On 2007-08-10, Ian Iveson wrote:
Arny wrote:

So why do x-ray tubes have rotating tungsten anodes?

Not all.

When they do, its for high energy, long duration runs.

How high is high, and how long is long? What about those used in
medical x-ray photography, for example?

But, back on point, it takes 16 Kv or more to pry X-Rays loose from
that anode, rotating or not.

Fair enough, but where do you get the 16kV from? I have read various
figures for the minimum voltage to produce x-rays. How is it
calculated?

This is how I remember it from solid state theory education nearly 30
years ago.

1. Formally X-rays have a frequency (v) of 30 to 30,000 PHz (PHz =
10^15 Hz). They lie above UV and below gamma rays.

2. Using Planck's constant (h) in convenient units of electron-Volt.s
(h = 4x10^-15 eV.s), and Planck's black body law (E=hv), an X-ray photon
has an energy of between 120 eV and 120 keV.

3. Thus "white" X-rays, produced by bremsstrahlung, can in theory be
produced by decelerating electrons at an anode of just 120 V potential
up to 120 kV. There's no threshold. However 120 eV X-rays are fairly
"soft" and only just above UV.

4. However "characteristic" X-rays are produced by electron transistions
between energy states of the element(s) in the anode stimulated by
an electron of sufficient energy. In this case there is an energy
(anode voltage) threshold. IIRC, these "X-ray lines" range from about
8 keV to 20 keV for Cu, Zn, Mo, W anodes, so 16kV could correspond to
the anode voltage threshold for a specific transition in the
electron shell of a particular element. I'm not going to look up the
possibilities!

5. X-ray production is very inefficient (0.01%-ish IIRC) so anodes
do need to be cooled.

Atoms have got much more complicated than when I was at school.

Not really. It's the unification of the small and the large scale
theories (and the incorporation of gravity) that makes even the
Hilbert-space quantum theory look simple,

John
--
John Phillips