ASNT

equations E = h ν = hc / λ . These interactions dominate when the X-rays are incident upon atoms with higher Z, higher atomic number, and the energy of the photons is less than 1 MeV. b. Compton (incoherent) scattering is the elastic collision between a photon E = h ν and an electron mass m . The conservation of momentum and energy applies to the collision, and well-known equations relate the energy and momentum transferred to the electron and the angles at which the photon and electron are seen emerging from the collision (Kaplan 1955). For energies higher than 500 keV, compton scattering becomes important. After the collision, the incident photon has smaller energy and therefore longer wavelength, and it moves off in a direction different from the original. The electron gains energy and momentum and acquires a different trajectory as well. c. Pair production occurs when the energy of the photon produces a pair of particles (an electron and a positron) in the field of the nucleus. Larger nucleus (higher Z ) and higher photon energy enhance pair production. Since two particles of mass 0.511 MeV each are created, the photon energy must exceed this amount (1.02 MeV). The attenuation law of X-rays by matter is expressed in an exponential relationship between the incident X-ray f lux I o , the final or detected f lux I , where m is the linear attenuation coefficient for the material and t is the material thickness. The linear attenuation coefficient, in units of cm -1 is based on the mass attenuation coefficient ( m / r ) in units of cm 2 /g multiplied by r, where r is the material density in units of g/cm 3 . There are tabulated values of m / r for different elements, common materials, and mixtures as a function of energy maintained online by the National institute of Standards and Technology (NIST) (Hubbell and Seltzer 2004) and in Chapter 17 of this Handbook. Mixtures are treated as weighed additives in the m / r coefficients of the different elements of the mixture in the exponent argument (Eq. 3) I = I 0 e − t

where w i is the weight percent of the i th component and ( m / r ) i is the mass attenuation coefficient of the i th component. Note: In Chapter 17 of this handbook, the mass attenuation coefficient, µ/ ρ , is also known as µM; and the linear coefficient is also known as µ L . The energy-dependent absorption of X-rays is not uniform, nor is it smoothly varying. This nonuniform absorption can be used in radiographic inspection by seeding gaps, cracks, and spaces with material exhibiting markedly stronger absorption characteristics (higher µ/ ρ ) than the surround- ing material. The same effect can be achieved in radiographic inspection of parts if the total thickness × density does not obscure the effect. At energies higher than 85 keV, there are no elements exhibiting K-edge discontinuities in absorption. ANGULAR DISTRIBUTION OF X-RADIATION EMERGING FROM THE TARGET The bremsstrahlung portion of X-radiation generated in thick-target X-ray sources is the large fraction of the total radiation emitted. In the tens to hundreds keV range of energy, characteristic X-rays comprise a fraction of the total, depending upon electron energy and target material. At lower energies, below the tens keV range, low target Z characteristics supply a significantly higher fraction (Behling 2016). This feature and filters made of thin coatings of materials can be used to produce nearly monochromatic sources (keV to tens keV wide). The nearly isotropic distribution of X-rays emerging is affected noticeably by the absorption for long path length X-ray trajectories at small angles with respect to the target surface as shown in Figure 6 (CASINO 2007). This is less severe for industrial inspection tubes that use 45º target angles where the effect of absorption is small; departure from isotropic angular distribu- tion becomes more evident at emission angles that are much larger than normal angles. In the realm of higher energy radiography, 1 to 10 MeV, drop-off at the edge of radiation fields is due to the forward- peaked radiation pattern discussed previously.

⎞ ⎠ ⎟

⎛ ⎝ ⎜

ρ

ρ

i ∑

w i

=

(Eq. 4)

i

CHAPTER 2

45

Part 2