THE NEUTRON-PROTON RATIO OF FAST NUCLEON EMISSION IN ANTIPROTON ANNIHILATION ON NUCLEI

The measurement of fast protons and neutrons emitted after antiproton annihilation at rest on U-238 and Cu-63 reveals a large neutron/proton ratio R. Its value for Cu-63 is larger than expected from the convent ional model of intranuclear pion rescattering. A value of R essentiall y constant over the whole range of mass number, from C-12 up to U-238, is also announced by the experimentalists. It is shown that, on the c ontrary, the conventional scheme predicts a regular increase of R with mass number. Alternative explanations of the effect are looked for. W ithin the usual scheme, an excess of negative pions, leading to more e mitted neutrons, is not compatible with final pion multiplicities. The influence of meson resonances on the neutron/proton ratio is consider ed, as well as the possible occurrence of annihilations on two nucleon s. They cannot warrant an increase of R at low mass targets sufficient to explain a constant value over the whole range of target masses.