[ RadSafe ] X-ray Production in Power Supplies

[bobcherry at cox.net]

The physics is simple: If you have a large enough potential difference ("large enough" is usually defined at greater than 10 kV), you will produce x rays when electrons jump across that potential difference. The potential difference can be in a medical x-ray tube, some rectifiers and transformers, and the cathode-ray tube in your TV, for example. The spectrum of x rays depends on the target material as well as the potential difference. Air in the gap will slow down the electrons and affect the spectrum, too (e.g., "gassy" x-ray tubes).

Oh, and those x rays from your CRT travel at 300,000,000 m/s in a vacuum, just like those x rays from the imploding neutron star. In all inertial reference frames.

Physicist Bob C

> 
> From: "Richard L. Hess" <lists at richardhess.com>
> Date: 2005/02/22 Tue AM 11:46:08 EST
> To: "Redmond, Randy (RXQ)" <redmondrr at y12.doe.gov>,  radsafe at radlab.nl
> Subject: Re: [ RadSafe ] X-ray Production in Power Supplies
> 
> Hi, Randy,
> 
> As a resident non-health physicist, grab your salt shaker. I have played 
> around with electronics for many years and have some understanding of this. 
> Here is what I recall.
> 
> My basic assumption is that X-rays are produced when electrons hit 
> something. That's why the X-ray tubes have a target at 45 degrees, sending 
> the x-rays out where you want them, right?
> 
> If this assumption is broadly correct, then what do we have that could 
> produce this?
> 
> Cathode Ray Tubes (CRTs) produce some ionizing radiation and that is why 
> there is lead in them. The current emissions are extremely low. The higher 
> the 2nd anode voltage, as I understand it, the more likely the device is to 
> produce radiation. I must say that I've never seen an indication using a 
> working (according to the check source) CDV-700 survey meter on a computer 
> monitor. I don't think I ever tried my 25" TV. The larger the TV, the 
> higher the required 2nd anode voltage.
> 
> In old TVs, there was a high-voltage rectifier tube and I believe that was 
> also the cause of radiation emissions. It had a large gap (to avoid 
> arcing), as I recall.
> 
> Other than that, I don't think there is a mechanism to produce ionizing 
> radiation from contemporary solid-state power supplies.
> 
> There are other issues with contemporary solid-state power 
> supplies,  however, that your questioner may be getting confused about:
>   (a) they can produce electrostatic and electromagnetic fields
>   (b) the high frequency (as in above audibility, but under 1MHz typically) 
> switching
>        regulators can produce  hash. A worst-case example of that was the 
> Sylmar
>        terminus of the Pacific DC Intertie where there are solid-state 
> switches running
>        at 60Hz. They produced enough hash (high-order harmonics) to 
> interfere with nearby
>       AM radio reception that the whole facility was covered by a mesh 
> shield. This is
>       a +/-500,000 DC link from the Columbia River to the LA basin.
>   (c) Some of this hash from power supplies can be conducted back into the 
> powerline
>        contaminating that.
>    (d) Some power supplies have a poor power factor, but that is currently 
> being regulated
>         in the EU and that problem will disappear.
> 
> One annoyance of magnetic fields is that over 10mG they will cause CRTs to 
> "swim" 2-10mG is considered by some to be the safe threshold (fortunately 
> no one is proposing an LNT theory for that, but 2mG is very low).
> 
> So there may be issues with power supplies, but without knowing the 
> topology of the supply and the power levels, I can't provide a better 
> answer. Have you asked the manufacturer?
> 
> Good Luck!
> 
> Cheers,
> 
> Richard
> 
> 
> At 10:34 AM 2/22/2005, Redmond, Randy (RXQ) wrote:
> >Have any of you run across any articles on, or actually measured, X-rays
> >being produced by power supplies, for example, an X-ray power supply?  I have
> >not been able to figure out how this could be possible, if it is possible.
> >It's a long story.
> >
> >
> 
> Richard L. Hess                           email: richard at richardhess.com
> Vignettes 
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> 
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