Performance of YB 66 Double-crystal Monochromator for Dispersing Synchrotron Radiation The crystal growth technique of YB 66 was developed originally by Tanaka and Kamimura’s group of the National Institute for Materials Science (NIMS) [1]. YB 66 has a lattice plane (4 0 0) that is large (0.586 nm) and suitable for dispersing soft X-ray [2], which cannot be covered easily by grating monochromators. Synchrotron radiation technology has developed rapidly and has been introduced as a third-generation light source for the past ten years. Although the problem of a high heat load from third- generation light sources has not yet been overcome in the dispersion by a YB 66 crystal, a great demand for executing an experiment using a high-resolution and high-brilliance soft X-ray light source still remains. In addition, one of the important specifications of beamline BL15XU , which has been developed by NIMS, is the capability of obtaining monochromatic light of optional wavelength on a sample between 0.5 keV and 60 keV. YB 66 is known to have an energy resolution ( ∆ E/E) of about 5 10 -4 and is more resistant to synchrotron radiation damages than other crystals (Beryl, Quartz, InSb) [2] applicable in the same energy region. Thus, the beamline monochromator with YB 66 is the most suitable for BL15XU [3] at SPring-8, which is a third-generation light source in the 1 - 2 keV range and the first of its kind in the world. The YB 66 crystal (10 mm H 20 mm V 1 mm T ) used in this study was a commercial product (Crystal Systems Inc.). The crystal surface which is parallel to a diffracting plane was polished by a lapping machine and by hand. An indirect cooling system using a Ni- coated holder made of Cu was used as shown in Figs. 1(a) and 1(b) [4]. The YB 66 crystal was set on the holder using liquid InGa after pretreating of the contact surface, ultrasonic washing for 15 min with acetone and preheating for 2 hours at 110 C. The soft X-ray used in this study was generated by a helical undulator. This type of undulator has the characteristics that higher harmonics distribute mostly outside an axis of X-ray, and thus, the heat load of the crystal due to higher order lights can be reduced to some extent. In this study, the characteristics of synchrotron radiation dispersion from the helical undulator operating at an electron energy of 8 GeV and an injection current of 100 mA were studied. The schematic drawing of the experimental configuration is shown in Fig. 2. For the estimation of the source condition which dominated the heat load at the crystal surface, thermal analysis by the finite element method was performed. R ocking curves were measured by rotating the ∆θ axis of the first crystal, and the photon flux passing through this monochromator was measured by an Si PIN photodiode. The most suitable energy region for the YB 66 crystal was the 1 - 2 keV range. It is also possible, but not easy, to use this crystal up to 3 keV or higher, because there are absorption edges of Y ( L 1 -edge 23 7 3 eV, L 2 -edge 2156 eV and L 3 -edge 2080 eV). From the results of measurement, the F W HM of the 4 0 0 reflection is plotted as a function of photon energy ranging from 0.15 eV at 1.1 keV to 0.42 eV at 2.1 keV as shown in Fig. 3. The F W HM of the YB 66 crystal obtained is superior to that obtained at SSRL [5]. The footprint on the crystal at SSRL, 1.5 mm H × 15 mm V [5], was significantly larger than that obtained by us, 0.0 7 mm H × 1 mm V . The effective divergence of our beam originating from the helical undulator being smaller than that of SSRL is the main reason that our F W HM value was superior to that obtained at SSRL. SR YB 66 No.1 Cu holder coated by Ni Thermo-couple area Water flow pipe Water flow pipe SR Spring board YB 66 (b) (a) Fig. 1. (a) Installation of the YB 66 c r y s t a l o n t h e h o l d e r o f t h e monochromator in BL15XU. First, YB 66 crystal is set on the Ni - coated Cu holder. InGa is placed in the interface between the YB 66 and Ni-coated Cu holder. A thermo-couple is attached to the front surface of YB 66 . Ni-coated Cu holder is cooled by water. (b) Top view of the YB 66 crystal. The size of YB 66 crystal is 10 mm H 20 mm V 1 mm T . The front surface near the edge was pressed down by two points using the spring board of the holder. 115 Masaru Kitamura a and Sei Fukushima b (a) Toshiba Co., Manufacturing Engineering Center (b) Harima Office, National Institute for Materials Science E-mail: himajin @spring8.or.jp References [1] A.G. Slack et al. : P h y s. R e v . B 4 (1 97 1) 1 7 1 4 . [ 2 ] T. Tanaka et al. : J . Cr y st. Gro w th 73 (1 9 8 5 ) 3 1. [ 3 ] M. Kitamura, H. Y oshika w a, T. Mochi z uki, A.M. V laicu, A. Nisa w a, N. Y agi, M. Okui, M. Kimura, T. Tanaka and S. Fukushima: Nucl. Instrum. Meth. P h y s. R es. A 497 ( 2003 ) 550 . [ 4 ] M. Kitamura et al. : S P ring-8 U ser E x perimental R eport No. 7 ( 200 1A), J AS R I, C 0 1A1 5XU - 2000 N, p. 236 . [ 5 ] J . W ong et al. : J . S y nchrotron R ad. 6 (1 999 ) 1 0 8 6 . Fig. 3. Energy resolution obtained from the FWHM of the rocking curve of the YB 66 4 0 0 reflection as a function of photon energy ranging from 0.15 eV at 1.1 keV to 0.42 eV at 2.1 keV. Storage Ring 8 GeV, 100 mA Water cooled graphite filter (0 ~ 300 μ m) wall Double crystal monochromator (Tanden type: YB 66 mode) Be Filter (0 ~150 μ m) Water cooled movable Be window (25 μ m) Optical Hutch Exp. Hutch 1 Au mesh I o monitor Si PINPD Undulator (Helical mode) 30 m wall 29 m 16 m 6.9 m SPring-8 13 m YB 66 No.2 θ wall ∆θ A 1400 mm 1500 mm Front End Front End slit YB 66 No.1 Sample position (experimental position) The photon flu x passing through the monochromator of BL 1 5XU measured at the sample position at a stored current of 1 00 mA b y a Si P IN photodiode under v acuum is sho w n in Fig. 4 . The photon flu x ranged from 6 1 0 8 photons / sec / 1 00 mA at 1.1 ke V to Fig. 2. Outline of the experimental configuration to the first experimental hutch of beamline BL15XU at SPring-8. A tandem type monochromator consists of two monochromators which can select the analyzed crystal from Si, YB 66 , and multilayer. In the 1 - 2 keV range, we adopted the YB 66 crystal for dispersing synchrotron radiation. F i g . 4 . P h o t o n f l u x p a s s i n g t h r o u g h t h e monochromator at the ring current of 100 mA. It was measured at an experimental position by a Si PIN photodetector as a function of photon energy for the front end slit size and filter conditions as parameters. Photon Energy (eV) FWHM (eV) 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Condition of front end slit aperture and filter 1000 ~ 1500 eV: FE 0.06 mm × 0.4 mm, Be 25 μ m 1500 ~ 2000 eV: FE 0.05 mm × 0.4 mm, Be 25 μ m 2000 ~ 3000 eV: FE 0.05 mm × 0.4 mm, Be 25 μ m, Be 150 μ m 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Photon Energy (eV) Photon Flux (photons /s / 100 mA) FE 0.06 mm × 0.4 mm, Be 25 μ m FE 0.05 mm × 0.4 mm, Be 25 μ m FE 0.05 mm × 0.4 mm, Be 25 μ m, Be 150 μ m 10 8 10 9 10 10 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 5 1 0 9 photons / sec / 1 00 mA at 2 ke V , almost the same as that obtained at SS RL [ 5 ]. E v en w e attenuated the thermal load from the undulator, a photon flu x as high as 1 0 9 photons / sec / 1 00 mA measured at the sample position w as obtained. 116
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