Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Development of β-NMR with spin-aligned beam
H. Nishibata,∗1,∗2 A. Gladkov,∗1 H. Ueno,∗1 Y. Ichikawa,∗1 A. Takamine,∗1 H. Yamazaki,∗1 M. Tajima,∗1K. Kawata,∗1,∗3 M. Mihara,∗4 T. Sato,∗1 K. Doi,∗1,∗5 W. Kobayashi,∗1,∗5 M. Sanjo,∗1,∗5 T. Asakawa,∗1,∗5Y. Sasaki,∗1,∗5 T. Yada,∗1∗5 and K. Asahi∗1Nuclear electromagnetic moments, namely magnetic
dipole (µ) and electric quadruple (Q) moments, are oneof the most sensitive probes to address a single-particle
conﬁguration and nuclear deformation, respectively.
So far, nuclear moments for ground states have beenmeasured by the method of β-ray-detected nuclear
magnetic resonance (β-NMR) combined with a spinpolarized radioactive beam. However, it is diﬃcult
to apply β-NMR to far-unstable nuclei produced by
BigRIPS because the spin polarization is signiﬁcantly
reduced for reactions with the removal of many nucleons and/or with a beam energy >100 MeV/nucleon. Inorder to overcome this diﬃculty, we have been developing a new method that combines the β-NMR techniqueand highly spin-aligned beam production with the twostep fragmentation method by using BigRIPS.1) This
report presents the recent results of a demonstration
experiment using spin-aligned 13 B (T1/2 = 17.3 ms, I π= 3/2− ) with the known ground-state µ and Q moments.3)
The experiment was performed at the Heavy Ion
Medical Accelerator in Chiba (HIMAC). A primary
15N beam with an energy of 100 MeV/nucleon was accelerated by the synchrotron. The beam irradiation
time was 20 ms in every 3.3 s. A radioactive 13 B beam
was produced by the two-proton-removal reaction of15
N with a 1-mm-thick Be target, which was locatedat the entrance of the secondary beamline SB2. The
13B beam was separated by two dipole magnets and awedge-type energy degrader (3-mm thickness, 6◦ angle) at the dispersive focal plane F1. The spin alignment was obtained by selecting a moment oﬀ the center of the momentum distribution by −4–−3%. Subsequently, the spin-aligned 13 B beam was delivered tothe end of the beamline F3, where the β-NMR apparatus is located, and was implanted to a TiO2 singlecrystal.
Since the Q moment of the implanted 13 B interacts
with an electric ﬁled gradient at the implantation site
in the crystal and causes unequal Zeeman splitting between the individual sub-levels, a partial spin reversal using the adiabatic fast passage (AFP) method2)
becomes possible and convert the spin alignment of
the implanted 13 B into the spin polarization. Consequently, the angular distribution of the emitted β rays
becomes anisotropic. For the AFP method, a static
∗1
∗2
∗3
∗4
∗5Fig. 1. Obtained NMR spectrum for spin-aligned 13 B.
The ordinate of this spectrum shows the double ratio
RRFon /RRFoﬀ , where RRFon (RRFoﬀ ) is the ratio of theβ-ray counts detected by the 0◦ and 180◦ telescopeswith (without) the oscillating magnetic field. The dotted line is the spectrum expected from the literature
values of µ = +3.1778(5) µN and Q = +36.6(8) mb3)
assuming a spin alignment of 4.5%.

magnetic ﬁeld of 0.2 T was applied to the crystal perpendicular to the beam axis in the horizontal plane,
and an oscillating magnetic ﬁeld was applied by a pairof coils perpendicular to the static magnetic ﬁeld. Theβ rays from the β decay of 13 B were detected by twotelescopes placed at 0◦ and 180◦ with respect to thedirection of the static magnetic ﬁeld. Each telescopeconsists of two 1.0-mm-thick plastic scintillators.
Figure 1 shows the obtained NMR spectrum forspin-aligned 13 B. The resonances were observed at3067 ± 84 kHz and 3394 ± 84 kHz, which correspondto the transitions between the Zeeman sub-levels withm = −3/2 and −1/2, +1/2, and +3/2, respectively.
From these values, the moments of 13 Bg.s. were deduced to be |µ| = 3.18 (9) µN and |Q| = 36(9) mb,
which are consistent with the literature values of µ= +3.1778(5) µN and Q = +36.6(8) mb.3) We havesuccessfully demonstrated our new method of β-NMR
with a spin-aligned beam.
References

1) Y. Ichikawa et al., Nat. Phys. 8, 918 (2012).
2) A. Abragam, The Principles of Nuclear Magnetism,(Clarendon, Oxford, 1961).
3) T. Nagatomo et al., Nucl. Phys. A 746, 509c (2004).

RIKEN Nishina Center
Department of Physics, Kyushu University
Center for Nuclear Study, University of Tokyo
Department of Physics, Osaka University
Department of Physics, Hosei University- 58 -