Role of the BaF2 scintillation detectors in investigations of particles production below the threshold energy

Heavy-ion collisions offer the unique possibility for laboratory studies of hot and dense nuclear matter. Production of subthreshold particles is a tool to investigate the initial phase of the collision. As the production probability of subthreshold particles is very low, their detection is an experimental challenge. The discovery of the fast scintillation in BaF2 crystal opened new chances for the spectroscopy of energetic photons and neutral mesons. This work describes the studies of the response function of BaF2 detectors to photons, neutrons, charged particles and cosmic radiation. The TAPS spectrometer, operating for photons in the energy domain of 1 MeV - 10 GeV, is described. In a series of heavy-ion experiments in the energy range of 30 -60 MeV/nucleon, production of energetic photons was measured. The photon spectra span a dynamic range of up to 5 times the beam energy per nucleon. The production of most energetic photons is explained as photoabsorption of π mesons produced below threshold in nuclear matter. In 180 A MeV Ar+Ca experiment, π0 mesons were detected in coincidence with energetic protons. The analysis of π0-proton correlations revealed the decay of the Δ+ resonance. Assuming isospin symmetry, the global ratio of produced Δ resonance to all π mesons equals 0.79±0.30(stat)±0.2(syst). (author)