell is divided into an analogue and a digital area, where the analogue part contains a preamplifier, discriminator and a 4-bit threshold adjustment. The digital side of the pixel consists of the counting and synchronization logic, and the pixel configuration registers. After the acquisition is completed, the contents of all 65k pixel counters are shifted out serially in about 8 ms. Hence a maximum rate of 125 Hz can be achieved.

Although the preamplifier output voltage is only linear up to 50k electrons, the discriminator output pulse width is linear up to larger input charges because of the constant discharge current in the preamplifier. The discharge slope of the preamplifier signal is controlled by the Krummenacher current (Ikrum). A higher Ikrum current decreases the ToT pulse width for a given input signal. This means that the resolution of the ToT measurement is determined by the frequency of the counting clock, which can be up to 100 MHz, and the Krummenacher current. Simulations have shown that the ToT dynamic range extends up to about 150k electrons. However, first measurements at LCLS described in section 4 have shown that the dynamic range is about 125k electrons.

Medipix: The Medipix Collaboration [online]. 2014 [cit. 2014-01-23]. Source: http://medipix.web.cern.ch

Parameters of Timepix:

The Timepix Chip was derived directly from the Medipix2 development. It shares many of the physical dimensions of the Medipix2 chip but has a different functionality at the level of the single pixel. The chip was designed at the request of the EUDet Collaboration. Here is a summary of its properties:

• The square pixel size of 55 µm side length makes the Tmepix chip competitive with most of the existing imaging devices in terms of spatial resolution.
• Direct X-ray conversion in a semiconductor sensor minimises image blurring and avoids an extra conversion stage from X-rays into visible light.
• The chip is designed to accept either positive or negative charge input in order not to restrict the choice of the sensor material (Si, GaAs, CdZnTe,...). Detector leakage current gets compensated pixelwise at the input.
• The chip can also be combined with a Gas Gain Grid to read out electron deposition in a gas detector.
• Timepix has a single energy threshold. The threshold can be adjusted pixelwise with 4 bits for uniform performance of the whole pixel matrix.
• Each pixel can be programmed to work on one of 3 modes: Single particle counting, Time over Threshold or Arrival time mode. In the case of ToT or Arrival time mode the clock ticks are counted.
• Exposure times can be chosen arbitrarily. Data is accumulated in a 13-bit counter per pixel. The pixel overflows at 11810 counts. In single particle counting mode each pixel can handle count rates of about 100 kHz of randomly arriving particles. Read-out is performed after exposure to avoid dead time.
• Parallel and serial read-out are realised.
• The Timepix chip has an active area of about 2 cm². 256 x 256 pixels form the pixel matrix.
• Larger area coverage is a big concern of the collaboration. Timepix will therefore be 3-side buttable.

Presentations about Timepix chip

Presentation 1: The design of the Timepix chip

Presentation 2: Quality test procedures for Timepix