Giant resonances in 12C

Detailed measurements on the reaction B¹¹(p, γ)C¹² are reported in the incident proton energy range $14\le E_p\le 22$ MeV. Previously reported absolute cross sections for this reaction have been found to be in question. Measurements have been made leading to the first (2⁺) and third (3⁻) states of C¹² in addition to the ground state. No evidence has been found for the predicted resonance at Eₓ=35 MeV, also reported in earlier experimental work. Detailed calculations based on the one-particle-one-hole model wave functions of Gillet and Vinh-Mau have been compared with the experimental data using a complete R-matrix formalism. Repetition of these calculations using pure j-j configurations yields comparable agreement. Evidence for the importance of many-particle-many-hole configurations in the radiative capture process has been found, particularly in the reaction leading to the first and third excited states, but also in the ground-state reaction data. The calculations are significant in the sense that no new parameters are introduced, nor are any significant approximations used which are not already in the model wave functions under test. The isospin mixing in C¹² is found to be very small, if the new absolute (p, γ₀) cross section reported here is used; this is in marked contrast to earlier estimates of this mixing.