Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Proton polarization in photo-excited aromatic molecule at room
temperature enhanced by intense optical source and temperaturecontrol†
S. Sakaguchi,∗1,∗2 T. Uesaka,∗1 T. Kawahara,∗1 T. Ogawa,∗3 L. Tang,∗4 T. Teranishi,∗2 Y. Urata,∗3 S. Wada,∗3and T. Wakui∗5
For the study of unstable nuclei with polarization
observables, we have constructed a solid polarized
proton target1) based on the electron polarization inphoto-excited aromatic molecules. Proton polarizationof about 20 percent has been obtained at a temperature of 100 K, which is much higher than that underconventional conditions. The target has been successfully applied to several RI-beam experiments2) carriedout at intermediate energies of several tens to a few
hundred MeV/nucleon. One of the future directions of
this research is conducting experiments at low energiesof several to a few tens of MeV/nucleon. This will open
up new possibilities such as resonant scattering and polarization transfer to embedded RIs. For such applications, the target should be placed in a vacuum environment and be polarized at room temperature. In ourprevious measurement at room temperature, a polarization of 4.8±1.2% was achieved by using p-terphenyl
as the material. The aim of the present work is toinvestigate the possibility of achieving a high proton
polarization of 30 percent for application to scatteringexperiments.
The magnitude of the polarization is expressed as
Pp = APe /(A + Γ). Here, Pe is the electron polarization, A is the build-up rate, and Γ is the relaxation
rate. In order to achieve a high polarization, we needto increase A and Pe or decrease Γ. To enhance thebuild-up rate A, we should increase the laser power forphoto-excitation. However, in Iinuma et al.3) reported
that a high laser power does not necessarily lead toa high polarization. This is considered to be becauseof the temperature rise of the sample. At temperatures higher than 300 K, the intrinsic relaxation rate
Γint rapidly increases4) because of the molecular motion. Thus, it is expected that the polarization can beenhanced by using an intense optical source with thesample temperature controlled at T ∼300 K.
The sample used in the present work is a single crystal of p-terphenyl doped with pentacene molecules witha concentration of 0.005 mol%. The target temperature was monitored with a platinum resistance thermometer. The temperature of the crystal was con†
∗1
∗2
∗3
∗4
∗5

trolled by ﬂowing cold N2 gas. For the light source, weused an Ar ion laser with a wavelength of 514 nm.
The polarization was measured under two diﬀerentconditions. The ﬁrst condition (averaged laser powerof 80 mW, w/o temperature control) is the same asthat used in our previous measurement, where a polarization of 4.8% was achieved. The other condition(1.5 W, w/ temp. control), referred to as “present,”
was also considered. The time evolution of the polarization is shown in Fig. 1. In the present condition, thepolarization is enhanced by a factor of three as compared with the polarization under the ﬁrst condition.
Although the absolute measurement is to be done in
the future, the magnitude of achieved polarization corresponds to about 15% if we assume the polarizationof 4.8% in the previous condition.

Fig. 1. Time evolution of proton polarization.

Finally, it should be mentioned that optimization of
the time structure of the laser pulse would enhance
the polarization rate APe by a factor of ∼5 without increasing the relaxation rate Γ (see Ref.5) ). Thus, such
optimization would almost directly improve the magnitude of polarization. Room-temperature polarizationof 30 percent would be realized by combining a highpower laser with a wavelength of ∼590 nm, throughoptimization of laser pulse structure, and employing a
sophisticated temperature control system.
References

Condensed from the article in Nucl. Instr. Meth. B 317(2013) 679-684
RIKEN Nishina Center
Department of Physics, Kyushu University
RIKEN Advanced Science Institute
CNS, University of TokyoCYRIC, Tohoku University- 190 -1) T. Wakui: Proc. of PST2005 (World Scientific, 2007).
2) S. Sakaguchi et al.: Phys. Rev. C 87, 021601(R)(2013), and references therein.
3) M. Iinuma et al.: Jour. Mag. Res. 175, 235 (2005).
4) T. Kawahara et al.: RIKEN Prog. Rep. 46, 164 (2013).
5) S. Sakaguchi et al.: Nucl. Instr. Meth. B 317 (2013)679-684