These webpages relate to research carried out between 2001 and 2004 into applying new micromachining techniques to the development of gas-avalanche type radiation detectors. Three-dimensional microstructures fabricated with SU-8 photosensitive epoxy showed promise in overcoming electrical breakdowns and other problems encountered in high radiation fluxes with the early generation of gas-avalanche micropattern radiation detectors. The motivation behind the work was to produce stable detectors with fast response principally for X-ray imaging and synchrotron diffraction applications. The work was funded by a Marie Curie Fellowship awarded by the European Community.



Gas microstrip detectors

The gas microstrip detector (MSGC) introduced by Anton Oed at ILL in 1988 represented a major development in the evolution of position sensitive, gas-based radiation detecors. The operating principle of radiation detectors using gas as the detecting and signal amplification medium can be described as follows. A fast charged particle moving through the gas causes ionisation along its path, creating ion pairs. An electric field across the gas volume causes electrons thus produced to drift toward an anode and positive ions toward a cathode, so developing an electrical signal. Uncharged particles such as photons are detected when a primary ionising interaction with a gas molecule takes place and a fast electron is produced which then causes further ionisation along its path. Various electric field configurations enable gas filled detectors to be operated in different modes accorded to the requirements of the system. The MSGC belongs to the class of detectors known as proportional detectors, which are operated such that the signal read out from the device is proportional to the incident particle energy. In proportional gas filled detectors, the electric field near the anode (usually a thin wire or strip) is sufficiently high for the electrons drifting towards it to accelerate to a point where they themselves are capable of causing further ionisation, causing a cascade or avalanche of electrons, which amplify the original signal. The increase in the number of electrons detected at the anode for each secondary electron entering the avalanche region is known as the gas gain.

Section of a typical microstrip plane
Section of a typical microstrip plane

The gas microstrip detector consists of a gas-filled chamber containing an insulating substrate on which alternating anode and cathode strips are fabricated using microlithograpic patterning technolgies developed for the microelectronics industry. Anode widths are typically of the order of 10 microns, cathode widths somewhat larger, around 100 microns. The distance between the anode and cathode strips can be reduced to less than 100 microns, with typical feature pitches of 200 - 500 microns. The drift field is defined by an electrode placed a distance typically 3-5 mm above the microstrip plane and held at a negative high voltage, so free electrons produced in the gas volume will drift along the field lines towards the microstrips. They then initiate avalanches in the high field regions close to the anode. The count rate capability of microstrip detectors is high since the proximity of the cathode strips to the avalanche region ensures that a large proportion of the positive ions generated in the avalanches are removed relatively quickly from the avalanche region. This means that the electric field reverts to the pre-avalanche configuration more quickly than is the case with other detectors, giving the detectors a lower inherent dead time.

Microstrip cathode ends passivated using SU-8 photosensitive epoxy
Microstrip cathode ends passivated using SU-8 photosensitive epoxy



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