Decay ofCa39

The "doubly closed shell±one nucleon" mirror transition, ${\mathrm{Ca}}^{39}\left({\beta }^{+}\right){\mathrm{K}}^{39}$, has been investigated in order to obtain a more precise $\mathrm{ft}$ value. The beta spectrum was measured with a magnetic, thin-lens spectrometer and has a maximum energy of 5.490±0.025 Mev. A search for gamma rays was made with a NaI(Tl) scintillation spectrometer, and an upper limit of 0.12% was placed on transitions to the lowest known excited levels in ${\mathrm{K}}^{39}$ between 2.5 and 3.5 Mev. A half-life of 0.88±0.01 second was determined. The $\mathrm{ft}$ value calculated from these results is 4320±100 seconds.