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

John wrote:

[snip lots of good stuff]

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,


Interesting and informative, thanks.

When I was at school electrons were like planets in neat circular
orbits. The only mystery was that there was no in-between. Later they
were spinning and occupying spirograph patterns. When they became
uncertain I lost the plot, like most folk, probably.

The distinction between invention and discovery isn't as easy to
define as it seemed to be. I'm quite drawn to the view that science
has finished.

cheers, Ian

On 2007-08-11, Ian Iveson wrote:
John wrote:

[snip lots of good stuff]

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,


Interesting and informative, thanks.

Actually from looking up on the web this morning I see that as a
consequence of the production of deceleration X-rays (Bremsstrahlung)
not converting anywhere near all of the electron energy into X-rays, there
is something close to a voltage threshold (but not in the quantum sense).

See http://hyperphysics.phy-astr.gsu.edu...tum/xrayc.html (the
bottom of the page) for how the intensity varies with anode voltage from
20 kV up.

--
John Phillips

"John Phillips" wrote:
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.

As you know, but forgot to make clear, the distinction between
X-rays and gamma rays is not one of frequency/wavelength, but one of
origin: Gamma rays are (mostly) produced by nuclear energy transitions
or by interactions of subatomic particles, whereas X-rays aren't.
Gamma rays are not _necessarily_ more energetic than X-rays. The name
simply indicates the mechanism by which the radiation was produced.

A very long time ago (maybe 10 years or more), I think I remember
reading on somewhere like sci.physics, someone implying that there
were even such things as _visible_ gamma rays; i.e. there were
situations in which nuclear transitions could produce radiation
of such low energy that its wavelength fell into the visible range!
However, I've only read that once and a very long time ago, so take
that with several tons of Saxo.

Another vaguely interesting thing I read on Wikipedia quite recently:
X-rays can (presumably indirectly, but nevertheless) be visible to
the human eye! Not something you want to try at the levels required,
but apparently true:

http://en.wikipedia.org/wiki/X-rays#..._the_human_eye


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie