Total isovector M1 radiative width of the 16.6 to 16.9 MeV doublet in 8Be

The isovector M1 radiative widths of the strongly isospin-mixed 2+ levels at 16.6 and 16.9 MeV excitation is 8Be for a transition to the 2.94-MeV 2+, T = 0, first excited state of 8Be were measured through the radiative capture reaction 4He (4He,γ1)8Be*. This experiment was undertaken as part of a search for evidence of second-class weak interaction currents in the mass-8 system. A 4He beam from the Wright Nuclear Structure Laboratory MP tandem accelerator passed through a 100-keV-thick 4He gas target. The resulting gamma rays were detected by an anticoincidence-shielded lead-collimated 30 cm x 30 cm NaI(Tl) spectrometer, placed at a backward angle of 1250. A p-γ coincidence measurement using the reaction 10B(3He,p)12C* (15.11)(γ)12C provided a photopeak efficiency curve for the spectrometer. An excitation function over the range of 16.4 to 17.1 MeV in 8Be was measured at 1250. This was an almost direct measurement of the total M1 width, which was determined by fitting the excitation function to a curve generated from 2-level R-matrix theory. The isovector M1 width GAMMA/sub M1//sup Δ tau=1/ was found to be 6.91 +- 0.85 eV; there was no evidence for a direct interaction contribution. An angular distribution at 16.9 MeV gave a rough value for the E2/M1 mixing ratio. When the resulting weak magnetism form factor, which is proportional to the square root of GAMMA/sub M1//sup Δ tau=1/, was compared with the results of a recent β-α angular correlation measurement on the beta decays of 8Li and 8B to the same final state, no evidence was found for the existence of second-class currents in the mass-8 system