RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-9. Instrumentation

Design of a thin-film polarized proton target system for low-energy
RI beam experiments
E. Milman,∗1,∗3 S. Sakaguchi,∗2 S. Chebotaryov, ∗1,∗3 W. Kim,∗3 K. Tateishi,∗1 and T. Uesaka ∗1
The search for unknown resonances of unbound 10 N
via 9 C+p elastic resonant scattering is proposed. Theoretically, four broad and overlapping low-lying 10 N
resonances that may not be clearly identifiable in the9C+p excitation function are expected.The vector analyzing power is measured to determine these broadresonances1) . The level information obtained in theexperiment is useful for discussing resonances in 10 Li
since 10 N and 10 Li are mirror nuclei that are expected
to have a similar structure. The 10 Li structure provides us with valuable information for constructing thethree-body model of the borromean 11 Li nucleus.
A polarized target is required for the measurementof the analyzing power. A polarized proton solid target
for low-energy beam experiments2) has been designed
based on an existing system for intermediate energies
3). In the target, proton polarization is obtained bytransferring the electron polarization of photo-excitedtriplet state of pentacene to protons of p-terphenyl.
Polarization transfer is done by microwave irradiationthat satisfies the condition that the Rabi frequency ofelectron spin is matched to the Larmor frequency ofthe proton2) .
We select a magnetic field strength of 0.2 T. Thisvalue is a tradeoff between the magnitude of proton
polarization and magnetic field effect for the trajectoryof recoil particles. At this field strength, the resonancefrequency of electron spins is ∼4.8 GHz and the proton
resonance frequency is ∼8.5 MHz.
For the detection of low-energy recoil protons emitted with respect to ±22.5◦ where the analyzing poweris expected to be the largest, the microwave resonatormust be sufficiently short to avoid interference with thetrajectory of scattered protons.A three-loop two-gap
resonator4) (LGR) was chosen as the microwave resonator, as shown in Fig. 1 (Left). Its structure allows
us to sufficiently shorten the resonator and to implement NMR coil close to the resonator without affecting
its performance4) .
We conducted a simulation of the LGR using theelectromagnetic field simulation software CST Microwave Studio. In the simulation, the LGR was placed
inside a brass shield to reduce microwave radiation
losses, as shown in Fig. 1 (Right). The shield wasdesigned to allow beam particles and recoil protons topass through the resonator freely. The LGR was excited by an antenna placed close to one of edge loops.
The resonance frequency of LGR and the strength of
∗1
∗2
∗3

RIKEN Nishina Center
Department of Physics, Kyushu University
Department of Physics, Kyungpook National University

Fig. 1. Three-loop two-gap resonator (LGR) (Left). LGR
placed in the shield (Right).

the oscillating microwave field were simulated at 1-W
incident power. The power required for optimal polarization transfer was evaluated. Simulations were performed for different central loop diameters. We foundthat the resonator with 18-mm diameter and 5-mm
length was the tradeoff between the target size andrequired incident power of 20 W.
The thickness of the target was chosen to be 110 µm
to cover the range of the 9 C beam with an energy of 5.6
MeV/nucleon. A novel thin crystal production method
is proposed. In this method, the crystal prepared by
the Bridgmann method is placed in a vacuum tube
and heated. The crystal sublimates and its thickness
slowly decreases. The sublimation speed is controlled
by the heater temperature. A test sample prepared by
the sublimation method is shown in Fig. 2.The sample
thickness is approximately 700 µm.

Fig. 2. Sample prepared by the sublimation method.

In conclusion, the microwave resonator was designed
for the thin-film polarized proton target at 0.2 T.
The sublimation method was proposed to produce thincrystals. In the next fiscal year, the resonator will bebuilt and tested. A thin crystal with a thickness ofapproximately ∼100 µm will be produced by the sublimation method.
References
1) T. Teranishi, S. Sakaguchi, T. Uesaka et al.: AIP Conf.
Proc. 1525, 552 (2013).
2) S. Sakaguchi et al.: Nucl. Instrum. Methods Phys. Res.
B 317, 679 (2013).
3) T. Uesaka et al.: Nucl. Instrum. Methods Phys. Res. A
526, 186 (2004).
4) R. L. Wood, W. Froncisz, J. S. Hyde: J. Magn. Reson.
58, 243 (1984).

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