The Medipix1 chip is a prototype (digital) CMOS imaging chip that emerged from particle detection in high energy physics experiments. It was designed at CERN following specifications from the Medipix1 Collaboration (CERN, the Universities of Freiburg and Glasgow and the Universities and INFN of Napoli and Pisa). The present contribution aims at demonstrating the applicability of Medipix1 Si and GaAs pixel devices for alpha particle detection. In addition, tests were carried out with X-ray radionuclide sources and Xray tube. This work was done within the framework of the Medipix2 Collaboration which carry out the design and evaluation of the new Medipix2 chip (for more info see: http://www.cern.ch/MEDIPIX).

The Medipix1 chip is formed by 64×64 square pixels of 170 μm side-length. The Medipix chip is bump-bonded to an equally segmented Si or GaAs sensor (so-called pixel detector) which provides direct conversion of the deposited energy from single quanta of ionising radiation. The analog front-end of Medipix1 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 performs linearly within its large dynamic range of 15 bits per pixel. It permits to achieve high count statistics per pixel what enables to minimize Poissonian fluctuations and in such a way to reach a high signal-to-noise ratio.

Experiments were realized using Medipix1 assemblies connected via a readout system interface board – called MUROS-1 (Medipix1 re-Usable Read-Out System) – to two commercial National Instruments (NI) cards AT-AO-10 and PCI-DIO-32HS inside a personal computer. The MUROS-1 hardware with NI cards is controlled by dedicated software, called Medisoft 3 written in LabWindows/CVI. (The LabWindows/CVI software and AT-AO-10, PCI-DIO-32HS boards are products of National Instruments Corporation, http://www.ni.com.) Medisoft 3 is designed to perform all basic operations needed to control the Medipix circuit as well as special tasks like threshold equalization or basic image processing operations. The Medipix chip is usually glued on a printed circuit board (PCB), directly connected to the MUROS-1 interface. An external high voltage power supply is needed to bias the semiconductor sensor. No other hardware components are needed in the standard configuration (no additional external power supply, no pulse generator).

Measurements were performed with Si and GaAs detectors using standard spectrometric radionuclide 241Am alpha source and special point alpha source. An X-ray 241Am roentgen fluorescence source and X-ray tube were used to demonstrate the capability of the pixel device for X-ray imaging. In all cases, the detector was illuminated from the backside.

A brief description of several measurements performed follows. Charge sharing between adjacent pixels in the case of alpha particles can be easily demonstrated with uncollimated 241Am alpha source. It is possible to observe single alpha hits and corresponding clusters of responding pixels. Clusters typically consists of 5 to 9 adjacent pixels.

A special point alpha source has been produced by the technique of electrostatic collection of 220Rn daughter products (with alpha particle energies of 6.1 MeV and 8.8 MeV) onto a tip of a metallic needle. The source emits also beta particles and photons. By collimating alpha particles from such point source, an alpha particle beam was obtained. We have available at present collimators of about 500, 300 and 125 μm in diameter. These point sources have been used to study charge sharing between adjacent pixels and to determine spatial resolution of the device for alpha particles.

In experiments with X-ray tube we are using X-ray device with maximum anode voltage of 35 kV. This is product of Phywe Systeme GmbH, ref. no. 09058.99 (for more info see: http://www.phywe.com). Measurements presented was performed with Mo anode without monochromatization of X-rays. Cadmium filter (thickness 10 mm, apertures of 0.5 mm in diameter, distance between apertures 1 mm) was irradiated by X-ray tube in one of our experiments. Image of Cd filter with very good image contrast is observed. We can see also very small charge sharing between neighboring pixels in the case of X-ray energies used. In the another X-ray tube measurement lead plate (thickness 1 mm, aperture 0.5 mm in diameter) overlapped by lead foil (thickness 100 μm) was irradiated. In this case Laue diffraction pattern on crystal lattice of Pb (crystal plane (1 1 1)) was observed.

An X-ray 241Am fluorescence ring source (ring diameter 30 mm, energy of X-rays 60 keV) was used as another example to demonstrate the capability of the pixel silicon device for X-ray imaging. An image of the ring source was obtained by using a simple “pin hole“ camera geometry (hole of 1 mm diameter in a 1 mm thick lead sheet) with the pixel device as the position sensitive sensor.

In conclusion, performance of pixel device for position sensitive detection of heavy charged particles and X-ray photons with precision of about 150 μm was demonstrated. Response to a single alpha hit in the form of a cluster of adjacent pixels can be effectively used to improve a precision of a position of incident particle. This will be also subject of our further research. Experiments mentioned above tend to application of pixel device for position sensitive detection of slow neutrons and for surface ion beam analysis, e.g. Rutherford Back Scattering (RBS), channeling etc. Further tests with X-rays will tend to an application of pixel device as a sensor for X-ray defectoscopy.