Annihilation of Antiprotons At Rest In He-3 and He-4

At LEAR of CERN the annihilation of antiprotons, stopping in He-3 and He-4 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 pi(-) production in pbar He-3 and pbar He-4 annihilation events have been obtained. The ratio between the pbar annihilation probability on the neutron and the proton for He-3 and He-4 has been deduced to be about half the value obtained for H-2 in bubble chamber experiments. The analysis of the results shows that this difference cannot be due only tfo the pion final-state interaction or to the shadow effect of the nucleons of the nuclei. The probability of pbar 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.