PLASTIC DETECTORS FOR MEDIUM ENERGY NEUTRONS

Three neutron detectors made up of a total of fifty-three vertical plastic scintillator bars BC412 (equivalent to NE110) with 8x20 cm2 section and a sensitive height varying from 40 to 130 cm are used in the PS199 experiment at the CERN Low Energy Antiproton Ring (LEAR). Monitoring of the stability of the counters during data taking is done by a laser system. The TOF is calibrated with gamma rays from the ppBAR --> pi0's --> gamma's reaction. A new method to calibrate the amplitude of the counters at low energy using 4.4 MeV gamma rays from an Am-241/Be source is described, as well as a high energy calibration using cosmic muons. The absolute neutron detection efficiency of the counters is measured for neutron energies varying from 33 MeV to 204 MeV. Measurements are performed at five counter thresholds of 1.5, 2.5, 4.5, 6.5 and 8.5 MeV(ee). A comparison of the experimental measurements with the predictions of a Monte Carlo computer code is presented.