The HIBEAM/NNBAR Calorimeter Prototype

The HIBEAM/NNBAR experiment is a free-neutron search for n → sterile n and n → 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 - 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 → 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 in situ neutron background studies.