Re: glass vacume gaps x-rays

        Re: glass vacume gaps x-rays
        Mon, 14 Apr 1997 06:06:17 -0400
        Tesla List <tesla-at-pupman-dot-com>

I believe that you need 10^-6 torr and 50kv to get xray production.
are the vacuum and voltage levels commercial xray tubes operate at.

At 12:10 AM 4/14/97 -0500, you wrote:
>        Re: glass vacume gaps

>>Could high voltage flowing thru a vaccume create X rays?
>It could provided that electrons were emitted in some fashion from one
>the electrodes. (thermionic, high field emission, photoelectric, etc.) 
>this does happen, it always occurs at the negative electrode.  The anode
>(+) electrode material is important to the emission as well as the
>voltage across the tube.
>Richard Hull, TCBOR
>X-rays are the direct result of a change in the kinetic energy of an
>orbiting electrons.
>In order to generate x-rays, you need to have three things,
>1. A sufficiently high voltage gradient in the tube (or similar
>so that the electrons are moving sufficiently fast (perhaps possible in
>some very hv tesla coil designs)
>2. A target which will be amenable to the generation of x-rays.  Ie, the
>electrons when they strike the target must have a sufficient change in
>kinetic energy so that the resulting photons will have a sufficiently
>frequency.  By conservation of energy, if the electron's kinetic energy
>drops drastically (collision), the energy must be somewhere: it goes
>heat, light, uv and x-ray production  (one reason gaps are fair uv
>3.  Enough electrons of 1 and 2 so that any x-rays will be detectable.
>There are two main x-ray types: Bremsstrahlung and Characteristic.  The
>first type is due to the multiple kinetic energy loss of electrons
>a solid/liquid/gaseous target.  As the electron burrows into the target,
>suffers collisions with the orbiting electrons of the target's atoms. 
>can't get around this without careful material choices.
>The second type is due to the shell transitions in the target material:
>tungsten, molyb, etc.  This can be controlled by simply choosing low
>molecular density metals.
>Usually, you need voltages on the order of 40kv dc and above in the tube
>(70kv+ is much better) for x-ray production with the ususal materials
>(tungsten, thorium etc).  However, you can have "soft" x-ray generation
>(35-45 kv dc) a la breast mamogram machinery but the targets are usually
>berillium which is expensive, toxic, hard to acquire and not used in
>A good reference: "Aspects of medical radialogy" by Christensen.  It
>x-ray production from tubes and the like.
>So, to avoid x-rays, use tube voltages under 40 kv dc or make certain
>have about 6-8+ mm of aluminum shielding around your high voltage tube
>attenuate the possible x-rays (this is done in the medical community).
>(The usual email disclaimer)