II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

High mass resolving power and isomeric state separation at
SLOWRI/ZD-MRTOF system
W. Xian,∗1,∗2 M. Rosenbusch,∗2 S. Chen,∗1,∗2 Y. Hirayama,∗2 D. Hou,∗3,∗2 S. Iimura,∗5,∗4 H. Ishiyama,∗4Y. Ito,∗6 S. Kimura,∗4 J. Liu,∗3 H. Miyatake,∗2 S. Nishimura,∗4 T. Niwase,∗7,∗4,∗2 P. Schury,∗2 A. Takamine,∗4M. Wada,∗2 Y. X. Watanabe,∗2 H. Wollnik,∗8 and S. Yan∗9With high precision and accuracy, as well as shortmeasuring times, the multi-reﬂection time-of-ﬂight mass
spectrograph (MRTOF-MS) has become competitive
with Penning traps for measuring masses of short-livednuclei.1) In 2013, the ﬁrst MRTOF built at RIKEN wastested online by measuring 8 Li ions. The mass excess of
8
Li was accurately reported to be 20947.6(15)(34) keV(∆m/m ∼ 6.6 × 10−7 ). The mass resolving power
achieved at that time was Rm > 150k.2) In 2020, anext-generation device of similar design has been put
into operation at the ZeroDegree spectrometer of RIBF
with the goal of measuring very exotic nuclides andtheir isomers with a higher resolving power.
In the MRTOF reﬂection chamber, ions are conﬁned
and reﬂected back and forth between a pair of ion
reﬂection mirrors, allowing the ions to have a ﬂight
path of up to or beyond one kilometer. In any ﬁeld-freedrift region, more energetic ions have a shorter time-ofﬂight, but in the reﬂection region inside the ion mirrors,
this can change because of the shape of the potential
distribution. It is possible to make more energetic
ions penetrate deeper into the mirrors to achieve alonger ﬂight path that compensates for their higher
energy and equilibrates the time-of-ﬂight with thatof less energetic ions. The better this compensation
works, the narrower the ﬁnal TOF distribution canbe as ions with diﬀerent energies can simultaneously
reach the detector.3) However, this requires a nontrivial search for complex mirror potentials and thoseof ion-optical focusing elements located in the central
drift tube section. In our case, this would require asearch in 11-dimensional parameter space.
In a ﬁrst-order approach, every possible potential
distribution is the generator of a function mapping
the TOF of an ion to a kinetic energy TOF(E). In
recent oﬄine experiments using the new ZD-MRTOF
system, by use of a pulsed drift tube between theion trap and MRTOF, we measured these response
functions in a much wider energy range than done
before. This measurement was repeated after changing
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9

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

279 keV(AME2016)

Fig. 1. TOF spectrum of 134g Sb+ and 134m Sb+ in the online
experiment of the ZD-MRTOF system.

some voltage of each ion mirror. In this manner,the eﬀect of changing each mirror potential on thisfunction could be studied. This allowed for an improvedlocal search around the present conditions to fulﬁll thecondition ∂TOF(E)/∂E ≈ 0 for a particular numberof reﬂections. The diﬀerent shapes of the TOF(E)functions have been added in an appropriate manner,which resulted in a new voltage conﬁguration withexcellent energy compensation.
In the corresponding oﬄine tests, by using 39 K+ ionsfrom a thermal ion source to perform the procedure
mentioned above,a mass resolving powerRm ∼ 700k was achieved for the ﬁrst time at RIBF,
with a ﬂight time of ∼9 ms after 490 laps.
The ﬁrst online commissioning experiment of theMRTOF at ZeroDegree terminal was conducted at theend of 2020. Even under online conditons, we weregenerally able to maintain a mass resolving powerin excess of 500k during the commissioning. Thishigh mass resolving power allowed us to demonstrate
an isomeric separation of 134m, g Sb+ , which has anisomeric state with an excitation energy of 279(1) keV[T1/2 = 10.07(5) s] above the ground state [T1/2 =0.78(6) s]. As shown in Fig. 1, we could successfully
identify the ground state (134g Sb+ ) and the isomericstate (134m Sb+ ) in the TOF spectrum owing to the very
high mass resolving power that the ZD-MRTOF systemachieved.
References

1) Y. Ito et al., Phys. Rev. C 88, 011306(R) (2013).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335, 39 (2014).
3) R. N. Wolf et al., Int. J. Mass Spectrom. 313, 8(2012).

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