Timing resolution from beam tests on thin LGADs down to 16.6 ps

The paper reports on the timing resolution achieved with Low-Gain Avalanche Diodes (LGADs), optimised for extreme-fluence conditions, at the DESY Test Beam Facility using 4~GeV/c electrons. The LGADs adopt an $n$-in-$p$ technology with a $p^{+}$-type boron gain implant, co-implanted with carbon to mitigate acceptor deactivation due to irradiation. The substrate thickness of the sensors varies from 20 to 45~\micron, with an active area spanning from 0.75~$\times$~0.75 to 1.28~$\times$~1.28~mm$^{2}$. The experimental setup consisted of a 45~\micron-thick trigger sensor with an active area of 3.6~$\times$~3.6~mm$^{2}$, two device-under-test (DUT) planes, and a Photonis micro-channel plate photomultiplier tube (MCP) as a time reference. Data taking was performed at the ambient temperature of the facility, at 18$^{\circ}$C. The gain was measured between 7 and 40 across all non-irradiated sensors in the study. The timing resolution was calculated from a Gaussian fitting of the difference in times of arrival of a particle at the DUT and the MCP, using the constant fraction discriminator technique. A timing resolution of 26.4~\ps was achieved in 45~\micron-thick sensors, down to 16.6~\ps in 20~\micron-thick sensors, in the non-irradiation study. The combination of two 20~\micron-thick LGADs reached a timing resolution of 12.2~\ps. A set of 30~\micron-thick sensors irradiated with neutrons at fluences between 0.4~$\times$~10$^{15}$ and 2.5~$\times$~10$^{15}$~\mevneut were tested in the beam. These irradiated sensors achieved a gain between 7 and 30 using a similar apparatus but cooled with solidified CO$_{2}$ to -42$^{\circ}$C. A timing resolution of 20~\ps was obtained in these irradiated sensors.