Alpha-cluster knockout in the O-16(e,e 'alpha)C-12 reaction

The knockout of a-clusters induced by electrons has been used to study four -nucleon correlations in O-16. For the transition to the ground state of C- 12 the (e, e ' alpha) cross section has been measured for three values of t he momentum transfer and four values of the missing momentum. The momentum- transfer dependence gives evidence for a quasi-elastic alpha -knockout mech anism. The shape of the measured missing-momentum distribution is reasonabl y well described by shell-model and cluster-model calculations, but the the oretical curves overshoot the data by a factor three to four. (C) 2001 Else vier Science B.V. All rights reserved.