Kinetic energy of shakeoff atomic electrons from $^{37}$K $\beta^+$ decay

We have measured the kinetic energies from 0 to 30 eV of atomic shakeoff electrons from the $\beta^+$ decay of $^{37}$K. Despite much experimental and theoretical work on the distribution of final ion charge states, shakeoff electrons from $\beta^-$ decay have only been measured with energies above 150 eV [Mitrokhovich, Nucl. Phys. Atom. Energy, 11 125 (2010)]. We use our magneto-optical trap's time-varying magnetic quadrupole field combined with a uniform electric field as a spectrometer. Our result has more 15 eV electrons than a model using the sudden approximation and hydrogenic wavefunctions [Levinger Phys. Rev. 90 11 (1958)]. The total energy carried away by electrons is, as expected, a negligible correction to superallowed Ft values. Understanding the energy of these low-energy electrons is important for their use in precision $\beta$ decay to select events coming from trapped atoms and start time-of-flight for the recoil ions. Our results could provide a benchmark for shakeoff electron calculations used for biological radiation damage [Lee, Comp. Math. Meth in Medicine doi:10.1155/2012/651475].