Analyzing power distribution in the 12C(p, pα)8Be(g.s.) reaction at an incident energy of 100 MeV

The analyzing power distribution in the 12C(p,pα)8Be(g.s.) reaction at an incident energy of 100 MeV is investigated. The experimental coincidence data measured under quasifree kinematic conditions are in remarkable agreement with the trend of free p+4He elastic scattering, and also with a distorted-wave impulse approximation prediction. Thus the projectiles interact with the clusters as if they were free entities, with the rest of the target nucleons acting merely as spectators to the reaction. This result provides compelling and direct evidence for the existence of preformed α-clusters in 12C, as has often been postulated for this nucleus.