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Back scatter imaging

Collection & imaging of scattered radiation

Digital Technique where access to both

sides not required Imaging from one side

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Scattering

Takes place through interaction of X or gamma photonswith electrons or nucleus

For imaging interaction with electrons is important

Other than Rayleigh /elastic/ coherent scattering, thereis transfer of energy

Types of scattering Coherent scattering

Photo-electric absorption -fluorescence Compton scattering

Resonance fluorescence

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Applications related to scattering

Elastic scattering measurement of thin coatings

Photoelectric process Basis for chemical analysis tools

Compton scattering

-basis for back-scatter imaging Resonance process

Examination of thick & dense structures

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Imaging Techniques

Pin hole

Measurement of source dimensions

Multiple aperture collimator

Moving slit

Flying spot

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Multiple aperturecoolimator

Instead of single-hole ,multiple conical holesradially drilled in a Pb sheet

Masks deigned to selectvolume of elementrepeatedly on the detector

The specimen illuminatedfrom side

Photons scattered at thegeometric centre of

collimator are collected

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Moving slit

Slits are configured to provide depth resolution

Configuration I

Slit & detector moved related to surface under study Configuration II

Source & source slit stationary

Detector & imaging slit moved across the surface to

collect scatter from progressively deep-layer

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Flying spot

Technique A rotating collimator disk

,in front of a slitcollimator forms a movingmask to limit incidentbeam falling on object

The un-collimateddetectors detect scatteredphotons

High throughput Detector solid angle 2phi

steredian

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Applications

Checking of fuses & explosives in artillery shells Made up of organic materials , the composition scatters well

Show no contrast in film RT as absorption is little aircraftbulk-heads

Mine detection High density low Z needed to make explosives suitable for

detection by back-scattered techniques ion

Aircraft corrosion Need for detection permits one side access only

Use depth scans to find out thickness of subsurface layer &determine remaining metal thickness , as forming corrosionproducts separate metal structure members

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Applications

Between lap joints corrosion causes swellingbetween rivets called pillowing

Testing of baggage for explosives &contraband

Certain amount of chemical analysis

Estimation of Z of material under inspection