Towards high-precision mass measurements on $^{74}$Rb for a test of the CVC hypothesis and the unitarity of the CKM matrix

At the highest possible precisions, atomic-mass measurements can be used to perform fundamental studies. Examples for such studies are a check of the conserved-vector-current (CVC) hypothesis and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, both postulates of the Standard Model. The comparative half-lives $Ft$ of superallowed $\beta$-decays constitute the nuclear-physics access to these tests. The $Q$ value of the $\beta$-decay of $^{74}$Rb, one of the three experimentally accessible parameters that enter into the $Ft$ values, has been measured with the ISOLTRAP experiment at ISOLDE /CERN. The ultimate mass precision requirement and the way to achieve it are outlined.