Beam-test evaluation of pre-production Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector

TL;DR

The study addresses the HGTD need for precise timing with radiation-tolerant LGADs for HL-LHC. It uses beam tests at CERN and DESY with carbon-enriched LGAD pre-production sensors, irradiated up to $2.5\times 10^{15}$ n$_{eq}$/cm$^2$, and analyzes charge collection, time resolution, and hit efficiency using a CFD-based timing method and a track-based analysis framework. Results show unirradiated sensors exceed $15\ \mathrm{fC}$ charge and achieve $<40$ ps timing, while irradiated devices maintain $>4\ \mathrm{fC}$ and $<50$ ps timing up to end-of-life fluence, with efficiency $>95\%$ across the tested bias range; charge collection improves with angle while timing remains robust. These findings confirm the radiation tolerance and timing performance required by HGTD, enabling progression to full-size sensors and final ASIC integration for HL-LHC operations.

Abstract

The High Granularity Timing Detector (HGTD) will be installed in the ATLAS experiment as part of the Phase-II upgrade for the High Luminosity-Large Hadron Collider (HL-LHC). It will mitigate pile-up effects in the forward region, and measure per bunch luminosity. The design of HGTD is based on Low Gain Avalanche Detector (LGAD) sensors. This paper presents the results of beam-test campaigns conducted at CERN and DESY in 2023 and 2024 on single LGADs from HGTD pre-production test structures, before and after neutron irradiation up to fluences of $2.5 \times 10^{15}~\mathrm{n_{eq}/cm^2}$. The tested LGADs can meet HGTD requirements in terms of charge collection, time resolution, and hit efficiency, even under HL-LHC end-of-life conditions, supporting their deployment in the final detector.

Figures (17)

• Figure 1: Layout of the single-pad LGAD of the QC-TS from the USTC-IME and IHEP-IME designs.
• Figure 2: Leakage current as a function of the bias voltage measured at $-30^\circ$C for all tested sensors.
• Figure 3: Pictures of the test beam set-up at CERN(a) and at DESY(b).
• Figure 4: Picture of the QC-TS single pad LGAD mounted on a readout board.
• Figure 5: Simplified schematic of the DAQ system used during the test-beam campaigns at CERN and DESY. The TLU receives triggers from the FEI4 HitOr signal (grey line). All DUTs are interfaced to the TLU via LVDS signals (magenta lines). The oscilloscope is connected through an adapter board that converts LVDS to TTL and splits the trigger and busy signals (orange lines). The green lines represent the sensor and MCP signals connected to the oscilloscope. Red lines indicate all connections routed through the internal network. Finally, the blue line shows the connections between the high-voltage (HV) and low-voltage (LV) power supplies and the sensor boards.