Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Hyperpolarization of thin films with dynamic nuclear polarization
using photoexcited triplet electrons†
K. Tateishi,∗1 M. Negoro,∗2 A. Kagawa,∗2 T. Uesaka,∗1 and M. Kitagawa∗2With dynamic nuclear polarization using the photoexcited triplet electron spin (triplet-DNP) of pentacene,1) nuclear spins can be hyperpolarized morethan 10% even in a low magnetic ﬁeld at room temperature. DNP is a means of transferring spin polarizationfrom electrons to nuclei. In the ﬁeld of experimental
nuclear physics, a nuclear spin polarized target in alow magnetic ﬁeld (<0.5 T) opens new research possibilities. The polarized target used presently in RI
beam experiments is 1-mm thick.2) The relatively large
target thickness prevents us from applying the target
to low-energy experiments. In the experiments, beamand scattered particles have energies as low as several
MeV/nucleon and cannot penetrate the 1-mm thickmaterial. In order to perform low-energy scatteringexperiments, a polarized ﬁlm with a thickness of lessthan 100 µm is desirable. However, it is diﬃcult to cut
the fragile organic crystals to such a thickness from alarge crystal prepared by the conventional method.1,2)
In this work, we succeeded in polarizing 1 H spins in athin ﬁlm of p-terphenyl and trans-stilbene doped withpentacene fabricated with a new method.
In order to achieve the ideal sample thickness andpentacene concentration, we applied the cell method
to the sample preparation.3,4) The method is as follows: A powder mixture of p-terphenyl doped withpentacene was melted and absorbed into a gap between two quartz plates adjusted with copper foils.
When this arrangement was cooling at room temperature, a single crystal, shown in Fig. 1(a), was grownin one minute. After removing one of the plates, wemeasured the thickness of the sample using a surface
proﬁler. The result is shown in Fig. 1(b). From experiments, we determined that the ﬁlm plane is thecleavage plane. Optimizing the sample thickness andthe pentacene concentration, we obtained 1 H spin polarization of 12.9% in a 7-µm thick ﬁlm of p-terphenyldoped with 0.05 mol% pentacene in 0.4 T and at room
temperature. The slightly lower value of the achieved
polarization compared to that for the bulk crystal
grown by the conventional method may be due to thedegradation of the crystal quality.
The advantage of the cell method is not only in theadjustability of the thickness over a wide range but
also in a short crystal growth time. trans-Stilbene
cannot be grown up to a single crystal doped with†∗1∗2

Condensed from the article in J. Phys. Soc. Jpn. 82, 084005(2013)RIKEN Nishina Center
Department of Electronics and Materials Physics, Osaka
University- 192 -Fig. 1. (a) Photograph of p-terphenyl doped with 0.05
mol% pentacene grown by the cell method. (b) Typical sample thickness measured using surface profiler.

pentacene by the conventional method because melted
trans-stilbene decomposes pentacene immediately. By
adopting the cell method, we succeeded in avoiding
considerable decomposition, preparing a single crystalof trans-stilbene doped with pentacene, and polarizing
it with triplet-DNP for the ﬁrst time. The achieved
polarization was improved to 3.9% at 150 K in a 60µm thick ﬁlm of trans-stilbene doped with 0.05 mol%pentacene.
The polarized thin ﬁlms also enable new applicationsof triplet-DNP in general NMR spectroscopy with therealization of Tycko s proposal,5) which claims thatthe use of multi layered structures of hyperpolarizing
layers and any material of interest allows polarizationtransfer through the interfaces. This will open newopportunities in various ﬁelds such as chemistry, life
science, and medical science.
References
1) K. Takeda: Triplet State Dynamic Nuclear Polarization(VDM Verlag, 2009).
2) T. Uesaka et al.: Phys. Rev. C 82, 021602(R) (2010).
3) S. Hashimoto et al.: Jpn. J. Appl. Phys. 27, 726 (1988).
4) T. Aoki-Matsumoto et al.: Phys. Status Solidi C 6, 228(2009).
5) R. Tycko: Solid State Nucl. Magn. Res. 11, 1 (1998).