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Precise Measurements of Six Chromium Isotopes

ISOLTRAP delivers information about shell closure and shape of the exotic nuclides


An international research team has for the first time at ISOLDE, the Isotope Separator On Line Device at CERN, succeeded in creating six chromium isotopes and measuring their masses up to 300 times more precisely than ever before using the ISOLTRAP ion trap. The ISOLTRAP experiment was largely constructed by scientists from GSI and has been constantly supported by significant contributions from GSI since that time. The new measurement results have enabled scientist to make the first-ever statements concerning trends of the shape and binding energies of these and neighboring isotopes. 

The masses of the exotic chromium nuclides were measured more accurately than ever before by the experimenters at CERN in the Penning trap mass spectrometer ISOLTRAP. The binding energies can be derived from the results. When the physicists plot the binding energies of the six isotopes they can draw a trend line through the points and from this line they can see whether a shell closure occurs in this region or the nuclear shape suddenly changes between one isotope and the next. The measurement uncertainties were previously too large to enable reliable statements. “Thanks to the new, extremely precise measurements we can now state with confidence that the abrupt change of shape that had previously been speculated about does not occur in the case of these isotopes,” said Frank Herfurth, a scientist from GSI who participated in the experiment. “The new, more exact data shows us a slow change of shape away from the symmetrical form. Thanks to the joint efforts of experimenters and theorists we have been able to compare our results with an ab initio model for the first time. These are special nuclear models, the calculations of which are essentially based on the interactions of protons and neutrons and thus are less dependent on intuitive approximations. Two out of four nuclear structure models confirm our observation, the other two don’t. The experiment results are a valuable help to test the assumptions that underlie the different models.”

ISOLTRAP is a Penning trap mass spectrometer combined with a multi-reflection time-of-flight (MR-ToF) mass separator. This structure enables the masses of especially rare isotopes to be measured directly. The combination of two Penning traps enables precise and clean measurements unaffected by contaminants. The most exact mass measurements of exotic, short-lived nuclei can thus be carried out using penning trap spectrometers.

ISOLTRAP is a forerunner of the Penning trap precision experiment for exotic ions. Technology, software, and hardware that has been and is being developed for ISOLTRAP is in use at SHIPTRAP, HITRAP, and is also planned for use at the FAIR experiment collaboration MATS within the NUSTAR collaboration. At FAIR, the particle accelerator facility that is under construction at GSI, similar experiments with even more exotic nuclei are planned.

The ISOLTRAP experiment, initiated by the former head of Atomic Physics at GSI, Prof. Kluge, is the result of a collaboration over many years between GSI, Johannes Gutenberg University Mainz, the University of Greifswald, and the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg. In recent years MPIK’s Prof. Blaum has taken over the leadership of the collaboration, and the Institute continues to be supported in this by various GSI/FAIR departments such as Experiment Electronics, Atomic Physics, and Decelerators.

More information

Original publication in Physical review Letters: Precision Mass Measurements of 58–63Cr: Nuclear Collectivity Towards the N=40 Island of Inversion

The cave of ISOLTRAP at CERN
The Penning trap mass spectrometer ISOLTRAP
Photo: Frank Wienholtz