Polarized Solid Proton Target for RI Beam
Experiments
T. Wakui∗1, M. Hatano† , H. Sakai†∗∗ , A. Tamii† and T. Uesaka∗∗
∗
RIKEN, Wako, Saitama 351-0198, Japan
Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
∗∗
Center for Nuclear Study, University of Tokyo, Wako, Saitama 351-0198, Japan
†
Abstract. A polarized solid proton target system that can be used for radioisotope beam experiments has being developed. A high-power Ar-ion laser has been installed to improve proton polarization. With the laser, proton polarization of 36.8±4.2% has been achieved in 0.3 T at 77 K. The
new target system has been constructed toward the first nuclear physics experiment scheduled in
2003.
INTRODUCTION
For the purpose to study nuclear structure of unstable nuclei with radioisotope (RI)
beam, we are developing a polarized solid proton target that can be operated in a
magnetic field lower than 0.3 T at a temperature higher than 77 K [1]. In the modest
operating condition, low energy recoil protons can be detected in p-RI elastic scattering
experiments that carried out under the inverse kinematics condition. This allows us to
descriminate true events from backgrounds and also to achieve high angular resolution.
As a target material, a crystal of aromatic molecules such as naphthalene doped with
pentacene is used. Protons in the crystal can be polarized by means of a pulsed dynamic
nuclear polarization [2]. In this method, pentacene molecules are excited to the lowest
triplet state by laser irradiation [3]. A population difference appears spontaneously in
the triplet state [4, 5]. Subsequently, the population difference is transferred to proton
polarization by a cross polarization technique.
For the excitation of pentacene molecules, a flushlamp-pumped dye laser has been
used as a light source. Iinuma et al. has successfully obtained proton polarization of 32%
with the dye laser [6]. However, the dye laser is not suited for application in the nuclear
physics experiments because of the rather short lifetime of the dye. It is necessary to
change the dye once a day during the experiments to maintain the laser power. This
causes that the proton polarizing process is interrupted by the changing of the dye for a
few hours. Thus, we decided to use other type of laser, Ar-ion laser, which demands less
maintenance.
1
Present address : Center for Nuclear Study, University of Tokyo, Wako, Saitama 351-0198, Japan
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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In this paper we describe the recent progress in our polarized proton target project,
including upgrade of the laser and construction of the new target system.
PROTON POLARIZATION WITH AR-ION LASER
A prototype of proton polarizing system has been constructed and used to study the
effectiveness of Ar-ion laser as a light source for excitation of pentacene. By using this
system, we obtained proton polarization of 18.4 ±3.9% in 0.3 T at 100 K [7]. In the
experiment, a continuous wave Ar-ion laser having the maximum power of 4.2 W in
the multiline operation was used for excitation of pentacene. Since the lifetime of the
most populated sublevel, m s =0, is shorter than that of other levels, m s =±1, as shown in
Fig. 1, the laser beam was mechanically pulsed by an optical chopper to obtain a large
population difference. The pulse width and the repetition rate were 20 µ s and 1 kHz,
respectively. The resulting average power was 84 mW.
Singlet state Triplet state
100
S1
2% T1
20
20 n
100
S0
FIGURE 1. Energy levels of pentacene. The lifetime of m s =0 state is 20 µ s, while that of m s =±1 states
are 100 µ s.
In order to improve proton polarization, we have installed a high-power Ar-ion laser.
The laser has the maximum power of 25 W in multiline operation and has a standard
power specification for the wavelengths ranging from 454.5 nm to 514.5 nm. Operation
modes of the laser can easily be changed between multiline and single-line operation.
For excitation of pentacene, the laser is used in the single-line operation at wavelength of
514.5 nm. This is because pentacene molecules in the lowest triplet state have absorption
maxima at the wavelengths of 490.0 nm and 457.0 nm. The absorption will cause
relaxation of proton polarization.
Figure 2 shows a result of the proton polarization as a function of time during
the buildup process. We have succeeded in polarizing protons up to 36.8±4.2% in a
magnetic field of 0.3 T at a temperature of 100 K. The laser power was 10 W, which
corresponds to the average power of 200 mW. The crystal was naphthalene doped with
0.01% pentacene. The size of the crystal was 4 × 4× 3 mm3 . The error in the polarization
is mainly dominated by the uncertainty in the calibration measurement. This proton
polarization is comparable to that obtained with the dye laser, which shows that an
ordinary Ar-ion laser can be a reasonable light source for the optical pumping.
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FIGURE 2. Proton polarization as a function of time in 0.3 T at 100 K. The obtained polarization is
36.8±4.3% and the extrapolated maximum proton polarization is 39.3±4.6%.
NEW TARGET SYSTEM
Figure 3 shows a schematic of the newly constructed target system. The magnetic field
is produced by a C-type magnet, which can generate the maximum field of 0.7 T. The
measured inhomogeneity of magnetic field is 1.1 × 10 −3 in central 10 mmφ sphere at
0.3 T. Between the pole gap, a scattering chamber is mounted. The scattering chamber
has a double-layered structure to cool the target sample. A cooling chamber, in which
the target sample is placed, is mounted in a vacuum chamber that is connected to a beam
line. Both of the cooling and vacuum chambers have glass windows for laser irradiation
and Kapton windows in the path of the RI beam and recoil protons.
FIGURE 3. The new target system. The scattering chamber mounted between the pole gap consists
of a vacuum chamber and a cooling chamber. The target sample is placed inside a copper-film loop-gap
resonator.
In the prototype system, a cylindrical microwave cavity is used to produce oscillating
magnetic fields. The cavity is enclosed by thick walls that prevent low energy recoil
protons from reaching to detectors. Instead of the microwave cavity, a copper-film loopgap resonator (LGR) [8] is introduced to the new system so that recoil protons can reach
to detectors. Figure 4 shows a schematic view of the LGR. The LGR is made of 25-µ m
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µ
µ
FIGURE 4. Schematic view of a copper-film loop-gap resonator. The resonator is used to generate
oscillating magnetic fields.
thick Teflon sheet coated on both sides with 4.4-µ m thick copper metal. The copper is
etched to create capacitive gaps in the overlapping regions of strips. The etched sheet
is formed into a cylindrical loop which act as an inductive element. The LGR and a
microwave circuit are inductively coupled by a coupling coil. The radius of the LGR is
8 mm and the axial length is 10 mm. The resonance frequency of the LGR is 3.2 GHz,
which is the ESR frequency in the magnetic field of 85 mT. An NMR coil is wound
around the LGR. The NMR frequency is 3.6 MHz in 85 mT. A pair of coils are placed
for magnetic field sweep and it generates the magnetic field of 6 mT by applying a
current of 50 A.
Figure 5 shows a recent result of buildup of proton polarization with the new system. We have obtained an enhancement of proton polarization. To provide the absolute
polarization caliblation factor, a measurement of proton polarization in the thermal equilibrium is required. For the measurement, we are planning to improve NMR sensitivity
by installing a movable device on which the LGR is mounted. The LGR, which reduces
NMR signal intensity, will be removed from the target position, when NMR measurements are carried out.
FIGURE 5. Enhancement of proton polarization in 85 mT at 90 K. The absolute value of polarization
is not calibrated.
SUMMARY
A high-power Ar-ion laser has been installed to improve proton polarization. With the
laser, proton polarization of 36.8±4.2% has been obtained in 0.3 T at 100K. The result
shows that an Ar-ion laser can provide proton polarization comparable to that obtained
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with the dye laser.
We have also begun to construct the new target system that will be used in nuclear
physics experiments. An enhancement of proton polarization has been observed with
the LGR. An improvement of NMR sensitivity to measure the polarization in thermal
equilibrium is under way. The first experiment, the measurement of a vector analyzing
power in the elastic scattering of p-6 He at 71 MeV/A, will be carried out with the new
target system and a 6 He beam from the projectile fragment separator RIPS in 2003.
ACKNOWLEDGMENTS
The authors would like to thank Dr. I. Tanihata and Dr. T. Suda at RIKEN for their
constant support of this study. One of us (T.W.) would like to acknowledge the Special
Postdoctral Researchers Program.
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