Annihilation of antiprotons at rest in 3He and 4He

At LEAR of CERN the annihilation of antiprotons, stopping in 3He and 4He filling a self-shunted streamer chamber in a magnetic field, has been studied. The charged-particle multiplicities have been measured and the relative probabilities of π- production in anti p3He and anti p4He annihilation events have been obtained. The ratio between the anti p annihilation probability on the neutron and the proton for 3He and 4He has been deduced to be about half the value obtained for 2H in bubble chamber experiments. The analysis of the results shows that this difference cannot be due only to the pion final-state interaction or to the shadow effect of the nucleons of the nuclei. The probability of anti p annihilation at rest on a proton bound in the nucleus results to be twice as high as that on a bound neutron, showing the dominance of annihilation in the I=0 isospin states. (orig.)