MR-TOF-MS for improving the sensitivity of CRIS measurements at ISOLDE-CERN

The collinear resonance ionisation spectroscopy experimental setup (CRIS) is a powerful apparatus that provides measurements of nuclear properties (spin, electromagnetic moments and charge radii) for exotic nuclei with high resolution and efficiency. Nuclear information is obtained from studying the study of atomic hyperfine structure, which is measured by detecting resonantly ionised atoms (or molecules) as a function of laser frequency. This can be achieved by applying a multi-step ionisation scheme in which the atomic bunch is overlapped with several laser beams to resonantly excite the atom. This 'resonant' step is followed by an ionisation step induced by a high-power laser. The ion is then separated from the neutral particles in the beam using deflectors and deflected towards an ion detector. This method allows the detection of a low background signal and thus a very high sensitivity. However, there may be a reduction in sensitivity due to the contribution of a non-resonant ionised particle in the ion beam. The solution is to couple a Multi-Reflection Time-Of-Flight Mass Spectrometer (MR-TOF-MS) with CRIS. Ions sent into such a device are trapped and bounce repeatedly between two sets of electrostatic mirrors for a given trapping time. This method helps to separate ions with different masses as they move at different speeds in the ion trap. After the given trapping time, the ion bunch can be ejected toward an ion detector to perform a mass analysis. In this way, completely background-free detection of the specific ion of interest is possible, which should improve the sensitivity of the CRIS experiment by several orders of magnitude. This will allow studies of the most exotic species near the cornerstones of the nuclear chart, the doubly- magic 100Sn and 78Ni.