  The HIBEAM/NNBAR experiment is a free-neutron search for $n \rightarrow$
sterile $n$ and $n \rightarrow \bar{n}$ oscillations planned to be installed at
the European Spallation Source under construction in Lund, Sweden. A key
component in the experiment is the detector to identify $n-\bar{n}$
annihilation events, which will produce on average four pions with a final
state invariant mass of two nucleons, around $1.9\,$GeV. The beamline and
experiment are shielded from magnetic fields which would suppress $n
\rightarrow \bar{n}$ transitions, thus no momentum measurement will be
possible. Additionally, calorimetry for particles with kinetic energies below
$600\,$MeV is challenging, as traditional sampling calorimeters used in HEP
would suffer from poor shower statistics. A design study is underway to use a
novel approach of a hadronic range measurement in multiple plastic scintillator
layers, followed by EM calorimetery with lead glass. A prototype calorimeter
system is being built, and will eventually be installed at an ESS test beam
line for \textit{in situ} neutron background studies.
