Lobster-Eye Hard X-Ray Telescope Mirrors

Transcription

1 Lobster-Eye Hard X-Ray Telescope Mirrors Victor Grubsky, Michael Gertsenshteyn, Keith Shoemaker, Igor Mariyenko, and Tomasz Jannson Physical Optics Corporation, Torrance, CA Mirror Technology Days 007 Albuquerque, NM July 31, 007

2 NASA is developing a hard X-ray telescope (HXT) for Constellation-X mission HXT Goals: 1. Study of black hole evolution. Investigation of the Cosmic X-ray Background (CXRB) 3. Detection of hard X-rays in stellar flares 4. Observation of nuclear transition lines

3 Baseline Constellation-X HXT requirements Band Pass: kev Minimum effective area: kev Minimum angular resolution: 30 Field of View: 5 X 5 (10 X 10 goal) 3

4 Conventional X-ray telescopes employ nested parabolic mirrors (Wolter I geometry) Disadvantages of this technology: Hard to achieve high surface quality Each mirror element has to individually manufactured (expensive!) Requires expensive multilayer coatings to achieve high collection efficiency (Ni is not an efficient hard X-ray reflector) 4

5 POC is developing a hard X-ray telescope based on the optical principle of a lobster eye 5

6 Lobster eye developed for efficient light detection in dark deep-sea environment Lobster eye consists of square channels (~30x30x60 m in size) with reflecting walls 6

7 Image is formed by corner reflections from adjacent channel walls focal surface corner reflection M.F. Land & D-E Nilsson, Animal Eyes 7

8 Lobster eye resembles a spherical mirror Important difference: Lobster eye Spherical mirror produces real image produces virtual image 8

9 Natural lobster eye uses multilayer organic reflective coatings, which are perfectly suited for working in visible light. To adopt this technology for focusing hard X- rays, we need to make the following modifications: 1) Use very flat mirrors with excellent surface quality; ) Apply a coating made of a material with high X-ray reflectivity (high-density, high-z metals). 9

10 X Y Numerical simulation of an X-ray Lobster Eye lens Z One-dimensional ray-tracing for a parallel beam incident on the lens Two-dimensional view of the focal plane intensity distribution shows a characteristic cross pattern X Simulation parameters: E = 40 kev, R = m, channel angle 0 = 0.5 mrad, Au mirrors 10

11 Lobster-Eye lenses are manufactured out of semiconductor-grade silicon wafers AFM surface profiling of 1 wafers Commercial 1 double-sidepolished Si substrate Typical surface parameters: RMS roughness ~ 5 Å Bow/Warp < 30 m (possible to achieve ~ 5-10 m) 11

12 Lobster-eye lenses are assembled from pairs of male and female elements Female element Male element Au coating is applied to Si substrates for enhancing X-ray reflectivity 1

13 Male and female elements are inserted into each other to produce the square channel structure of a lobster-eye lens Female elements Partial assembly Full assembly Male elements 13

14 Lobster-eye lens assembly station Simulation in Pro/E Photo of the assembly process 14

15 y, mm y, mm (c) Experimental X-ray focusing demonstration using a parallel-channel lens 10 POC X-ray Lobster-Eye lens made of Au-coated Si wafers Measured image of the X-ray point source produced by this lens for 10-0 kev photons x, mm 10 Channel width = 0.9 mm 5 0 Simulated image x, mm 15

16 X-ray reflectivity R Theoretical X-ray reflectivity of gold mirrors for various X-ray energies kev 15 kev 0 kev 30 kev 40 kev 60 kev 80 kev Grazing angle i, deg R ~ 1 only for c i c, where decreaseswith photon energy c criticalgrazing angle. c ~ 1. E 16

17 POC successfully tested Lobster-eye lens elements at Argonne National Lab Gold-coated silicon lens element Wide portion: σ Au = 15.8 Å θ c Incidence angle θ i, deg. Finger area: σ Au = 0.9 Å Experimental setup for measuring hard X-ray reflectivity of lens elements θ c Demonstrated R>90% for θ i <θ kev! Incidence angle θ i, deg. 17

18 Coatings for hard X-ray mirrors Element Z Density g/cm 3 Critical kev, deg. Effective coll. area (rel. to Au)* Price, US$ /troy ounce Rhenium Osmium Iridium Platinum Gold Nickel Negligible Uncoated silicon *Effective collecting area is proportional to θ c. Iridium-coated mirrors will provide the best combination of performance, material price, and deposition cost. 18

19 Flatness of mirror substrates is crucial for achieving sharp X-ray focusing δθ θ i + δθ h θ i Bow and warp of the silicon substrates distorts the directions of X-ray reflections, which can significantly reduce the angular resolution of a Lobster-eye telescope. Angular blurring due to the bow and warp: ref 6h L 19

20 Profilometry measurements on 100-mm wafers Scan along X-axis: Bow/warp h 8 m Scan along Y-axis: Bow/warp h 5 m Expected angular resolution~ 6h L mm 100mm 0.5 mrad 1.7 arcmin 0

21 Characterization of silicon mirror flatness Microfocus X-ray source θ i Si mirror z Reflected bright line (width W) X-ray camera D = 1.8 m L = 0.1 m D = 1.8 m Reflection line width for a perfectly flat mirror: W ideal L tan i Lz D L Surface waviness will typically produce a broader line: W measured > W ideal We define flatness parameter: W W measured ideal 1 ε > 0: de-focusing ε < 0: focusing Good mirror will have ε << 1 1

22 Relationship between ε and bow/warp h 3, 3 6 Angular blurring L D z L h z L L D h L h L D Lz L D W ideal For example, for D = 1800 mm, L = 10 mm, and z = 3.5 mm, we get: 9.7 μm h

23 ~0 mm Reflections from three wafers: The Good, the Bad, and the Ugly Bad Good Ugly ε ~ ε < 0.5 ε ~ 7 Wafer 1 Wafer Wafer 3 All wafers have the same length (1 cm) 3

24 Typical X-ray reflection images for various vertical positions of a wafer z = -3.6 mm z = -1.7 mm z = +1.7 mm z = +3.6 mm ε = 1.5 ε =.75 ε = ε = 0.5 Focus broadening is almost 3X its natural width BAD! 4

25 Fabrication of LE telescope prototype with two Si mirror stacks Si mirror thickness = Spacer thickness = 400 m 5

26 y, mm Experimental setup for testing LE lenses X-ray source X-ray camera X-ray source D m D m Lobster-eye lens 1.8 m 1.8 m Lobster-eye lens 10 5 H y 0 X-ray camera -5 Angular resolution ~ H~ x,y H / x,y (/D m) x, mm H x 6

27 Images of a point source produced by the prototype lobster-eye lens Horizontal stack, 0 kv Vertical stack, 0 kv Combined, 0 kv Combined, 50 kv Combined, 90 kv Measured FWHM of the peak = 1. mm, theoretical FWHM = 0.4 mm 7

28 Blurring of the focal spot is due to the mirror waviness Teoretical resolution Channel D width 0.4 mm 1800mm 0.7 arcmin Measured resolution FWHM of D peak 1. mm 1800mm arcmin Bow/warp resolutionlimit 6h L mm 10mm.6 arcmin (assuming a typical bow/warp of 15 m across a 10-mm mirror) Measured angular resolution is in good agreement with theoretical estimates 8

29 Mirror waviness has to be reduced in order to achieve better performance Common semiconductor silicon wafer bow/warp spec: 30 m across a 300-mm wafer Angular resolution of arcmin Available high-quality silicon polishing: 5 m across a 300-mm wafer Angular resolution of 0 arcsec Further improvements: mirror segmentation, MRF polishing, material selection by inspection Angular resolution of <5 arcsec 9

30 Lobster-eye hard X-ray telescope for NASA s Constellation-X mission ALEX module detail Constellation-X spacecraft Precision silicon mirrors with gold or iridium coating 8x8 array of adjustable lobster-eye modules 30

31 Future hard X-ray telescope will be an adjustable 8x8 array of Lobster-Eye lenses Parameters of each lens: Radius of curvature R = 0 m Number of channels N = Channel angle 0 = = 7" Channel spacing = 700 m Mirror length l = 8 cm Mirror thickness d = 100 mm Lens dimensions = 8 cm 8 cm 8 cm Lens weight = 1.1 kg. Overall dimensions = 64 cm 64 cm 8 cm Overall weight = 70 kg 31

32 Effective area of the telescope in four different FOV/sensitivity configurations Use of Au-coated Si mirrors is assumed 3

33 Lobster-Eye Telescope Parameter Number of channels Timeline of lobster-eye hard X-ray telescope development July (parallel) NASA HXT Requirement N/A Angular resolution 10 arcsec 30 arcsec 7 arcsec 7 arcsec <30 arcsec Field of view ~0 arcmin 15 arcmin 15 arcmin 10 arcmin 5 arcmin Effective collecting 40 kev ~0.5 cm cm >5 cm 000 cm >150 cm Size, cm Weight ~10 g ~150 g ~1 kg 70 kg <50 kg POC telescope performance will exceed NASA HXT specs 33

34 Advantages of POC s lobster-eye technology over conventional X-ray telescopes Higher performance: Higher resolution, wider FOV, larger collecting area Simplicity of manufacturing: All telescope optics can be assembled from only parts: male and female mirror elements Proven technology: Lobster-eye components will be manufactured using standard materials and techniques of the optical and semiconductor industries Lower weight: Lobster-eye telescope will be manufactured out of lightweight silicon wafers, resulting in two- to threefold weight reduction Lower cost: POC technology does not need substantial material R&D because we use standard materials and methods. 34

35 Conclusion 1. We demonstrated a technology for fabricating lobstereye hard X-ray optics based on assembling the lens from semiconductor-quality silicon wafers coated with gold.. A prototype lens demonstrated hard X-ray focusing (~30-40 kev) with angular resolution ~ arcmin. 3. Further improvements in performance can be achieved by fabricating larger optics using high-quality silicon material with reduced waviness. 4. We designed a hard X-ray telescope (HXT) for NASA Constellation-X mission with expected performance exceeding NASA specs. 35

36 Acknowledgements This work was supported by NASA SBIR funding. 36