After the initial work done with silicon bump-bonded to the Medipix chip, one thought of the possibility to post-process the chips with grids on top to create a gaseous detector with a multiplication stage just above the chip's surface.
A lot of work has been done and even a specific chip for these kinds of detectors was designed with timing capabilities; the Timepix chip, closely related to the MXR Medipix chip.
The line of research that emerged focuses on the development and application of Gridpix detectors. GridPix detectors are micro time projection chambers. With a time projection chamber one can reconstruct a track in three dimensions by collecting the electrons that drift in a gas from the ionisation origin to a position sensitive detector. The x-coordinate and the y-coordinate are provided by the position on the detector while the drift time gives the z-coordinate.
- Ingrid detector system with detail of grid in the lower right corner
- Helix tracks of electrons drifting through gas volume under the influence of a magnetic field
The electron detector of a GridPix detector is a TimePix chip read out by e.g. a Muros system and the Pixelman software. The chip was developed at CERN and is based on the Medipix2 design. The TimePix chip has 256 x 256 square pixels with a pitch of 55 micron. Each individual element of the pixel-matrix is connected to a pre-amplifier, discriminator and digital counter (100 MHz) integrated on the readout chip.
Since a bare TimePix chip is not able to measure single electrons, an Ingrid has been constructed on top of the chip. The grid is formed by a layer of 1 micron thick aluminium with holes etched in it. The Ingrid placed on top of 50 micron tall spacers. Between the Ingrid and the chip the electric field is large enough to create avalanches. First, a Si3N4 protection layer is grown over the pixel matrix surface to avoid damage from possible discharges. The individual drift electrons from the ionisations are focused into the Ingrid-holes by the electric field and the subsequent significant amount of electrons created by avalanches are measured by the pixels of the chip. The time that it takes for electrons to drift to the Ingrid is measured by an internal counter that can be started by an external trigger. The counter can also be used to do a time over threshold measurement.
The advantages of GridPix detectors:
- Sensitive for single electrons
- Very high track efficiency
- High position and angle resolution
- High energy resolution
- Extremely high radiation tolerance
- Very low material budget
