Defeating Optical Tempest will be easy...

Major Variola (ret) mv at
Mon Jul 21 16:22:21 PDT 2003

At 07:49 PM 7/21/03 +0100, Peter Fairbrother wrote:
>a_b_sorbed. Absorb is a widely used word meaning 3to drink in, to soak
>both literally and figuratively. Adsorb is a specialized technical
>meaning only 3to collect a condensed gas or liquid on a surface.2

Thank you.  Have a hard time keeping them straight.  Probably a textual
clue that will undermine my pseudo-anonymity some day :-)

>The glass of CRT's absorbs so much of the X-rays that it might be hard
>detect a signal at all at any distance, but then the signal is not
>by noise from the not-immediately-illuminated areas, unlike the optical


Yes but anything that picks up the weak X-ray will be sensitive to other

normal background ionizing.  With a proportional counter like a
combo, where you could discriminate different types of radiation on the
basis of pulse height,
but you'll be down in the photomultiplier tube's noise.  And as a cosmic
ray secondary
slows down it can generate x-rays.

Maybe if the Adversary is allowed cryogenic detectors in the next room
over... he still has to deal with the attenuating
coefficient for drywall, etc.  And again, I think basically nothing gets
through the glass.

>"0.5 milliroentgens per hour at a distance of five (5) centimeters from
>point on the external surface of the receiver" is the US legal
limit[*], and
>low voltage (and thus very low x-ray emission) crt monitors are common
>if not a de-facto standard.

That's pretty hot, actually.   A glass vial of 5 gms U Acetate is only
twice that,
a few mm from the alpha-window of a GM detector.
And the broad face of a CRT means 1/R^2 doesn't apply until you
get some distance away... more like 1/R for an infinite slab.

>However, I expect shot noise to be a limiting factor here.
>the Roentgen is such a wierd unit it's not that easy to convert it to
>photons and do the math!

Since the signal is rastering at MHz, you can't very well integrate
ionizing radiation over long times,
as you could to say detect the betas coming out of a jar of salt
substitute (potassium).

Roentgens are defined as producing a certain amount of ionization in dry
The photons doing the ionizing would range from the 10's of keV for
to MeV for gammas.  (Careful with that brightness control Eugene!)  An
pair takes about 37 eV to form.  Compare with visible light's very small
blue to red.

>I use 180:210:210[**] (r:g:b) text on a 255:255:255 window background
>present, with very light wallpaper, though I speckle both slightly.
It's a
>little hard to read, but much better than some other suggested

I hope you don't do this all the time...

>[*]< Probably far too high for safety! Originally for TV's, where the
>viewing distance is much higher. But most modern monitors will emit
>less than that. I hope! >

As do TVs.  Nowadays its the Radon daughters attracted to the charged
that will be giving you your RDA (Radiation Daily Allowance.. RDA is a
Yank FDA pun)

Also some of the TV radiation was from HV tubes inside the box; that was
solved by first
using leaded glass (hack!) and then more elegantly by getting rid of

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