Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Dependence of spin-polarized proton target performance onmicrowave resonator thickness parameter and operation temperature
S. Chebotaryov, ∗1,∗3 S. Sakaguchi, ∗1,∗2 W. Kim, ∗3 E. Milman, ∗1,∗3 K. Tateishi, ∗1 and T. Uesaka ∗1

∗1
∗2
∗3

RIKEN Nishina Center
Department of Physics, Kyushu University
Department of Physics, Kyungpook National University- 174 -Fig. 1. Schematics of the loop-gap resonator.

tion signal intensity √P as a function of the square rootof microwave power PM W supplied to the LGR froman RF ampliﬁer. The magnetic√ inside√ ﬁeld generated
the LGR is proportional to PM W : H1 ∝ PM W .
Measurements were performed at −40 ◦C, −80 ◦C, and−173 ◦C using two types of LGRs with copper layer
thicknesses of 4.4 μm and 36 μm with all other conditions being equal. In Fig. 2 the results of the measurements are presented.
−40 ◦ C
Spin polarization (a.u.)

A spin-polarized proton target provides opportunities to experimentally study spin-dependent interactions with unstable nuclei. Such a target is required because unstable nuclei are short-lived species and haveto be provided as an RI-beam. Such an experimental study has not been possible until recently owing
to the lack of a polarized target that is applicable to
RI-beam experiments. Conventional polarized targets
use a high magnetic ﬁeld of a few tesla and sub-kelvintemperatures to attain high polarization. However, inexperiments the energy of recoil protons can be as low
as few tens of megaelectronvolts. Thus the maximum
magnitude of the magnetic ﬁeld used has to be constrained to several tens of millitesla to prevent signiﬁcant distortion of proton trajectories. The Center for
Nuclear Study, University of Tokyo, and the RIKEN
group has developed a spin-polarized proton target system suitable for use in RI-beam experiments,1) which
made it possible to conduct an experimental study ofspin-dependent interactions in unstable nuclei.
The method of generating spin polarization employed in the target system is based on the crosspolarization technique2) , where the polarization of anelectron system is transferred to the protons in thepresence of an oscillating magnetic ﬁeld. The ﬁeld
is generated as a standing electromagnetic wave using a microwave resonator. The type of resonator employed in our system is the so-called loop-gap resonator(LGR); its schematics is presented in Fig. 1. This resonator is made of a sheet of CuFlon material, which
consists of a copper metal clad on both sides of a thin
Teﬂon sheet.3) .
In this report, we study the polarization performance of the target system at diﬀerent temperatures,
speciﬁcally how performance depends on temperaturedependent parameters of an LGR used for creating theoscillating magnetic ﬁeld. The goal is to choose theoptimal parameters of the LGR to obtain maximum
polarization at a target normal operating temperatureof −173 ◦C. As for LGR, the use of a thicker copper
layer tends to give better results in terms of resonator
performance, while for target operation, a thinner copper layer is preferable as it disturbs the trajectories ofrecoil protons less signiﬁcantly. For 10 MeV protons,
the angular resolution will vary from ±0.5◦ to ±1.6◦(sigma), for an LGR with copper layer thicknesses of
4.4 μm and 36 μm, respectively.
We measured a series of dependences of the polariza-400−80 ◦ C−173 ◦ C36 um
4.4 um

300
200
10000 1 2 3 4 0 1 2 3 4 0 1 2 3 4√√Microwave amplitude − P MW ( W )Fig. 2. Dependence of polarization on microwave amplitude at diﬀerent temperatures (measurement error iscomparable to symbol size).

At a temperature of −173 ◦C, the thickness of theLGR copper layer does not play an important role,
while at higher temperatures measurable diﬀerence
starts to appear. In the case of the 4.4 μm LGR, adecrease in polarization performance vs. temperature
is more pronounced in comparison to the 36 μm one. If
the temperature is above −173 ◦C, to maximize polarization performance, the use of the LGRs with a thicker
copper layer is preferable. To better understand theperformance of the polarized target at diﬀerent temperatures and to be able to build a quantitative model,a more detailed study of magnetic ﬁeld dependence
from temperature in the LGR itself is required.
References1) T. Wakui et al., Nucl. Instr. Meth. Phys. Res. A 550,521 (2005).
2) A. Henstra et al., Chem. Phys. Lett. 165, 6 (1990).
3) B. Ghim, et al., J. Magn. Reson., Ser A 120, 72 (1996).