ABSORPTION OF NEUTRAL PIONS PRODUCED IN SUBTHRESHOLD NUCLEUS-NUCLEUS COLLISIONS

摘要

Neutral pions produced in Ar-induced reactions at 95A MeV on C, Al, Ag and Au targets were identified through their γγ decay photons, measured with TAPS spectrometer installed at GANIL Caen. Angular distributions of neutral pions are reasonably well described within a phenomenolog-ical model of pion reabsorption, taking into account momentum-dependent absorption length. The angular distribution of primary neutral pions (before absorption) seems to be independent of the beam energy and mass numbers of colliding nuclei. This distribution is described by the formula: 1 + A_2P_2(cos v) with A_2 = 0.33 ± 0.05, determined from the comparison to all available angular distributions of neutral pions. The transverse momentum spectra of neutral pions can be fitted within the thermal model only for momenta above 60 MeV/c. Assuming, that the model of pion re-absorption is correct, the transverse momentum spectra of primary pions were reconstructed. These spectra are surprisingly well described by the thermal model with temperatures about 22 MeV.