Technology

Hybrid pixel imaging detector

ASI’s imaging technology is based on the hybrid pixel imaging technology developed within the international Medipix collaboration. The detection principle is based on the direct conversion of radiation into charge in a sensitive layer that can be electronically collected, detected and processed by an imaging ASIC. For example, absorption of X-rays in a silicon sensor leads to the generation of electron and hole pairs in the sensor with a charge proportional to the X-ray energy. The applied electric field allows fast signal collection with small spread of the signal. Therefore, high sensitivity and high spatial resolution can be achieved. The readout principle is based on pixelized imaging ASICs. Each sensor pixel is bump bonded to an ASIC pixel by a miniature bump bond. The basic architecture of each ASIC pixel consists of a signal amplifier, a discriminator and a counter. The threshold level can be controlled by the user through software and only incoming pulses above the threshold are counted. The digital detection provides noise free detection of X-rays and because of the energy threshold also spectral resolution. Other advantages of hybrid pixel imaging detectors are that they can achieve high frame rates with zero dead time because of the digital processing and parallel readout of the ASIC. The layout of the ASICs and readout boards allows a modular approach such that larger detector areas can be achieved.
Imaging technology

Imaging ASICs

From the effort of the Medipix collaboration ASI provides detectors based on three types of ASICs.

Readout chip
Timepix
The Timepix ASIC can be read out with a maximum frame rate of 120 frames/s. The shutter can be controlled by software or with an external trigger. The Timepix can operate in three different modes:
• counting: all pulses above the threshold are counted

• Time-over-threshold: counter keeps increasing while the pulse is above the threshold. The larger the absorbed energy in each pixel, the larger the Time-over-threshold.
• Time-of-arrival: the time of arrival of each incoming signal can be determined with a time resolution down to 20 ns.

Timepix3
The Timepix3 ASIC is a successor of Timepix and has a timing accuracy of 1.6 ns. Besides frame read out it offers data driven or list mode read out of pixels above the threshold. This data stream provides Time-over-threshold, time-of-arrival information and the pixel coordinate of each pixel above threshold.

Medipix3RX
The Medipix3RX ASIC operates in counting mode and has an increased frame rate, up to 1800 frames/s, and can operate in zero dead time (continuous read/write mode). Besides speed the Medipix3RX ASIC has improved spectral performance with respect to other imaging ASICs. Selection on the pulse threshold and spectral resolution can suffer from the charge spread problem. Depending on impact position, energy of the incoming particle and choice of the sensor material, the generated charge can arrive on multiple neighbouring pixels of the ASIC. When the threshold is on a fixed level events which have high charge spread can be lost. The Medipix3RX ASIC has a special inter pixel circuit that assigns the sum of the charge to the pixel with the highest charge. This is called the charge summing mode. The Medipix3RX also has an enhanced spectral configuration when bump bonding is done with 110 µm pitch instead of 55 µm. In this configuration and with charge summing mode activated, particles can be counted with four user settable thresholds. This allows simultaneous detection in multiple spectral ranges.

Sensors

A big advantage of hybrid pixel detectors is the possibility to use different sensor materials or radiation detector mechanisms. For X-ray, electron and other charged particle imaging and diffraction sensor materials like Si, GaAs, CdTe are possible. The last two materials have a high density and absorb more efficiently higher energy particles. For mass spectrometry, ions, neutrons and XUV micro channel plates (MCP) can be applied. With the MCP and a high voltage, single events generate electron avalanches that can be detected with the bare ASIC.


Vacuum

ASI has designed and offers solution for in vacuum detection of electrons and ions (mass spectrometry). For these applications, the detector is temperature stabilized with water cooling. Other possible applications are neutron detection, XUV and optical photon detection.