Direct mass measurements beyond the proton drip-line

Abstract

At GSI in Darmstadt the Penning trap spectrometer SHIPTRAP was set up to perform precision experiments with low-energetic radioactive ions produced in fusion-evaporation reactions at the velocity filter SHIP. First high-precision measurements were performed determining the masses of 18 neutron-deficient isotopes in the terbium-to-thulium region with relative uncertainties of about 10-8. The masses of nine nuclides were measured for the first time. Four of them (144Ho, 145Ho, 147Tm and 148Tm) were determined to lie beyond the proton drip-line. The data were included into the Atomic Mass Evaluation (AME) and showed generally good agreement with previous measurements. The implication of the results on the nuclear structure and on the location of the proton drip-line is discussed by analyzing the two-neutron and one-proton separation energies. Further simulations and off-line measurements were performed to investigate the stopping efficiency in the gas cell, to optimize the contrast and the quality of the measurement and to determine the uncertainties arising from magnetic field fluctuations.