Convergence on the proton dripline in thulium
Phys. Rev. C 112, 024318 – Published 14 August, 2025
DOI: https://doi.org/10.1103/s8h6-f6kc
Abstract
Direct observation of proton emission for very small Q values is often unfeasible due to the long partial half-lives of the proton emission channel associated with tunneling through the Coulomb barrier. Therefore, proton emitters with very small decay energies may require the masses of both parent and daughter nuclei in order to establish them as proton unbound. Nuclear mass models have been used to predict the proton dripline of the thulium (Tm) isotopic chain , but until now the proton separation energy has not been experimentally tested. Mass measurements were performed using a multiple reflection time-of-flight mass spectrometer (MR-TOF-MS) at TRIUMF's TITAN facility to conclusively map the limit of proton-bound Tm. The masses of neutron-deficient and , combined with measurements of (which were found to deviate from literature by ), provide the first experimental confirmation that is the first proton-unbound nuclide in the Tm chain. Our measurements also enable determination of the strength of the neutron shell gap at the Tm proton drip line, providing evidence supporting its continued existence.
