The atomic number of the anode material in an X-ray tube is \( Z \). If the X-ray tube has an anode-to-cathode voltage of \( V \), the efficiency of X-ray production is proportional to:
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X-ray production is highly inefficient. At typical diagnostic voltages (e.g., \( 100 \text{ kV} \)) with a tungsten target (\( Z = 74 \)), the efficiency is roughly \( \eta \approx 10^{-9} \times 74 \times 10^5 \approx 0.0074 \), meaning less than 1% of the energy turns into X-rays while over 99% is wasted as heat!