Effect of multiple discharges on accumulated damage to the DLC anode layer of a resistive Well Electron Multiplier

Abstract

A prototype of the WEM (Well Electron Multiplier) detector with an active area of 10 × 10 mm2 and a resistive DLC anode was tested in terms of robustness to electrical discharges induced by highly ionizing particles (241Am alpha source). The perforated structure of the WEM detector was produced from a 500 μm thick FR4 with drilled holes of 200 μm in diameter and 500 μm in pitch. The resistive anode was made of 100 nm thick DLC layer with 30 MOhm/square sheet resistance deposited on the anode grid electrode. The anode grid electrode is used to distribute voltage to the resistive layer and provide fast charge evacuation. The detector was operated in Ar:CO2 (90:10) gas mixture at gas gain of 3,500. The alpha source was placed in the drift gap. The WEM detector with intrinsic capacitance of 34 pF did not show visible damage and changes in performance after 1 million accumulated discharges. To simulate a large area detector, we added a capacitance up to 1 nF in parallel with the test device. The results of the experiments with an additional capacitance revealed that a small WEM prototype can't be directly scaled to the dimensions more than 60 × 60 mm2 without losing the robustness to discharges. We assume that the observed damage could be caused by the design features of the prototype. The grid anode electrode with a thickness of 35 μm results in a gap between the perforated FR4 board and the resistive anode board. Simulations of the electric field distribution with Comsol Multiphysics software revealed a significant electric field strength in this gap. This could lead to electric discharge path bypassing the protective resistive DLC layer. A possible solution to this problem could be additional insulation of the anode grid electrode with a coverlay similar to that used in bulk MicroMegas production.