Under what condition does an X-ray source excite characteristic x-rays?

• A. The source energy is equal to the energy of the emitted characteristic x-ray
• B. The source energy is greater than the energy of the emitted characteristic x-ray
• C. The source energy is less than the energy of the emitted characteristic x-ray
• D. The source energy has no relation to the emitted energy

Answer: (B)

Exciting characteristic X-rays happens when the incoming X-ray has enough energy to knock an inner-shell electron out of the atom. That ejected vacancy is then filled by an electron from a higher shell, and the energy released in that transition is the characteristic X-ray energy. Because the energy required to eject the inner-shell electron (the binding energy) is larger than the energy of the emitted characteristic X-ray, the incident X-ray must have energy greater than the emitted line energy to cause that ionization and subsequent emission.

So, a practical way to think about it is: the source must supply more energy than the energy of the characteristic X-ray it will produce. For example, if a line is at 6.4 keV, the incident photons must be above the corresponding inner-shell binding energy (usually above that line energy) to create the vacancy and generate that emission.