Neutron Phase Contrast Neutron Phase Contrast RadiographyRadiography

Copyright, 1996 © Dale Carnegie & Associates, Inc.

OutlineOutline

• Who are we?Who are we?

• What is a Neutron?What is a Neutron?

• Where do we get neutrons?Where do we get neutrons?

• When do we get neutrons?When do we get neutrons?

• Why do we want neutrons?Why do we want neutrons?

• What are they good for?What are they good for?

• What is phase imaging?What is phase imaging?

• Why do we want to image with phase?Why do we want to image with phase?

• What is it good for?What is it good for?

Investigators and SupportInvestigators and Support

• Brendan AllmanBrendan Allman University of Melbourne, AustraliaUniversity of Melbourne, Australia

• Phillip McMahonPhillip McMahon University of Melbourne, AustraliaUniversity of Melbourne, Australia

• Keith NugentKeith Nugent University of Melbourne, AustraliaUniversity of Melbourne, Australia

• Muhammad ArifMuhammad ArifNIST Ionizing Radiation DivisionNIST Ionizing Radiation Division

• David L. JacobsonDavid L. Jacobson NIST Ionizing Radiation DivisionNIST Ionizing Radiation Division

• Samuel A. WernerSamuel A. Werner University of Missouri-Columbia/ NIST University of Missouri-Columbia/ NIST Ionizing Ionizing Radiation DivisionRadiation Division

• Work supported by Work supported by

– United States Department of CommerceUnited States Department of Commerce

• National Institute of Standards and TechnologyNational Institute of Standards and Technology– Ionizing Radiation Division Ionizing Radiation Division – Center for Neutron ResearchCenter for Neutron Research

– Australian Research CouncilAustralian Research Council

– National Science Foundation Grant No. PHY-9603559National Science Foundation Grant No. PHY-9603559

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The NeutronThe Neutron

•Weakly attenuated by many elements in the periodic table

•Strongly scattered by 1H but not 2H

•Strongly absorbed by several isotopes: 113Cd, 6Li, 3He, 157Gd

•Has refractive wave properties

•Magnetic moment of the neutron allows it to be used in studying magnetic properties of matter

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• Electrically neutral yet is composed of 3 charged quarks

•Neutron half life = 10 min. 14 sec

NIST Center for Neutron ResearchNIST Center for Neutron Research

• Located at the National Institute of Standards and Located at the National Institute of Standards and Technology in Gaithersburg, MarylandTechnology in Gaithersburg, Maryland

• The reactor achieved first criticality in 1967, began routine The reactor achieved first criticality in 1967, began routine operation at 10 MW in 1969, and was increased in power to operation at 10 MW in 1969, and was increased in power to 20 MW in 1985.20 MW in 1985.

• Thermal neutron sourceThermal neutron source

• Cold neutron sourceCold neutron source

• More information at www.ncnr.nist.govMore information at www.ncnr.nist.gov

Thermal Neutron Reactor

Cold Neutron Guide Hall

Cherry Trees

Thermal Neutron SourceThermal Neutron Source

• It is fueled with uranium, contained in 30 MTR It is fueled with uranium, contained in 30 MTR type elements of unique split-core design with type elements of unique split-core design with a 18 cm unfueled gap at the center plane. a 18 cm unfueled gap at the center plane.

• The reactor is cooled, moderated and The reactor is cooled, moderated and reflected by D2O, producing a peak thermal reflected by D2O, producing a peak thermal flux at the reactor centerline of 4 x 10 flux at the reactor centerline of 4 x 10 1414 neutrons/cm2/s at the rated power level of 20 neutrons/cm2/s at the rated power level of 20 megawatts. megawatts.

• The reactor is operated on a seven week The reactor is operated on a seven week cycle, with approximately 38 continuous days cycle, with approximately 38 continuous days at full power (20 MW) operation followed by at full power (20 MW) operation followed by 11 days for refueling and maintenance. 11 days for refueling and maintenance.

1000 2000 3000 4000 50000

Neutron Velocity (m/s)

5 20 50 80 130

Neutron Energy (meV)

0

Cold Neutron SourceCold Neutron Source

• Liquid hydrogen cold sorceLiquid hydrogen cold sorce

• Neutron Maxwell-Boltzman Neutron Maxwell-Boltzman distribution now peaks at ~ 0.43 distribution now peaks at ~ 0.43 nm or nm or

• Neutron guides coated with Ni-58 Neutron guides coated with Ni-58 provide integrated fluence rates provide integrated fluence rates as much as 2x10as much as 2x1099 neutrons/cm neutrons/cm22 s s

1000 2000 3000 4000 50000

Neutron Velocity (m/s)

5 20 50 80 130

Neutron Energy (meV)

0

Traditional Radiography (forget about Traditional Radiography (forget about waves)waves)

• Contrast is due to attenuation of radiationContrast is due to attenuation of radiation

• Scattering density of material can be Scattering density of material can be extractedextracted

• N - density of sample atoms per cmN - density of sample atoms per cm33

• II00 - incident neutrons per second per cm - incident neutrons per second per cm22

- neutron cross section in ~ 10- neutron cross section in ~ 10-24-24 cm cm22

• t - sample thicknesst - sample thickness

0ItNeII 0

Sample

t

Material necessary to reduce 0.18 nm neutron beam intensity by 1/2

0

1

2

3

4

5

6

7

8

9

10

11

12

13

0 4 12 16 21 25 29 33 38 42 47 51 56 75 79

Atomic Number Z

Cen

tim

eter

s

Material necessary to reduce 0.18 nm neutron beam intensity by 1/2

0

1

0 4

Atomic Number Z

Cen

tim

eter

sB

Be

Li

D20

H2O

Cross-Section DensitiesCross-Section Densities

Computer

Collimating guide

Neutron beam

Lens to magnify or minify image to fit on CCD chip

Light tight box

Thermal electric cooler

Axis

of

rota

tion

Mirror

CCD chip

Rotary table

Neutron to light converter

Setup for Traditional Setup for Traditional Radiography/TomographyRadiography/Tomography

• Neutron beam (Neutron beam (shown in shown in redred) is incident on object.) is incident on object.

• Converter screen (Converter screen (shown in shown in greengreen) ) 66Li embeded in Li embeded in ZnS(Cu) absorbs neutrons ZnS(Cu) absorbs neutrons and produces a charge and produces a charge particle, which induces the particle, which induces the ZnS to scintillate.ZnS to scintillate.

• Lens focuses the Lens focuses the scintillation light on a CCD scintillation light on a CCD chip.chip.

• CCD chip views the beam CCD chip views the beam from a 45° mirror. This from a 45° mirror. This prevents radiation from prevents radiation from destroying the CCD.destroying the CCD.

• Image collected by CCD is Image collected by CCD is downloaded to a computer downloaded to a computer for further processingfor further processing

All Slice

Reconstructions

3D Reconstruction

TomographyTomographyRadiographs

SliceReconstructi

on

ApplicationsApplications

• Determination of hydrogen distribution in materials. Determination of hydrogen distribution in materials.

• Study of structural defects in materials.Study of structural defects in materials.

• Coking determination in gas turbine engine nozzles.Coking determination in gas turbine engine nozzles.

• Investigation of hydrogen distribution in polymer electrolyte fuel Investigation of hydrogen distribution in polymer electrolyte fuel cells. cells.

• Study of lithium ion conductor motion in lithium batteries.Study of lithium ion conductor motion in lithium batteries.

• Visualization of porosity of oil containing shale.Visualization of porosity of oil containing shale.

• Determination of time dependent migration of hydrocarbons in Determination of time dependent migration of hydrocarbons in rocks.rocks.

• Study of hydrocarbons under pressure.Study of hydrocarbons under pressure.

• Nondestructive evaluation of archeological specimens.Nondestructive evaluation of archeological specimens.

• Visualization of liquid metal flow.Visualization of liquid metal flow.

• Examination of solar cell arrays.Examination of solar cell arrays.

• Contraband detection and identification in sealed containers.Contraband detection and identification in sealed containers.

• Visualization the motion of water in plants.Visualization the motion of water in plants.

Recent Interest in Phase ContrastRecent Interest in Phase Contrast

• Within the last ten years CCD image detection technology has made it feasible to easily image the scattered radiation from various objects.

• Inexpensive high speed computers have given way to sophisticated processing of images.

• Image resolution with x-rays can be as much as 50 nm

• Neutron image resolution 100 m, but may be improved to as low as 10 m.

The neutron as a wave The neutron as a wave

• Neutrons are massive particles with a wavelength given by Neutrons are massive particles with a wavelength given by the DeBroglie relation:the DeBroglie relation:

• Typical wavelengths for thermal neutrons are 0.1 nm to 0.3 Typical wavelengths for thermal neutrons are 0.1 nm to 0.3 nm.nm.

• This range is similar to most crystal lattice spacings.This range is similar to most crystal lattice spacings.

• Wavelengths for Cold neutrons are 0.2 nm to 1.0 nm.Wavelengths for Cold neutrons are 0.2 nm to 1.0 nm.

• These longer wavelengths allow studies at as much as 50 These longer wavelengths allow studies at as much as 50 nm length scales.nm length scales.

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Thermal Neutrons 0.1-0.3 nmCold Neutrons 0.2-1.0 nm

0.43 nm

ColdNeutrons

0.18 nm

ThermalNeutrons

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4Wavelength (nm)

82 21 9.1 5.1 3.3 2.3 1.7 1.3 1.0 0.82 0.67 0.57 0.48 0.42Energy (meV)

>bullet speeds<

Neu

tron

flu

ence

rat

e (n

/cm

2 s

)

speed of sound

4000 2000 1320 990 790 660 570 500 440 400 360 330 300 280

Velocity (m/s)

MeV neutrons from reactor

Neutron Flux DistributionNeutron Flux Distribution

Neutron Refractive IndexNeutron Refractive Index

• Neutrons have a DeBroglie Neutrons have a DeBroglie wavelengthwavelength

• Wavelength ranges:Wavelength ranges:

– Thermal (0.1nm - 0.3nm)Thermal (0.1nm - 0.3nm)

– Cold (0.2nm - 1.5 nm)Cold (0.2nm - 1.5 nm)

• Index of RefractionIndex of Refraction

mv

h

• Index of refraction range for (1-n):Index of refraction range for (1-n):

– Thermal (10Thermal (10-6-6 - 10 - 10-5-5))

– Cold (10Cold (10-5-5 - 10 - 10-4-4))

• Note that for visible light n ranges Note that for visible light n ranges from 1-3from 1-3

speed in mediumnspeed in vacuum

1

2

Snell’s lawn1sin1=n2sin2

n1~1

n2<1

• Unlike the refractive powers of glass for Unlike the refractive powers of glass for visible light, neutron refractive powers visible light, neutron refractive powers are very weak.are very weak.

• Light refractive indices vary from 1-3.Light refractive indices vary from 1-3.

• Neutron refractive indices are typically Neutron refractive indices are typically less than 1 and differ from 1 by parts in less than 1 and differ from 1 by parts in 101055 or 10 or 1066..

• This low refractive power owes to the This low refractive power owes to the rather high energy or short wavelength rather high energy or short wavelength of the neutron.of the neutron.

• Much longer wavelength neutrons, Much longer wavelength neutrons, called very cold neutrons, are currently called very cold neutrons, are currently difficult to produce in large enough difficult to produce in large enough quantities to be useful for imaging.quantities to be useful for imaging.

Refractive Power of NeutronsRefractive Power of Neutrons

Attenuation v.s. Phase ContrastAttenuation v.s. Phase Contrast

• Amplitude objects are objects that Amplitude objects are objects that attenuate the intensity or attenuate the intensity or amplitude of the wave front like amplitude of the wave front like the motor seen previously.the motor seen previously.

• Are transparent objects invisible?Are transparent objects invisible?

• Not quite, actually they distort a Not quite, actually they distort a wavefront that passes through wavefront that passes through them redistributing intensity them redistributing intensity downstream of the object.downstream of the object.

• This is called phase objectThis is called phase object

• Therefore transparent phase Therefore transparent phase objects can be rendered visible.objects can be rendered visible.

(r,t) =Ar e-i(kr-t)

•Wave amplitude decreases as 1/r

|(r,t)|2 =A2

r2

•Energy propagates perpendicular to the wavefront and decreases as 1/r2

Energy redirected by lens

Example: Lens redistribution of Example: Lens redistribution of intensityintensity

• Cold neutron beam Cold neutron beam polychromatic or polychromatic or monochromaticmonochromatic

• Point source that produces a Point source that produces a very coherent image (here very coherent image (here 0.4mm dia.)0.4mm dia.)

• Sample placed downstream Sample placed downstream about 2 meters awayabout 2 meters away

• Image planes are: Image planes are:

– position 1) directly behind the position 1) directly behind the samplesample

– position 2) downstream about position 2) downstream about 2 meters away2 meters away

Sample

Horizontal scale 1 m

Contact imagePhase contrast image

Cold neutron beam

Neutrons from pointsource (0.4 mm dia)

2d detectorposition 1

2d detectorposition 2

Typical Phase Typical Phase Contrast SetupContrast Setup

400 m PinholeApperture

1.8 m

Contact Image

PolychromaticNeutrons

Phase Contrast of a Lead SlugPhase Contrast of a Lead Slug

1.8 m

Phase Contrast Image

1.8 m

Contact Image

1.8 m

Phase Contrast Image400m Pinhole Apperture

MonochromaticNeutrons

• Neutron are monochromaticNeutron are monochromatic

• Pinhole aperture is 0.4 mm in diameterPinhole aperture is 0.4 mm in diameter

• Details of delicate organ structure is visible Details of delicate organ structure is visible to phase contrastto phase contrast

Phase Contrast of a WaspPhase Contrast of a Wasp

   

Reconstructed Neutron Optical Reconstructed Neutron Optical Density of Lead Slug Density of Lead Slug

Reconstructed Phase of Lead Reconstructed Phase of Lead SinkerSinker

Turbine Blade Phase ImageTurbine Blade Phase Image

Why do it?Why do it?

• Low attenuation contrast.Low attenuation contrast.

• Smaller radiation dose.Smaller radiation dose.

Neutron Interferometer and Optics Facility

CoherenceCoherence

• Beam characteristics Beam characteristics

– High transverse coherenceHigh transverse coherence

– High intensityHigh intensity

• BeamsBeams

ConclusionConclusion

• Thermal and cold neutrons have short wavelengths, which Thermal and cold neutrons have short wavelengths, which results in very low refractive powers in matter. In spite of this results in very low refractive powers in matter. In spite of this fact coherent wave scattering can be utilized as an image fact coherent wave scattering can be utilized as an image contrast mechanism.contrast mechanism.

• Beams with suitable coherence require small pinhole apertures Beams with suitable coherence require small pinhole apertures resulting in very low neutron flux. We need more flux.resulting in very low neutron flux. We need more flux.

• Real time imaging capabilities allow for study of time Real time imaging capabilities allow for study of time dependent phenomenondependent phenomenon

• Beam lines designed specifically for this application would be Beam lines designed specifically for this application would be able to optimize flux. We need better resolution.able to optimize flux. We need better resolution.

• Dedicated higher flux beam lines are available at NIST and will Dedicated higher flux beam lines are available at NIST and will be developed in the near future.be developed in the near future.

• Neutron detector technology improvements (more, faster, Neutron detector technology improvements (more, faster, better):better):

– Higher flux detectors 10Higher flux detectors 1066 neutrons / cm neutrons / cm22 / s / s

– Near real time image captureNear real time image capture

– Higher resolution ~ 10 Higher resolution ~ 10 m might be possiblem might be possible