Gas detectors have definite advantages over other types of detectors: large-sensitive area, flexible geometry, low cost, radiation resistance etc. With these merits as well as low density they are very attractive for tracking devices in high-energy physics experiments and other applications. Nevertheless they suffer from some problems: dead zone effect, electrical breakdown, and angular dependence.
In two-dimensional gas avalanche pixel detector, the dead zone effect is most predominant because of the drift field perturbation by the readout bus. It is found that the conductive coating onto the substrate is the best method to minimize this effect when the ratio between the surface coating conductivity and the substrate bulk conductivity is greater than 0.1 in unit of centimeter. Otherwise, avoiding the serious dead zone effect, and simultaneously allowing good spatial resolution and large avalanche signal, the 100 ㎛-pitch detector is optimum.
In order to avoid the permanent damage of electrical breakdown due to sparking, the charge preamplification method is most desirable because it can share the avalanche gain to the preamplifying structure and the electrodes of detector so it reduces the operation voltage. However, the conventional gas electron multiplier (GEM) as the preamplifying structure shows an unstable signal output as a function of time, which is due to the charging effect. A new GEM having very steep wall sides and high-aspect ratio has been made by the LIGA process, and which can lead to stable signal output and larger avalanche signal. First experimental results show a large avalanche gain (lower limit to $3×10^3)$, and a good time stability (~ 2% during 30 min).
A columnar cesium iodide (CsI) layer was initially intended as a dominant ionization source in gas avalanche microdetectors to overcome the angular dependence of the detector performance upon the incident radiation. However, based upon the results from the electric field simulation and experiments, it is revealed that the electric field does not penetrate sufficiently in the space between CsI columns to produce avalanches. Therefore there is no improvement with this. As an alternative structure, the LIGA technique has been proposed, because the micro-hole array made by this method can define the electric fields by introducing electrodes between the top and the bottom sides. From the feasibility test calculation, the detection efficiency is about 95% and the improvement of spatial resolution is a factor of 5 compared with a conventional microstrip detector accounting for 30° angle of the incident radiation.
