X-ray Tube Spectrum
What actually comes out of an X-ray tube — a continuous Bremsstrahlung spectrum with the anode’s characteristic lines standing on it. The continuum stops dead at λmin = hc/eU, which depends on the accelerating voltage and on nothing else at all.
- You supply
- An anode and a tube voltage in kV. A current in mA as well, if you want the power and the heat load.
- Reading it
- The limit is exact — hc/e is arithmetic, not a measurement. The continuum’s shape is Kramers’ idealisation and a real tube peaks nearer 1.5λmin. The excitation figure is a lower bound, not the threshold: this site ships no absorption-edge energies, so what it can say is that the edge lies above Kβ.
Worked examples: a copper tube at 40 kV · molybdenum at 50 kV · silver at 20 kV — no lines · the same tube at 50 kV
Input
The short-wavelength limit does not depend on the anode. Change it and the continuum stays exactly where it is; only the sharp lines move.
What comes out of an X-ray tube
An X-ray tube accelerates electrons through a few tens of kilovolts and stops them in a metal anode. What comes off is two things at once: a continuous spectrum from electrons braking in the field of the nuclei — Bremsstrahlung — and the characteristic lines of whatever the anode is made of.
The continuum has a hard edge on the short-wavelength side, and it is the one number here you can write down exactly:
λmin = hc / eU
An electron cannot emit a photon carrying more energy than it has. So the shortest wavelength in the beam is set by the accelerating voltage and by nothing else — not by the anode, not by the current. Everything a diffraction experiment uses sits somewhere to the right of that edge.
Choose an anode and a voltage. The page draws the spectrum, says which characteristic lines that voltage can excite, and works the limit out in full.
Try it
a copper tube at 40 kV · molybdenum at 50 kV · silver at 20 kV — no lines · the same tube at 50 kV