Semidirect Isospin Mixing in Deuteron-Induced Reactions

A new mechanism which leads to substantial violation of isotopic invariance in "direct" processes involving deuterons is suggested. The $T$-forbidden ($d, \alpha$) cross section resulting from this mechanism is nearly comparable in magnitude with that from the formation of isotopically impure compound intermediate states. This new mechanism involves transitions from the $S=1\mathrm{}$ state of the deuteron to the $S=0\mathrm{}$, $T=1\mathrm{}$ "state." These transitions occur because the nucleon-target interaction contains a spin-orbit part; however, the Coulomb distortion of the proton wave function in the intermediate states causes an asymmetry in the spin-orbit potentials seen by the neutron and proton. Thus the spin of one nucleon is flipped more often than that of the other. Because the electromagnetic interaction merely provides the asymmetry factor (which is due to the long-range nature of the Coulomb potential), while it is the nuclear forces which actually flip the spins, this source of $T=1\mathrm{}$ admixture is far more important than two other "direct" mechanisms which have been suggested, namely, Coulomb polarization of the deuteron internal wave function, or preliminary (allowed) deuteron pickup leading to a $T=0\mathrm{}$ state of the residual nucleus, followed by Coulomb excitation to a $T=1\mathrm{}$ final state. Estimates of the forbidden reaction cross sections resulting from either the preferential spin-flip or the Coulomb-excitation mechanism are presented, and an experimental test of the new hypothesis is proposed. Finally, it is concluded from this investigation that the preferential spin-flip mechanism can explain the recent observations of Meyer-Schützmeister et al., both qualitatively and quantitatively.