Measurement of thel-forbidden Gamow-Teller branch of37K

The $\beta$ decay of ${}^{37}$K is studied in detail. Several new $\beta$ decay branches and $\gamma$-ray transitions are assigned to this nucleus. The $l$-forbidden ${0d}_{3/2}\to {1s}_{1/2}$ Gamow-Teller branch to the first excited state in ${}^{37}$Ar is observed for the first time and its branching ratio measured to be $(42.2\mathrm{}\pm 7.5)\times {10}^{-6}$. Such $l$-forbidden branches are very sensitive to extranucleonic effects such as meson-exchange currents and $\Delta$-isobar excitations. A calculation of these as well as core-polarization and relativistic effects, predict branching ratios of $8\times {10}^{-6}$ with universal $s$-$d$ wave functions and $52\times {10}^{-6}$ with Chung-Wildenthal wave functions.