Characteristic X-rays are emitted when

• A. Nuclear transitions
• B. Electrons move between shells after inner-shell ejection
• C. Thermal vibrations
• D. Photon-phonon interactions

Answer: (B)

Characterist X-ray lines come from the atom itself relaxing after an inner-shell vacancy is created. When an inner electron is removed (often by an incoming X-ray or particle during analysis), a hole appears in a low-energy shell. An electron from a higher shell drops into that vacancy, and the energy difference between the two levels is emitted as a photon in the X-ray region. These photons have specific energies that depend on the element, so they are called characteristic X-rays and are used to identify which elements are present.

Nuclear transitions produce gamma rays, not these atomic-shell X-rays, so option involving nuclear transitions isn’t correct. Thermal vibrations relate to heat and lattice motion, not emission of characteristic X-rays. Photon-phonon interactions describe scattering or coupling phenomena without the discrete atomic-shell transitions that generate characteristic X-rays.