Thursday, December 19, 2013

What good are lead lined rooms?

The recent 60 Minutes report on the NSA contains many factual errors (like those about BIOS). One thing that looks like an error is the description of "lead lined rooms". Are rooms at the NSA really lined with lead? Or is this just a mistake reflecting the common misconception about lead stopping radiation?

We all grew up with the stories that Superman's X-Ray vision can see through any substance other than lead, and that lead is used to block radiation in nuclear reactors. These stories aren't really true.

What stops X-Rays is mass. The heavier the object between you and the X-Ray source, the better, but the material doesn't really matter. Lead is often chosen because it's dense and cheap, not because it has any special blocking capabilities. A 1-inch thick lead wall is equivalent to 2.5 inches of steel, or 6-inches of concrete. I'm pretty sure building walls out of concrete and steel would be a better choice for the NSA, since they can bear their own weight, than lining them with lead.

As a corollary, unlike how you see in the movies where any lead, no matter how thin, stops Superman's vision, the real issue is the amount. Twice as much mass stops X-rays twice as good. Thin lead foil in your undies to protect your modesty around Superman isn't going to work -- there's just not enough mass.


The major threat to the NSA isn't X-rays, but radio waves. The NSA wants to stop signals from getting out (TEMPEST), and radio waves from getting in (EMP). To stop these things, what you really want are electrical conductors. Sure, lead is a conductor, but copper and aluminum are far better. These are the metals you want to line your room with, rather than lead.

A minor threat is sound, preventing people from eavesdropping. Most sound protection is done by controlling the echoes, as in an anechoic chamber. But mass also helps, so lead is sometimes used for deadening sound, often in foams that both distort and block the sound. I'm not sure if this has any bearing on the NSA "lead lined rooms".


What I think happened here is that the NSA talked about SCIFs, which have to be TEMPEST hardened against leaking electromagnetic radiation, and the reporters at CBS just assumed this meant "lead" without checking the facts.

So my question for anybody is this: are rooms at the NSA actually lined with lead? And if so, why?

4 comments:

Steve Syfuhs said...

Just a thought, but I wonder how that works with things like nuclear bunkers. Assuming that's an attack they actually built to defend against. Not the fallout side of things, but from a blast. I don't know if lead would be a good material for that either though.

Dan Anderson said...

It would be copper. But it's not overly commonly used in SCIFs - most SCIFs are TEMPEST "hardened" by fences and guards that enforce distance.

altmode said...

I lived nearby when the newer NSA buildings (the ones with the black glass sides) were being constructed in the 80s, and before the glass went up, entire building (except for considerably smaller actual windows) were covered in copper. They were very pretty. Who knows, there may be internal shielding as well, but the outer shield is copper. For good RF shielding, you want good conductivity.

Probably for reasons of cost, I have seen SCIFs constructed of welded steel plate as well. Not quite the conductivity of copper, but structurally sound. But lead just doesn't make sense.

Borepatch said...

Used to work in those buildings. Not only were they wrapped in copper foil, but the windows had a fine mesh of wires in between the two glass panes.

Not sure if all SCIFs have to be done like that - a lot rely on distance and the inverse square law to make signals degrade.