Direct Mass Measurements on the Superallowed Emitter ⁷⁴Rb and Its Daughter ⁷⁴Kr: Isospin-Symmetry-Breaking Correction for Standard-Model Tests

The decay energy of the superallowed β decay ⁷⁴Rb(β⁺)⁷⁴Kr was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and was found to be Q=10 416.8(4.5)   keV. The exotic nuclide ⁷⁴Rb, with a half-life of only 65 ms, is the shortest-lived nuclide on which a high-precision mass measurement in a Penning trap has been carried out. Together with existing data for the partial half-life as well as theoretical corrections, the decay energy yields a comparative half-life of Ft=3084(15)   s for this decay, in agreement with the mean value for the series of the lighter nuclides from ¹⁰C to ⁵⁴Co. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction δ_C.