Multilayer Supermirrors Research Articles

Morphology control in Ni/Ti multilayer neutron mirrors by ion-assisted interface engineering and B4C incorporation

The optical contrast and minimum layer thickness of Ni/Ti broadband neutron multilayer supermirrors is usually hampered by an interface width, typically 0.7 nm, caused by nanocrystallites, interdiffusion, and/or intermixing. We explore the elimination of nanocrystallites in combination with interface smoothening by modulation of ion assistance during magnetron sputter deposition of 0.8 to 6.4 nm thick Ni and Ti layers. The amorphization is achieved through incorporation of natural B4C where B and C preferably bond to Ti. A two-stage substrate bias was applied to each layer; -30 V for the initial 1 nm followed by -100 V for the remaining part, generating multilayer mirrors with interface widths of 0.40-0.45 nm. The results predict that high performance supermirrors with m-values as high as 10 are feasible by using 11B isotope-enriched B4C combined with temporal control of the ion assistance.

Development of a Broadband Double Monochromator based on Multilayer Supermirrors for Hard X-Ray Spectroscopy on High-Intensity Beams

Development of a Broadband Double Monochromator based on Multilayer Supermirrors for Hard X-Ray Spectroscopy on High-Intensity Beams

Design and development of an in-line sputtering system and process development of thin film multilayer neutron supermirrors.

Neutron supermirrors and supermirror polarizers are thin film multilayer based devices which are used for reflecting and polarizing neutrons in various neutron based experiments. In the present communication, the in-house development of a 9 m long in-line dc sputtering system has been described which is suitable for deposition of neutron supermirrors on large size (1500 mm × 150 mm) substrates and in large numbers. The optimisation process of deposition of Co and Ti thin film, Co/Ti periodic multilayers, and a-periodic supermirrors have also been described. The system has been used to deposit thin film multilayer supermirror polarizers which show high reflectivity up to a reasonably large critical wavevector transfer of ∼0.06 Å(-1) (corresponding to m = 2.5, i.e., 2.5 times critical wavevector transfer of natural Ni). The computer code for designing these supermirrors has also been developed in-house.

Manufacturing Process Evolution Method of Neutron Ellipsoidal Mirror Simulation Using Measured Point-Set

Small angle neutron scattering (SANS) analysis using ellipsoidal neutron mirrors are highly demanded for basic researches of high tensile strength steel sheets, proteins and polymer materials. Although the mirror was conventionally assembled from multilayer neutron super mirrors deposited on ground and polished glass segments, it was difficult to align segmented mirrors with high accuracy and it took several months for the grinding and polishing process. An optimized manufacturing process is proposed utilizing optical function evaluation based on measured mirror surface profile. Optical aberration caused by surface form error and misalignment is evaluated by ray-tracing using mirror surface profile measured by non-contact high precision profilometer.

Analysis and design of multilayer structures for neutron monochromators and supermirrors

A relatively simple and accurate analytical model for studying the reflectivity of neutron multilayer monochromators and supermirrors is proposed. Design conditions that must be fulfilled in order to reach the maximum reflectivity are considered. The question of the narrowest bandwidth of a monochromator is discussed and the number of layers required to build such a monochromator is derived. Finally, we propose a new and efficient algorithm for synthesis of a supermirror with specified parameters and discuss some inherent restrictions on an attainable reflectivity.

Characterization of a Hard X-ray Telescope at Synchrotron Facility SPring-8

Space-borne astronomical instruments require extensive characterization on the ground before launch. In the hard X-ray region however, it is difficult for a laboratory-based beamline using a conventional X-ray source to provide a capability sufficient for pre-flight high-precision calibration. In this paper, we describe an experiment to characterize a hard X-ray telescope at a synchrotron facility, mainly on the basis of experimental setup and examples of measured results. We have developed hard X-ray telescopes consisting of Wolter-I grazing incidence optics and platinum-carbon multilayer supermirror coatings. The telescopes have been characterized at the synchrotron facility SPring-8 beamline BL20B2. The measurements at BL20B2 have great advantages such as extremely high flux, large-sized and less-divergent beam, and monochromatic beam covering the entire hard X-ray region from 8 to over 100 keV. The telescope was illuminated by monochromatic hard X-rays, and the focused image was measured by high resolution hard X-ray imagers. The entire telescope aperture was mapped by a small beam, and the effective area and the point spread function were obtained as well as local optical properties for further diagnostics of the characteristics of the telescope.

Hard X-ray imaging mission NeXT

One of the major fields, which should be explored following Chandra and XMM-Newton is the hard X-ray sky up to several tens of keV with certain imaging capabilities. A new X-ray telescope mission or Non-thermal Energy eXploration Telescope (NeXT) in Japan is the first satellite mission proposed for a hard X-ray telescope using multilayer supermirror. The hybrid X-ray imager will be placed on the focal plane to observe hard X-ray images with CdTe pixel detector underneath a thin CCD for soft X-ray imaging. For high-resolution spectroscopy, TES type calorimeters are considered without cryogen. Non-imaging hard X-ray and soft gamma-ray detector is studied to achieve unprecedented sensitivity. NeXT will be a sort of pathfinder of hard X-ray astronomy not only technically but also scientifically.

Design and Fabrication of Ni/Ti Multilayer for NeutronSupermirror

In the applications of neutron guides and focusing devices, by usingthe Ni/Ti multilayer supermirrors (SM), the neutron flux issignificantly enhanced, because the critical reflective angle ofsupermirrors increases m times compared to the one of naturalbulk Ni. We design and fabricate the Ni/Ti multilayer supermirrors byconsidering the effect of the interfacial imperfection, such asinterface roughness and diffusion, and by using the direct currentmagnetron sputtering technology. The reflective performances of thesesupermirrors are measured on a V14 neutron beam line at the BerlinNeutron Scattering Centre (BENSC), Germany. The measurement datasuggest that the critical angles of the supermirrors are 1.5 and 2.2times that of bulk Ni, respectively.

Performance of back supportless CCDs for the NeXT mission

New X-ray Telescope (NeXT) will be the next Japanese X-ray astronomical satellite, which will be launched around 2010. A multilayer super mirror will give NeXT a large effective area across the 0.1–80 keV band. This wide bandpass requires a focal plane detector that is also sensitive along the entire energy band. As the focal plane detector for NeXT, we have been developing a Wideband hybrid X-ray Imager (WXI) consisting of X-ray CCDs and pixelized CdTe detector. The X-ray CCD of the WXI is required to maintain high quantum efficiency up to the high-energy band while allowing hard X-rays to pass undetected through the depletion (sensitive) layer. In order to meet these requirements, we have been developing a back supportless CCD (BS-CCD) which has a thick depletion layer, a thinned Si wafer and a back supportless structure. As the first step, we manufactured a test model of BS-CCD in order to (1) learn the handling and thinning process and (2) confirm that there is no change of the performance. For the mechanical strength and the safe handling of the wafer, we decided to thin the wafer to ∼ 200 μ m . We verified the thickness of the depletion layer and wafer of the BS-CCD to be about 70 and 190 μ m , respectively. The energy resolution at 5.9 keV of 144 eV and the read-out noise of 7 electrons (rms) are equal to those of an un-thinned CCD, hence, we confirmed that our thinning process has no effect on the performance.

Development of Pt/C multilayer supermirrors for hard X-ray optics

Pt/C multilayer supermirrors with the periodic length of 29– 130 A and number of layer pairs of 25–65 were fabricated and evaluated to get higher reflectivity in the hard X-ray range of 20– 40 keV by making use of the Bragg reflection. They were assembled in a grazing incidence X-ray telescope with the diameter of 40 cm and focal length of 8 m using 2040 pieces in total. The effective area and angular resolution were obtained to be 50 cm 2 at 30 keV and 2.4 arcmin ( HPD) , respectively. The performance of this hard X-ray telescope was confirmed by a balloon-borne observation of a celestial object.

Development of Pt/C multilayer supermirrors for hard X-ray optics

Pt/C multilayer supermirrors with the periodic length of 29– 130 A and number of layer pairs of 25–65 were fabricated and evaluated to get higher reflectivity in the hard X-ray range of 20– 40 keV by making use of the Bragg reflection. They were assembled in a grazing incidence X-ray telescope with the diameter of 40 cm and focal length of 8 m using 2040 pieces in total. The effective area and angular resolution were obtained to be 50 cm 2 at 30 keV and 2.4 arcmin ( HPD ) , respectively. The performance of this hard X-ray telescope was confirmed by a balloon-borne observation of a celestial object.

Towards 0.1-mm spatial resolution in neutron diffractometry

A design goal for VULCAN, the SNS engineering neutron diffractometer, is to enable spatial mapping with 0.1-mm resolution. Because the targeted applications often involve the use of large samples or special environments, slits cannot be used for this purpose. In this paper, methods to achieve 0.1-mm spatial resolution are outlined. For the incident beam, a new compact focusing device is proposed. The device is made of a stack of bent silicon wafers, each having a reflective multilayer (supermirror) deposited on one side and a neutron-absorbing layer on the other side. The optimal design to minimize the optical spatial aberrations is discussed and Monte Carlo simulation results are presented. For the diffracted beam, imaging devices made from thick packets of diffracting bent silicon wafers (known as Bragg mirrors) could be used. The requirements to achieve sharp imaging together with a large phase-space acceptance window are discussed and preliminary testing results are presented.

A hard X-ray telescope with multilayered supermirrors for balloon observations of cosmic X-ray sources

We have been developing a hard X-ray telescope utilizing multilayer supermirrors. The international focusing optics collaboration for μ-Crab sensitivity (InFOCμS) project is for balloon observations of cosmic hard X-ray sky with this new type of telescope. For the first flight scheduled in the next summer, we are preparing one full telescope with outermost diameter of 40 cm, focal length of 8 m, and energy band witgh of 20 – 40 keV as a first step. At the focal plane a CdZnTe imaging detector will be prepared. In this paper, the science objectives of this project, current status of the development of gondola, X-ray telescope and hard x-ray detector, and expected performance are described.

X-Ray Engineering and Its Application. Present Status and Future Prospects of Hard X-Ray Imaging Optical Systems.

The total external reflection of a single-layer mirror is no longer useful for grazing incidence optics in the hard x-ray region (10-100keV), whereas the Bragg reflection of multilayers is more advantageous in this region. Recently, a hard x-ray telescope was tested for practical use in the energy band 24-36 keV by making use of multilayer supermirrors. The focused image was measured by a newly developed position-sensitive CdZnTe solid-state detector. Further development of hard x-ray optical systems is proceeding in orderto extend the energy region up to 100keV and to improve the image resolution. These developments may open upnew research fields in the hard x-ray region.

Present Status and Future Prospect of Hard X-ray Imaging Optical Systems

The total external reflection of a single-layer mirror is no longer useful for grazing incidence optics in the hard x-ray region (10-100keV), whereas the Bragg reflection of multilayers is more advantageous in this region. Recently, a hard x-ray telescope was tested for practical use in the energy band 24-36 keV by making use of multilayer supermirrors. The focused image was measured by a newly developed position-sensitive CdZnTe solid-state detector. Further development of hard x-ray optical systems is proceeding in orderto extend the energy region up to 100keV and to improve the image resolution. These developments may open upnew research fields in the hard x-ray region.

Fabrication and characterization of multilayer supermirrors for hard X-ray optics.

Multilayer supermirrors stacked with three sets of Pt/C combinations have been fabricated on a flat float-glass and conical replica foil mirror using a magnetron DC sputtering system, and applied to X-ray optical systems in the hard X-ray region. The design of the supermirror is optimized to obtain the highest integrated reflectivity in the energy band and at the grazing angle concerned. X-ray reflectivities of 30% in the 25-35 keV band at an incidence angle of 0.3 degrees were obtained.

Multilayer X-Ray Optics for Future Missions

Multilayers have a great potentiality to improve the image quality, spectral resolution and energy coverage of x-ray optical systems. The angular resolution of a normal incidence telescope aims at approaching the diffraction limit in the soft x-ray region. Multilayer supermirror makes it possible to fabricate a grazing incidence telescope with high sensitivity in hard x-ray region. Multilayer coated gratings are also useful dispersive elements with high efficiency and spectral resolution in the 2-10keV region. The application of multilayers is expected to open up a new field in astronomical imaging and spectroscopic observations which are not accessible by present telescopes.

Third-generation synchrotron radiation X-ray optics

In addition to these main research and development activities, a number of other developments have been followed up that cannot be described here in detail. It is, however, important to mention the very precise and stable mechanical support structures, in particular the bending devices for mirrors, multilayers and single crystals that are used on many beamlines for meridional and sagittal focusing. As an example, the performance tests of a monolithic bender based on flexure hinges showed a residual slope error of 1.1 μrad when focusing an X-ray beam by a multilayer supermirror coated on a 10 mm thick silicon substrate. A novel bimorph bending system for dynamical focusing by mirrors, single crystals and multilayers, based on piezoelectric materials, yielded a focal spot size of 11 μm. Focusing methods for high-energy X-rays have been developed, for instance for the medical beamline, and here typical focal spot sizes are in the range 5–150 μm, depending on magnification and other parameters.If smaller focal sizes are needed, BFO (Bragg–Fresnel optics) is mainly used, and tapered glass capillaries are also being developed for this purpose. Both single-crystal and multilayer-based BFO have been studied extensively for many years. They consist of micrometer-scale Fresnel structures machined into Bragg reflectors. Micron spot sizes in one or two directions can currently be obtained with both techniques. So-called Kirkpatrick–Baez systems deliver almost identical spatial resolution.By analogy to what is known from visible-light optics, half-wave or quarter-wave plates can also be made for X-rays using diamond, silicon or beryllium crystals. They permit the production and analysis of the polarization state of X-rays. This has recently been applied to magnetic diffraction and circular magnetic X-ray dichroism. Polarization efficiencies in excess of 95% can be achieved and it is expected that many beamlines will be equipped with such phase-shifting optics.

RITA: The reinvented triple axis spectrometer

Since the pioneering work of Brockhouse on inelastic neutron scattering the triple-axis spectrometer has been the instrument of choice for studies of dynamics, both magnetic and structural, in single crystals. In the years since the late fifties improvements in source brightness, beam optics, and detector technology have resulted in increased sensitivity and capabilities of the basic triple-axis design. Work has begun at Risø National Laboratory on a new spectrometer design, RITA, the reinvented triple axis, which attempts to incorporate more recent innovations such as multilayer supermirrors and microstrip proportional counters into a rethinking of the triple-axis spectrometer. By optimizing the beam optics using supermirrors and extending the analyser to map regions of (Q, ω) space using an array of independently controllable pyrolytic graphite crystals focussed on an area detector, we hope to increase the efficiency of single-crystal inelastic experiments by about a factor of 20.

X-ray supermirrors for BESSY II

X-ray multilayer supermirrors for the energy range up to 20 keV have been theoretically studied and experimentally measured with synchrotron radiation. A multilayer mirror with 50 W/Si bilayers with different thicknesses on the Si substrate has a smooth reflectivity of up to 32% in the whole energy range from 5 to 22 keV at a grazing incidence angle of 0.32° which is considerably larger than using total external reflection.