  The High Granularity Timing Detector (HGTD) will be installed in the ATLAS
experiment to mitigate pile-up effects during the High Luminosity (HL) phase of
the Large Hadron Collider (LHC) at CERN. Low Gain Avalanche Detectors (LGADs)
will provide high-precision measurements of the time of arrival of particles at
the HGTD, improving the particle-vertex assignment. To cope with the
high-radiation environment, LGADs have been optimized by adding carbon in the
gain layer, thus reducing the acceptor removal rate after irradiation.
Performances of several carbon-enriched LGAD sensors from different vendors,
and irradiated with high fluences of 1.5 and 2.5 x 10^15 neq/cm2, have been
measured in beam test campaigns during the years 2021 and 2022 at CERN SPS and
DESY. This paper presents the results obtained with data recorded by an
oscilloscope synchronized with a beam telescope which provides particle
position information within a resolution of a few um. Collected charge, time
resolution and hit efficiency measurements are presented. In addition, the
efficiency uniformity is also studied as a function of the position of the
incident particle inside the sensor pad.
