Mass measurements of neutron-rich nuclei are critical for understanding heavy-element synthesis during the rapid neutron-capture (r) process. Over the past few decades Penning traps have been the most reliable tool for accurate and precise mass determinations, however the traditional time-of-flight measurement technique is not optimal for studying the most exotic radioactive ion beams. A modern phase-imaging method (PI-ICR) has recently been adopted by the Canadian Penning Trap (CPT) mass spectrometer at the CARIBU facility of Argonne National Laboratory where beams of neutron-rich nuclei are created from fission fragments of Cf-252. In combination with a multireflection time-of-flight mass separator this technique has improved the experimental sensitivity of the CPT by more than two orders of magnitude enabling measurements of nuclei which have previously been inaccessible. I will describe the technique and discuss the astrophysical implications of several recent measurements of rare-earth nuclei near N = 100.
