Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 52 (2019)

SHE-Mass-II: an MRTOF-MS for Super Heavy Nuclei
M. Wada,∗1 P. Schury,∗1 S. Kimura,∗2 S. Ishizawa,∗2,∗5 T. Niwase,∗2,∗3 Y. Ito,∗4 M. Rosenbusch,∗2 D. Kaji,∗2K. Morimoto,∗2 H. Haba,∗2 H. Miyatake,∗1 X. Y. Watanabe,∗1 H. Hirayama,∗1 J. Y. Moon,∗7,∗1 A. Takamine,∗2T. Tanaka,∗3,∗2 K. Morita,∗3,∗2 H. Ishiyama,∗2 and H. Wollnik∗6
The SHE-Mass facility was a system with a multireﬂection time-of-ﬂight mass spectrograph (MRTOFMS)1) and a cryogenic gas catcher coupled with theGARIS-II at RILAC. With this setup, we have measured the masses of more than 80 fusion-evaporated
products including several trans-fermium nuclides.2–5)
After the shutdown of GARIS-II at RILAC, we rebuilt a similar setup, i.e., the SHE-Mass-II, at the newGARIS-II in the E6 experimental room.
The new setup has a single stage triplet ion trapsystem, which directly connects the gas catcher to theMRTOF to achieve high eﬃciency. The whole setup
including vacuum pumps, a gas cylinder, and electric
circuits is mounted on a single base plate and the plate
is movable on rails to ensure quick coupling and decoupling to the GARIS-II. The gas catcher and ion
trap chambers are mounted on a two-dimensional railsystem that enables easy maintenance of the internal
structures of the setup.
A drawback of the new setup is that more contaminant ions could be expected owing to a diﬃculty ofintermediate mass selection. In the previous setup, we
had a 5 m-long beam line between the two ion traps
where we could place a Bradbury-Nielsen (BN) iongate6) to select a single mass number. However, thenew setup has only a 50 cm distance between the trapand MRTOF where a BN gate can be installed. We
developed a 10 times ﬁner gate device made of 13 µthick wires with 130 µ intervals, which can deﬂect a
1 keV/q ion beam by 65 mrad with a ±100 V pulse.
Such ﬁne structure makes sharp cut on the ion beam.
In a preliminary test, a resolution of 2 mass units wasachieved.
Another improvement in the new setup is the replacement of a sextuple ion beam guide (SPIG) madeof rigid molybdenum rods with a segmented micro
quadrupole ion beam guide (SµQPIG) made of 2 mmwide, four-layer printed circuit boards. These devices interface the gas catcher with the triplet ion trap
through diﬀerential pumping sections. In the previoussetup, we needed to run an ion source in the gas catcher
to have a ‘space-charge cloud’ which pushed the ionsin the SPIG, while in the new setup, a DC gradientpotential in the SµQPIG carries the rare ions without
∗1
∗2
∗3
∗4
∗5
∗6
∗7

KEK, Wako Nuclear Science Center
RIKEN Nishina Center
Department of Physics, Kyushu University
JAEA
Department of Physics, Yamagata University
Department of Chemistry and Biochemistry, New Mexico
State University
Institute for Basic Science, Rare Isotope Science Project- 136 -Fig. 1. SHE-Mass-II setup.

support from the space-charge cloud. The new scheme
provides cleaner ion beams to the MRTOF.
We also replaced the time-of-ﬂight detector with a
newly developed α-TOF detector, which records thetime-of-ﬂight signal and the successive α-decay, simultaneously. The ﬁrst online commissioning of the SHEMass-II setup was performed in December 2018 andwe found that the α-TOF detector signiﬁcantly reduced the background level.7) An oﬄine experimentusing 252 Cf ﬁssion source is also in progress.
We are ready to start the direct mass measurementof hot-fusion super heavy elements, 288 Mc and 284 Nh,with the SHE-Mass-II setup.
References
1) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335 39, (2014).
2) P. Schury et al., Phys. Rev. C 95, 011305(R) (2017).
3) S. Kimura et al., Int. J. Mass Spectrom. 430, 134 (2018).
4) M. Rosenbusch et al., Phys. Rev. C 97, 064306 (2018).
5) Y. Ito et al., Phys. Rev. Lett. 120, 152501 (2018).
6) N. E. Bradbury, R. A. Nielsen, Phys. Rev. 49, 388 (1936).
7) T. Niwase et al., RIKEN Accel. Prog. Rep. 51, 157 (2018).