Observation of Enhanced Monopole Strength and Clustering in 12Be

In a recent breakup-reaction experiment using a Be-12 beam at 29 MeV/nucleon, the 0(+) band head of the expected He-4 + He-8 molecular rotation was clearly identified at about 10.3 MeV, from which a large monopole matrix element of 7.0 +/- 1.0 fm(2) and a large cluster-decay width were determined for the first time. These findings support the picture of strong clustering in Be-12, which has been a subject of intense investigations over the past decade. The results were obtained thanks to a specially arranged detection system around zero degrees, which is essential in determining the newly emphasized monopole strengths to signal the cluster formation in a nucleus.