Development and Characterization of a Time Projection Chamber Prototype for Neutron Oscillation Searches at the European Spallation Source
A compact time projection chamber (TPC) prototype was studied for charged-pion tracking in the annihilation detector proposed for the HIBEAM experiment at the European Spallation Source. HIBEAM aims to search for neutron-antineutron oscillations, where a produced antineutron would annihilate on a carbon target and produce a pionic final state. A tracking algorithm was developed and validated using cosmic-particle measurements expected to be dominated by atmospheric cosmic muons, proton elastic-scattering data from the Cyclotron Centre Bronowice, and simulated TPC event data. Cosmic measurements provided a minimum-ionising reference for pion-like tracks, while proton-scattering data tested reconstruction in a more highly ionising environment. Simulated crossing-track events were used as a stress test of complex topologies. The algorithm performed reliably for well-separated tracks, while crossing or overlapping tracks and dense proton-scattering events reduced performance. In experimental data, reconstructed tracks showed sub-millimetre residual widths, robust charge sharing supporting the zigzag-shaped readout, and uniform measured dE/dx across the drift volume. The truncated-mean track-level dE/dx distribution in cosmic data showed an approximately Gaussian peak with a relative width of about 16%, demonstrating how energy-loss information can be used for particle identification. Tilted proton tracks showed broader residuals in yz, because the pad row signal was extended in time. A sub-centroid refit technique was proposed, using the internal ADC time profile within a pad row instead of a single charge-weighted time position. With four equal-y sub-centroids per pad row, the yz residual width was reduced from 1.30 mm to 0.55 mm. These results validate the initial tracking capabilities of the algorithm and prototype, and provide feedback for the next-generation prototype.