Proton decay from the giant dipole region in carbon 12

Proton decay of the giant dipole resonance in 12C has been studied via the reaction 12C(e,e' p) at incident electron energies of 258 and 315 MeV. The electrons were produced by the Bates-MIT Linear Accelerator. The scattering electrons were detected in the Medium Energy Pion Spectrometer on beamline B at Bates. Decay protons from the giant resonance were detected in coincidence with the electrons in eight charged particle telescopes, each containing four silicon surface barrier detectors of thicknesses 50, 150, 1000 and 1000 microns, respectively. Protons were distinguished from other charged particles on the basis of their energy loss in the thin detectors. Proton decay to the ground and first excited states in 11B was observed. The (e,e' p) cross section was measured as a function of the proton decay angle determined with respect to the axis of momentum transfer bar q. Data were taken for proton angles between 0 and 240 degrees with two settings of the goniometer on which the proton telescopes were mounted. The ring of telescopes was tilted 135 degrees out of the scattering plane. The angular correlations were fit to an unconstrained Legendre polynomial expansion and compared to similar fits from previous work at Mainz at lower q. The authors studied the angular correlations and Legendre fits as a function of momentum transfer and excitation energy in 12C. They find that the strength of the open-quotes shoulderclose quotes state at 25.5 MeV grows as q increases, attaining an amplitude roughly equal to that of the main giant resonance at 22.5 MeV in 12C. They verify that the shoulder state prefers to decay via p1 in comparison to the po decay. These observations confirm that this state is dominated by the predicted spin-flip giant dipole resonance. Other trends in the data as a function of q are shown and discussed