C12(γ, p)B11Angular Distributions In and Just Above the Giant Resonance

The energy and angular distributions of photoprotons have been observed with 30.5- and 32.1-MeV electron bremsstrahlung using the $\Delta E\times E$ coincidence technique. The angular-distribution anisotropy parameter $A_2$ has the average value -0.56 in the giant resonance peak from 21.8 to 23.7 MeV and varies less than ±0.05 from that value. From 24.4 to 28 MeV, variations in $A_2$ of about ±0.2 were detected and correlate somewhat with bumps seen in the total cross section. Above 28 MeV, $A_2$ decreases toward -0.9. No model yet exists which predicts correctly the anisotropy observed above the giant resonance. The probable nature of the particle-hole states at and above 25 MeV is deduced. The asymmetry parameters $A_1$ and $A_3$ are everywhere nonzero, have opposite signs, and exceed 0.3 in magnitude above 29 MeV, indicating moderate electric-dipole-quadrupole interference. This is interpreted as being consistent with one-particle-one-hole quadrupole excitation of ≲1% relative to the dipole photoproton emission strength.