[Paper Review] A spark-resistant bulk-micromegas chamber for high-rate applications

J. Burnens, R. De Oliveira+5 more|arXiv (Cornell University)|Nov 24, 2010

Particle Detector Development and Performance1 references4 citations

TL;DR

This paper presents a spark-resistant bulk-micromegas chamber design using resistive strips on a printed circuit board to suppress high-voltage breakdowns in high-rate environments. By isolating readout strips with a resistive layer connected to ground through high-value resistors, the chamber achieves stable operation at gas gains up to 30,000 and withstands neutron fluxes up to 1.5×10⁶ cm⁻²s⁻¹ without HV breakdowns, even under extreme particle rates of 1 MHz/cm².

ABSTRACT

We report on the design and performance of a spark-resistant bulk-micromegas chamber. The principle of this design lends itself to the construction of large-area muon chambers for the upgrade of the detectors at the Large Hadron Collider at CERN for luminosities in excess of 10**34/cm2/s or other high-rate applications.

Motivation & Objective

• To develop a spark-resistant bulk-micromegas chamber for high-rate applications in high-luminosity LHC upgrades.
• To address the risk of HV breakdown caused by ionizing debris from neutron interactions and minimum-ionizing particles.
• To maintain high spatial resolution and detection efficiency under extreme particle fluxes and high gas gains.
• To enable stable operation in environments with particle rates exceeding 1 MHz/cm² and neutron fluxes up to 1.5×10⁶ cm⁻²s⁻¹.
• To demonstrate that spark-induced HV breakdowns no longer limit detector performance in high-rate scenarios.

Proposed method

• A resistive protection layer (64 µm thick, 1–10 MΩ/cm resistivity) is deposited on top of 18 µm copper readout strips on a PCB.
• Resistive strips (150 µm wide, 250 µm pitch) are patterned to match the readout strips and connected to ground via 15–50 MΩ resistors.
• A 128 µm-distant stainless steel mesh (400 lines/inch, 18 µm wire) is supported by 400 µm-diameter photoimageable coverlay pillars at 2.5 mm intervals.
• The drift electrode is a 5 mm distant stainless steel mesh (350 lines/inch, 22 µm wire), forming a 4–5 mm drift gap.
• The chamber is sealed with an O-ring and Kapton foil windows to maintain gas tightness.
• The design prevents charge spreading and limits discharge energy by confining current to individual resistive strips.

Experimental results

Research questions

• RQ1 Can a resistive strip layer on the readout plane effectively suppress HV breakdowns in bulk-micromegas chambers under high-rate particle irradiation?
• RQ2 How does the resistive layer design affect the chamber's ability to maintain high gas gain while preventing spark propagation?
• RQ3 What is the maximum particle rate and neutron flux the chamber can withstand without HV breakdown or performance degradation?
• RQ4 Does the resistive protection scheme preserve spatial resolution and detection efficiency compared to non-resistive micromegas?
• RQ5 How does the choice of gas mixture (e.g., 80:20 vs. 93:7 Ar:CO₂) influence the spark rate under neutron irradiation?

Key findings

• The resistive chamber design successfully prevented HV breakdowns during exposure to 120 GeV/c pion beams at rates up to 30 kHz over 2–3 cm², with no breakdowns observed over several days.
• The chamber operated stably at gas gains up to 30,000 and maintained signal integrity with less than 30% signal degradation at particle rates up to 1 MHz/cm².
• Under neutron irradiation at 1.5×10⁶ cm⁻²s⁻¹, the spark rate per incident neutron was reduced to a few ×10⁻⁸ in 93:7 Ar:CO₂ gas, representing a fivefold reduction compared to 80:20 Ar:CO₂.
• Sparks in the resistive chambers produced currents below 300 nA, and the mesh voltage dropped only transiently at gains above 1.5×10⁴, indicating effective energy dissipation.
• No significant differences in spatial resolution, efficiency, or data quality were observed between the three resistive chambers tested, confirming consistent performance.
• The non-resistive chamber suffered repeated HV breakdowns under identical beam conditions, confirming the effectiveness of the resistive protection scheme.