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

RIKEN Accel. Prog. Rep. 54 (2021)

Removing non-isobaric ions from an MRTOF-MS by periodic electricpulses
S. X. Yan,∗1,∗2 M. Rosenbusch,∗2 W. D. Xian,∗3,∗2 S. D. Chen,∗3,∗2 Y. Hirayama,∗2 D. S. Hou,∗4,∗2 S. Iimura,∗5,∗6H. Ishiyama,∗5 Y. Ito,∗7 S. Kimura,∗5 J. J. Liu,∗3,∗2 H. Miyatake,∗2 S. Nishimura,∗5 T. Niwase,∗8,∗5,∗2 P. Schury,∗2A. Takamine,∗5 M. Wada,∗2 Y. X. Watanabe,∗2 and H. Wollnik∗9Lens

Drift tube

Ions inLens

Eject mirror
To detector

Inject mirror 5, 6, 7, 8

Pulse on (600 V)
Pulse off (0 V)

Fig. 1. Schematic of cleaning mode and potential distribution along the axis in MRTOF-MS.
∗1
∗2
∗3
∗4∗5
∗6
∗7
∗8
∗9

Institute of Mass Spectrometry and Atmospheric Environment, Jinan University
Wako Nuclear Science Center(WNSC), IPNS, KEK
Department of Physics, The University of Hong Kong
Institute of Modern Physics, Chinese Academy of Science
RIKEN Nishina Center
Department of Physics, Osaka University
Advanced Science Research Center, JAEA Ibaraki
Department of Physics, Kyushu University
Department of Chemistry and Biochemistry, New Mexico
State UniversityCounts103102101

CountsIn the recent SLOWRI/ZD-MRTOF1) commissioning
run, radioactive ions were extracted with stable molecular ions spanning a wide mass range from impurities
in the helium gas cell. Owing to the periodic natureof an MRTOF mass spectrometer (MRTOF-MS), thisleads to an overlapping of ions with ions of varying
A/q performing diﬀerent numbers of laps in the TOF
spectra. Unwanted stable molecular ions with higher
abundance than the ions of interest prevented theidentiﬁcation of exotic isotopes from BigRIPS. Thus,
it was necessary to devise a new puriﬁcation method
to remove contaminant ions as much as possible in ouronline measurement. An in-trap mass separation system
for the ZD-MRTOF system was not yet installed duringcommissioning runs. Such systems are scheduled forthe future based on existing knowledge, e.g., selection
methods called in-trap potential lift technique2) andin-trap deﬂector technique.3) Both techniques apply
electric pulses to eject unwanted ions, where a single
pulse on the order of a kilovolt is used at the moment ofion ejection (only applicable for isobaric ion separation)in the ﬁrst method, and periodic low-voltage pulses areapplied to a deﬂector inside the MRTOF device forthe wideband ejection of ions in the second method.
Here, we implement a new mass ﬁlter method based on
periodic 600 V pulses of 2–4 µs duration using mirror
electrodes that were on the ground potential previously.
This periodic rectangular pulse is applied to injectionside mirror electrodes 5–8 (see graphic and simulated

10210115.7080

15.7085

15.7090

15.7095TOF [ns]15.7100

15.7105

15.7110(×106 )

Fig. 2. Measured TOF spectrum without and with thecleaning mode; spectral peak marked in red is 82 Ge.

potentials in Fig. 1).
The√ ﬂight time depends on the mass-to-charge ratiot ∝ (A/q) of the ions. Diﬀerent ion species of equal
initial energy separate during ﬂight in the MRTOF
analyzer according to their A/q. The “on”/“oﬀ” stageof the pulses is synchronized with the wanted A/qvalue, and therefore, it is ensured that the ions ofinterest always experience the “oﬀ” stage when crossing
the pulsed electrodes. In turn, the unwanted A/qions are not synchronized, and they cross the pulsed
region randomly in time. Being aﬀected by the pulses
once or several times changes their kinetic energy andadditionally scatters the ions in a radial direction, whichleads to unstable trajectories and ejection of ions. Thisejection scheme was ﬁrst tested in a simulation, and thenapplied in the experiment. After about 100–200 laps,ions of undesirable A/q crossed this region several times,
and they could mostly be removed from the system. In
our online experiments, the cleaning mode was applied;
a comparison without and with the cleaning is shownin Fig. 2. Only the selected mass range is retained,
and other mass areas are considerably clean in thespectrum. Although a very slight peak shift may occur
due to the ﬂuctuation of mirror potentials, this canbe measured and corrected in the mass analysis. The
new cleaning method was successful and enabled ourﬁrst online measurements to determine low count rate
nuclides.
References1) M. Rosenbusch et al., Nucl. Instrum. Methods Phys. Res.
B, 463, 184 (2020).
2) F. Wienholtz et al., Int. J. Mass. Spectrom., 421, 285 (2017).
3) P. Fischer et al., Rev. Sci. Instrum. 91, 023201 (2020).

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