Longitudinal momentum distributions of ${}^7\mathrm{Be}$ fragments following the dissociation of ${}^8\mathrm{B}$ on heavy, highly charged target nuclei show forward-aft asymmetries, the result of interference of electric quadrupole $\mathrm{}\left(E2\mathrm{}\right)\mathrm{}$ transitions with the dominant $E1\mathrm{}$ excitation process. These asymmetries can therefore be used to gain insight into the $E2\mathrm{}$ contributions to the breakup process. To assess the sensitivity of these $E2\mathrm{}$ interference terms to the assumed reaction mechanism, in particular, the role of higher-order coupling effects at medium energies, coupled discretized continuum channels (CDCC) calculations are carried out for ${}^8\mathrm{B}$ breakup at 44 and 81 MeV/nucleon on heavy targets. The effects of higher-order processes due to both Coulomb and nuclear breakup mechanisms can be estimated. In line with earlier work we find that the asymmetries produced by the calculations are reduced when including the higher-order couplings, reflecting an effective quenching of the $E2\mathrm{}$ contributions. The full CDCC calculations show less asymmetry than the available experimental data, suggesting that the structure or reaction model now contains insufficient $E2\mathrm{}$ strength. This contrasts with the results of lowest-order reaction theories that conclude that the ${}^8\mathrm{B}$ model $E2\mathrm{}$ amplitudes are too large.

• Received 11 April 2002

DOI:https://doi.org/10.1103/PhysRevC.65.064619