Energy-sharing distributions have been measured at five quasifree angle pairs for the reaction ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$(α,2α${)}^5$He at an incident energy of 197 MeV. In order to extract absolute spectroscopic factors, elastic scattering cross sections for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$(α,α${)}^4$He were also measured at approximately the same incident energy, between center-of-mass angles of 40° and 90°. The knockout data are compared with distorted-wave impulse approximation theory. The extracted cluster spectrosopic factors are found to be in agreement with a theoretical estimate and also with existing results from (p,pα) studies. Consequently, this new result is in strong contrast to those from previous (α,2α) studies at lower incident energies which yield spectroscopic factors significantly in excess of the expected values. The radial localization of the coincidence cross section in the target nucleus suggests that the observed incident energy dependence of the spectroscopic factors is related to the cluster-core nuclear density. Thus the cluster formation probability is enhanced in the region of very low nuclear density which is preferentially sampled by the (α,2α) reaction at low incident energies.

• Received 13 May 1994

DOI:https://doi.org/10.1103/PhysRevC.50.2449