Electroexcitation of isoscalar and isovector magnetic dipole transitions in C-12 and isospin mixing

The excitation of the J(pi);T = 1(+) ;0 and 1(+);1 states at E-x = 12.71 an d 15.11 MeV in C-12 has been studied at momentum transfers q < 0.5 fm(-1) i n inelastic electron scattering. The corresponding ground state radiation w idths and transition radii for the isoscalar and isovector transition are d educed as Gamma(gamma 0) = 0.32(2) eV, R-tr = 2.95(9) fm a nd Gamma(gamma 0 ) = 35.9(6) eV, R-tr = 2.63(4) fm, respectively. For the evaluation of Gamm a(gamma 0) and R-tr for the transition to the 15.11 state (e,e'), data of t he present work have been combined with older data from other low energy el ectron accelerators, The results support the strong isospin mixing of the t wo 1(+) states, and in the frame of a two-state model a Coulomb matrix elem ent [HC] = 118(8) keV is derived. This value agrees with other experiments using electromagnetic or pionic probes but is much more accurate. Furthermo re, the measured difference in the transition radii for the isoscalar and i sovector transitions allows for the Dst time a unique determination of the sign of (Hc). (C) 2000 Elsevier Science B.V. All rights reserved.