Accurate mass determination of short-lived isotopes by a tandem Penning-trap mass spectrometer

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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ have been achieved, corresponding to about 20 keV.