Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 47 (2014)

β-NMR measurement of unstable nuclei with cross-polarization
techniqueT. Kawahara,∗1 K. Tateishi,∗1 R. Kojima,∗2 S. Sakaguchi,∗1,∗3 E. Milman,∗1,∗4 S. Chebotarov,∗1,∗4T. Wakui∗1,∗5 and T. Uesaka∗1
A polarized solid proton target for RI beam experiments has been developed at RIKEN and the Centerfor Nuclear Study, University of Tokyo.1) By means ofelectron polarization in photo-excited triplet states ofpentacene, proton polarization of approximately 20%
has been achieved in a low magnetic ﬁeld of 0.1 T andat a high temperature of 100 K. The target has been
applied to RI beam experiments for several times.2,3)
One of the next directions in the research is the polarization of unstable nuclei. If the polarization of protons
can be transferred to unstable nuclei stopped in thetarget, measurements of magnetic moments would become possible with the β-NMR method. The polarization condition of high temperature and low magnetic
ﬁeld, which is the distinct advantage of the target, isindispensable in such low-energy beam experiments.
In this article, we report on our attempt of transferring proton polarization to 13 C nuclei contained in thesample.
As a sample, we used a single crystal of p-terphenyl
doped with pentacene molecules. Most of 1 H nuclei inp-terphenyl molecules were replaced by deuterium toobtain a higher 1 H polarization. The abundance of the1
H was 2%. The weight of the sample was 28 mg. The
crystal was irradiated by the pulsed laser light with awavelength, an average power, pulse width, and repetition rate of 514 nm, 0.3 W, 13 µs, and 7.5 kHz,respectively. The sample temperature was controlled
at 293 K by ﬂowing cold nitrogen gas. The optimum
power of the microwave was 3 W. Under these conditions, a proton polarization of 6.2±1.2% was obtained.
In the next step, the obtained 1 H polarization wastransferred to the 13 C system by the cross-polarization
method. The 13 C (or 1 H) spin rotates along the static
magnetic ﬁeld at a certain Larmor frequency. In thecross-polarization method, we apply a transverse magnetic ﬁeld rotating with the Larmor frequency. This rotating ﬁeld, produced by radio-frequency (RF) waves,eﬀectively changes the level gap between spin up/downstates. When the eﬀective level gaps of 1 H and 13 C areequal, these systems couple to each other and polarization transfer takes place. The level gap is given ash̄ωR = γh̄HRF , where ωR and γ are the Rabi frequency
and gyromagnetic ratio, respectively. The HRF is thestrength of the rotating ﬁeld, which is proportional tothe square root of the RF power.
∗1
∗2
∗3
∗4
∗5

RIKEN Nishina Center
Center for Nuclear Study (CNS),University of Tokyo
Department of Physics,Kyushu University
Department of Physics, Kyungpook National University
Cyclotron and Radioisotope Center, Tohoku University

In the present case, the Larmor frequencies of 13 C
and 1 H are 3.167 and 12.59554 MHz, respectively, in astatic ﬁeld of 0.3 T. By irradiating these two RF waves
at the same time and by tuning their powers to satisfy
HC
the Hartmann-Hahn condition, γ H h̄HRF= γ C h̄HRF,one can realize the polarization transfer between twosystems. Here, the superscripts “H” and “C” represent
1C
H and 13 C, respectively. By changing HRFwith ﬁxed
HHRF , we searched the point where the HartmannHahn condition is satisﬁed. The result is shown inFig. 1 .

Fig. 1. The RF power dependence of 13 C polarization

As seen in the ﬁgure, the 13 C polarization was successfully obtained for the RF power of ∼160 W. The
magnitude of 13 C polarization was 0.12 ± 0.05%. The
polarization-transfer eﬃciency, which is the 13 C polarization divided by 1 H polarization (6.2%), is found tobe 1.9%. While this value is not high, it is reasonablebecouse the sample is deuterated and the abundance of1
H is 2%. If the sample is not deuterated, the numberof 1 H nuclei to which 13 C couples becomes 50 times
larger. In that case, a polarization transfer eﬃciencyof close to 100% would be obtained.
In conclusion, we obtained a high proton polarization of 6.2% bia temperature control and the use ofa deuterated p-terphenyl crystal. By transferring theproton polarization, the 13 C polarization was successfully obtained with the cross-polarization method. The
next step would be the polarization transfer to unstable nuclei stopped in the target. As the gyromagnetic
ratio of the unstable nuclei is not precisely known,
the parameter search for the Hartmann-Hahn condition will become more diﬃcult. Finding an eﬃcientmethod of the search is a challenge for the future.
References
1) T. Uesaka et al.: Eur. Phys. J. 150 (2007) 71.
2) T. Uesaka et al.: Phys. Rev. C. 82 (2010) 021602.
3) S. Sakaguchi et al.: Phys. Rev. C. 87 (2013) 021601.

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