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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 ,l8 Cs-137 Cs. A resolving power of over 10 6 and an accuracy of 1.4x 10 ~7 have been achieved, corresponding to about 20 keV.There has been a decade-long effort to determine atomic masses ever more precisely and, surprisingly, a persistent demand to do it even better. This is because the atomic mass is a gross property which embodies all the effects of the interplaying forces in the nucleus.Today, the nuclear binding energy of stable isotopes is generally known with an accuracy of the order of keV. l For unstable nuclei, the errors in mass determination increase rapidly for isotopes further from the valley of ft stability. Yet, it is the atomic masses of those short-lived isotopes with unusual Z/N ratios that provide stringent tests of nuclear models, enable the discovery of new phenomena such as nuclear shape transitions, or are cornerstones in the interpretation of unexpected results such as the increased matter radius of n Li. 2 Furthermore, atomic masses are needed as input data for mass formulas which allow extrapolation to the masses of even more exotic isotopes.The masses of most of the radioactive isotopes have been provided by nuclear reactions or the determination of Q values in the nuclear decay. A major step forward was taken in the 1970s when magnetic spectrometers were put on-line with accelerators so as to measure the masses directly. 3 During recent years the demand for precise masses of short-lived isotopes, or exotic particles, has prompted the development of new techniques for direct mass spectrometry. A general feature of these new devices is an evolution from measurements of voltage ratios to time and frequency. Time-of-flight mass spectrometry is performed by the facilities TOFI at Los Alamos 4 and SPEG at GANIL, 5 and is planned at ESR at GSI. 67 Cyclotron resonance is another powerful technique first demonstrated for light masses 8,9 and currently used at the Low Energy Antiproton Ring at CERN for a determination of the mass of the antiproton 10,11 and at the on-line isotope separator ISOLDE at CERN for the measurement of masses of short-lived isotopes. 12 The present Letter reports on the performance of the ISOLDE tandem Penning-trap mass spectrometer and the results obtained on neutron-deficient Cs isotopes.The principle of the experiment is the determination of the cyclotron frequency co c = (q/m)B of an ion (mass ra, charge q) stored in a Penning trap. The superimposed homogeneous magnetic field B and the electric quadrupole field in a Penning trap confine the ion to a precessing circular motion with an axial oscillation. The precessional frequency, called the magnetron frequency co~, is usually slow compared to the ...

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