A new monolithic proportional gas detector with a micro-gap electron multiplier structure directly integrated on top of a finely pixelated readout ASIC

Abstract

We describe the design and preliminary test of a new generation of Gas Pixel Detector where the amplification structure is built directly on top of a CMOS ASIC. In this concept the chip works at the same time as readout electronics, collection plane and electron multiplication structure. We use an ASIC from the XPOL family, successfully operating on-board the IXPE polarimetric and imaging space telescope. It comprises 100k $50\mu m$ pitch hexagonal pixels and a large area analog readout, combined with a very fine pitch 1-D or 2-D gas proportional charge amplifying structure based on the concept of the micro-gap chamber. The charge multiplier is built directly on the uppermost metal layer of the chip, i.e. on the metal pads which act as the charge collecting electrodes each connected to the input stage of their respective individual pre-amplifier. This is achieved by adding two thin (about $2\mu m$ thick) finely patterned layers, one insulator and one metal, as a post-processing micro-fabrication step. These three layers together constitute the anode-cathode gap of a micro-gap like charge multiplication structure. The goal is to exploit the intrinsic sub-micron precision of the process and the extremely small exposed dielectric material to improve the compactness, space and energy response uniformity and the gain stability over time. Preliminary tests validating this concept are discussed in this paper.