Subthreshold π0-production in the reactions 24Mg(α,π0)X at 43 MeV .A and 24Mg(16O,π0)X at 24 and 33 MeV .A

The subthreshold production of neutral pions was studied in the reactions 24Mg(α,π0)X at 43 MeV .A and 24Mg(16O,π0)X at 24 and 33 MeV .A. The energies and emission angles of the two coincident photons from the π0-decay were measured with a lead glass shower detector. The cross sections of 3.5 and 6.6 nb from the 16O-induced reactions are compatible with other experiments in this energy region. For the α-induced reaction a production cross section of 1.3 nb was found. Here, only 22 MeV above the absolute threshold, the pion emission from an excited Δ-nucleon hole state is strongly suppressed. The differential cross sections at very backward angles are by an order of magnitude bigger than the yields at 90 pointing to a strong absorption of the produced pions by the larger target nucleus. (orig.)