Parity-transfer reaction for study of spin-dipole 0− mode

M. Dozono,∗1 K. Fujita,∗2 N. Fukuda,∗3 M. Ichimura,∗3 N. Inabe,∗3 S. Kawase,∗1 K. Kisamori,∗1 Y. Kiyokawa,∗1

K. Kobayashi,∗4 M. Kobayashi,∗1 T. Kubo,∗3 Y. Kubota,∗1 C. S. Lee,∗1 M. Matsushita,∗1 S. Michimasa,∗1

H. Miya,∗1 A. Ohkura,∗2 S. Ota,∗1 H. Sagawa,∗1,∗5 S. Sakaguchi,∗2 H. Sakai,∗3 M. Sasano,∗3 S. Shimoura,∗1

Y. Shindo,∗2 L. Stuhl,∗3 H. Suzuki,∗3 H. Tabata,∗2 M. Takaki,∗1 H. Takeda,∗3 H. Tokieda,∗1 T. Uesaka,∗3

T. Wakasa,∗2 K. Yako,∗1 M. Yamagami,∗5 Y. Yanagisawa,∗3 J. Yasuda,∗2 R. Yokoyama,∗1 K. Yoshida,∗3 andJ. Zenihiro∗3

The spin-dipole (SD) 0− excitation has recently at-tracted theoretical attention owing to its strong rele-vance in the tensor correlations in nuclei. For example,self-consistent HF+RPA calculations in Ref.1) predictthat the tensor correlations produce a strong harden-ing (shifting toward higher excitation energy) effect onthe 0− resonance. It is also predicted that the effect issensitive to the magnitude of the tensor strength. Thusexperimental data of the SD 0− distribution enable usto examine the tensor correlation effects quantitatively.Despite this importance, experimental information on0− states is limited because of the lack of the experi-mental tools suitable for 0− studies.

We propose a new probe, the parity-transfer(16O, 16F(0−, g.s.)) reaction, for 0− studies2). Theparity-transfer reaction selectively excites unnatural-parity states for a 0+ target nucleus, which is an advan-tage over the other reactions used thus far. In order toestablish the parity-transfer reaction as a new tool for0− studies, we measured the 12C(16O, 16F(0−, g.s.))12Breaction. We demonstrate the effectiveness of this re-action by identifying the known 0− state at Ex =9.3 MeV in 12B.

The experiment was performed at the RIKEN RIBeam Factory (RIBF) by using the SHARAQ spec-trometer and the high-resolution beam line. Figure 1shows a schematic layout of the experimental setup. Aprimary 16O beam at 250 MeV/nucleon and 107 pps

Fig. 1. Schematic layout of the experimental setup.

∗1 Center for Nuclear Study, University of Tokyo∗2 Department of Physics, Kyushu University∗3 RIKEN Nishina Center∗4 Rikkyo University∗5 Center for Mathematics and Physics, University of Aizu

from the superconducting RING cyclotron (SRC) wastransported to the S0 target position. The beamline to the spectrometer was set up for dispersion-matched transport. We used a segmented plastic scin-tillation detector as an active 12C target. This detec-tor consisted of 16 plastic scintillators with a size of30 mm× 5 mm× 1 mm, and it was used to determinethe x-position of the beam on the target. The outgoing15O + p particles produced by the decay of 16F weremeasured in coincidence. The particles were momen-tum analyzed by using the SHARAQ spectrometer.The 15O particles were detected with two low-pressuremulti-wire drift chambers (LP-MWDCs) at the S2 fo-cal plane, while the protons were detected with twoMWDCs at the S1 focal plane.

We reconstructed the relative energy Erel betweenthe 15O and the proton. A preliminary result is shownin Fig. 2. The obtained Erel resolution was 150 keV inFWHM at Erel = 535 keV, and the 0− ground stateof 16F was clearly separated from other excited states.In order to identify the 12B(0−, 9.3 MeV) state, dataanalysis for obtaining the 12C(16O, 16F(0−, g.s.)) spec-trum and its angular distributions is in progress.

Fig. 2. Preliminary result of the relative energy between

the 15O nucleus and the proton from the decay of 16F.

References1) H. Sagawa and G. Colo: Prog. Part. Nucl. Phys. 76, 76

(2014).2) M. Dozono et al.: RIKEN Accel. Prog. Rep. 45, 10

(2012).

Direct mass measurements of neutron-rich Ca isotopes beyondN = 34

M. Kobayashi,∗1 S. Michimasa,∗1 Y. Kiyokawa,∗1 H. Baba,∗2 G.P.A. Berg,∗3 M. Dozono,∗1 N. Fukuda,∗2

T. Furuno,∗4 E. Ideguchi,∗5 N. Inabe,∗2 T. Kawabata,∗4 S. Kawase,∗1 K. Kisamori,∗1,∗2 K. Kobayashi,∗6

T. Kubo,∗2 Y. Kubota,∗1,∗2 C.S. Lee,∗1,∗2 M. Matsushita,∗1 H. Miya,∗1 A. Mizukami,∗7 H. Nagakura,∗6

D. Nishimura,∗7 H. Oikawa,∗7 S. Ota,∗1 H. Sakai,∗2 S. Shimoura,∗1 A. Stolz,∗8 H. Suzuki,∗2 M. Takaki,∗1

H. Takeda,∗2 S. Takeuchi,∗2 H. Tokieda,∗1 T. Uesaka,∗2 K. Yako,∗1 Y. Yamaguchi,∗6 Y. Yanagisawa,∗2

R. Yokoyama,∗1 and K. Yoshida∗2

The shell evolution in nuclei far from stability is oneof the main subjects of nuclear physics. Nuclear massis one of the most fundamental quantity providing in-formation on the shell structure. The neutron numbersof 32 and 34 have been suggested to be candidates ofnew magic numbers in the Ca isotopes1). Recentlythe masses of 53Ca and 54Ca were measured, and theshell closure at N = 32 was established2). The presentwork aims at studying the nuclear shell evolution atN = 32, 34 by direct mass measurements of neutron-rich nuclei in the vicinity of 54Ca.

The experiment was performed at the RIKEN RIBeam Factory (RIBF). The masses were measured di-rectly by the TOF-Bρ method. Neutron-rich isotopeswere produced by fragmentation of a 70Zn primarybeam at 345 MeV/nucleon in a 9Be target. The frag-ments were separated by BigRIPS, and transported inthe High Resolution Beam Line to the SHARAQ spec-trometer. The beam line and SHARAQ were operatedin the dispersion matching mode allowing a momen-tum resolution of 1/14700.

Fig. 1. Schematic view of the beamline and the detectors

used in the experiment.

∗1 Center for Nuclear Study, University of Tokyo∗2 RIKEN Nishina Center∗3 Department of Physics, University of Notre Dame∗4 Department of Physics, Kyoto University∗5 RCNP, Osaka University∗6 Department of Physics, Rikkyo University∗7 Department of Physics, Tokyo University of Science∗8 NSCL, Michigan State University

A schematic view of the beamline with the detec-tors used in the experiment is shown in Fig. 1. TheTOF was measured with newly developed diamonddetectors3) installed at BigRIPS-F3 and the final fo-cal plane of SHARAQ (S2). The flight path lengthbetween F3 and S2 is 105 m along the central ray.We installed two low pressure multi-wire drift cham-bers (LP-MWDCs)4) at both F3 and S2 to correct theflight pass lengths using the tracking information onan event-by-event basis. The Bρ value was measuredby a parallel plate avalanche counter (PPAC) locatedat the target position of SHARAQ (S0). At S2, wemounted two silicon strip detectors for identificationof the atomic numbers of the fragments. To identifythe isomers, which leads to a systematic shift towardshigher masses, we placed a plastic stopper downstreamof S2 and a γ-detector array consisting of 2 Ge cloverand 16 NaI(Tl) detectors. Details of this system canbe found in Ref.5).

Figure 2 shows the preliminary particle identifica-tion of the secondary beams. The total yield of 55Cawas on the order of several thousands. Many species ofreference nuclei over a broad range of A and Z were ob-served, which were used in the mass calibration. Fur-ther analysis is in progress.

Fig. 2. Particle identification of the secondary beams.

References1) T. Otsuka et al.: Phys. Rev. Lett. 87 082502 (2001).2) F. Wienholtz et al.: Nature 498 349 (2013).3) S. Michimasa et al.: Nucl. Instr. Meth. B 317 305 (2013).4) H. Miya et al.: Nucl. Instr. Meth. B 317 317 (2013).5) Y. Kiyokawa et al.: in this report.

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