Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of 41 S spin polarization
H. Shirai,∗1 Y. Ichikawa,∗2 H. Ueno,∗2 Y. Ishibashi,∗2,∗3 T. Suzuki,∗1 T. Furukawa,∗4 A. Yoshimi,∗5 Y. Abe,∗2,∗3
K. Asahi,∗1 J. M. Daugas,∗6 T. Fujita,∗7 M. Hayasaka,∗8 K. Imamura,∗2,∗9 S. Kishi,∗8 S. Kojima,∗1 D. Nagae,∗3A. Nakao,∗2 Y. Ohtomo,∗1 T. Sagayama,∗8 Y. Sakamoto,∗1 and T. Sato∗1Spectroscopic data have indicated the erosion of theN = 28 shell gap in several studies1–4) . In particular, the isomeric state of 43 S at 320 keV is suggested
to have a quasi-spherical shape with a spin-parity of
7/2−5,6) . On the other hand, the spin-parity of theground state of 43 S has been neither confirmed nor
predicted uniquely5,7,8) . In order to investigate themechanism of the N = 28 magicity loss through thedetermination of the spin parity of the ground stateof 43 S, we aim to measure systematically the groundstate electromagnetic moments for 41,43 S.
The electromagnetic moments of nuclei in their
ground states are measured by combining the technique to produce spin-polarized RI beams9) and themethod of β-ray-detected nuclear magnetic resonance(β-NMR). In this scheme, the RIs are stopped in acrystal, which provides spin-lattice relaxation times T1
that are longer than the β-decay halflife of the RI. In
order to find out optimum conditions for the β-NMR
measurement, the T1 measurements were carried out
for stopper crystal candidates, such as Si, ZnS, andCaS. In the measurements, an RI beam of 41 S, forwhich a large yield was expected, was used, insteadof 43 S, to measure the relaxation time T1 .
The experiment was carried out at the RIPS11) facility at RIBF. The RI beam of 41 S was produced by
the fragmentation of 48 Ca projectiles at an energy of
E = 63 MeV/nucleon on a 0.52 mm-thick 9 Be target.
The intensity of the 48 Ca beam at the target was typically 200 pnA. The isotope separation of the 41 S beam
was conducted by the RIPS beam line, in which theemission angle θF and momentum pF of the fragment
were selected so as to realize 41 S spin-polarization. Under the condition of pF = p0 × (1.015 ± 0.025) andθF > 1◦ , where p0 represents the central momentum of the fragment 41 S, the 41 S beam was obtained
from RIPS with a purity of 47% and an intensity of
1.6 × 104 particles/s.
The 41 S beam was then transported to the final focal plane and implanted into a stopper crystal locatedat the center of the adiabatic field rotation (AFR) device12) . The AFR device enables us to extract the∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9

asymmetry of β-ray emission without relying on theNMR technique, by only rotating a pair of Nd permanent magnets adiabatically. The β rays emitted from41
S were counted by plastic scintillators, two of whichwere set above the crystal and two others were set below it. The measurement was conducted according to
the following sequence of cycles: beam irradiation for
2,900 ms, rotation of the AFR magnets for 150 ms,waiting time of 200 ms, and the β ray counting for
2,900 ms. The irradiation and counting time periods
were chosen to be comparable with the meanlife of 41 S.
The waiting margin was inserted in order to avoid spurious effects that might arise from a tiny vibration of
the magnets following the rotation. The value of AP
was deduced from β rays counts obtained in the following four different configurations with the field directions up/down and the magnet rotation true/false(hence, the spin is flipped/not flipped). Here, A and P
denote the asymmetry parameter for the β-ray emission and the degree of polarization of 41 S, respectively.
From the results of the AFR measurement, we obtained AP = −0.14(4)% with the CaS multi-crystalstopper of 0.5 mm thickness, and T1 was found to belonger than 4,600 ms in 1σ confidence level.
Following the T1 and AP measurements, the g-factor
search by means of the β-NMR method was carried out
using the spin-polarized 41 S with AP = −0.14% andthe CaS crystal. Because the range within which theg-factor of 41 S is predicted theoretically is quite wide,a fast switching system for changing the tank-circuit
frequency13) has been used. The results of the NMR
measurement are under analysis.
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Department of Physics, Tokyo Institute of Technology
RIKEN Nishina Center
Department of Physics, Tsukuba University
Department of Physics, Tokyo Metropolitan University
Research Core for Extreme Quantum World, Okayama University
CEA
Department of Physics, Osaka University
Department of Physics, Tokyo Gakugei UniversityDepartment of Physics, Meiji University- 39 -