What percent of electron energy is converted into heat during production of X-rays?: (A) 99%, (B) 94%, (C) 89%, (D) 84%
Answer is A. The purpose of the target in an x-ray tube is to convert the kinetic energy of the colliding electrons into x-ray photons. The conversion of the kinetic energy of the electrons into x-ray photons is an inefficient process, with more than 99% of the electron kinetic energy converted to heat.
The target is made of tungsten, an element that has several characteristics of an ideal target material, including the following:

High atomic number (74), allows for efficient x-ray production.
High melting point (3422°C), to withstand heat produced during x-ray production.
High thermal conductivity(173 Wm-1K-1), to dissipate the heat produced away from the target.
Low vapor pressure at the working temperatures of an x-ray tube, to help maintain vacuum in the tube at high operating temperatures.

Key Concept:
The conversion of the kinetic energy of the electrons into x-ray photons is an inefficient process, with more than 99% of the electron kinetic energy converted to heat.
White and Pharoah's Oral Radiology 8th EDITION