A novel approach for the readout of a TPC at the future linear collider is to use a CMOS pixel detector combined with some kind of gas gain grid. A first test using the photon counting chip Medipix2 with GEM or Micromegas demonstrated the feasibility of such an approach. Although this experiment demonstrated that single primary electrons could be detected the chip did not provide information on the arrival time of the electron in the sensitive gas volume nor did it give any indication of the quantity of charge detected. The Timepix chip uses an external clock with a frequency of up to 100 MHz as a time reference. Each pixel contains a preamplifier, a discriminator with hysteresis and 4-bit DAC for threshold adjustment, synchronization logic and a 14-bit counter with overflow control. Moreover, each pixel can be independently configured in one of four different modes: masked mode: pixel is off, counting mode: 1-count for each signal over threshold, TOT mode: the counter is incremented continuously as long as the signal is above threshold, and arrival time mode: the counter is incremented continuously from the time the first hit arrives until the end of the shutter. The chip resembles very much the Medipix2 chip physically and can be readout using slightly modified versions of the various existing systems. This paper presents the main features of the new design, electrical measurements and some first images.

Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements

After passage through matter, the energy spectrum of a polychromatic beam of x-rays contains valuable information about the elemental composition of the absorber. Conventional x-ray systems or x-ray computed tomography (CT) systems, equipped with scintillator detectors operated in the integrating mode, are largely insensitive to this type of spectral information, since the detector output is proportional to the energy fluence integrated over the whole spectrum. The main purpose of this paper is to investigate to which extent energy-sensitive photon counting devices, operated in the pulse-mode, are capable of revealing quantitative information about the elemental composition of the absorber. We focus on the detection of element-specific, K-edge discontinuities of the photo-electric cross-section. To be specific, we address the question of measuring and imaging the local density of a gadolinium-based contrast agent, in the framework of a generalized dual-energy pre-processing. Our results are very promising and seem to open up new possibilities for the imaging of the distribution of elements with a high atomic number Z in the human body using x-ray attenuation measurements. To demonstrate the usefulness of the detection and the appropriate processing of the spectral information, we present simulated images of an artherosclerotic coronary vessel filled with gadolinium-based contrast agent. While conventional systems, equipped with integrating detectors, often fail to differentiate between contrast filled lumen and artherosclerotic plaque, the use of an energy-selective detection system based on the counting of individual photons reveals a strong contrast between plaque and contrast agent.

https://iopscience.iop.org/article/10.1088/0031-9155/52/15/020/pdf

K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors.

The study of astronomical objects using electromagnetic radiation involves four basic observational approaches: imaging, spectroscopy, photometry (accurate counting of the photons received) and polarimetry (measurement of the polarizations of the observed photons). In contrast to observations at other wavelengths, a lack of sensitivity has prevented X-ray astronomy from making use of polarimetry. Yet such a technique could provide a direct picture of the state of matter in extreme magnetic and gravitational fields1,2,3,4,5,6, and has the potential to resolve the internal structures of compact sources that would otherwise remain inaccessible, even to X-ray interferometry7. In binary pulsars, for example, we could directly ‘see’ the rotation of the magnetic field and determine if the emission is in the form of a ‘fan’ or a ‘pencil’ beam1,8. Also, observation of the characteristic twisting of the polarization angle in other compact sources would reveal the presence of a black hole9,10,11,12. Here we report the development of an instrument that makes X-ray polarimetry possible. The factor of 100 improvement in sensitivity that we have achieved will allow direct exploration of the most dramatic objects of the X-ray sky.

http://cds.cern.ch/record/511427/files/0107486.pdf

An efficient photoelectric X-ray polarimeter for the study of black holes and neutron stars

We discuss design issues related to the extensive use of Enclosed Layout Transistors (ELT's) and guard rings in deep submicron CMOS technologies in order to improve radiation tolerance of ASIC's designed for the LHC experiments (the Large Hadron Collider at present under construction at CERN). We present novel aspects related to the use of ELT's: noise measured before and after irradiation up to 100 Mrad (SiO/sub 2/), a model to calculate the W/L ratio and matching properties of these devices. Some conclusions concerning the density and the speed of IC's conceived with this design approach are finally drawn.

/pdf/radiation-tolerant-vlsi-circuits-in-standard-deep-submicron-fc6y1p74q4.pdf

Radiation tolerant VLSI circuits in standard deep submicron CMOS technologies for the LHC experiments: practical design aspects

A prototype pixel detector readout chip has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture neighbouring pixels communicate with one another. At the corner of each pixel summing circuits add the total charge deposited in each sub-group of 4 pixels. Arbitration logic assigns a hit to the summing circuit with the highest charge. In the case where incoming X-ray photons produce fluorescence-a particular issue in high-Z materials-the charge deposited by those fluorescent photons will be included in the charge sum provided that the deposition takes place within the volume of the pixels neighbouring the initial impact point. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are 4 times greater in area than the readout pixels. In the latter case event-by-event summing is still possible between the larger pixels. As well as this innovative analog front-end circuit, each pixel contains comparators, logic circuits and two 15-bit counters. When the larger detector pixels are used these counters can be configured to permit multiple thresholds in a pixel providing spectroscopic information. The prototype chip has been designed and manufactured in an 8-metal 0.13 mum CMOS technology. First measurements show an electronic pixel noise of ~ 72 e-rms (Single Pixel Mode) and ~ 140 e-rms (Charge Summing Mode).

/pdf/the-medipix3-prototype-a-pixel-readout-chip-working-in-1exr3zqww3.pdf

The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode with Improved Spectrometric Performance

The Medipix2 chip is a pixel-detector readout chip consisting of 256 /spl times/ 256 identical elements, each working in single photon counting mode for positive or negative input charge signals. Each pixel cell contains around 500 transistors and occupies a total surface area of 55 /spl mu/m /spl times/ 55 /spl mu/m. A 20-/spl mu/m wide octagonal opening connects the detector and the preamplifier input via bump bonding. The preamplifier feedback provides compensation for detector leakage current on a pixel by pixel basis. Two identical pulse height discriminators are used to create a pulse if the preamplifier output falls within a defined energy window. These digital pulses are then counted with a 13-b pseudorandom counter. The counter logic, based in a shift register, also behaves as the input-output register for the pixel. Each cell also has an 8-b configuration register which allows masking, test-enabling and 3-b individual threshold adjust for each discriminator. The chip can be configured in serial mode and readout either serially or in parallel. The chip is designed and manufactured in a 6-metal 0.25-/spl mu/m CMOS technology. First measurements show an electronic pixel noise of 140 e~ root mean square (rms) and an unadjusted threshold variation around 360 e~ rms.

Medipix2: A 64-k pixel readout chip with 55-/spl mu/m square elements working in single photon counting mode

A radiation tolerant pixel detector readout chip has been developed in a commercial 0.25 /spl mu/m CMOS process. The chip is a matrix of two columns of 65 identical cells. Each readout cell comprises a preamplifier, a shaper filter, a discriminator, a delay line and readout logic. The chip occupies 10 mm/sup 2/, and contains about 50000 transistors. Electronic noise (/spl sim/220 e rms) and threshold dispersion (/spl sim/160 e rms) allow operation at 1500 e average threshold. The radiation tolerance of this mixed mode analog-digital circuit has been enhanced by designing NMOS transistors in enclosed geometry and introducing guardrings wherever necessary. The chip, which was developed at CERN for the ALICE and LHCb experiments, was still operational after receiving 3.6/spl times/10/sup 13/ protons over an area of 2/spl times/2 mm. Other chips were irradiated with X-rays and remained fully functional up to 30 Mrad (SiO/sub 2/) with only minor changes in analog parameters. These results indicate that careful use of deep submicron CMOS technologies can lead to circuits with high radiation tolerance.

R. Dinapoli

Papers

Medipix2: A 64-k pixel readout chip with 55-/spl mu/m square elements working in single photon counting mode

A pixel readout chip for 10-30 Mrad in standard 0.25 /spl mu/m CMOS