High-Accuracy Mass Determination of Unstable Rb, Sr, Cs, Ba, Fr and Ra Isotopes with a Penning Trap Mass Spectrometer

Abstract

The majority of masses of radioactive isotopes has been measured by determination of Q-values in nuclear reactions or in nuclear decay. For a long time the use of direct mass determination has been limited to stable isotopes or isotopes close to stability. This changed in the 70’s with magnetic spectrometers put on-line to isotope separators. The Orsay group (Audi et al., 1986) succeeded in measuring the masses in long isotope chains of alkali elements. They impressively demonstrated the possibilities embedded in direct mass determination of isotopes far from stability. The persisting demand for more precise masses of short-lived isotopes (or exotic particles) has prompted during recent years the development of new techniques. One powerful approach is the determination of the cyclotron frequency in a Penning trap, first demonstrated for light stable masses and now applied to unstable isotopes in the experiment reported here. We report on the technique and performance of a tandem Penning trap mass spectrometer at the on-line mass separator ISOLDE/CERN and summarize the very accurate results obtained on Rb, Sr, Cs, Ba, Fr and Ra isotopes.