The Forbidden Transition ofYttrium91andCesium137

${\mathrm{Y}}^{91}$ decays by the emission of a single beta-particle with a maximum energy of 1.54 Mev, while ${\mathrm{Cs}}^{137}$ decays in two ways, (1) beta-decay (maximum energy=0.518 Mev) to ${\mathrm{Ba}}^{137*}$ followed by a gamma-transition to the ground state, and (2) beta-decay (maximum energy=1.2 Mev) directly to the ground state. Probably not more than 5 percent of the ${\mathrm{Cs}}^{137}$ nuclei decay directly to the ground state. Conventional (allowed transition) Kurie plots of ${\mathrm{Y}}^{91}$ and the low energy group of beta-particles of ${\mathrm{Cs}}^{137}$ display curvature which is concave toward the energy axis at high energies and concave upward at low energies. If it is assumed that Gamow-Teller selection rules govern the beta-process, the Fermi function $F(Z,W)\mathrm{}$ for allowed spectra must be multiplied by a factor $G=(W^2-1)+{(W_0-W)}^2$ in a beta-transition for which there is a change of parity and for which the spin change is two units. If these conventional Kurie plots of ${\mathrm{Y}}^{91}$ and ${\mathrm{Cs}}^{137}$ are modified by the factor $G$, the resulting plots are approximately straight lines. This indicates that the beta-decay of ${\mathrm{Y}}^{91}$ and the low energy beta-decay of ${\mathrm{Cs}}^{137}$ involve a spin change of two units and a parity change. These results contribute evidence for the validity of the Fermi theory of beta-decay and the Gamow-Teller selection rules.