Investigation of nuclear structure via high-precision mass measurements

High-precision mass measurements and spectroscopy give an insight into the variation of the nuclear structure and stability across the nuclear chart. Systematic measurements along isotopic chains and the observed shell structure of nuclei, reflected by increased binding energies at the so-called magic neutron and proton numbers, led to the development of the current shell model. From precise mass values, important differential quantities, such as the proton and neutron separation energies can be calculated, which highlight different nuclear structure effects.

Approaching the limits of nuclear binding, the structure and properties of nuclei is of great interest and attracts large attention, theoretically and experimentally. It has become evident that the nuclear shell structure can change towards the drip-line. New effects such as shell quenching, weakening or disappearance of shells around the classical magic numbers and appearance of new magic numbers have been theoretically predicted and observed in experiments.

In recent work we investigated the evolution of the N=32 neutron shell closure, that forms in neutron-rich Sc, Ca and K isotopes. Here we performed mass measurements of neutron-rich Ti and V isotopes at TITAN, TRIUMF, Vancouver, with a new multiple-reflection time-of-flight mass-spectrometer (MR-TOF-MS) in combination with the established TITAN Penning Trap system. The new MR-TOF technology has shown to be particular suitable for low count rate measurements of short lived isotopes with relative uncertainties on the ~ 10-7 scale, sufficient for nuclear structure and nuclear astrophysics investigations.
Our direct measurements, together with new state-of-the-art ab-initio shell model calculations, showed the transitional character of Ti, right between “no shell effects” and “strong shell effects”. The results challenge modern ab-initio theories and serve as an excellent testing ground for new shell model calculations. We plan to exploit the new experimental capabilities and push towards an investigation of the N=34 neutron shell closure, currently under discussion in Ti. Further we aim to expand our studies across the mid-shell region between the proton shell closures at Z=20 and 28 in Cr, Mn and Fe isotopes along the neutron-rich side of the nuclear chart, as well as investigate the evolution of the N=82 neutron shells on the neutron-deficient side.