ASNT

absence of any X-ray focusing lenses or similar concentrating optics, the area of the sphere subtended by the radiation increases as A sphere = 4 π r 2 . The number of photons/area decreases as the radius or distance from the source increases and scales as 1/r 2 . Doubling the distance from a source to a detector will cut the count rate to 25% of the original count rate.

Inverse Square Law The distance-dependent decrease in intensity of radiation in nonabsorbing media is related to the number of photons per unit area. A source of electron-impact X-radiation of a few square millimeters effective area behaves like a point source when imaging objects of tens of cm size located at 1 m source-to-detector distance. In the

CHAPTER 2

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Part 2