Recent high-precision atomic mass measurements from medium-mass to heavy isotopes using MRTOF-MS at RIKEN/RIBF*

For reliable predictions of processes in nuclear astrophysics, atomic masses belong to the most essential experimental data together with beta half-lives, decay modes, and deacy branching ratios. For atomic mass measurements, multi-reflection time-of-flight (MRTOF) mass spectrometry [1] has become a breakthrough-technology considering the required duration of a measurement and the small number of (rare) events needed to reach a relative mass precision of $delta m/m leq 10^{-7}$. This technology has been developed at RIKEN's RIBF facility for about two decades in combination with gas-filled ion catchers for low-energy access. In the recent past, three independent systems, operating at different access points to radioisotopes at RIBF, provided substantial mass data in the medium-mass and heavy region of the nuclear chart, reaching even out to the superheavy nuclides.

Recent achievements like high mass resolving power [2], installations like ⍺/β-TOF detectors [3], and in-MRTOF ion selection have tremendously increased the selectivity of the systems and improved the reduction of background. This makes us capable to distinguish between a rare radioactive ions and unwanted molecules or dark counts.

In this contribution, I will give an overview about recent MRTOF atomic mass measurements achieved at RIBF focusing on astrophysical aspects. The results include new (and updated) mass values for 35 neutron-rich isotopes between Ga and Br pinning down astrophysical reaction rates. Furthermore, I will discuss the discovery of the isotope $^{241}$U using the KISS facility [4], mass measurements using a fission source, and the present status of MRTOF mass measurements of superheavy nuclides downstream of the GARIS-II separator [5].

[1] H. Wollnik, M. Przewloka, Int. J. Mass Spectrom. Ion Proc. 96, 267 (1990).

[2] M. Rosenbusch et al., Nucl. Instrum. Meth. A 1047, 167824 (2023).

[3] T. Niwase et al., Theo. Exp. Phys. 2023(3), 031H01 (2023).

[4] T. Niwase et al., Phys. Rev. Lett. 130, 132502 (2023).

[5] P. Schury et al., Phys. Rev. C 104, L021304 (2021).