Convergence on the Proton Drip-Line in Thulium

TL;DR

This study used advanced mass spectrometry to experimentally confirm the proton drip-line in thulium isotopes, providing new insights into nuclear stability and shell structure at the limits of nuclear existence.

Contribution

First experimental measurement of the proton drip-line in thulium, confirming $^{149}$Tm as proton-unbound and analyzing the N=82 shell gap at the drip-line.

Findings

$^{149}$Tm is the first proton-unbound isotope in Tm.

Mass measurements deviated from literature by ~150 keV.

Evidence supports the continued existence of the N=82 shell gap.

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 drip-line of the thulium (Tm) isotopic chain ($Z=69$), 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, $^{149}$Tm and $^{150}$Tm, combined with measurements of $^{149m,g}$Er (which were found to deviate from literature by $\approx$150 keV), provide the first experimental confirmation that $^{149}$Tm is the first proton-unbound nuclide in the Tm chain. Our measurements also enable the strength of the $N=82$ neutron shell gap to be determined at the Tm proton drip-line, providing evidence supporting its continued existence.