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Really Fast X-ray Imaging Instruments

John OertelTeam Leader for Diagnostic Engineering and Operations

Presented toLANL Critical Skills program

June 26, 2002

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Outline

• Why do you need a <100 ps x-ray imager?• The facilities they operate at• Block diagram a gated x-ray imager• Some specifications• X-ray imaging basics• X-ray gating basics• Characteristics• Developments• Applications• The future

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Why?

In order to minimize motional blurring and freeze material velocities of > 10 7 cm/s, spatial resolution of 5 m and gate times of 50 ps are required.

One way to gate images from an x-ray pinhole camera is to propagate a high-voltage gate pulse across a microwave transmission deposited on the front surface of a Microchannel Plate.

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The Facilities

NovaOmegaTridentNIFOthers…

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Block Diagram

Connector includes:1 - 10 volt trigger3 - 15 volt DC power3 - fiberoptics2 - Nitrogen or water cooling lines1 - High bandwidth monitor1- Ground line15 - multi-wire twisted pair

Film pac withCCD camera option

4 kV phosphorbias

Impedancematchingcircuit 50 Ω to6.25 Ω

Pulsers @ 6 chan.of 4.5 kV, variable pulsewidth,and 50 ohm

6 channeldelay circuitTrigger circuit

Monitor circuit

DC bias forMCP+/- 1000 V

TIM

Connector

Impedancematchingcircuit 6.25Ω to50 Ω

Front-endFiltersMCP modulePhosphor& fiberoptictaper

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Typical Specifications

40 mm MCP typical4 Au transmission lines16 data channels, 4 images/ stripVariable gainFiltering12x, 8x, 4x and 2x magnificationsInsertable or flange mountedFilm or CCD image capture100 eV to 10 keV sensitivity5 m spatial resolution80 ps temporial resolution, some are adjustable a few nsFilm or CCD readout

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Imaging X-rays with Pinholes has Advantages and Disadvantages

• Pros: – Simple

– Good signal to noise ratio– Broadband energy transmission– Inexpensive technology for multiple channels

• Cons:– Low flux due to small solid angles– Have to get pinhole close to TCC for best

resolution

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Airy pattern consists of a bright central disk surrounded by a system of concentric alternately dark and bright rings. The

first zero occurs at kasin = 3.83

1.0

kasin

3.83

-3.8

3 0

I/I(0)

a

qR

L

kasin= 3.832asin = 3.83, sin = q/R, k = 2 2a = d

q = 1.22 R/d

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Geometric Optics

L1 L2

r

L1 L2

object plane image plane

d/2

d/2q

q

tanq/ L1+L2 = d/2L2

q = d/2(L1 + L2/L2)

Putting the diffraction and geometric parts togethersetting q = 0 and taking derivative with respect to d gives:

The geometric part

0 = (L1/L2+1)1/2 - 1/d^2(1.22 L1)

The optimum pinhole diameter is therefore:

d = 2.44L1 (M/M+1)

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Spatial Resolution Depends Upon Diffraction and Geometric Factors

Far field (Fraunhofer) D>> vs. Near field (Fresnel) D~= 1.54 x 10 -10 m, D = 5 x 10-6 m

q1 = 1.22L1/d, diffraction of circular aperture

q2 = d/2(L1/L2+1) geometric optics

Minimized source produces a optimum aperture for given E

Q(d) = q1 + q2

d = 2.44L1(M/M+1)

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Pinhole Resolution vs X-ray Energy

0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10

X-ray Energy (keV)

Pin

hole

resolu

tion

(u

m)

12X, 1.38"

8X, 2.00"

4x, 3.60"

2x, 6.00"

Most x-ray imagers are designed to get the MCP as close as possible to TCC without clipping beams

OPHD 2.44LM /M 1

Re solution OPHD 2

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Be light seal

MCPs 3 - 105mmX35mm6 - strips @ 13 mm wide

50Ω to 6.25Ω taperedtransmission lines

fiberoptic faceplatewith P-11 phosphorand vacuum seal orfiberoptic w/ P-20 for CCD

Micro Channel Plate Module

All components are off-the-shelfand interchangeable.

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Gating is provided by launching a short (<200 ps) ~1kV voltage pulse across a microstrip transmission line coated on the MCP. A photoelectron signal produced at the front surface of the MCP is then only amplified during the transit time of the voltage pulse across a given point on the microstrip.

50

25

25

12.5 DC Bias

- 2500 Volts, < 200 ps FWHM

6.6 mm

1000 Gold5000 Copper50 Chromium

Pin E = Pout

Rout (Vin)^2 E

Rin

Vout =

0 to -300 VDC

With E = 90%V in = 2500 VR in = 50 Rout = 12.5

V out = 1185 volts

Vp =c

Vp = 1.5 cm/100 ps

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Temporal Gate Limited by Electron Transit Time

MCP L/D = 40

Fiberoptic Faceplate with500 /cm2 InSnO2 and0.7 mg/cm2 P-11 phosphor

Kodak2484 35 mmfilm

x-ray

3 kV-300 V

10 m

500 m

8o

e -

e -

e -

t tr = m L LeV D

Electron transit time in a MCP channel

t tr ~ 250 ps w/ V = 1 kV and L/D = 40

G ~ V

Electron Gain in a MCP is very non-linear

kn n = # of dynodes (26)k = 0.5

Minimum temporal gate limited by electron transit time.

If applied voltage pulse begins to compare in width to t trit is no longer possible to extract the full gain from a channel.

For usable output gains minimum optical gate time tends to be 1/3 t tr.

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To drive a MCP stripline we use fast high-voltage pulsers

4.04 kV149.82 ps FWHMtF = 87.75 ps

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High Voltage Pulser similar to a Marx bank

4 kV, < 200 ps Output

2.2 kVDC

• From 2 kV, 200 ps to 4 kV, 150 ps better differentiation techniques better interstage capacitors improved surface conditions

• More reliable than previous pulsersbetter interstage caps saves transistors

• Improved pulse shape consistencyOff-board differentiation simplifies faband allows for variable pulse lengths

• Reduced pulser drift1% zeners

other stages andtrigger

50 pF

8.2 pF

interstage cap

DC blocking cap

shorted stub reverses polarityand reflected signal adds into original resulting in FWHM < 200 ps

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GXI-3 Resolution Grid ShotTrident Shot T3040704

7 m wire dia. 25 m spacing12x mag. 5 m pinholes10 mils Be filtering- 200 VDC bias

50 J, 2, 1 ns square

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GXI-2 Flatfield Data for NOVA shot # 24042613

GXI-2 Flatfield Data Shot#24042614

0.00E+00

5.00E-02

1.00E-01

1.50E-01

2.00E-01

2.50E-01

3.00E-01

3.50E-01

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

Position (mm)

Expo

sure

(erg

s/cm

2)

Stripline 1Stripline 2Stripline 3Stripline 4

Time

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Linearity plot

Saturation

LLNL

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2 ns electrical pulse provides a 450 ps FWHM optical gate

LLNL

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Optical vs Electrical FWHM

LLNL

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Gain vs Bias, 2 ns PFM

LLNL

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MCP Output vs Coating

250

1250

2250

3250

4250

5250

6250

7250

8250

9250

10250

11250

12250

13250

14250

15250

16250

17250

0 500 1000 1500 2000 2500 3000

Coating thickness (Å)

500 V

600 V

700 V

800 V

900 V

MCP A

MCP B

MCP C

1000Å

1250Å

1500Å

1750Å

3 MCP’s all from same bouleAll coating were Au 250Å to 2500ÅAluminum - DC source @ 1.6 keVPreliminary results suggest we have not peaked!

Does it act the same for a different wavelength?

Detector cathode optimization requires further study

What is the optimum cathode coating for a MCP?

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P-11 Phosphor Areal Density vs Intensity

y = -107.36x6 + 1442.3x5 - 7601.8x4 + 19722x3 - 25595x2 + 14205x

0

500

1000

1500

2000

2500

3000

3500

0 0.5 1 1.5 2 2.5 3 3.5 4

Areal Density (mg/cm2)

Resolution vs. Phosphor Areal Density

y = 2.9769x + 74.032

60

65

70

75

80

85

90

95

0 0.5 1 1.5 2 2.5 3 3.5 4

Areal Density (mg/cm2)

P-11 Phosphor optimization

What phosphor coating density yields the greatest flux? 0.6 mg/cm2

What phosphor coating density yields the best spatial resolution?

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Applications

25563

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Rayleigh-Taylor instability growth studied in cylindrical convergent geometry using “indirect-drive” on the NOVA laser

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The Future… GXD for the National Ignition Facility

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GXD Electronic Block Diagram

MCPModule CCD CCD

P.S.

PC104computer

Housekeepingsensors

PulserCards

& Power suppliespfn

DelayCircuits

TrigCircuits

Imp. Mis-match

Trigger in

Imp. matching

Cooling

28VDC

Ethernet

DC bias

Monitor

phos

Adder circuit

pcd

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Gated X-ray Imager for the NIF

52” - 58”

Ethernet unit

CCD power supply2x2x3”, 2 ea.

Kentech Electronics•HV pulser units (3.5 kV - 4 kV)•Power supplies•Delay generators (50 nsmax.)•Triggering•Monitor•Command/Control

CCDMCP Module

4”

10-12”

30” - 36”

Cooling in external base of airbox

Embedded Computer

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MCP Module

Phosphor HV

MCP Striplines

Intensifier Block Terminator Circuit Wing

CCD Cooling lines

SMA Connectors

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Fiber Optically coupled

Vacuum Enclosure

Pump Out

Cooling Block

PowerReadoutTE

Cooling Lines

Spectral Instruments CCD Camera

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Kentech ElectronicsPulse Out

Pulse In

MCP Bias +1kV/ -500V

Phosphor Bias +4kV

PCD +400V

Trigger In

Monitor Out

+28VDC

Computer Interface

30” - 36”

Dimension based on performance requirements

Pulser Units4 each1 1/8” W x 4.92” H

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PC 104+ Embedded Computer

CCD Power Supply2x2x3”, 2 each

Embedded ComputerPC104/PC104+Support Cards:Frame GrabberKentech InterfaceHousekeeping

Ethernet:Electrical/Fiber opticFiber/Fiber