During an x-ray exposure, an incident electron with energy 90 keV interacts to produce a photon of 30 keV. What is the approximate energy of the electron as it exits the atom?

• A. 0 keV
• B. 60 keV
• C. 30 keV
• D. 90 keV

Answer: (B)

Energy is conserved in the interaction: the electron’s initial energy must equal the sum of the emitted photon energy and the electron’s remaining kinetic energy. If the incident electron has 90 keV and emits a 30 keV photon, the electron retains 90 keV − 30 keV = 60 keV of energy as it exits. This is the remaining kinetic energy, while the photon carries away the 30 keV. The other options would imply distributing energy differently (for example, all energy to the photon or no energy to the photon), which isn’t what this scenario shows.