Gaseous Detector Laboratory

Miro Pin Array (MIPA)

A technique for electron multiplication in gas is being developed using ithographic techniques, with the potential of easy fabrication. Moreover, the electrical characteristics of the resulting device are very stable. Comprising an array of anode pins that are recessed into a closely-packed hexagonal cathode, the MIcro-Pin Array (MIPA) represents a promising addition to micro-pattern detectors with two-dimensional sensitivity.

The MIPA was produced by sequential patterning of a UV-sensitive photoresist known as SU-8. This resist is quite transparent in the UV wavelength range used for exposure, and microstructures with aspect ratios as high as 15:1 can be patterned using a contact mask aligner. After UV exposure with an appropriate mask and annealing, the epoxy-based resist cross-links to form a rigid microstructure with good mechanical properties. Vacuum evaporation is used to metalize the surfaces of the anode and the cathode arrays. A small region at the bottom of the cathode structure is left uncoated to provide the electrical insulation between anodes and cathodes.

Above the array substrate, typically at a distance of about one cm, there is an entrance window made from conductive material. The volume between the substrate and the entrance window contains the proportional counter gas, tests so far being performed with 80%Ar/20%CO₂. The detector is usually operated with the cathode at ground potential, the anodes at a potential of about +600 Volts, with the entrance window at negative potential. For a wide range of values of electric field between the window and substrate, all electrons created by ionizing radiation within the gas volume drift to the hexagon-pin microstructure. Within the structure itself, the electric field directs the electrons toward the tip of the pin anode where avalanche multiplication takes place. Multiplication is limited to only a small region around the tip of the anode pin, because of the rapid decrease of electric field with distance from the center of the pin. The small extent of the electron multiplication region allows this detector to reach high values of gas gain in a stable amplification mode.

The figure above shows the 5.9 keV X-ray spectrum of Fe55 obtained from full irradiation of the 2.5x2.5 cm² area of the detector. The FWHM of the photo-peak is about 20% and the photo- and the escape-peaks are well separated, again comparing favorably with some other micro-pattern detectors.

Reference:

P. Rehak, G.C. Smith, J.B. Warren, and B. Yu, "First Results from the Micro Pin Array (MIPA)," IEEE Trans. Nucl. Sci. NS-47, No. 4, August (2000) 1426-1429.