What is the primary by-product of a coherent scatter event?

• A. scattered photon
• B. free electron
• C. characteristic photon
• D. recoil electron

Answer: (A)

In coherent (Rayleigh) scattering, the X-ray interacts with the entire atom without transferring energy to electrons, so the process is elastic. The atom remains in its ground state, and the observable by-product is the photon that has been scattered in a new direction but retains essentially the same energy as the incident photon. That’s why the scattered photon is produced as the primary by-product.

The other possibilities don’t fit because a free electron would require energy transfer to eject an electron (as in Compton scattering), a characteristic photon comes from subsequent transitions after inner-shell ionization, and a recoil electron is a product of Compton scattering, not coherent scattering.