8 - Ideal gas electron multipliers (GEMs) for X-ray polarimeters

Summary

We have developed gas electron multipliers (GEMs) for space science applications, in particular for X-ray polarimeters. We have employed a laser etching technique instead of the standard wet etching for the GEM production. Our GEMs showed no gain increase after applying high voltage and kept the gain for more than two weeks at a level of 2% (RMS). We show the gain properties and the results of some acceleration tests to mimic a two-years low-Earth-orbit operation in this paper.

Introduction

The GEM is one of the recently developed micro-pattern gas detectors. A dense pattern of through-holes is drilled in an insulator substrate, which is typically polyimide, sandwiched by thin copper foils. The surface and cross-section micrographs of a GEM are shown in Figure 8.1. When high voltage is applied to the copper electrodes in an appropriate gas, the GEM works as an electron multiplier. GEMs are used in many fields such as high energy and nuclear physics, X-ray imaging, etc. In astrophysics, photoelectric X-ray polarimeters, in which the GEM is a key device to multiply an electron cloud whilst retaining its shape, are the most interesting application.

We have produced GEMs since 2002 for X-ray polarimeters. The standard method to produce GEMs is a wet etching technique, while our method is laser etching, which has many advantages. Cylindrical holes are easily formed with the laser etching. The capability to drill cylindrical holes helps in forming finer-pitch holes on a thicker substrate.