1. Slow Neutron Physics and Neutron Scattering

概要

INTRODUCTION: Magnetic multilayer mirror con- sisting of ferromagnetic layers and nonmagnetic layers is useful to polarize neutron beam. As a polarizing neutron device, larger saturation magnetization of the ferromag- netic layers with lower applied magnetic field is very important. In this sense, pure iron (Fe) is good material. Fe-Co alloy, known as Permendur, is very good material and it had used for many neutron research establishments. Recently brightness of neutron beam has been really in- creased, and magnetic multilayer contained cobalt is not suitable due to the activation of the element for modern intense neutron facility. The radioactivation of Fe due to neutron irradiation is less serious than the other ferro- magnetic materials. Silicon (Si), germanium (Ge) and these compounds are generally used for the nonmagnetic layers because the neutron scattering length density (SLD) of Fe for down spin neutrons. We also have al- ready succeeded in fabricating m~5 Fe/SiGe polarizing neutron supermirrors with high reflectivity using an ion beam sputtering instrument (KUR-IBS) installed to the Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS) [1,2]. These polarizing su- permirrors have been used as polarizer and analyzer at neutron resonance spin echo spectrometer at BL06 in Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC). The SiGe layer is obtained by simultaneous deposition of Si and Ge target. The composition ratio of Si and Ge is possible to be controlled by changing geometrical con- figuration of Si and Ge target. Recently detailed investi- gation of Fe/Si and Fe/Ge multilayer fabricated by ion beam sputtering was reported [3]. The fine-tuning of scattering length density of nonmagnetic material is still very important for high performance polarizing super- mirror. With use of the KUR-IBS, SiGe layer tends to be slightly rough comparing with Si and SiC layers by using X-ray reflectometer. Carbon (C) is very good element because they have low incoherent and absorption cross section with less radioactivation. The composition ratio of Si and C is also possible to be controlled by changing geometrical configuration of silicon and carbon target.

EXPERIMENTS:
Figure 1 shows measured hysteresis loops for the Fe/SiC(Si) polarizing supermirror with typical two sam- ple geometry, A and B. After the measurement of A, the sample was rotated by 90 degrees and the measurement of B was conducted. The sample size is about 8 mm in square and the magnetization measurement was con- ducted with vibrating sample magnetometer of the KURNS. It indicates that there is no in-plane anisotropy and applied magnetic field of 30 mT is possible to satu-rate magnetically the polarizing supermirror.

RESULTS: Figure 2 shows up and down spin neutron reflectivities by the Fe/SiC(Si) polarizing supermirror under an external field of 60 mT. The reflectivity with high-m area (m>3) was relatively high although the flip- ping ratio was not so high. The reflectivity of up spin neutrons was rapidly reduced around low-m area (1

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