Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Precision mass measurements of proton-rich nuclei in A ∼ 60−80
region with the multireﬂection time-of-ﬂight mass spectrograph
S. Kimura,∗1,∗2 Y. Ito,∗2 D. Kaji,∗2 P. H. Schury,∗3 M. Wada,∗3,∗2H. Haba, T. Hashimoto,∗4 Y. Hirayama,∗3 M. MacCormick,∗5 H. Miyatake,∗3 J. Y. Moon,∗4K. Morimoto,∗2 M. Mukai,∗1,∗2 I. Murray,∗5,∗2 A. Ozawa,∗1,∗2 M. Rosenbusch,∗2 H. Schatz,∗6A. Takamine,∗2 T. Tanaka,∗7,∗2 Y. X. Watanabe,∗3 H. Wollnik,∗8 and S. Yamaki∗9,∗2
∗2

∗1
∗2
∗3

∗4
∗5
∗6

∗7∗8∗9Counts / dose (arb.u.)

Nuclear masses of nuclei near the proton drip line
up to 100 Sn are crucial in determining the rp-process
pathway which drives explosive astronomical phenomena called type I X-ray bursts (XRB). In order to compare diﬀerent XRB models meaningfully, the relative
mass uncertainties must be improved. Precisions of
the order of δm/m  10−7 are necessary for current
rp-process calculations1) . Half lives of the key nuclei inthe rp-process are of the order of several tens to several
hundreds of milliseconds. The multireﬂection time-ofﬂight mass spectrograph (MRTOF) satisﬁes the experimental requirements for these conditions2) .
We performed mass measurements of the protonrich nuclei in the A ∼ 60−80 region by utilizing theMRTOF combined with a gas-ﬁlled recoil ion separator GARIS-II3) via a gas-cell and an ion transport system. To produce the proton-rich nuclei the fusionevaporation reaction nat S(36 Ar,X) was used. In thisreaction, it was expected that the inadequate separation in GARIS-II between the evaporation residues
and the primary beam would lead to breakage of thegas cell and the GARIS-II bulkhead thin mylar windows due to irradiation damage. Therefore, we installed two independent beam stoppers4) . We also installed a double-layered plastic scintillator combinedwith copper energy degraders to suppress the low energy β-rays (Eβ  4 MeV) at the GARIS-II focal plane
for the β-activity search. The energy and maximum
intensity of the 36 Ar10+ beam were 3.30 MeV/nucleon
and 3 particle μA, respectively. The average target
thickness of nat S2 Mo on Ti backing was 1.9 mg/cm2 .
Figure 1 shows the intensity distribution of βactivities as a function of magnetic rigidity. Mass measurements were performed with two diﬀerent GARISII settings, Bρ = 0.86 Tm and Bρ = 1.01 Tm. The
settings corresponded, respectively, to the sulfur reaction products and unexpected reaction products on

110

100

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

Bρ ( Tm )Fig. 1. Intensity distribution of β-activities as a functionof magnetic rigidity.

Fig. 2. Summary of the mass measurements. The bold-line
boxes indicate the stable isotopes. The dashed lines indicate the proton drip line. The color-coded boxes indicate the mass precision of measurements: a precisionof 10−7 is indicated by blue and a precision of 10−8 isindicated by green. The isotopes labeled ”R” are used
as isobaric references. The red line represents the rpprocess pathway, which has a mass fraction greater than10%5) .

the titanium backing. We found two dozen isotopes
in the time-of-ﬂight spectra of the MRTOF with clearpeaks. The nuclear masses were determined by thesingle-reference method; thus, in each isobaric series,
we utilized an isotope as a mass reference. The summary of the preliminary results is shown in Fig. 2.
We successfully measured nuclear masses with therequired precision under the worse separation condition in GARIS-II. We must now proceed to the more
proton-rich side to improve the understanding of therp-process in XRBs.

Institute of Physics, University of Tsukuba
RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)
Institute for Basic Science
Institut de Physique Nucléaire, Orsay
Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University
Department of Physics, Kyushu University
New Mexico State University
Department of Physics, Saitama University

References
1) H. Schatz, IJMS 349-350, 181 (2013).
2) P. H. Schury et. al., NIM B 335, 39 (2014).
3) D. Kaji et. al., NIM B 317, 311 (2013).
4) S. Kimura et. al., in this report.
5) H. Schatz et. al., PRL 86, 3471 (2001).

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