The subthreshold production of neutral pions was studied in the reactions 24 Mg(α,π 0 )X at 43 MeV·. A and 24 Mg( 16 O,π 0 )X at 24 and 33 MeV·. A . The energies and emission angles of the two coincident photons from the π 0 -decay were measured with a lead glass shower detector. The cross sections of 3.5 and 6.6 nb from the 16 O-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.
No description provided.