ISOLTRAP is a tandem Penning trap mass spectrometer at the On-Line Isotope Mass Separator at CERN, very similar to SMILETRAP. The facility plays a leading role in the field of high precision mass spectrometry of radioactive ions. The masses of more than 200 short-lived nuclides have been measured with a relative uncertainty of typically dm/m ~ 1x10−7 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 ~1x10−8. In particular, a linear radiofrequency quadrupole (RFQ) trap and more recently a multi-reflection time-of-flight (MRTOF) mass spectrometer 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. For this purpose, a laser-ablation source of carbon-cluster ions is installed at ISOLTRAP.

Accurate mass measurements of short-lived nuclides are of high interest for a number of reasons. From the measured atomic masses it is possible to compute nuclear binding energies, which are sensitive to 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.