Mass measurements on superallowed β-emitters using Ramsey's excitation method at ISOLTRAP

The Penning trap mass spectrometer ISOLTRAP, installed at the ISOLDE/CERN facility, is dedicated to high-precision mass measurements on short-lived radionuclides. One of the very active fields, where mass values with uncertainties at the level of 10-8 and even below are required, is the test of the electroweak part of the standard model. Here, the conserved-vector-current (CVC) hypothesis postulates that the vector-current part of the weak interaction is unaffected by the strong interaction. Thus, the decay strength of all superallowed 0+→0+β-decays is independent of the nuclei except for theoretical corrections. The 12 best known superallowed Fermi-type β-decays ranging from 10C to 74Rb yield an average Ft value of Ft = 3072.7 (0.8)s, which confirms the CVC hypothesis at a level of 3 . 10-4. Now the masses of a few more candidates for further tests have been measured, two of them at ISOLTRAP, with a novel technique in Penning trap mass spectrometry: time separated oscillating fields, known as the Ramsey method and recognized with the Nobel prize in 1989, were used for the excitation of the stored ions' motion to determine the masses of 26Al and 38Ca. The method as well as the results will be presented. (orig.)