Ricardo J. Bruni

Development of a Prototype Nickel Optic for the Constellation-X Hard-X-Ray Telescope

The Constellation-X mission planned for launch in 2015-2020 timeframe, will feature an array of Hard X-ray telescopes (HXT) with a total collecting area greater than 1500 cm at 40 keV. Two technologies are being investigated for the optics of these telescopes, one of which is multilayer-coated Electroformed-Nickel-Replicated (ENR) shells. The attraction of the ENR process is that the resulting full-shell optics are inherently stable and offer the prospect of better angular resolution which results in lower background and higher instrument sensitivity. We are building a prototype HXT mirror module using an ENR process to fabricate the individual shells.This prototype consists of 5 shells with diameters ranging from 15 cm to 28 cm with a length of 42.6 cm. The innermost of these will be coated with iridium, while the remainder will be coated with graded d-spaced W/Si multilayers. The assembly structure has been completed and last year we reported on full beam illumination results from the first test shell mounted in this structure. We have now fabricated and coated two (15 cm and 23 cm diameter) 100 micron thick shells which have been aligned and mounted. This paper presents the results of full beam illumination X-ray tests, taken at MPE-Panter. The HEW of the individual shells will be discussed, in addition to results from the full two shell optic test.

Hard x-ray multilayers: a study of different material systems

Multilayer structures with depth-graded spacing can show a high reflectivity in a broad energy passband for hard X-rays if the interface roughness/diffuseness is controlled and minimized. We present a study of several multilayer systems deposited by DC magnetron sputtering on <111> silicon wafers and superpolished fused silica substrates. The material combinations discussed are W/Si, WSi2/Si, W/C, Pt/C, Ni/C, Ni/B4C, and Mo/Si. The deposition method used was DC magnetron sputtering at low argon pressures (1.5 to 5 mT). The characterization methods used were: Atomic Force Microscopy in tapping mode, stylus profilometry, Rutherford backscattering, cross sectional TEM, and specular X-ray reflectivity (XRR) scans at 8.05 keV. Different process parameters were varied in order to optimize the interface roughness/diffuseness (sigma) that was measured by XRR scans.

Development of light weight replicated x-ray optics, II

NASA’S future X-ray astronomy missions will require X-ray optics that have large effective area while remaining lightweight, and cost effective. Some X-ray missions, such as XMM-Newton[1] , and the upcoming Spectrum-Röntgen- Gamma[2] mission use an electroformed nickel replication (ENR) process[3] to fabricate the nested grazing incidence X-ray telescope mirror shells for an array of moderate resolution, moderate effective area telescopes. We are developing a process to fabricate metal-ceramic replicated optics which will be lighter weight than current nickel replicated technology. Our technology development takes full advantage of the replication technique by fabricating large diameter mirrors with thin cross sections allowing maximum nesting and increase in collecting area. This will lead to future cost effective missions with large effective area and lightweight optics with good angular resolution. Recent results on fabrication and testing of these optics is presented.

From X-Ray Telescopes to Neutron Focusing

In the case of neutrons the refractive index is slightly less than unity for most elements and their isotopes [1]. Consequently, thermal and cold neutrons can be reflected from smooth surfaces at grazing-incidence angles. Hence, the optical technologies developed for x-ray astronomy can be applied for neutron focusing. The focusing capabilities of grazing incidence neutron imaging optics have been successfully demonstrated using nickel mirrors. The mirrors were fabricated using an electroformed nickel replication process at Marshall Space Flight Center. Results of the neutron optics experiments and current status of the multilayer coating replication technique development are presented.

New developments in light material overcoating for soft x-ray reflectivity enhancement

X-ray Wolter focusing telescopes concentrate the light by means of reflection on smooth surfaces at small grazing angle (below a couple of degrees). The traditional coatings for these kind of applications are heavy materials that, due to their high density, present a high critical energy for total reflection. Recent works have shown how a thin layer of a light material, like carbon, on top of a traditional reflecting coating, can enhance the reflectivity in soft x-ray spectral region (below 5 keV), without degrading the performances for higher energies. We presented at SPIE 2007 some experimental results about the reflectivity measurement at very low energies (200 eV) and rather large angles (1-2 deg). In the present work we extend the former study, by the realization of a new set of samples with coatings made of different materials (Pt, Au, W, Ir) and the measurement of their reflectivity for the typical angles (< 1°) and energies (1-10 keV) employed in astronomical grazing incidence telescopes.

Application of multilayer coatings to replicated substrates

We are engaged in a program to develop focusing hard x-ray telescopes in a double conical or Wolter 1 geometry that function up to 100 keV by employing small graze angles and multilayer coatings. Directly polished substrates are not an option because they are too thick to be nested efficiently. The only alternative is to fabricate the very thin substrates by replication. Our objective is the production of integral cylindrical substrates because they should result in better angular resolution than segmented foil geometries. In addition, integral cylinders would be more resistant to possible stress from deep multilayer coatings than segmented ones. Both electroforming of nickel (method of SAX, JET-X, and XMM) and epoxy replication are under consideration. Both processes can utilize the same types of mandrels and separation agents. While electroforming can produce substrates that are thin, the high density of the nickel may result in high weight optics for some missions. For convenience, experimentation with replication and coating is being carried out initially on flats. Our replication studies include trials with gold and carbon separation agents. This paper reports on our efforts with epoxy replicated optics.

Characterization and calibration of a multilayer coated Wolter optic for an imager on the Z-machine at Sandia National Laboratories

The need for a time-resolved monochromatic x-ray imaging diagnostic at photon energies >15 keV has motivated the development of a Wolter optic to study x-ray sources on the Z-machine at Sandia National Laboratories. The work is performed in both the LLNLs x-ray calibration facility and SNLs micro-focus x-ray lab. Characterizations and calibrations include alignment, measurement of throughput within the field of view (FOV), the point-spread function within the FOV both in and out of focus, and bandpass in the FOV. These results are compared with ray tracing models, showing reasonable agreement.

Monitoring program for the coating of the AXAF flight optics

The Mission Support Team SAO Reflectivities studies laboratory was responsible for the verification of the coating performance specification during the coating of the eight AXAF flight optics. Prior to the start of the coating of the flight optics, it was necessary to verify the scaleup of the coating chamber parameters from the test chamber to the flight optic coating chamber as well as to verify repeatability of coating quality. Immediately prior to the coating of each flight optic, witness samples were coated to verify the coating specification for each particular geometry. Similar witness samples were coated to verify the coating specification for each particular geometry. Similar witness samples were also coated with each flight optic. An overview of this monitoring program is presented along with a description of how the measurements are made, what tests are used to verify performance and a description of the witness sample deployment. Preliminary data on coating uniformity will also be presented.

Correlation between x-ray reflectivity measurements and surface roughness of AXAF coated witness samples

One of the specifications used to polish the AXAF witness samples was that the rms surface roughness be <EQ 5 angstrom as measured by optical profilometry. This specification was chosen based on the cost of polishing and the necessity to keep scatter to a minimum. However, it is not necessarily the best indication of the expected performance of the soft x-ray reflectivity of the surfaces. In particular, the reflectivity data from the AXAF flight optic witness samples indicate sample to sample differences of a few percent which do not correlate with the optical profilometry results for these samples. Further investigations were carried out to measure rms surface roughness using atomic force microscopy (AFM). The differences shown by AFM surface roughness measurements correlates to differences found in reflectivity for these same samples. One-dimensional power spectral density data is presented from both AFM and WYKO measurements along with the reflectivity results at 8 keV for the AXAF witness samples. The results indicate that to obtain accurate prediction of x-ray performance it is necessary to look at the scanning probe metrology data provided by the AFM, in addition to the optical profilometry data.

Mandrel replication for hard x-ray optics using titanium nitride

X-ray astronomy grazing incidence telescopes use the principle of nested shells to maximize the collecting area. Some of the more recent missions, such as XMM-Newton, have used an electroformed nickel replication process to fabricate the mirror shells. We have been developing coatings to simplify and improve this electroforming process. This paper discusses our most recent results from studies using TiN as a mandrel hardcoat in the electroforming process of fabricating nickel shell optics. The results indicate that nickel replicas separate easily from the TiN coated mandrel, and little (if any) degradation of the mandrel occurs after more than 20 replications. AFM characterization of the mandrel and replica surfaces is shown. Preliminary results are also included from studies which use this same process to replicate multilayer coatings; these results indicate no change in the multilayer stack after separation from the mandrel.