A mass spectrometer consisting of two Penning traps has been set up for short-lived isotopes at the on-line mass separator ISOLDE at CERN. The ion beam is collected and cooled in the first trap. After delivery to the second trap, high-accuracy direct mass measurements are made by determining the cyclotron frequency of the stored ions. Measurements have been performed for ${}_{}{}^{118}{}_{}{}^{}–_{}^{137}{}_{}{}^{}$Cs. A resolving power of over ${10}^6$ and an accuracy of 1.4×${10}^{\mathrm{-}7}$ have been achieved, corresponding to about 20 keV.

• Received 30 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.3104