Accurate mass measurements of short-lived nuclides are of high interest for a number of reasons: They allow direct observation of nuclear structure effects like the location of shell and sub-shell closures, pairing, or the onset of deformation. In combination with a precise study of super-allowed beta emission they provide tests of the standard model. Additionally, masses of unstable nuclei are the most critical nuclear physics parameters for reliable nucleosynthesis calculations in astrophysics.
The tandem Penning trap mass spectrometer ISOLTRAP at the on-line isotope separator ISOLDE at CERN/Geneva plays a leading role in this field. The masses of more than 400 short-lived nuclides have been measured with a relative uncertainty of typically δm/m ~ 5*10-8 and even almost up to one order of magnitude lower in some special cases. Recently, the performance of the Penning trap mass spectrometer ISOLTRAP has been considerably enhanced. Major technical improvements were implemented to increase the range of accessible nuclei to those that are produced in minute quantities of only 100 ions/s and to nuclei with half-lives down to ~50 ms as well as to decrease the typical relative uncertainty down to ~1*10-8. In particular, a linear radiofrequency quadrupole (RFQ) trap and a carbon-cluster source were added to the ISOLTRAP spectrometer. Since the unified atomic mass unit is defined as 1/12 of the mass of 12C the calibration of the magnetic field with carbon clusters allows absolute mass measurements. More detailed information can be found in the latest ISOLTRAP overview (M. Mukherjee et al., ISOLTRAP: An on-line Penning trap for mass spectrometry on short-lived nuclides, Eur. Phys. J. A 35, 1-29 (2008)).