lobster eye hard x-ray mirrors
DESCRIPTION
x Ra Focusing micro channel plateTRANSCRIPT
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Mirror Technology Days 2007
Albuquerque, NM
July 31, 2007
Victor Grubsky, Michael Gertsenshteyn, Keith
Shoemaker, Igor Mariyenko, and Tomasz Jannson
Physical Optics Corporation, Torrance, CA
Lobster-Eye Hard X-Ray Telescope Mirrors
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NASA is developing a hard X-ray telescope
(HXT) for Constellation-X mission
1. Study of black hole evolution
2. Investigation of the Cosmic X-ray Background (CXRB)
3. Detection of hard X-rays in stellar flares
4. Observation of nuclear transition lines
HXT Goals:
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Baseline Constellation-X HXT
requirements
Band Pass: 10 - 40 keV
Minimum effective area: 150 cm2 @ 40 keV
Minimum angular resolution: 30”
Field of View: 5’ X 5’ (10’ X 10’ goal)
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Conventional X-ray telescopes employ nested
parabolic mirrors (Wolter I geometry)
• 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)
Disadvantages of this technology:
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POC is developing a hard X-ray
telescope based on the optical
principle of a lobster eye
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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
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Image is formed by corner reflections
from adjacent channel walls
focal surfacecorner
reflection
M.F. Land & D-E Nilsson, Animal Eyes
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Lobster eye resembles a spherical mirror
Lobster eye
produces real image
Spherical mirror
produces virtual image
Important difference:
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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;
2) Apply a coating made of a material with high
X-ray reflectivity (high-density, high-Z metals).
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Numerical simulation of an X-ray
Lobster Eye lens
One-dimensional ray-tracing for a
parallel beam incident on the lens
-6
-4
-2
0
2
4
6
X
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
Z
Two-dimensional view of the focal
plane intensity distribution shows a
characteristic “cross” pattern
6
4
2
0
-2
-4
-6
Y
86420-2-4-6
X
6000
5000
4000
3000
2000
1000
0
Simulation parameters: E = 40 keV, R = 2 m,
channel angle 0= 0.25 mrad, Au mirrors
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Lobster-Eye lenses are manufactured out
of semiconductor-grade silicon wafers
Commercial 12” double-side-polished Si substrate
Typical surface parameters:
RMS roughness ~ 5 Å
Bow/Warp < 30 m (possible to
achieve ~ 5 - 10 m)
AFM surface profiling of 12” wafers
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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
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Male and female elements are inserted into
each other to produce the square channel
structure of a lobster-eye lens
Male elements
Female elementsPartial assembly
Full assembly
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Lobster-eye lens assembly station
Simulation in Pro/EPhoto of the assembly
process
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Experimental X-ray focusing demonstration
using a parallel-channel lens
(c)
-10
-5
0
5
10
y, m
m
-10 -5 0 5 10
x, mm
-10
-5
0
5
10
y,
mm
-10 -5 0 5 10
x, mm
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
10-20 keV
photons
Simulated
image
Channel width
= 0.9 mm
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Theoretical X-ray reflectivity of gold
mirrors for various X-ray energies
0.001
2
4
6
80.01
2
4
6
80.1
2
4
6
81
X-r
ay r
efle
ctivity R
0.60.50.40.30.20.10.0
Grazing angle i, deg.
10 keV 15 keV 20 keV 30 keV 40 keV 60 keV 80 keV
.1~energy photon with decreases
angle. grazing critical where,for only 1~
E
R
cc
cci
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POC successfully tested Lobster-eye
lens elements at Argonne National Lab
Experimental setup for measuring hard
X-ray reflectivity of lens elements
Gold-coated silicon
lens element
Incidence angle θi, deg.
Incidence angle θi, deg.
θc
θc
Wide portion:
σAu= 15.8 Å
“Finger” area:
σAu= 20.9 Å
Demonstrated R>90% for θi<θc @55 keV!
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Coatings for hard X-ray mirrors
Iridium-coated mirrors will provide the best combination of performance,
material price, and deposition cost.
Element ZDensity
g/cm3
Critical angle
@30 keV, deg.
Effective coll.
area (rel. to Au)*
Price, US$
/troy ounce
Rhenium 75 21.0 0.160 1.09 5950
Osmium 76 22.6 0.165 1.16 380
Iridium 77 22.4 0.164 1.15 425
Platinum 78 21.5 0.161 1.11 1312
Gold 79 19.3 0.153 1.00 667
Nickel 28 8.9 0.114 0.56 Negligible
Uncoated
silicon
14 2.3 0.059 0.15
*Effective collecting area is proportional to θc2.
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Flatness of mirror substrates is crucial
for achieving sharp X-ray focusing
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:
L
6href
θi
δθ
θi + 2 δθ
h
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Profilometry measurements on 100-mm wafersScan along X-axis:
Scan along Y-axis:
Bow/warp h ≈ 8 m
Bow/warp h ≈ 5 m
arcmin 1.7mrad 5.0mm 100
mm 008.066~resolutionangular Expected
L
h
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Characterization of silicon mirror flatness
Microfocus
X-ray source Si mirror
X-ray camera
D = 1.8 mD = 1.8 m
Reflected
bright line
(width W)
L = 0.12 m
θi
2
2tan2
LD
LzL iidealWReflection line width for a
perfectly flat mirror:
1ideal
measured
W
WWe define flatness parameter:
ε > 0: de-focusing
ε < 0: focusing
Good mirror will have |ε| << 1
z
Surface waviness will
typically produce a broader
line: Wmeasured > Wideal
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Relationship between ε and bow/warp h
2
2
2
2
2
23
2,
2
23
6
2
2
2
blurringAngular
LD
zLh
zL
LDh
L
h
LD
Lz
LD
Wideal
For example, for D = 1800 mm, L = 120 mm, and z = 3.5
mm, we get:
mμ 7.9h
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Reflections from three wafers:
The Good, the Bad, and the Ugly
GoodBad
Ugly
All wafers have the same length (12 cm)
Wafer 1Wafer 2
Wafer 3
~2
0 m
m
ε ~ 2 ε < 0.5 ε ~ 7
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Typical X-ray reflection images for
various vertical positions of a wafer
-8-4
04
8
2015
-8-4
04
8
2015
-8-4
04
8
2015
-8-4
04
8
2015
z = -3.6 mm z = -1.7 mm z = +1.7 mm z = +3.6 mm
ε = 1.25 ε = 2.75 ε = -0.13 ε = 0.25
Focus broadening is almost 3X its natural width BAD!
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Fabrication of LE telescope prototype
with two Si mirror stacks
Si mirror thickness = Spacer thickness = 400 m
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Experimental setup for testing LE lenses
-10
-5
0
5
10
y,
mm
-10 -5 0 5 10
x, mm
X-ray source
Lobster-eye lens
X-ray camera
2 m 2 m
Hy
Hx
Angular resolution ~ Hx,y / (2 m)-10
-5
0
5
10
y,
mm
-10 -5 0 5 10
x, mm
X-ray source
Lobster-eye lens
X-ray camera
2 m 2 m
Hy
Hx
Angular resolution ~ Hx,y / (2 m)
X-ray
source
Lobster-eye
lens
X-ray
camera
1.8 m 1.8 mD D
Angular resolution ~ Hx,y /D
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Images of a point source produced by the
prototype lobster-eye lens
12
84
0-4
-8
-8 -6 -4 -2 0 2 4 6 8
1500
1000
500
0
-8-4
04
8
12840-4-8
1500
1000
500
0
Horizontal stack, 20 kV-8
-40
48
-8 -4 0 4 8
3500
3000
2500
2000
1500
1000
500
0
-8-4
04
8
-8 -4 0 4 8
3000
2500
2000
1500
1000
500
0
-8-4
04
8
-8 -4 0 4 8
3500
3000
2500
2000
1500
1000
500
0
Vertical stack, 20 kV
Combined, 20 kV Combined, 50 kV Combined, 90 kV
Measured FWHM of the peak = 1.2 mm, theoretical FWHM = 0.4 mm
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Blurring of the focal spot is due to the
mirror waviness
arcmin 2mm 1800
mm 1.2peak of resolution Measured
D
FWHM
arcmin 2.6mm 120
mm 015.066limit resolution Bow/warp
L
h
(assuming a typical bow/warp of 15 m across a 120-mm mirror)
arcmin 7.0mm 1800
mm 0.4 widthChannelresolution Teoretical
D
Measured angular resolution is in good agreement with
theoretical estimates
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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 2 arcmin
•Available high-quality silicon polishing: 5 m across a 300-mm wafer Angular resolution of 20 arcsec
•Further improvements: mirror segmentation, MRF polishing, material selection by inspection Angular resolution of <5 arcsec
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Lobster-eye hard X-ray telescope for
NASA’s Constellation-X mission
Constellation-X
spacecraft
8x8 array of adjustable
lobster-eye modules
ALEX module detail
Precision silicon
mirrors with gold
or iridium coating
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Future hard X-ray telescope will be an
adjustable 8x8 array of Lobster-Eye lenses
• Radius of curvature R = 20 m
• Number of channels N = 115 115
• Channel angle 0 = 3.3 10-5 = 7"
• Channel spacing = 700 m
• Mirror length l = 28 cm
• Mirror thickness d = 100 mm
• Lens dimensions = 8 cm 8 cm 28 cm
• Lens weight = 1.1 kg.
• Overall dimensions = 64 cm 64 cm 28 cm
• Overall weight = 70 kg
Parameters of each lens:
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Effective area of the telescope in four
different FOV/sensitivity configurations
Use of Au-coated Si mirrors is assumed
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Timeline of lobster-eye hard X-ray
telescope development
Lobster-Eye
Telescope
Parameter
July 2007 2008 2009 2010-2012NASA HXT
Requirement
Number of channels13 13
(parallel)30 30 111 111 888 888 N/A
Angular resolution 120 arcsec 30 arcsec 7 arcsec 7 arcsec <30 arcsec
Field of view ~20 arcmin 15 arcmin 15 arcmin 120 arcmin 5 arcmin
Effective collecting
area @ 40 keV~0.5 cm2 2 cm2 >25 cm2 2000 cm2 >150 cm2
Size, cm 1 1 12 2 2 20 8 8 28 64 64 28
Weight ~10 g ~150 g ~1 kg 70 kg <250 kg
POC telescope performance will exceed NASA HXT specs
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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 2 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.
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Conclusion
1. We demonstrated a technology for fabricating lobster-eye hard X-ray optics based on assembling the lens from semiconductor-quality silicon wafers coated with gold.
2. A prototype lens demonstrated hard X-ray focusing (~30-40 keV) with angular resolution ~2 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.
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Acknowledgements
This work was supported by NASA
SBIR funding.