Kristin Madsen

X-ray study of W/Si multilayers for the HEFT hard x-ray telescope

This paper outlines an in-depth study of the W/Si coated mirrors for the High Energy Focusing Telescope (HEFT). We present data taken at 8, 40 and 60 keV obtained at the Danish Space Research Institute and the European Synchrotron Radiation Facility in Grenoble. The set of samples were chosen to cover the parameter space of sample type, sample size and coating type. The investigation includes a study of the interfacial roughness across the sample surface, as substrates and later as coated, and an analysis of the roughness correlation in the W/Si coatings for N = 10 deposited bilayers. The powerlaw graded flight coating for the HEFT mirrors is studied for uniformity and scatter, as well as its performance at high energies.

Coating of the HEFT telescope mirrors: method and results

We report on the coating of depth graded W/Si multilayers on the thermally slumped glass substrates for the HEFT flight telescopes. The coatings consists of several hundred bilayers in an optimized graded power law design with stringent requirements on uniformity and interfacial roughness. We present the details of the planar magnetron sputtering facility including the optimization of power, Ar pressure and collimating geometry which allows us to coat the several thousand mirror segments required for each telescope module on a time schedule consistent with the current HEFT balloon project as well as future hard X-ray satellite projects. Results are presented on the uniformity, interfacial roughness, and reflectivity and scatter at hard X-ray energies.

Investigation of new material combinations for hard x-ray telescope designs

The materials chosen for depth graded multilayer designs for hard x-ray telescopes (10 keV to 80 keV) have until now been focusing on W/Si, W/SiC, Pt/C, and Pt/SiC. These material combinations have been chosen because of good stability over time and low interface roughness, However both W and Pt have absorption edges in the interesting energy range from 70 - 80 keV. If looking at the optical constants Cu and Ni would be good alternative high-Z candidates since the k-absorption edges in Cu and Ni is below 10 keV. We have investigated both of these materials as the reflecting layer in combination with SiC as the spacer layer and give the performance in terms of roughness, minimum obtainable d-spacing and stability over time as deposited in our planar magnetron sputtering facility. Likewise we review the same properties of WC/SiC coatings which we have previously developed and which allow for very small d-spacings. The combination of WC/SiC or the well established W/SiC with the above mentioned Cu and Ni-containing multilayers in the same stack allows for novel telescope designs operating up to and above 100 keV without the absorption edge structure.

Novel multilayer designs for future hard x-ray missions

Current multilayer designs for 10-80 keV hard X-ray telescope missions have focused primarily on the proven properties of W and Pt based multilayer coatings. Recently a number of new material combinations and coating capabilities have emerged which allows for more elaborate designs that can further extend the energy band of current mission designs as well as avoid some of the unwanted absorption edge effects in the effective area near potentially important line emission energies. These new design possibilities are investigated for current hard X-ray mission designs. The new material combinations to be considered are recently proven capabilities of enhanced NiV/C coatings and NiV/SiC coatings in conjuction with the well-established W based coatings.

Hard X-ray Optics: From HEFT to NuSTAR

Focusing optics are now poised to dramatically improve the sensitivity and angular resolution at energies above 10 keV to levels that were previously unachievable by the past generation of background limited collimated and coded-aperture instruments. Active balloon programs (HEFT), possible Explorer-class satellites (NuSTAR - currently under Phase A study), and major X-ray observatories (Con-X HXT) using focusing optics will play a major role in future observations of a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters. These instruments call for low cost, grazing incidence optics coated with depth-graded multilayer films that can be nested to achieve large collecting areas. Our approach to building such instruments is to mount segmented mirror shells with our novel error-compensating, monolithic assembly and alignment (EMAAL) procedure. This process involves constraining the mirror segments to successive layers of graphite rods that are precisely machined to the required conic-approximation Wolter-I geometry. We present results of our continued development of thermally formed glass substrates that have been used to build three HEFT telescopes and are proposed for NuSTAR. We demonstrate how our experience in manufacturing complete HEFT telescopes, as well as our experience developing higher performance prototype optics, will lead to the successful production of telescopes that meet the NuSTAR design goals.

Production and calibration of the first HEFT hard x-ray optics module

Complete hard X-ray optics modules are currently being produced for the High Energy Focusing Telescope (HEFT), a balloon born mission that will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented layers are made of thermally-formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. Our novel mounting technique involves constraining these mirror segments to successive layers of precisely machined graphite spacers. We report the production and calibration of the first HEFT optics module.

W/SiC and Pt/SiC multilayers for the NuSTAR hard x-ray telescope

This paper will discuss the coatings for the Nuclear Spectroscopic Telescope Array (NuSTAR) and describe the updates of the coating facility at the Danish National Space Center, necessary to make all the coatings in the required time frame. The inner part of the three NuSTAR telescopes will be coated with Pt/SiC and the outer part with W/SiC. To understand the roughness of the flight coatings, we will present results from 10 bilayer constant d-spacing coatings for both types of flight coatings. Also, data showing the homogeneity over the octant mirror segments as well as X-ray data from realistic depth graded coatings will be presented. The long time stability and stress in the coatings will be discussed.

Thin-shell plastic lenses for space and laboratory applications

We have identified an inexpensive, readily available, mechanically stable, extremely smooth, elastic, and mechanically uniform plastic suitable for thin film X-ray optics. Polyethylene terephthalate (PET) is easily deformed without losing its elastic properties or surface smoothness. Most important, PET can be coated with mono- or multilayers that reflect X-rays at grazing incidence. We have used these properties to produce X-ray optics made either as a concentric nest of cylinders or as a spiral. We have produced accurately formed shells in precisely machined vacuum mandresl or used a pin and wheel structure to form a continuously wound spiral. The wide range of medical, industrial and scientific applications for our technology includes: a monochromatic X-ray collimater for medical diagnostics, a relay optic to transport an X-ray beam from the target in a scanning electron microscop0e to a lithium-drifted silicon and microcalorimeter detectors and a satellite mounted telescope to collect celestial X-rays. A wide variety of mono- and multilayer coatings allow X-rays up to ~100 keV to be reflected. Our paper presents data from a variety of diagnostic measurements on the properties of the PET foil and imaging results form single- and multi-shell lenses.

Effective area calibration of the nuclear spectroscopic telescope array (NuSTAR)

The Nuclear Spectroscopic Telescope ARray (NuSTAR) has been in orbit for 6 years, and with the calibration data accumulated over that period we have taken a new look at the effective area calibration. The NuSTAR 10-m focal length is achieved using an extendible mast, which flexes due to solar illumination. This results in individual observations sampling a range of off-axis angles rather than a particular off-axis angle. In our new approach, we have split over 50 individual Crab observations into segments at particular off-axis angles. We combine segments from different observations at the same off-axis angle to generate a new set of synthetic spectra, which we use to calibrate the vignetting function of the optics against the canonical Crab spectrum.

Arcus: the x-ray grating spectrometer explorer (Conference Presentation)

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50 A bandpass with unprecedented sensitivity: effective areas of >350 cm^2 and spectral resolution >2500 at the energies of O VII and O VIII for z=0-0.3. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Arcus will be launched into an ~ 7 day 4:1 lunar resonance orbit, resulting in high observing efficiency, low particle background and a favorable thermal environment. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned. The baseline science mission will be completed in <2 years, although the margin on all consumables allows for 5+ years of operation.