A magneto-optic trap (MOT) can provide a well-polarized, backing-free, localized source of radioactive atoms for $\beta$-decay experiments. We have trapped approximately 6000 atoms of ${}^{38}{\mathrm{K}}^m$ ( $t_{1/2}=0.925\mathrm{}\mathrm{s}$) and 2000 atoms of ${}^{37}\mathrm{K}$ (1.226 s) produced at the TRIUMF on-line separator TISOL in a vapor-cell MOT. We have measured optical isotope shifts and deduced the nuclear charge radii, which show an unusual lack of change at the neutron number $N=20\mathrm{}$ shell closure. Plans include a search for scalar contributions to the ${\beta }^{+}$- $\nu$ correlation in the $0^{+}\to 0^{+}$ decay of ${}^{38}{\mathrm{K}}^m$.

• Received 3 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.375