Mass measurements of neutron-rich nuclides for the astrophysical r process using the Canadian Penning trap mass spectrometer

Mr. Dwaipayan Ray
University of Manitoba, Canada/Argonne National Laboratory

The formation of about half of the elements heavier than iron is attributed to the astrophysical r process (rapid neutron capture process) [1]. Observations from the gravitational wave event GW170817 [2] and its electromagnetic counterpart AT2017gfo [3] have provided evidence of the r process occurring in neutron star mergers, but do not fully elucidate the r-process sites. In order to do so, predicted elemental abundances from r-process simulations, using astrophysical models and nuclear physics inputs, are compared to the solar r-process abundance pattern. Therefore, precise nuclear data for these neutron-rich nuclides are of utmost importance. Masses, in particular, are extremely significant as they provide avenues into probing nuclear properties like separation energies, and neutron-capture, photodissociation, and β-decay rates [4]. However, such data exist in limited quantity due to the difficulties in accessing those nuclides. The CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory produces such neutron-rich nuclides, utilizing the spontaneous fission of a ²⁵²Cf source. The fission fragments are collected, bunched, and mass separated using a multi-reflection time-of-flight (MR-TOF) mass separator, and sent to the Canadian Penning Trap (CPT) mass spectrometer where mass measurements are conducted using the phase-imaging ion-cyclotron-resonance (PI-ICR) technique. The CPT currently focusses on probing the rare-earth peak (REP) in the r-process abundance pattern. PI-ICR in combination with the MR-TOF have vastly improved the sensitivity of the CPT, allowing for mass measurements of long-lived low-lying isomers as well as some of the more weakly produced nuclides from ²⁵²Cf fission, while regularly achieving a precision of δm ≤ 10 keV/c² , well within the precision required to constrain r-process calculations in the REP region [4]. A brief description of the apparatus, along with some highlights and recent results will be presented.

This work is supported in part by NSERC, Canada under Application No. SAPPJ-2018-00028, and the U.S. DOE, Office of Nuclear Physics under Contract No. DE-AC02-06CH11357.

References
[1] M. Arnould et al. Phys. Rep. 450 (2007) 97.
[2] B.P. Abbott et al. Phys. Rev. Lett. 119 (2017) 161101.
[3] M. Nicholl et al. Astrophys. J. Lett. 848 (2017) L18.
[4] M.R. Mumpower et al. Prog. Part. Nucl. Phys. 86 (2016) 86.

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