Design, simulation and performance of the resistive-anode PICOSEC Micromegas detector

TL;DR

This work tackles achieving tens of picoseconds timing in MPGD-based detectors under high-rate operation by deploying a resistive DLC layer on Kapton in the PICOSEC Micromegas. It combines analytical modeling, FEM, and Garfield++ simulations to quantify rate-dependent gain loss and to dissect signal formation, including delayed components from the resistive layer. Experimental validation with single-channel prototypes and 150 GeV muon beams shows that a surface resistivity around $20\ \Omega/\Box$ yields manageable gain drop while preserving the electron-peak leading edge, and timing resolutions comparable to the metallic reference (11.5–11.9 ps with CsI, ~28–32 ps with DLC) across the pad area. The results demonstrate that resistive-anode PICOSEC can deliver robust, high-precision timing and inform design choices for scalable multi-channel implementations, with potential future improvements based on local charge-evacuation schemes for large-area coverage.

Abstract

The PICOSEC Micromegas detector is a Micro-Pattern Gaseous Detector concept developed to achieve tens of picosecond timing resolution for charged particle detection by combining a Cherenkov radiator with a two-stage Micromegas amplification structure. To improve operational robustness, a resistive anode has been implemented using a DLC layer deposited on a Kapton substrate. While this design enhances detector stability, the resistive layer may influence rate capability, signal formation, and detector capacitance, altering timing performance. This work presents a comprehensive study of a resistive design, including an analytical model and finite-element simulations to quantify rate-dependent gain reduction due to ohmic voltage drop on the resistive layer. An analytical solution for the voltage across a finite-size resistive layer is derived, and a numerical model is developed to evaluate gain suppression under intense particle fluxes. The impact of the resistive layer on signal formation is investigated using time-dependent weighting fields and the Garfield++ simulation framework. The contribution of signal components induced by the resistive layer is quantified, and preservation of the signal leading edge is found for surface resistivities above 100 kohm per square. Single-channel resistive-anode prototypes were designed, constructed, and experimentally characterized. Laboratory measurements using single photoelectrons and power spectral density analysis show the predicted reduction in signal amplitude while preserving the leading edge. Muon beam tests with CsI and DLC photocathodes demonstrate a time resolution of 11.5 ps for CsI, comparable to 11.9 ps for the metallic-anode device, showing the suitability of the resistive design for precision timing applications.

Figures (21)

• Figure 1: Top: Operating principle of the PICOSEC MM detector. Bottom: Typical PICOSEC MM signal at the output of the amplifier.
• Figure 2: Cross-section at $y = 50$ mm of the potential drop across a 10 x 10 cm$^2$ grounded resistive layer with surface resistivity R with an impressed total current $I$ across a disk centered at $(x_c,y_c = 5 \text{ cm})$ with radius $r = 1$ cm (full lines) and $r = 2$ cm (dashed lines).
• Figure 3: Electric potential $\phi(x,y)$ across the resistive layer due to the injection of a current density coming from a $15$ mm wide circular beam with a flux of $\Phi = 5.66\cdot10^5$ cm$^{-2}$s$^{-1}$ and effective detector gain $G_e = 10^6$.
• Figure 4: Computed relation between the gain $G_e$ of the externally applied current density $j_e$ on the resistive layer as a function of the average voltage drop $V_a + \bar{\phi}$ on the anode. Furthermore, the measured gain is depicted, represented as $G_0$, for diverse (pre-)amplifications, indicating the data points with markers, where a 5% error is assumed. The full lines signify the exponential fits for the two gain curves, employing Eq. \ref{['eq: G vs Va']}. The equilibrium state can be identified as the point of intersection between $G_0$ and $G_e$.
• Figure 5: Calculated normalized gain for the resistive PICOSEC as a function of the applied anode voltage $V_a$ for different surface resistivities and cathode voltage of $-475$ V (left panel) and $-500$ V (right panel).