Two-dimensional localization of neutrons in a large-area liquid-scintillator cell

ORION is a sectorized 4π neutron detector, part of it being used as a time-of-flight spectrometer. In a sector, six phototubes collect the light created during the neutron slowing down. The amount of charge integrated by each one strongly depends on the impact position of the neutron. Several tests have been performed with cosmic rays to study in detail the relation between the collected charge distribution and the particle impact position. The results are presented and compared with a simulation based on simple assumptions of light propagation in this particular medium. The detailed analysis has revealed a rather good resolution, better than 5 cm, and at the same time, regions of ambiguity where the correct position could not always be extracted. A new detector prototype, TOURNESOL, allowing one to overcome the present limitations is under construction.