RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

β-NMR measurement of 41 S
Y. Ichikawa,∗1 Y. Ohtomo,∗1,∗2 Y. Ishibashi,∗1,∗3 T. Suzuki,∗2 T. Sato,∗2 K. Imamura,∗1,∗4 T. Fujita,∗5T. Furukawa,∗6 K. Asahi,∗2 T. Egami,∗7 C. Funayama,∗2 M. Hayasaka,∗8 C. Hirao,∗2 S. Kojima,∗2 T. Komine,∗2M. Matsumoto,∗8 Y. Sakamoto,∗2 A. Takamine,∗9 D. Tominaga,∗7 H. Yamazaki,∗1 and H. Ueno∗1∗1∗2∗3∗4∗5∗6∗7∗8∗9RIKEN Nishina Center
Department of Physics, Tokyo Institute of Technology
Department of Physics, University of Tsukuba
Department of Physics, Meiji University
Department of Physics, Osaka University
Department of Physics, Tokyo Metropolitan University
Department of Physics, Hosei University
Department of Physics, Tokyo Gakugei University
Department of Physics, Aoyama Gakuin University

300
280

43260

Energy loss (MeV)

The erosion of N = 28 shell gap has been suggested
from several spectroscopic experimental data.1–4) In
particular, the 43 S nucleus is of considerable interest because shape coexistence is expected to occur,
which is key to understanding the evolution of shellgaps far from stability. The isomeric state of 43 S at
320 keV is suggested to have a shape close to sphericalwith a spin-parity of 7/2− ,5,6) but both the spin-parityand deformed parameter of the ground state have notbeen determined directly. To investigate the mechanics
leading to such an anomalous nuclear structure, we aim
to measure the ground-state nuclear moment of 41,43 S.
First, µ of 41 S was measured using the β-ray detectednuclear magnetic resonance (β-NMR) method,7) combined with a technique to produce spin-polarized RI
beams.8)
The experiment was carried out at the RIPS facility
at RIBF. The RI beam of 41 S was produced by thefragmentation of a primary beam of 48 Ca at an energyof E = 63 MeV/nucleon on a primary target of 9 Be
with a thickness of 0.52 mm. The typical intensity of
the 48 Ca beam at the target was 200 pnA. To realize
the spin polarization in 41 S, an emission angle of θF >1◦ and a momentum window of pF = p0 × (1.015 ±0.025) were selected, where p0 represents the central
momentum of the fragment 41 S. Under this condition,
the particle identification of the secondary beam wasperformed on an event-by-event basis with information
regarding time of flight (TOF) and energy loss (∆E) as
shown in Fig. 1. The beam was pulsed with durationsof beam-on and beam-off periods of 2.9 s and 2.9 s,
equally.
The 41 S beam was then transported to the final focal plane and implanted into a stopper crystal of CaS
with which AP = −0.14% was observed previously,9)where A and P denote the asymmetry parameter forthe β-ray emission and the degree of polarization of41S, respectively. The CaS stopper was mounted between the poles of a dipole magnet that produces anexternal magnetic field of B0 = 0.5 T. β rays emitted
from the stopper were detected using plastic scintillator telescopes located above and below the stopper.
An oscillating radio-frequency field B1 was applied per-Cl

240
22020041S

180160140120100

101520253035

Time of flight (ns)Fig. 1. Particle identification of 41 S. The horizontal andvertical axes represent TOF between the plastic scintillators at F2 and F3, and ∆E taken at the silicondetector at F2, respectively.

pendicular to B0 using a pair of coils. The frequencyof B1 was swept over a certain region, and spin reversal occurred when the region included the Larmor
frequency. The spin reversal was detected through thechange of the up/down ratio R of the β-ray counts atthe two telescopes. Because the range within which
the g-factor of 41 S is predicted theoretically is quitewide, a fast switching system for changing the tankcircuit frequency10) was used. In this experiment,
the g-factor search was conducted in the region where0.2 < g < 0.8. The results of the NMR measurements
are under analysis.
References
1) R. W. Ibbotson et al.: Phys. Rev. C 59, 642 (1999).
2) F. Sarazin et al.: Phys. Rev. Lett. 84, 5062 (2000).
3) Zs. Dombrádi et al.: Nucl. Phys. A727, 195 (2003).
4) S. Grévy et al.: Eur. Phys. J. A 25, 111 (2005).
5) L. Gaudefroy et al.: Phys. Rev. Lett. 102, 092501(2009).
6) R. Chevrier et al.: Phys. Rev. Lett. 108, 162501(2012).
7) K. Sugimoto et al.: J. Phys. Soc. Jpn. 21, 213 (1966).
8) K. Asahi et al.: Phys. Lett. B 251, 488 (1990).
9) H. Shirai et al.: RIKEN Accel. Prog. Rep. 47, in print.
10) N. Yoshida et al.: Nucl. Instrum. Meth. B 317, 705(2013).

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