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An integrated readout system for drift chambers: The application of monolithic CMOS pixel sensors as segmented direct anode
Abstract
A small TPC has been read out by means of a MediPix2 readout chip as direct anode. A Micromegas foil was placed 50 μm above the chip, and electron multiplication occurred in the gap. With a He/Isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90%. We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as δ-rays.
The Medipix2 Collaboration was started officially in September 1999 with the aim of disseminating hybrid pixel detector technology from High Energy Physics to other fields. The Collaboration was initially composed of 13 European research institutes. Over the ensuing 10 years the Collaboration expanded to reach a peak of 17 member institutes. Although our main scientific focus has been the development of the Medipix2 and Timepix single photon counting pixel detector readout chips the Collaboration members have expanded the range of applications for the technology to many more scientific fields than initially foreseen. We have signed a number of Technology Transfer Agreements during that time, most notably with PANalytical, whose commercially available PIXcel detector is based on the second version of the Medipix2 chip. This paper will review the history of the Collaboration covering as much as possible the main technical highlights. The success of the Collaboration is testimony to the willingness of a large number of groups and individuals to pool efforts for a common purpose. The paper will also cover some of those aspects and summarize the lessons learnt.
The functioning of a high-resistive, hydrogenated amorphous Silicon layer as a protection against discharges for Micoomegas-based pixel readout gaseous detectors, has been investigated. Chips, protected with a 3 mum thick layer, still broke, but a 20 mum thick layer has proven to be adequate. Images from discharge events disclose their geometrical parameters, enabling to further optimize the discharge protection.
We present here preliminary tests of Medisoft 4, a software procedure for the control and the readout of the single photon counting radiation imaging systems based on the Medipix2 chip (256×256 pixels, 55 μm pitch). The system has been developed in the framework of the Medipix2 collaboration. This hardware and software system is the successor of the system based on the Medipix1 photon counting chip (64×64 pixels, 170 μm pitch). Following the Medipix system evolution, Medisoft 4 allows the user to access the new implemented features such as the higher resolution, the faster data communication rates, the daisy-chain multichip mode, the energy windowed acquisition, the continuous acquisition mode, the double signal polarity (holes and electrons), etc. The readout of the data from the Medipix2 chip is presently via the chip serial bus through a MUROS2 interface board and a commercial input/output board, but a parallel readout via a is also foreseen. The current version (Medisoft 4.0), here the subject of preliminary tests connected via MUROS2 to a Medipix2 chip not bump-bonded to any detector , reads out only single chips and features a reduced set of functionalities. Future versions will read up to eight chips in daisy chain and provide full system performance, including high frame rate acquisitions and spectroscopic imaging. After a description of the Medipix2 chip, its readout interfaces and the Medisoft 4 software architecture, we show the results of preliminary software tests on serial communication protocol and speed with MUROS2 and Medipix2, analogue test input, internal digital analog converters calibration and threshold uniformity.
The Medipix2 chip is a pixel-detector readout chip consisting of 256 × 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 μm × 55 μm. A 20-μm 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-μm 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.
We have designed an interface board between the Medipix2 chip and a general-purpose commercial PCI-based acquisition card, making the Medipix2 fully controllable from a PC. The main component on the board is an FPGA that implements the data transmission between the chip and the PC, as well as a number of internal registers to control the operation of the chip. Besides the FPGA, the board also includes a number of data converters for different purposes, a timing source, power supply regulators to generate the power supply voltage needed by the chip, and some level converters to accommodate the different logic levels at the PC, FPGA and Medipix2 chip.The board has been designed to interface with a chip-board containing a maximum of eight Medipix2 chips. The Medipix2 chips are read out via their serial data interface using the LVDS standard.We will describe the design of the board, its operational characteristics and show how the board has been used to characterize the Medipix2 chip.
We describe a novel structure for a gaseous detector that is under development at Saclay. It consists of a two-stage parallel-plate avalanche chamber of small amplification gap (100 microm) combined with a conversion-drift space. It allows a fast removal of positive ionsproduced during the avalanche development. Fast signals (3/4 1 ns) are obtained duirng the collection of the electron avalanche on the anode microstrip plane. The induced positive ion signal has a rise time of 100 ns. The fast evacuation of positive ions combined with the high granularity of the detector provide a high rate capability. Gas gains of up to $10^5$ have been achieved.
The readout of a GEM or Micromegas equipped TPC by means of the MediPix2 CMOS sensor as direct anode
- P Colas
- A P Colijn
- A Fornaini
- Y Giomataris
- H Van Der Graaf
- E H M Heijne
- X Llopart
- J Schmitz
- J Timmermans
- J L Visschers