RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Research activities of KEK RNB group at RIBF
H. Miyatake ∗1
How are gold and platinum elements synthesized inthe universe? This is one of the important questions
in fundamental science of the 21st century. The KISS
project aims at ﬁnding the astrophysical origin of suchheavy-element synthesis through the experimental nuclear physics.1) This is a challenging and next-step
project based on the scientiﬁc expertise of the Tokai Radioactive Ion Accelerator Complex (TRIAC), which was
the ﬁrst-generation Radioactive Nuclear Beam (RNB)
facility of the post-acceleration scheme operated atthe KEK Tokai campus.2) The project was encouraged to proceed in the international review committee held in FY2009. Since then, KEK Isotope Separation System (KISS) was installed in experimental
halls of the Nishina building at RIBF and has been utilized various experiments under the collaboration between RNC/RIKEN and IPNS/KEK. The ﬁrst ceremonial meeting for this collaboration was held in October
2011.
According to the scenario of the rapid neutron capture (r-)process,3) waiting nuclei as progenitors of thestable heavy elements around A = 195 can provide
crucial information on the astrophysical circumstancethrough their masses, lifetimes, decay schemes, and neutron capture rates,4) although these nuclei in the vicinity of the waiting region are still far to access.
We have proposed multi-nucleon transfer (MNT) re-actions of intense neutron-rich heavy-ion beams as production tools.5) Produced nuclei are stopped immediately in the KISS argon gas cell to be neutral atoms forfurther A- and Z-separation by applying the laser resonance ionization and dipole magnetic ﬁeld (Fig. 1).6)
The commissioning of KISS was performed from 2011 to2014 to conﬁrm the A- and Z-separation of RIs reasonably extracted from the gas cell.7) Apart from the KISS
R&D, In-Gas Laser Ionization and Spectroscopy network (IGLIS-net 8) ) has been launched in 2012 as an international information network of the physics subjects
and techniques related to IGLIS with 15 groups and institutes worldwide. Further, the Wako Nuclear Science
Center (WNSC) was established as a formal branch ofIPNS, KEK at the RIKEN Wako campus in April 2015for supporting user activities.
A new research activity on the nuclear mass measurements has been started in our group in FY2015 based on
the joint R&D works with RNC of the Multi-Reﬂection
Time-Of-Flight mass spectrograph (MRTOF).9) The resolving power of MRTOF can reach the level of approximately 105 even for heavy-isotope measurements withina relatively short analyzing time less than 10 ms. It isa powerful tool not only for nuclear astrophysics but
also for nuclear physics in a broad range of the nuclearchart.
As the ﬁrst series of measurements (called the SHEmass project), MRTOF was set at the focal plane

Fig. 1. Schematic drawing of KISS.
∗1

Head of Wako Nuclear Science Center, IPNS, KEK (2015–present).

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Fig. 2. Schematic drawing of the MRTOF system set atGARIS II.

Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Fig. 3. Hyperﬁne (Hf-) structures of 199m, 199g Pt obtainedwith β-transitions of 199g Pt. Short and long dotted lines
indicate contributions of 199g Pt and 199m Pt, respectively.
The Hf-structure of 199m Pt is independently determinedby the internal γ-transition (392 keV) from 199m Pt shownin the inset.

of GARIS II to determine the masses of evaporation
residues in nuclear fusion reactions,10) as shown inFig. 2.
Thus far, a number of experimental subjects havebeen approved at RIBF-PAC, and their experiments
have been performed with KISS and MRTOF devices.
Those are RRC29, RRC37, RRC40, RRC41, RRC44,
and RRC45 for the KISS experiments and LINAC07,
LINAC23, and LINAC24 for the MRTOF experiments.
KISS experimental subjects approved thus far include
lifetime measurements of nuclei in the vicinity of thewaiting region and decay- and laser-spectroscopies ofisomeric and ground states in neutron-rich heavy nuclei.
Subjects using MRTOF are devoted to the mass measurements of very heavy elements and proton-rich nuclei
with astrophysical interests concerning X-ray bursters.
Figure 3 shows the ﬁrst result of the hyperﬁne structures of 199g Pt and 199m Pt ﬁrstly measured in the recent KISS experiment (RRC41).11) The magnetic moment and shift of the charge-radii of each state wereclearly obtained. This experiment has been successfully
performed under the collaboration between IBS andKEK, in which we have constructed a gamma-ray measurement system with super-clover Ge detectors fromIBS and a mass measurement system with MRTOF inFY2016. The former system is already available, andthe latter will be ready in FY2017 for KISS experiments
and at the RAON facility 12) in future.
Figure 4 shows the ﬁrst result of the SHE-mass
project (LINAC07),13) which reveals unique MRTOF
characteristic as a mass spectrograph having a certain
mass range with high sensitivity for a small amount ofRIs. A typical mass-determination uncertainty is approximately 100 keV/c2 for 100 detected ions in thismass region. Thus far, measurements have been performed for more than 80 isotopes, including new mass
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Fig. 4. Time-of-ﬂight spectrum measured in the nuclear fusion reactions of 169 Tm and 40 Ar. Isotopes shown inthe ﬁgure are clearly identiﬁed by comparison with tofspectrum of a diﬀerent number of ﬂight laps, n = 224.
RI of 205 Bi was clearly identiﬁed within an error of 1
MeV/c2 by two detected events, as shown in the inset.

determinations of six isotopes of mendelevium (Z =101) and einsteinium (Z = 99) elements.14)
Based on the current research activities, WNSC has
proposed an upgrade plan of the KISS project, which
is KISS Stage II for a comprehensive study of element
synthesis with precise nuclear spectroscopy covering thewhole region of the nuclear chart.15) It can be realized
by the present KISS upgrade for accepting intense HIbeams and by installation of a pure RI spectroscopy line
at the SLOWRI facility being commissioned.16)
References1) S. C. Jeong et al., KEK Report 2010-1 (2010).
2) H. Miyatake et al., Nucl. Instrum. Methods Phys. Res.
B 204, 746 (2003); S. C. Jeong et al., KEK Progress
Report 2011-1 (2011).
3) E. M. Burbidge, G. R. Burbidge, W. A. Fowler,
F. Hoyle, Rev. Mod. Phys. 29, 547 (1957).
4) M. R. Mumpower et al., Progr. Part. Nucl. 86, 86(2016).
5) Y. X. Watanabe et al., Phys. Rev. Lett. 115, 172503(2015).
6) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B 376, 52 (2016).
7) KEK RNB group, KISS outline, (2015). http://research.kek.jp/group/wnsc/facilities_and_devices/KISS_outline.pdf8) http://research.kek.jp/group/kekrnb/iglis-net/
9) H. Wollnik et al., Int. J. Mass Spect. 96, 267 (1990).
10) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
376, 425 (2016).
11) Y. Hirayama et al., accepted in Phys. Rev. C.
12) S. C. Jeong, New Physics: Sae Mulli 66 1458 (2016).
13) P. Schury et al., Phys. Rev. C 95, 011305(R) (2017).
14) Y. Ito et al., to be submitted.
15) H. Miyatake, at KEK-PIP Int. Rev. Comm. KEK(2016), unpublished.
16) M. Wada et al., AIP Conf. Proc. 865, 352 (2006).