We have accumulated a sample of ∼ 400 ${\eta }^{\prime }\mathrm{}\left(958\right)\mathrm{}$ decays in the modes (1) ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\gamma$ and (2) ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}{\eta }_N^0 (N=\mathrm{neutral}\mathrm{decay})$ from a 15.4-eV/μb study of $K^{-}p$ interactions at 2.89 $\frac{\mathrm{GeV}}{c}$. We find a branching ratio $R=\frac{\mathrm{}\left(1\right)\mathrm{}}{\mathrm{}\left(2\right)\mathrm{}}=1.11\mathrm{}\pm 0.18$, where (1) is entirely ${\eta }^{\prime }\to {\rho }^0\gamma$. A matrix-element analysis yields $J^P\left({\eta }^{\prime }\right)=0^{-},2^{-}$ as acceptable values for both samples. Production-decay angular correlations suggest that a $J^P=2^{-}$ assignment is unlikely.

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