The Medipix1 Chip (PCC)
The Medipix1 chip or PCC (photon counting chip) is a prototype (digital) CMOS imaging chip which emerged from particle detection in high energy physics experiments. It was designed at CERN in the micro-electronics group following specifications from the Medipix1 collaboration (CERN, University of Freiburg, Glasgow, Universities and INFN of Napoli and Pisa). The chip was received in 1997, characterised and bump-bonded to Si and GaAs sensors which provide direct charge conversion of photons and therefore minimum image blurring.
The design technology used for the Medipix1 ASIC was the 1 m SACMOS process. The analog front-end comprises a charge-sensitive pre-amplifier and a shaper. Incoming charge from a semiconductor sensor is amplified and compared with a threshold in a comparator. If the signal exceeds this threshold the event is counted. Medipix1 has the following properties (please consult our publications for more details):
- square pixels of 170 m side-length
- 64 x 64 pixels per chip
- sensitive to positive charge
- leakage current compensation at the input
- maximum countrate: ~2 MHz
- 1 comparator per pixel with 3 bits fine tuning for a homogeneous threshold distribution within the pixel matrix
- minimum threshold ~1500 e (~5.5 keV charge deposition in a Si sensor)
- 15 bit counter per pixel to store up to 32767 single events
- variable acquisition time
- read-out time: 384 s @ 10 MHz
- active area of the chip: ~1.2 cm
Medipix1 performs perfectly linear within its large dynamic range of 15 bits per pixel. Its noise performance follows the theoretical limits of Poissonian statistics which results in very high signal-to-noise ratios. Various applications like medical imaging should be able to profit from a reduction in dose (or measurement time) compared to traditional imaging systems at the same image information content.
Two different read-out systems were developed to control data acquisition and to read out Medipix1. The first one (called MRS) was VME based and designed by Laben S.p.A., Milano (I), together with INFN Pisa.
In order to reduce cost and to be more flexible in moving measurement setups the collaboration decided to develop another read-out system based on PCI standard. The Nikhef group took care of the design of a simple board called MUROS1 which interfaces two commercial Nuclear Instruments PCI cards and the Medipix1 chip-board. A description of MUROS1 and the hardware requirements can be found here.
Recently, the Napoli group developed a portable MUROS1 (pMUROS) board aimed primarily at autoradiography applications. The concept behind is to have an even simpler, portable measurement setup which is also cheap and lightweight. It comprises a laptop with a commercial digital IO board and a small box housing the pMUROS board with the detector mounted on a PGA package. Experiments with radioactive samples are now quickly prepared and straightforward with a pre-calibrated chip.
The Napoli group provides the software to control, test and read out Medipix1. It is written in C-code under LabWindows which provides a windows-based environment. Collaboration members have to contact the Napoli group to get a password with which they can download the most recent software version of medisoft here.