Abstract
A shell-model study is made of first-forbidden \ensuremath{\beta} decay in A=205--212 nuclei. A least-squares fit for eighteen \ensuremath{\Delta}J=0 and 1 decays gave a scaling factor for the rank-one contributions of 0.97\ifmmode\pm\else\textpm\fi{}0.06, i.e., agreement with experiment, and an enhancement factor ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{MEC}}$ for the rank-zero matrix element of ${\ensuremath{\gamma}}_{5}$ of 2.01\ifmmode\pm\else\textpm\fi{}0.05, indicating an enhancement by 100% over the impulse approximation. A 40% effect is predicted from meson exchange. Thus, a deficiency in the meson-exchange calculation or some further unforeseen effect is suggested.
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