X-ray Backscatter Detectors

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Topics
 Source of x-rays
 Interaction with matter
 Detection and Processing
 Fooling the Detector
 Health Concerns
Source: Towe and Jacobs 1981
Source of X-Rays
•Two guns used in airport detectors
•Low energy gun is equivalent to CAT scan source
•High energy gun is stronger than used in a mammogram
•Can either move the source or the object to be imaged
Source: Kaufman and Carlson 2010
Compton Scattering
 Uc = K1pnf(E) gives the Compton coefficient
 Ratio of penetration depths between two output fluxes is
given by x2 = x1 u2/u1ln(I2/I1)
 Penetration depth depends on energy, intensity, and
object density
Source: Towe and Jacobs 1981
Detecting Backscatter
Source: Herr et al 1994
Subtraction of Images
Source: Towe and Jacobs 1981
Modern Backscatter
Source: Kaufman and Carlson 2010
Fooling the Detector
 Monte Carlo simulation using parameters from real
detectors
 Cylindrical tissue represents the abdomen
 Ran simulation four times:




No added material
160 g of added tissue
190 g of added TATP explosive
320 g of added PETN explosive
 Formed cone of material 20 cm in diameter and 1 cm high
at center
Source: Kaufman and Carlson 2010
Source: Kaufman and Carlson 2010
Source: Kaufman and Carlson 2010
Health Concerns
 About 1 μSv dose per exposure.
 This results in increased cancer risk of 10-7 per round trip
 With 1 billion exposures per year, the population risk is no
longer minimal
 Millimeter wave imaging, which is non-ionizing, may
provide a viable imaging alternative for airport security
Source: Brenner 2011
Works Cited
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