X-ray production is about 99% heat and 1% x-rays
Key takeaways
- In Bushong, about 99% heat and about 1% x-rays.
- The x-ray share rises with kVp: 0.5% at 60 kVp, about 1% at 100 kVp.
- mA doesn't change the efficiency.
- No radiography textbook we hold gives the 99.8% and 0.2% split.
- Most diagnostic x-rays are bremsstrahlung.
About 99% of the electrons’ energy becomes heat, and about 1% becomes x-rays. Bushong gives more than 99% heat in one place and roughly 99 percent and 1 percent in another (11th ed., PDF pp.130, 144).
What about 99.8% and 0.2%?
An earlier version of this page taught that split as a standard value. We couldn’t support it:
- It isn’t in Bushong. A search of the full text finds no “99.8”. We couldn’t find it as a heat share in the other radiography textbooks we hold either.
- Bushong’s lowest figure in the diagnostic range is 0.5% x-rays at 60 kVp, which is more than 0.2%.
ARRT® doesn’t publish this number. Its registry outline lists “x-ray production” and “target interactions” under Safety, Radiation Physics and Radiobiology, which is 21 of the 200 scored questions in both the 2022 outline and the one that applies from March 1, 2027. We won’t tell you an exam uses any exact split. If a question offers both 99% and 99.8%, Bushong’s figure is about 99%, so check your program’s text. The rule to learn is that nearly all of it becomes heat.
Why isn’t there just one number?
The x-ray share depends on kVp (Bushong, PDF p.144):
| Tube potential | Share of energy that becomes x-rays |
|---|---|
| 60 kVp | 0.5% |
| 100 kVp | About 1% |
Efficiency doesn’t depend on mA. Doubling tube current doubles the heat, and the percentages stay put.
Where does the heat come from?
Most incoming electrons interact with outer-shell electrons of the target atoms. They excite those electrons without ionizing them. When those electrons drop back, they emit infrared radiation, which is heat.
What makes up the x-rays?
- Bremsstrahlung: an electron slows near the nucleus and gives off an x-ray. This makes most of the beam in the diagnostic range.
- Characteristic: an electron knocks out an inner-shell electron, and another drops in to fill the gap. For tungsten, K-characteristic x-rays need at least about 69 kVp. At 100 kVp, about 15% of the beam is characteristic.
Why does this matter for the tube?
All that heat is the design problem. It’s why the target is tungsten, why anodes rotate, and why heat units and rating charts exist.
For more, read the Radiation Physics chapter, then try the radiation physics practice questions. The Image Production guide covers the tube side.
Frequently asked questions
What percentage of electron energy becomes x-rays?
About 1% at 100 kVp and about 0.5% at 60 kVp, per Bushong. The rest becomes heat.
Is it 99% or 99.8% heat?
Bushong says about 99% in one place and more than 99% in another. It doesn't print 99.8%, and we couldn't find it as a heat share in the other radiography textbooks we hold. Don't treat 99.8% as an established fact.
Does raising mA make x-ray production more efficient?
No. Efficiency doesn't depend on tube current. Doubling mA doubles the heat and leaves the percentages the same. Raising kVp does raise efficiency.
Sources
Found an error? Email [email protected] with the page and the source.
- NIST X-ray Transition Energies, tungsten (repo copy; K edge about 69.5 keV) Official
- ARRT Radiography Examination Content Specifications, 2022 and 2027, pp.1 and 4 (question counts, x-ray production, target interactions) Official
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