Experimental Report
実験課題番号 Project No.
2014P0804
実験課題名 Title of experiment
Development and application of on-beam SEOP based 3He spin
filter at BL17
実験責任者名 Name of principal investigator
Takayuki OKU
所属 Affiliation
J-PARC Center

実験報告書様式（プロジェクト利用）
承認日 Date of Approval 2016/1/18
承認者 Approver Dai Yamazaki
提出日 Date of Report 2016/1/15
装置責任者 Name of Instrument scientist
Masayasu TAKEDA
装置名 Name of Instrument/（BL No.）SHARAKU / BL17
利用期間 Dates of experiments
2014/12/3-2014/12/8

１．研究成果概要（試料の名称、組成、物理的・化学的性状を明記するとともに、実験方法、利用の結果得られた主なデータ、考察、結論、図表等を記述してください。
Outline of experimental results (experimental method and results should be reported including sample
information such as composition, physical and/or chemical characteristics.
We have been developing an in-situ SEOP based polarized 3He neutron spin filter (in-situ SEOP3He NSF) as an analyzer for the polarized neutron reflectometer, SHARAKU (BL17). In 2012, we
performed demonstrative polarized neutron reflectivity measurements of Fe/Si magnetic
supermirrors using the in-situ SEOP NSF as an analyzer (Project No. 2012P0803). In 2014, a new
scattering chamber has been installed into SHARAKU. Then, we constructed a prototype of thein-situ SEOP 3He NSF for SHARAKU and set up it in the chamber and checked its performance interms of the maximum 3He polarization, the relaxation time constant of the 3He polarization andthe magnetic guide field for polarized neutron beam transportation.
Figure 1 shows the picture and schematic top view of the experimental setup at SHARAKU. We used amagnetic supermirror to polarize the incident neutron beam and a Drabkin spin flipper to change the polarizationof the neutron beam. Those devices were installed in the shield upstream of the sample position. A guide field
was applied along the neutron beam path to transfer neutrons with conserving the neutron spin state. The in-situ
SEOP 3He NSF was placed in the scattering chamber. The reflected neutrons were analyzed by using the in-situ
SEOP 3He NSF and 3He detector (Fig. 1).

Fig. 1 Picture and schematic top view of the experiment. The in-situ SEOP 3He NSF was installed in the vacuum
chamber of SHARAKU.

１．研究成果概要（つづき） Outline of experimental results (continued).

All of the in-situ SEOP 3He NSF components were put in a laser shield box made of black anodized aluminumboards for laser safety. The laser shield box has a dimension of 605 mm x 605 mm x 350 mm. The 3He cell was
installed in a solenoid coil with a double layered Permalloy magnetic shield. The size of the 3He spin filter cellwas 35 mm in diameter and 55 mm in length. The 3He-gas pressure length product of the cell was 11 bar-cm. The
magnetic shield was designed for the in-situ 3He NSF with a cell size of 100 mm in diameter and 100 mm inlength under a peripheral magnetic field of several tens Gauss. During the experiment, the scattering chamber
was open to the atmosphere.
We checked the magnetic guide field of SHARAKU for the polarized neutron beam transportation at first (Fig.
2). Two solenoid coils were placed at upstream of the sample stage and inside the scattering chamber. The
magnetic field strength of the solenoid coil was >10 G, which was enough strong to keep the polarization of theneutron beam. The direction of the guide magnetic field in the solenoid coil was parallel to the neutron beam axis
(z-axis)(Fig. 2). When the electromagnet on the sample stage was turned off, the magnet had a relict magnetic
field. The magnitude of the relict magnetic field inside the magnet was almost 50 G which was perpendicular to
the neuron axis (along the vertical direction, y-axis) (Fig. 2). We applied a magnetic field of 20 G onto the 3He
cell along the neutron beam axis. Next, we checked the connection of the magnetic guide field. It was confirmedthat the magnetic field between the guide coils and the electromagnet on the sample stage was strong enough totransport the polarized neutron beam adiabatically. Since the weak magnetic field region was found between thesolenoid coil and the in-situ SEOP 3He NSF, we installed an additional short solenoid coil in the region, which
increased the magnitude of the magnetic field from 4 G to larger than 10 G.
After the adjustment of the magnetic guide field, we started the optical pumping of the 3He gas to polarize it.
The time development of the 3He polarization was monitored by using the AFP-NMR as shown in Fig. 3. Here,the amplitude of the AFP-NMR signal is proportional to the 3He polarization degree. Thus, we can estimate the3
He polarization from the AFP-NMR signal by using the conversion parameter. In this experiment, we used aconversion parameter, which was determined for the same cell and the same pick-up coil previously. As theconversion parameter is affected by not only the cell shape and 3He gas quantity, but also the temperature,
relative position relation between the cell and the pick-up coil and etc, the 3He polarization degree, which isdetermined by the AFP-NMR, involves an ambiguity of several percent. Therefore, we determine the correct 3He
polarization degree by the neutron transmission measurements of 3He gas. To determine the correct 3He
polarization degree, we must compare the neutron transmissions of the polarized and completely unpolarized 3Hegas. We usually perform this measurements at the end of the experiment, since the data set should be taken
within a short time to measure them with condition as similar as possible for the accurate transmission
measurements.
The laser power supply for the optical pumping was stopped several times due to the laser diode temperatureerror. The SEOP box had a cooling fan to keep the temperature inside the box. However, since the temperature
within the scattering chamber, in which the SEOP box was located, rose with a time, the temperature error
occurred. We added the fan to lower the temperature of the scattering chamber. However, the system was stopped
several times even after the improvement. So we had to reduce the laser power down to the moderated level forthe stable operation.
The maximum 3He polarization P3He was determined to be 56.9% by the AFP-NMR (Fig. 3). By analyzing thetime development of the 3He polarization during the optical pumping, it is estimated that the maximum 3Hepolarization P3He reached about 62% (Fig.3).

Fig. 2 Schematic layout of the experimental setup at SHARAKU.

Fig. 3

The time development of 3He polarization measured by the AFP-NMR.

The incident neutrons were polarized with the magnetic supermirror (Fig.1) To check the polarization degree ofthe neutrons at the sample position, we put a Fe/Si magnetic supermirror at the sample position, and performed aflipping ratio measurement, where the in-situ SEOP 3He NSF was off the neutron beam axis (Fig.4). The
obtained neutron intensities with the spin flipper (SF) on and off, and the flipping ratio R are shown in Figs. 5(a)and (b).

Fig. 4

Schematic layout of the experimental setup to evaluate the neutron polarization at the sample position.

(a)(b)
Fig. 5 (a) The neutron intensities with the spin flipper on and off, and (b) the flipping ratio as a function ofneutron wavelength.

We expected that the neutron polarization degree was over 0.9 at the sample position, and the polarizationefficiency of the Fe/Si magnetic super-mirror was 0.9 or more for cold neutrons. With the expected condition, theflipping ratio should be about 20 or more. However, the obtained values of the flipping ratio were about 2 or less(Fig. 5(b)). We also observed neutron intensities oscillating as a function of the neuron wavelength with the spin
flipper on (Figs. 5(a) and (b)). These results indicated that neutrons were not well polarized at the sampleposition. The probable cause was thougt to be that 1) the polarizer did not work properly, 2) depolarization
occured before the sample position, and etc. We could not know the cause during the experiment. After theexperiment was finished, it was found out that the spin flipper did not work well due to a wrong electric wiring.
As the incident neutrons were not polarized well in the experiment, we decided to evaluate the 3He polarizationby using the unpolarized neutrons. We removed the polarizer and Fe/Si maginetic supermirror from the neutron
beam line in the setup illustrated in Fig. 1.We measured the intensities of neutrons which transmitted though the3He NSF with the polarized and unpolarized 3He gas conditions. The obtained neutron intensities are shown inFig. 6 (a). The 3He polarization degree was calculated to be 56.9 % based on the neutron transmission
measurements (Fig. 6 (b)). The result was well consistent with the value determined by the AFP-NMR.
In conclusion, we constructed a prototype of the in-situ SEOP NSF as the analyzer for SHARAKU, and set up it
within the new scattering chamber of SHARAKU. The laser for the in-situ SEOP NSF was stopped several timesdue to the laser diode temperature error, since the temperature within the scattering chamber rose up higher thanwe expected in the experiment. We must fix this problem by introducing an air circulator or something for thestable operation. It was found out that the relaxation property of the 3He polarization was good enough for thein-situ SEOP NSF. Thus, we can realize higher degree of the 3He polarization by fixing the temperatu reproblem. Next time, we plan to do a demonstrative experiment with the prototype of the in-situ SEOP NSF andproperly prepared polarized neutron devices.

(a)(b)
Fig. 6 (a) Intensities of the neutrons transmitted through the 3He cell with the polarized and depolarized 3He, and(b) 3He polarization determined by the neutron transmission measurements.

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