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.

Rate-induced streamer breakdown mechanisms

The most likely cause of discharges at high photon irradiation rates is the superimposition of two specific mechanisms leading to a critical value of electric field being reached at the cathode edge, resulting in a field emission initiated breakdown. Firstly, the probability of avalanches occurring at the anode near the site of a previous avalanche event (i.e. with a separation of the same order as the anode-cathode distance) before the original positive space charge has cleared to the cathode is greatly increased at higher irradiation rates. The electric field strength at the cathode is thus raised by the additive effect of the two charge clouds. The second effect becomes important at high values of drift potential, and manifests itself as an increase in avalanche size at higher irradiation rate *, provided that positive ion accumulation on the substrate surface is avoided, for example by lowering the surface resistivity. This increase in avalanche size relaxes the proximity required for two close avalanches to cause an excess in electric field strength at the cathode edge, thus further increasing the breakdown probability.

It is also possible that in some geometries and operating conditions, this mechanism may be alone responsible for raising the electric field at the cathode edge above the critical threshold. The interplay of the various effects leading to the breakdown is somewhat complex since a given gas gain may be achieved at a lower cathode potential by raising the drift potential, but the overall result is that negative streamers initiated by electrons extracted from the cathode edge via the field emission process are able to cross the anode-cathode gap, sustained in the early stages by the temporary high field conditions.

A low-resistance plasma channel is thus created between the anode and cathode through which a spark discharge can occur, damaging the electrodes and possibly the readout electronics. The explanation can be understood intuitively by considering that the maximum gain under low photon irradiation rate is governed by spontaneous field emission. When the maximum "safe" electric field that can be supported across the anode-cathode gap is exceeded, a streamer-mediated spark event can take place. Thus the maximum gain for a given photon energy is obtained by operating the detector just below this theshold (the value of the safe gain is governed by the nature of the incident radiation; the greater the number of secondary electrons, and so the mean avalanche size for a given gain, is determined by the energy of the incoming photon or charged particle).

This means that a safe gain limit is reached at low rates, at which the positive space charge remaining at the anode following the fast clearance of avalanche electrons is unable to cause a sufficient disturbance in the electric field to result in electrons being extracted and accelerated across the gap from the cathode; as the postive ions approach the cathode, it is reasonable to assume that negative streamers developing from the cathode edge are directed to the charge cloud itself, and are neutralised without reaching the anode. However, as the irradiation rate is increased, the probability of near avalanche events increases dramatically at fluxes of between 104 and 105 Hz mm-2 as illustrated in the figure below where the probabilities of 2 events occuring on the same anode strip within 100 ns of each other for avalanche separations up to 400 micron are shown for a 400 micron pitch microstrip detector.

electrical breakdown rates in microstrip detectors
Micrograph of microtrench detector with gold electrodes

Eventually, the maximum avalanche size in gas detectors is governed by the "Raether Limit" of approximately 108 ion pairs. At this point, the avalanche itself becomes self-propagating with positive and negative streamers growing outwards from the charge cloud. These streamers are then able to connect the respective electrodes forming a plasma channel spark precursor. Where a dielectric surface separates the anode and cathode, the positive streamer may be able to touch the surface thereby increasing the probability of connecting to the cathode aided by the surface polarization effect mentioned earlier. This process is especially important in the presence of highly ionizing particles e.g. alpha particles, where the large amount of primary charge deposited in the gas can lead to very large avalanches even at quite modest values of gain, and can be a critical factor in High Energy Physics applications. Actually reaching the Raether limit in micropattern detectors may often not be possible (depending on the structure) since the cathode field emission process occurs first, at lower avalanche charge values.

The increase in avalanche size observed at high values of drift field (5-10 kV/cm) with low resistivity substrates is due to a feedback mechanism whereby an increasing proportion of positive ions formed in the avalanche are directed towards the cathode at high rates. The avalanche charge has been shown to increase by around 60% between 103 and 105 Hz mm-2. Initially, due to the high value of the drift field, the majority of the ions formed in the avalanche are directed towards the drift electrode (approx 80% at 10 kV/cm), rather than the cathode. However, as the increased photon interaction rate results in more avalanches and therefore more positive ions travelling towards the drift electrode, the build-up of positive space charge in the gas volume begins to disturb the shape of the electric field close to the avalanche region with the result that more ions are directed towards the cathode rather than the drift electrode. The equilibrium in the recombination rate between secondary electrons drifting towards the avalanche region and positive ions drifting towards the drift electrode is therefore distorted and a progessively higher proportion of each secondary electron cloud reach the avalanche region with the result that the total charge produced in the avalanche is larger.

Note that the avalanche multiplication factor itself does not change significantly. With high resistivity substrates such as bare borosilicate glass, the accumulation of positive ions on the surface between the anode and cathode is the dominant rate dependent process, the effect being to reduce the avalanche multiplication factor. On these kind of substrates, the gas gain falls with higher rates of photon irradiation.

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