ASNT

PART 1 Atomic Structure

apparatus used in most radiographic imaging operates in low magnetic field (B-field) and is subject only to electric fields (E-fields) in the order of 6 to 10 kV/mm, so atomic structure details that arise from high E-fields or B-fields can be ignored. The targets used are in the solid state, though some analytical X-ray sources employ liquid metal targets; most are solid metal alloyed with small weight percent components for enhanced mechani- cal or thermal properties.

PROCESS FOR CREATING X-RAYS The most common X-ray source used in radiographic inspection employs an accelerated electron beam incident on a solid target. This is independent of whether the target is rotating or stationary. The spectrum of the radiation that emerges from a target depends upon the energy of the incident electrons and the identity of the elements that compose the target. The X-ray source

M shell

3d 3p

Incident electron

L shell

M shell

3s

K shell

2p

L shell

α 1 α 2

β 1 β 2

2s

Characteristic radiation

1s

K shell

α 1 α 2 β 1 β 2

Ejected electron

(a)

(b)

Figure 1 Schematic of incident electron on target atom: (a) K, L, and M electron shells in a bohr atommodel of electrons orbiting a nucleus; (b) electrons occupying the L-shell have three distinct energies corresponding to s-orbital, p-orbital occupancy (s designates l=0, and p designates l=1 with slight differences in total energy). Transitions of 2s to 1s not permitted due to angular momentum selection rules.

CHAPTER 2

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Part 1