  A search for long-lived particles, which have come to rest within the ATLAS
detector, is presented. The subsequent decays of these long-lived particles can
produce high-momentum jets, resulting in large out-of-time energy deposits in
the ATLAS calorimeters. These decays are detected using data collected during
periods in the LHC bunch structure when collisions are absent. The analysed
dataset is composed of events from proton-proton collisions produced by the
Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV and
recorded by the ATLAS experiment during 2017 and 2018. The dataset used for
this search corresponds to a total live time of 579 hours. The results of this
search are used to derive lower limits on the mass of gluino $R$-hadrons,
assuming a branching fraction $B(\tilde{g} \rightarrow q \bar{q}
\tilde{\chi}_1^0)=100$%, with masses of up to 1.4 TeV excluded for gluino
lifetimes of $10^{-5}$ to $10^3$ s.
