Precision Mass Measurements for the Astrophysical r process at Argonne National Laboratory

Abstract

The astrophysical r process must take place in a environment of high temperatures and neutron fluxes, to allow for the rapid captures of neutrons that take it far from the valley of stability. The multi-messenger neutron star merger GW170817/AT2017gfo has provided direct evidence of r-process nucleosynthesis, but this does not fully resolve the r-process site. A complete picture will require the comparison of modeled r-process abundances with observed abundances; a recent sensitivity study has shown that key among the data going into these simulations are nuclear masses [1]. The Canadian Penning Trap mass spectrometer (CPT) at Argonne National Laboratory measures such masses using the state-of-the-art phase-imaging ion-cyclotron-resonance technique, which pro-vides an increase in both precision and sensitivity to lowly-produced nuclei over the time-of-flight ion-cyclotron-resonance technique. The CPT is currently located at the CARIBU facility of Ar-gonne’s ATLAS accelerator, where it measures masses of interest to the formation of the rare-earth peak in the r-process abundance pattern which are produced from the spontaneous fission of CARIBU’s Californium-252 source [2, 3]. The next step in r-process studies for the CPT will be the measurement of masses of interest for the formation of the heaviest A ∼ 195 r-process abundance peak. Traditional particle-fragmentation, target-fragmentation, or fission production techniques will not efficiently produce these very neutron-rich nuclei around the N = 126 shell closure. Multi-nucleon transfer (MNT) reactions between two heavy ions, however, can produce these nuclei effectively [4]. The N = 126 factory currently under construction will use MNT reactions to produce such nuclei [5]. Due to the wide angular distribution of these reaction products, a large-volume gas catcher will be used to convert these reaction products into a low-energy continuous beam, and then a mass separating magnet, RFQ cooler-buncher, and MR-TOF will convert the high-emittance continuous beam into a low-emittance bunched beam that can be delivered to the CPT for mass measurements or to other experimental devices. All of these components are currently commissioning, and the facility as a whole is currently under construction and is expected to begin commissioning within the next year. This work is supported in part by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357; by NSERC (Canada), Application No. SAPPJ-2018-00028; and with resources of ANL’s ATLAS facility, an Office of Science User Facility.

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