Toshihiro Iwase

Studies of the moisture absorption of thin carbon fiber reinforced plastic substrates for x-ray mirrors

Abstract. We study a lightweight x-ray mirror with a carbon fiber reinforced plastic (CFRP) substrate for next-generation x-ray satellites. For tightly nested x-ray mirrors, such as those on the Suzaku and ASTRO-H telescopes, CFRP is the suitable substrate material because it has a higher strength-to-weight ratio and forming flexibility than those of metals. In flat CFRP substrate fabrication, the surface waviness has a root mean square (RMS) of ∼1  μm in the best products. The RMS approximately reaches a value consistent with the RMS of the mold used for the forming. We study the effect of moisture absorption using accelerated aging tests in three environments. The diffusivity of the CFRP substrate at 60°C and at relative humidity of 100% is ∼9.7×10−4  mm2·h−1, and the acceleration rate to the laboratory environment was 180 times higher. We also develop co-curing functional sheets with low water-vapor transmissivity on the CFRP substrate. Co-curing the sheets successfully reduced the moisture absorption rate by 440 times compared to the un-co-cured substrate. Details of the CFRP substrate fabrication and moisture absorption tests are also reported.

Recent progress in the ground calibration of the ASTRO-H Hard X-ray telescope (HXT-2)

The 6th Japanese X-ray satellite, ASTRO-H, equips two Hard X-ray Telescopes (HXTs) to perform imaging spectroscopy up to 70 keV. The 2nd flight module (HXT-2) had been completed in July, 2013. After some environmental tests were passed, the X-ray performance of the HXT-2 was measured at the SPring-8 BL20B2, 3rd generation synchrotron facility. The angular resolution defined with a Half Power Diameter (HPD) was 1:′9 at 30 keV and 1:′8 at 50 keV. This small energy dependence is considered to be caused by the difference in image quality of each foil; the inner mirror shells have better quality than outer ones. The effective area was found to be 178 cm2 at 30 keV and 82 cm2 at 50 keV, both of which exceed the requirement. Furthermore, the detailed energy dependence of the effective area was examined for a limited aperture in the 30{70 keV band with a pitch of 1 keV. We also measured the off-axis dependence of the effective area at 50 keV, and then determined the optical axis. The field of view of the HXT-2 was found to be 5:′6 (FWHM of the vignetting function), consistent with the simulation. In this paper, we also report the detailed analysis of the ground calibration data, which will be used for image reconstruction by a ray-tracing simulator.

Studies of print-through and reflectivity of x-ray mirrors using thin carbon-fiber-reinforced plastic

Abstract. We fabricated x-ray mirrors from carbon-fiber-reinforced plastic with a tightly nested design for x-ray satellites, using a replication method for the surfaces. We studied the effects of print-through on the mirror surface as a function of curing temperature. With room temperature curing, the root-mean-square value of the surface error was 0.8 nm. The reflectivity was measured using 8-keV x-rays, and the roughness was calculated as 0.5 nm by model fitting—comparable to that of the ASTRO-H/HXT mirror. We verified the long-term stability of the mirror surface over 6 months. We fabricated Wolter type-I quadrant-shell mirrors with a diameter of 200 mm and performed x-ray measurements at BL20B2 in the SPring-8 synchrotron radiation facility. We obtained reflection images of the mirrors using a 20-keV x-ray spot beam with a slit size of 10×1  mm in the radial and circumferential directions, respectively. The averaged half-power diameter (HPD) of the images in one mirror was 1.2 arc min in the circumferential center of the mirror and 3.0 arc min at the edge. In the spot images with a smaller slit size of 10×0.2  mm, we achieved an HPD of 0.38 arc min in the best case.

Studies of lightweight x-ray telescope with CFRP

We studies lightweight X-ray mirror with Carbon Fiber Reinforced Plastic (CFRP) substrate for next generation X-ray satellites. CFRP is suitable material as substrate for thin-foil highly nested X-ray mirrors like telescope of Suzaku, ASTRO-H since it has properties of higher strength-to-weight ratio and flexibility of forming than that of metals. In the current year we made flat panels for basic research and full/partial shell substrates by quasi-isotropic laminate with 8 ply prepregs, and performed reflector replication based on technique for the HXT mirror.

First result from a ground calibration of the Hard X-ray Telescope (HXT) onboard ASTRO-H satellite

We report a first result from a ground-based X-ray calibration of the ASTRO-H Hard X-ray Telescope (HXT) at a synchrotron radiation facility SPring-8. ASTRO-H, to be launched in 2015, is Japan’s sixth X-ray satellite mission following to Suzaku satellite. One of the features of ASTRO-H is a simultaneous observation between 0.3 keV to 600 keV with several instruments. ASTRO-H will carry two HXTs to cover hard x-rays up to 80 keV. HXT, which is one of the key instruments in ASTRO-H, is the conically approximated Wolter-I grazing incidence optics similar to the Suzaku X-ray telescope. Reflector surfaces are coated with depth-graded Platinum and Carbon multilayer to reflect hard X-rays efficiently. The integrations of the flight optics of HXT-1 and HXT- 2 were completed, and we performed a ground calibration of HXT-1 at a synchrotron facility, SPring-8 beamline BL20B2 to build a response function of HXT. We use a raster scan method with a pencil beam at the baseline length of 215m. A point spread function and effective area were measured at 30, 40, 50, 60, 70keV. From a preliminary analysis of the data, an angular resolution of 1.5 - 1.9 arcmin. was obtained at five energy band in the full telescope. The effective area is 170 cm2 at 30 keV and 82 cm2 at 50 keV, respectively. The effective area at 30 and 50 keV are about 13 % and 50 % larger than expected, respectively. We also measured the stray light from outside of field of view at 12’ and 20’ of-axis angle. We confirmed the effectiveness of pre-collimator to reduce the stray lights.

Development of an x-ray telescope using the carbon fiber reinforced plastic (CFRP)

We are developing an X-ray mirror using the carbon fiber reinforced plastic (CFRP) as a substrate in order to improve the angular resolution of tightly-nested thin-foil Wolter-I X-ray mirrors. We found that curing of the epoxy used in the replication process at the room temperature is effective to suppress the print through. We were able to make mirrors whose shape accuracy is 3 - 5 μm. Characterization at the synchrotron facility SPring-8 using the X-ray pencil beam of 20 keV showed that the angular resolution was 3 - 5 arcmin as a whole, but can reach to 20 arcsec locally.