On some specific features of the -p interaction with 9Be

This chapter shows how to experimentally ''filter'' out events where a residual nucleus has not undergone a final-state interaction (fsi) with annihilation debris, as is the case with 8Be. In order to study -p annihilation on 9Be, positronsensitive detectors are needed which are selective for low-energy, highly ionizing α's. 9Be consists of about 2.5% of a 8Be (/sub -/gs) core with a single, weakly bound neutron. Annihilation of p (stopped in 9Be) on this neutron can leave an intact 8Be (gs). The 8Be (gs) sample has a very specific and rather clean signature which can already be easily explored, namely the appearance of a maximal angle between the decay α particles