I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

On-line commissioning of the new SLOWRI/ZD-MRTOF system
M. Rosenbusch,∗1 S. Chen,∗2 Y. Hirayama,∗1 D. S. Hou,∗3 S. Iimura,∗4,∗5 H. Ishiyama,∗4 Y. Ito,∗6 S. Kimura,∗4J. Liu,∗3 S. Michimasa,∗7 H. Miyatake,∗1 S. Naimi,∗4 S. Nishimura,∗4 T. Niwase,∗8,∗1,∗4 P. Schury,∗1A. Takamine,∗4 M. Wada,∗1 Y. X. Watanabe,∗1 H. Wollnik,∗9 W. Xian,∗2 and S. Yan∗10The new combined gas cell and MRTOF spectrographdeveloped for high-precision atomic mass measurements
at BigRIPS has been ﬁnalized and transported to theF11 position behind the ZeroDegree spectrometer ofRIBF. The combination of the new cryogenic gas cell1)
with the MRTOF system was initially tested in October
2020, where stable 39 K+ ions were extracted from thegas cell and detected at the system’s downstream TOF
detector. The system was ready to operate on-line justbefore the start of the 2020 HiCARI campaign, which enabled the ﬁrst on-line commissioning of the new setup.
The commissioning run took place in parasitic on-line
operation during the HiCARI experiments (see HiCARI
APR publications in this volume). After passing the HiCARI target and detectors, the reaction products weretransported through the ZeroDegree spectrometer andstopped in the new cryogenic gas cell. Energy degraders
were used to reduce the beam energy to match the He
gas cell’s stopping power. The stopped reaction products were extracted mostly as singly-charged ions andtransported to the MRTOF setup2) where their masses
were determined with high precision and accuracy. The
2020 HiCARI campaign lasted throughout the month ofNovember with ﬁve separate experiments, and anothertwo experiments took place in December. During ourcommissioning, mass measurements covering four diﬀerent regions in the nuclide chart have been carried out(see Fig. 1). Total system eﬃciencies, determined bycomparing the incoming rate from F11 at the PID system with the detected rate after MRTOF, varied according to the presence of contaminants in the helium gas.
While some of the radioactive species were dispersed
across molecular sidebands (e.g. 55 ScOH+ ), very reasonable ion transport could be reached for many otherspecies, e.g. 85 As+ with 0.16% and 137 Te+ with 1.3% total eﬃciency. In sum, the commissioning campaign was
highly successful with more than 70 atomic masses measured during the HiCARI campaign. Notable results
have been achieved in four diﬀerent regions. Aroundneutron-rich Ti and V isotopes, our results include isotopes from Ca to Fe with 55 Sc, 58 Ti, and 59 V being
the most exotic, which improve nuclear masses very recently measured using the TOF-Bρ method at NSCL3)∗1

Wako Nuclear Science Center (WNSC), IPNS, KEK
Department of Physics, the University of Hong Kong
∗3 IMP Lanzhou∗4 RIKEN Nishina Center∗5 Department of Physics, Osaka University∗6 Advanced Science Research Center, JAEA Ibaraki∗7 Center for Nuclear Study, the University of Tokyo∗8 Department of Physics, Kyushu University∗9 Dep. of Chem. and Biochem., New Mexico State University∗10 Jinan University∗2Fig. 1. Nuclear mass measurements during the HiCARI campaign and the results by the ZD-MRTOF system. Backround color code illustrates the measured precision fromAME2016 (see Nucleus-win for reference).

and RIBF4) denoting the front line of nuclear mass studies in this region. In the neutron-rich region above Ni,nuclides have been studied reaching from Ga to Kr with84
Ga, 86 Ge, 89 As, and 91 Se as most exotic isotopes. In
this group the isotopes 88, 89 As have been measured forthe ﬁrst time while three other isotopes provide a signiﬁcant improvement of the previously performed measurements. Another region addressed with success is fromMo to Rh isotopes including the ﬁrst mass measurementof 112 Mo. In the fourth region addressed, i.e. near 132 Sn,
we have demonstrated the isomeric mass separtion of134g, mSb (W. Xian, APR same volume). In total, threeisotope masses have been measured for the ﬁrst timeand eleven other isotope masses improve the present uncertainty signiﬁcantly. By performing this ﬁrst on-line
commissioning of a gas stopper connected to BigRIPS
in a parasitic mode, we were able to eﬃciently identify
and resolve issues that could not be studied oﬄine. This
will allow for a very rapid improvement in performancegoing forward. In March 2021, the next commissioning
run takes place, wherein the neutron-rich Ni region will
be addressed by HiCARI group. Furthermore, two different beam times with the ZD-MRTOF system as main
experiment have been approved for 2021.
References

1) A. Takamine et al., RIKEN Accel. Prog. Rep. 53 (2018).
2) M. Rosenbusch et al., Nucl. Instrum. Methods Phys. Res.
B 463, 184 (2020).
3) Z. Meisel et al., Phys. Rev. C 101, 052801(R) (2020).
4) S. Michimasa et al., Phys. Rev. Lett. 125, 122501 (2020).

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