Satoshi Ichimaru

Characterization of the supermirror hard-x-ray telescope for the InFOCμS balloon experiment

A hard-x-ray telescope is successfully produced for balloon observations by making use of depth-graded multilayers, or so-called supermirrors, with platinum-carbon (Pt/C) layer pairs. It consists of four quadrant units assembled in an optical configuration with a diameter of 40 cm and a focal length of 8 m. Each quadrant is made of 510 pieces of coaxially and confocally aligned supermirrors that significantly enhance the sensitivity in an energy range of 20-40 keV. The configuration of the telescope is similar to the x-ray telescope onboard Astro-E, but with a longer focal length. The reflectivity of supermirrors is of the order of 40% in the energy range concerned at a grazing angle of 0.2 deg. The effective area of a fully assembled telescope is 50 cm2 at 30 keV. The angular resolution is 2.37 arc min at half-power diameter 8.0 keV. The field of view is 12.6 arc min in the hard-x-ray region, depending somewhat on x-ray energies. We discuss these characteristics, taking into account the figure errors of reflectors and their optical alignment in the telescope assembly. This hard-x-ray telescope is unanimously afforded in the International Focusing Optics Collaboration for muCrab Sensitivity balloon experiment.

Circular multilayer zone plate for high-energy x-ray nano-imaging

A circular multilayer zone plate (MZP) was fabricated and its focusing performance was evaluated using 20-keV x-rays. MoSi(2) and Si layers were alternately deposited by DC magnetron sputtering on a wire core; all the interfaces satisfied the Fresnel zone condition. The measured line spread function was converted to a point spread function by tomographic reconstruction. The results suggest that the MZP has the potential to realize the diffraction-limited resolving power, which is calculated to be 35 nm using the diffraction integral. Furthermore, scanning transmission microscopy using the MZP could resolve a 50-nm line-and-space pattern.

Optical Properties of MoSi2/Si Multilayer Laue Lens as Nanometer X-ray Focusing Device

In this study, we designed and fabricated a multilayer Laue lens (MLL) as a hard X-ray focusing device. MoSi2 and Si were chosen to form the layers by DC magnetron sputtering owing to their superior properties. The optical properties of the MLL were measured at BL24XU of SPring-8 for 20-keV X-rays. In order to confirm the effect of dynamical diffraction, far-field diffraction images were captured at various incidence angles and depths. The resultant intensity distributions showed a similar structure to those derived through calculations. An almost diffraction-limited size of 28.2 nm was obtained. The maximum local diffraction efficiency was 64.7%.

Multilayer coating for 30.4nm

A multilayer coating of Mo∕Si is usually used for space science in the spectral range of extreme ultraviolet, especially for He-II (30.4nm) radiation, because it is highly stable under vacuum and atmosphere. The fairly high reflectivity of 15%–20% is achieved. But the space science community needs a higher reflective coating at 30.4nm for the future satellite missions. In this work, we report the design of a multilayer, consisting of a pair of Mg and SiC, and its fabrication, and result of the reflectance.

Aging and thermal stability of Mg/SiC and Mg/Y2O3 reflection multilayers in the 25-35 nm region

Reflection measurements in the 25-35 nm region were made for Mg/SiC and Mg/Y2O3 multilayers kept in a low-humidity atmosphere for 4 or 5 years. Aged Mg/SiC multilayers keep their reflectances, and the reflectance value at 31.2 nm is 0.44 at 10 degrees of the normal angle of incidence. Aged Mg/Y2O3 multilayers change reflectance as top layer materials, and the best value at 30.1 nm is 0.40 at 10 degrees. Reflection measurements are also made for Mg-based multilayers that are annealed from room temperature to 400 degrees C at 50 degrees C intervals. Both multilayers keep their reflectance at annealing temperatures of 200 degrees C. These results suggest that both Mg-based multilayers can be applied to practical optics.

Development of Multilayer Laue Lenses; (1) Linear Type

A multilayer Laue lens (MLL) made from MoSi2/Si has been fabricated aiming at a sub‐10‐nm spatial resolution for hard x‐ray microscopy. A multilayer of 1000 alternating MoSi2 and Si layers with a layer thickness gradually increasing from 5 nm to 316 nm according to the Fresnel lens structure was deposited on a silicon substrate using DC magnetron sputtering, and then the substrate was diced, polished and thinned. The multilayer total thickness was about 10 μm. Optical properties were measured at the SPring‐8 beamline BL24XU using 20‐keV x‐rays. While observing changes in far‐field diffraction patterns by changing the tilt angle of the MLL, a dynamical diffraction effect of the MLL was visually confirmed. The achieved line focusing size was 13.1 nm, which is the best ever reported by using an MLL. The maximum local diffraction efficiency was measured to be 62%. Further, two MLLs were arranged in the KB configuration and applied to scanning transmission microscopy.

Development of Multilayer Laue Lenses; (2) Circular Type

A circular type multilayer Laue lens (MLL) has been designed and fabricated. A graded‐thickness multilayer was deposited on a tapered wire core by using a magnetron sputtering system. A combination of MoSi2 and Si was chosen as the multilayer materials owing to its superior properties of high diffraction efficiency and relatively sharp interfaces between layers. Optical properties of the circular type MLL were measured at BL24XU of SPring‐8 with 20‐keV x‐rays. It was confirmed that only the +first‐order diffraction was focused in the focal point owing to the wedged zone structure. Measured +first‐order diffraction efficiency of the multilayer part was as high as 52%. Applying the circular type MLL to scanning transmission microscopy, a 50‐nm line and space of a test chart was resolved.

New design concept of multilayer supermirrors for hard x-ray optics

It is important to enhance the reflectivity of multilayer supermirrors in 10-100 keV region used for hard x-ray optical systems. For this purpose design methods of multilayer supermirrors have been investigated at the grazing angle of 0.3 degrees by means of the x-ray etalon or phase matching configuration. It means that the 1st and higher order Bragg reflections emanated from different periodic lengths cooperatively enhance the reflectivity at energy bands concerned. The x-ray etalons method is useful for multi-band mirror with the band width of 5 keV or so, but becomes a bit difficult to make the energy band wider connecting gaps between isolated bands. Because heavy oscillation of reflectivity curve occurs due to adjacent destructive and constructive interference. The phase matching method is useful to get smooth reflectivity in the broad energy band and is possible to enhance the 2nd order Bragg reflection in higher energy region. We present the design of hard x-ray telescope sensitive in 25-40 keV region by means of multi-block supermirrors of Pt/C multilayers. The effective area was obtained to be more than 100 cm2.

Performance of Y2O3∕Al multilayer coatings for the He-II radiation at 30.4 nm.

We briefly report on the performance and stability of periodic multilayer mirrors containing Y(2)O(3) and Al layers designed for normal incidence reflection at the He-II emission line (30.4 nm). We found that Y(2)O(3)∕Al multilayer coatings had higher reflectivity (24.9%) at 30.4 nm and significantly lower reflectivity (1.3%) at 58.4 nm than the conventional coatings such as Mo∕Si. Furthermore, we investigated the temporal stability of the Y(2)O(3)∕Al multilayer coatings. Our sample was kept under vacuum, dry N(2) purge, and normal atmosphere for over three months, and there were no measurable changes in the reflectivity. These results suggest that we can use Y(2)O(3)∕Al multilayer coatings as standard mirrors for the He-II radiation.

Development of supermirror hard x-ray telescope and the results of first observation flight of InFOCuS flight observation

The development of hard X-ray focusing optics is widely recognized as one of key technologies for future X-ray observatory missions such as NeXT(Japan), Constellation-X(US) and possibly XEUS(Europe). We have developed hard X-ray telescope employing depth-graded multilayers, so-called supermirrors. Its benefit is to reflect hard X-rays by Bragg reflection at incidence angles larger than the critical angle of total external reflection. We are now continuously fabricating platinum-carbon(Pt/C) supermirror reflectors for hard X-ray observations. In this paper we focus on our development of the hard X-ray telescope for the first balloon flight observation (InFOCuS) and its results. InFOCuS is an international balloon-borne hard X-ray observation experiment initiated by NASA/GSFC. InFOCuS hard X-ray telescope have been jointly developed by Nagoya University and GSFC. The telescope is conical approximation of Wolter-I optics with 8m focal length and 40cm diameter. It consists of 255 nested ultra-thin reflector pairs with incidence angles of 0.10 to 0.36deg. Reflectors are coated with Pt/C supermirrors with periodic length of 2.9 to 10nm and bi-layer number of 25 to 60, depending on incidence angles. The effective area and imaging quality are expected as 100 cm2 at 30 keV and 2 arcmin in half power diameter, respectively. The InFOCuS experiment was launched on July 5, 2001, from National Scientific Balloon Facility in Texas, USA. We successfully observed Cyg X-1, chosen for a calibration target, in 20-40keV energy band. We are planning to carry out next flight for scientific observations as soon as additional telescopes, detectors, and upgraded gondola system are implemented.