= Characteristic X-ray
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A <Characteristic X-ray> comes from an <electronic transition> filling an inner <electron shell> vacancy. The emitted <photon> carries the difference between atomic <energies>. Vacancy creation by direct <Electron> impact requires <kinetic energy> above the shell's <binding energy>, exceeding the line <energy>. Higher tube <voltage> need not move the <spectral line>.<Diagnostic Radiology Physics: A Handbook for Teachers and Students/Chapter 5: X-Ray Production (teaching slides) by K. P. Maher>[Diagnostic Radiology Physics: A Handbook for Teachers and Students; IAEA Chapter 5, §§5.2.2–5.2.3: shell vacancy, binding-energy threshold and characteristic radiation]{ref}
An original stipulated shell model illustrates the distinction without using measured atomic data. Let positive <binding energies> be $B_K=9.0\,\mathrm{keV}$ and $B_L=1.2\,\mathrm{keV}$, ignoring relaxation corrections. Filling a K-shell vacancy from the L shell releases
$$
E_{KL}=B_K-B_L=7.8\,\mathrm{keV},\qquad
\lambda_{KL}=\frac{hc}{E_{KL}}\simeq0.159\,\mathrm{nm}.
$$
An $8.5\,\mathrm{kV}$ tube can energetically emit a $7.8\,\mathrm{keV}$ <bremsstrahlung> <photon>, yet cannot directly remove this K-shell <Electron>. A $12\,\mathrm{kV}$ tube can create that vacancy by <Electron> impact; if radiative filling occurs, the line is still $7.8\,\mathrm{keV}$, not $12\,\mathrm{keV}$. This argument concerns direct <Electron> impact and an initially unexcited target; it does not describe every possible excitation route.
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