Re: [Phys-l] x-ray machine

[Bernard Cleyet <bernardcleyet@redshift.com>]

Well, actually it is zero -- It's the Fermi - vacuum levels that  
create a high resistance, yes?

Once the electron is in the vacuum its movement is only limited by  
its inertia.  However, my second point, if there are other electrons,  
then one may have space charge limited current, as opposed to the  
emission limited that I previously mentioned.

Also it is interesting to note that the damper tube, in the examples  
I have, is lead shielded (W/  the warning of its poisonous nature)   
The HT diode is not.  that is because, I don't know, as its PD during  
the non conducting half cycle is high.  Someone help me here.  Of  
course now, well recently, Si devices are used.  No shielding, as the  
gap is so narrow, IIC?

Finally, that shadow mask is interesting -- it's invar!  for an  
obvious reason.

bc has a second thought about Si diode shielding.



On 2009, Jul 08, , at 05:47, chuck britton wrote:

> The 'resistance' of a vacuum can be pretty high ;-)
>
> An 'old fashioned' crt color TV set is a rather good x-ray machine.
> Maybe 25 kV instead of 60 kV.
>
> The electron beam hits the metal mask that separates the RGB color
> spots on the screen.
>
> Since the early '70's (IIRC) the front glass is heavily laced with Pb
> to shield the viewer.
>
> The current is limited by the circuit that supplies the electrons to
> the cathode.
>
> Ohm's Rule of Thumb is usually applied to devices that are a bit more
> simple that crt circuits.