VSI Seminar: Marco Rosenbusch and Toshika Nisake from RIKEN and KEK (Japan)

Marco Rosenbusch (KEK & RIKEN)

High-precision mass measurements with the SLOWRI /MRTOF-MS project at BigRIPS / ZeroDegree

Multi-reflection time-of-flight (MRTOF) mass spectrome try [1] has become a new powerful tool for fast and precise measurements of atomic masses. It is 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 dm/m < 10-7. In order to utilize the powerful isotope production capabilities of the RIBF facility and study nuclear masses for cutting-edge research at BigRIPS, a new MRTOF mass spectrograph (MRTOF-MS) has been assembled; it became operational for the first time in spring 2020 [2].

The new device has been coupled to a cryogenic gas cell to convert the radioisotopes produced at relativistic energies into a low-energy beam amenable to ion trapping. The setup underwent an initial on-line commissioning at the BigRIPS facility at the end of 2020, wherein more than 70 nuclear masses were measured.

In this talk, I will explain the MRTOF technique and the new setup at BigRIPS along with advances like decay-correlated mass spectroscopy. Also the technological challenges will be discussed, e.g. providing low-energy ion beams at RIBF and reaching high mass accuracy. The topics will include efforts to decrease mass-dependent effects coming from non-static electric fields in the mass spectrometer, and some studies to increase the resolving power of the MRTOF system.

Furthermore, a short summary of the first commissioning experiments and follow-up mass measurements will be presented. Among other measurements presented, these results include new masses of neutron-rich titanium and vanadium isotopes revealing a vanishing of a shell gap at N = 34, which is known to be pronounced in Ca isotopes [3].

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

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

[3] S. Iimura et al., Phys. Rev. Lett. 130, 012501 (2023).

Toshitaka Niwase (KEK)

Direct mass measurement of the transuranium isotopes with MRTOF via GARIS-II and KISS setups

The multi-reflection time-of-flight mass spectrograph (MRTOF-MS)[1] is one of the tool for high-precision direct mass measurement of the nuclides. We have operated several MRTOFs in the RIKEN RIBF facility. The SHE-Mass facility, which couples MRTOF-MS + α-TOF detector with gas-filled recoil ion separator GARIS- II[2], is working on the mass measurement of heavy and superheavy nuclides

produced in fusion reactions. The MRTOF -MS connected to the KEK isotope separation system KISS[3] allows the mass measuremen t of neutron -rich nuclides produced via multi-nucleon transfer reaction.

In the SHE-Mass facility, we have measured the mass of Dubnium isotopes (Z=105) produced by the fusion reaction[4], which are the heaviest nuclides directly measured to present. Also, in the KISS setup, we have performed the first investigation of the actinide region, which were performed with MNT products of the 238U + 198Pt system. We have succeeded in the direct mass measurements of nineteen neutron-rich actinide nuclides spanning from protactinium to plutonium, including the first identification of a new uranium

isotope (241U) since the 1970s[5].

In this talk, I will introduce the detail and result of the experiments, and our

future plans.

[1] P. Schury et al., Nucl. Inst. Meth. B 335, 39 (2014).

[2] D. Kaji et al., Nucl. Instrum. Meth. B 317, 311 (2013).

[3] Y. Hirayama et al., Nucl. Instrum. Meth. B 412, 11 (2017).

[4] P. Schury et al.,Phy. Rev. C 104, L021304 (2021).

[5] T. Niwase et al., Phy. Rev. Lett. 130, 132502 (2023).