NIST Non-Destructive Evaluation

David Jacobson

Muhammad Arif

Ionizing Radiation Division

Thermal Neutron Source• It is fueled with uranium, contained in 30 MTR

type elements of unique split-core design with a 18 cm unfueled gap at the center plane.

• The reactor is cooled, moderated and reflected by D2O, producing a peak thermal flux at the reactor centerline of 4 x 10 14 neutrons/cm2/s at the rated power level of 20 megawatts.

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

1000 2000 3000 4000 50000

Neutron Velocity (m/s)

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Neutron Energy (meV)

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NIST Thermal Neutron Imaging Facility

• Scheduled to go online in late summer 2002

• Predicted thermal flux >108 cm-2 sec-1

• Beam filtered by 10 cm LN cooled Bismuth single crystal

• High resolution L/d from 300 to 4000

sample positioning platform

beam stop

collimating drum

Neutron imaging camera

Neutrons from source

3-D cutaway view

Cold Neutron Source• Liquid hydrogen cold sorce

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

• Neutron guides coated with Ni-58 provide integrated fluence rates as much as 2x109 neutrons/cm2 s

1000 2000 3000 4000 50000

Neutron Velocity (m/s)

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Neutron Energy (meV)

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Traditional Radiography• Contrast is due to attenuation of radiation• Scattering density of material can be

extracted• N - density of sample atoms per cm3

• I0 - incident neutrons per second per cm2

- neutron cross section in ~ 10-24 cm2

0ItNeII 0

Sample

Collimating guide

Neutron beam

Lens to magnify or minify image to fit on CCD chip

Light tight box

Computer Thermal electric cooler

Axis

of

rota

tion

Mirror

CCD chip

Setup for Traditional Radiography/Tomography

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

• Converter screen (shown in green) 6Li embeded in ZnS absorbs neutrons and produces a charge particle, which induces the ZnS to scintillate.

• Lens focuses the scintillation light on a CCD chip.

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

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

Rotary table

Neutron to light converter

How can we use imaging?

• Real Time Imaging– Take radiographs every 1 second over long period

– Create movie of an operating object

• Tomographic Imaging– Create 3-d reconstruction of object

– Cut open/isolate any area in object

– Analyze area for damage

Radiography: X-Ray or Neutron?X-ray Contrast Mechanism

• Attenuation coefficients proportional to atomic number:– Low scattering/absorption for Hydrogen, etc.

– High scattering/absorption for Aluminum, Lead, etc

• Large difference in attenuation coefficients provides contrast for between on opposite ends on periodic table

• Low sensitivity to elements with low atomic numbers– Hydrogen

– Hydrogen compounds (H2O)

• High energy flux needed to pass through encasement (such as aluminum or steel)

• Similar problem in medical imaging – high energy flux needed to pass through bones

– Lack of sensitivity to tissue

Radiography: X-Ray or Neutron?Neutron Contrast Mechanism

• Attenuation coefficients not correlated with atomic number:– High scattering/absorption for Hydrogen

– Low scattering/absorption for Aluminum

• Sensitive to presence of Hydrogen and Hydrogen based compounds

• Metal encasements almost invisible to neutrons• Neutron absorption in element is proportional to 1/(velocity)

Radiography: X-Ray or Neutron?

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• Amount of material needed to attenuate the intensity of a thermal neutron beam by 1/2

Factors Influencing Image Quality

• Neutron beam flux– Image statistics increase as

number of neutrons increase– Reduced flux can be

compensated for by increasing image exposure time

• Dark Current– Tendency of a CCD sensor to

produce unwanted charge in each pixel over time.

– thermal electrons gain enough kinetic energy to tunnel into wells of CCD chip

Factors Influencing Image Quality

• Ideal Case: Point source– Each point is hit by only one

ray– One pixel represents one

point– Results in sharp edges

• Actual Case: Aperture– Each point is hit by multiple

rays– One pixel represents

multiple points– Results in unsharp edges– Geometric sharpness

decreases as diameter increases

– L/D Ratio

Image Processing• Pixel values proportional to number of neutrons

• Dark current correction

• Median filter (3x3 kernel, gets rid of salt and pepper noise)

• Divide by empty image to get I/I0

• Take -ln of value to get density values

Actual MotorActual Motor Radiograph (I)Radiograph (I) Transmission (Transmission (I/II/I00)) ) n n t t ((-ln[-ln[I/II/I00])]) )

tNeII 0

History of Tomography

• Principles discovered by Radon in 1917• Rediscovered as Computerized Tomography Cormack and

Hounsfield won Nobel Prize in 1979 • Used in CAT scans• CAT scan of iceman identified cause of death : arrow in

shoulder

Tomography Data Collection

• Dataset collected by rotating object by a step angle from 0 to 180 degrees.

• These images called projections, P

• Projections can also be represented as a line integral

• f(x, y) = N : attenuation coefficient

• After images have been collected, each slice of the object can be individually reconstructed

Filtered Back Projection Algorithm

• Conversion from rectangular to polar introduces Jacobian of |w| factor

• This is known as a ramp filter

• Based on the Fourier Slice Theorem

– the 1-D Fourier Transform of each projection is one slice of the overall 2-D Fourier Transform

Filtered Back Projection Algorithm

• Resolution (point spacing) much less at edges then at center

• Final equation is:

• Three Tomography StepsThree Tomography Steps

–1.1. Collect Projection DataCollect Projection Data

–2.2. Fourier transform Data and filterFourier transform Data and filter

–3. 3. Back project values across the image planeBack project values across the image plane

Filtered Back Projection Algorithm

Filtered Back Projection Algorithm

All Slice

Reconstructions

3D Reconstruction

TomographyRadiographs

SliceReconstructi

on

Tomography Software

• Tomography software written and optimized for c++

• Graphical User-Friendly Interface allows for graphical input of all selection

• Has ability to perform one step at a time, or complete all tasks at once

• Ideal for implementation with parallel processing

Applications

• Determination of hydrogen distribution in materials.

• Study of structural defects in materials.

• Coking determination in gas turbine engine nozzles.

• Investigation of hydrogen distribution in polymer electrolyte fuel cells.

• Study of lithium ion conductor motion in lithium batteries.

• Visualization of porosity of oil containing shale.

• Determination of time dependent migration of hydrocarbons in rocks.

• Study of hydrocarbons under pressure.

• Nondestructive evaluation of archeological specimens.

• Visualization of liquid metal flow.

• Examination of solar cell arrays.

• Contraband detection and identification in sealed containers.

• Visualization the motion of water in plants.

Neutron Phase Contrast Radiography

Copyright, 1996 © Dale Carnegie & Associates, Inc.

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

• Light refractive indices vary from 1-3.• Neutron refractive indices are typically

less than 1 and differ from 1 by parts in 105 or 106.

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

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

Refractive Power of Neutrons

Attenuation v.s. Phase Contrast

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

• Are transparent objects invisible?

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

• This is called phase object

• Therefore transparent phase objects can be rendered visible.

1.8 m

Contact Image

1.8 m

Phase Contrast Image400m Pinhole Apperture

MonochromaticNeutrons

• Neutron are monochromatic

• Pinhole aperture is 0.4 mm in diameter

• Details of delicate organ structure is visible to phase contrast

Phase Contrast of a Wasp

400 m PinholeApperture

1.8 m

Contact Image

PolychromaticNeutrons

Phase Contrast of a Lead Slug

1.8 m

Phase Contrast Image

Turbine Blade Phase Image

Conclusion• Thermal and cold neutrons have short wavelengths, which

results in very low refractive powers in matter. In spite of this fact coherent wave scattering can be utilized as an image contrast mechanism.

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

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

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

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

• Neutron detector technology improvements (more, faster, better):– Higher flux detectors 106 neutrons / cm2 / s– Near real time image capture– Higher resolution ~ 10 m might be possible