First Penning Trap Mass Measurements beyond the Proton Drip Line

The masses of six neutron-deficient rare holmium and thulium isotopes close to the proton drip line were determined with the SHIPTRAP Penning trap mass spectrometer. For the first time the masses of the proton-unbound isotopes ${}^{144,145}\mathrm{Ho}$ and ${}^{147,148}\mathrm{Tm}$ were directly measured. The proton separation energies were derived from the measured mass values and compared to predictions from mass formulas. The new values of the proton separation energies are used to determine the location of the proton drip line for holmium and thulium more accurately.

Figure 1

Left: Schematic overview of the SHIPTRAP experiment. The radioactive ions delivered by SHIP are stopped (gas cell section), accumulated and cooled (buncher section), and transferred (transfer section) into the 7 T superconducting magnet (trap section). After isobaric separation (purification trap) the cyclotron frequency is determined (measurement trap). Right: A time-of-flight ion-cyclotron-resonance for ${}^{147}\mathrm{Tm}$.

Figure 2

Proton separation energies versus mass number for the odd-$Z$ elements holmium ($Z=67$) and thulium ($Z=69$). Only the even-$N$ isotopes are plotted. The circles show the previous measurements (full gray circles) and estimations (empty circles) taken from 24. The data from this work are represented by black diamonds. Also shown are linear fits to the data. The inner box enlarges the interesting region around the drip line.