ASIC Readout Electronics

International Medipix collaboration based at CERN develops ASIC readout architectures for several applications, such as high energy physics, medical imaging and dosimetry. The Medipix family ASICs - Medipix2, Medipix3 and Timepix - have been recently offered for commercial applications. The Medipix technology has already been adopted for X-ray crystallography, education and small animal computed tomography.

The major difference of these devices compared to standard charge integrating imagers (CCDs, flat-panels) is the signal processing electronics integrated in each individual pixel. Sensors are pixelated arrays of reversely biased diodes. The impinging radiation generates charge in the sensor. The charge is collected to appropriate pixels, amplified and compared with a pre-set discrimination level. Then, depending on pixel settings, either the counter is increased, or the pulse height is measured or the time-of-arrival of the radiation is recorded.

There is no dark current measured in pixel thanks to the per-pixel processing electronics and discrimination threshold. Therefore, the noise in pixels corresponds only to the photon counting noise. All other sources of noise in pixels are eliminated. Another advantage of Medipix/Timepix detectors is the speed of readout that allows framerates up to 1000 frames per second.

The basic detector building block is a matrix of 256 x 256 pixels with 55 µm pitch. These chips can be thanks to the edgeless sensor technology used in our devices arrange into nearly arbitrarily shaped area. Therefore, we provide devices with sizes from 1.4 x 1.4 cm2 (single chip) up to 14 x 14 cm2 (array of 10 x 10 chips). However, rows of chips can be made as well (1 x N chips) for linear scanning applications.

The pixels register individual quanta of ionizing radiation depositing in the sensor energy higher than a configurable threshold. The minimum detectable energy is at level of 3-6 keV depending on sensor type. The maximum detectable particle rate is 100 000 particle per pixel per second. Hence, the whole chip is capable of registering up to 6.5∙109 particles per second in single detector chip.

The Timepix cameras are sensitive to any ionizing radiation such as X-rays, gammas, beta particles, alpha particles and other. It can be adapted for detection of neutrons too (slow or fast) using an appropriate convertor. Energetic particles create “tracks" in the recorded image (clusters of pixels are triggered). The shapes of such tracks are very often typical for each radiation type allowing measurement of composition of mixed radiation fields. The energy deposited by each particle can be recorded as well.

The particle-counting principle is very advantageous for imaging applications. Photon counting allows achieving arbitrary signal-to-noise ratio and contrast. Thus even low contrast structures, which are indistinguishable with standard cameras, can be visualized (such as structure of soft tissue or light composite materials). The adjustable energy threshold allows taking spectral (or “color”) radiograms allowing visualization of material composition of imaged samples.

Advacam s.r.o. has license to offer Medipix2 and Timepix based imaging camera products.