Alan Ibbetson wrote:
>
> At what anode voltage do valves begin to radiate dangerous X-rays? My
> schoolboy physics (Plank's constant, etc) suggests frighteningly modest
> voltages: around 2 Angstroms at only 5KV if there are suitable energy levels
> in the atoms of the anode material.
>
> I would guess that the self-shielding geometry of external anode valves
> reduces the problem, but the ceramic seals are still going to be transparent
> to X-rays.
I just spoke to a professor in my department (medical physics), whose
speciality is x-rays. He says below 10 kV, he would have no concerns,
even when I said the anode current would be several amps. 25 kV would
certainly be risky - apparently 25 kV is used for x-rays used for
mammograms.
Apparently, when running x-ray tubes at about 7-8 kV (he could not
recall the exact figure), with a tungsten anode, x-ray production will
definitely occur, as the chemistry of tungsten will show. However, this
is very difficult to detect outside the tube, as despite the fact they
will certainly be produced, the ceramic stops them. He feels the same
would happen with glass.
For people working in x-ray labs, like him, they all wear what is known
as a 'film badge' all the time. These are basically photograpic file,
which is developed periodically (weekly I think) and excessive x-ray
exposure will then show up as fogging of the film. The only problem in
using such a film badge very close to the tube would be the heat from
the tube. There is no doubt running a photographic film near a tube
would fog it, but an amateur could wear one to see if he is being
exposed. I don't know exactly what the film is, but I'm sure a search of
the web would find the answer, or I could ask my colleague again.
One colleague I know left her film badge in bag which was x-rayed by
customs. When developed, this showed a severe overdose!!
Perhaps when I get my YC156 amp up and running, I'll take a 'film badge'
home then get it developed, but my colleague feels below 10 kV would not
be an issue.
Dr. David Kirkby,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408
e-mail davek@medphys.ucl.ac.uk
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