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"I'm dealing from memory from Bill Porter's advanced physics class in 1960, but I was sure the U-238 was the nasty stuff, not the U-235. I know I'm right about the flourine gas diffusion, since it was such a fascinating way of solving a problem that had no chemical solution. "

No problem, if you don't work with it every day it's easy to forget. For all you would-be bombmakers: Actinide series, odd number of neutrons and a half-integral spin (Manhattan Project asked all US newspapers to voluntarily edit out even the word "actinide"- would they cooperate today?)

You mentioned sniffers...remember way back, someone sold a Co-60 machine to a Mexican scrap dealer who melted it and a bunch of other stuff into rebar and cafe table legs? We found out about it when a truckload of pretty warm rebar went over a geiger system buried in the roadway near Los Alamos. It created quite a scramble. Since then I've wondered where else the sensors may be. Certainly near Oak Ridge and as the poster above mentioned, near the White House.

You are correct about uranium hexafluoride gas being the stuff used in the diffusion plants, too,

"remember way back, someone sold a Co-60 machine to a Mexican scrap dealer who melted it and a bunch of other stuff into rebar and cafe table legs?

Remember it! Oh, yes indeed. I was living in El Paso then, and I recall that at least a couple of kids in Juarez were killed by this Cobalt-60: they were PLAYING with it, because it "glowed in the dark" and was "sparkly".

As I recall, though, quite a few of the cast iron table legs in which this stuff wound up were actually shipped into the Midwest, and were installed in at least one new fast-food restaurant.

So the radiation-monitoring system at the border was not all THAT good. Hope it is better now!

You are correct about uranium hexafluoride gas being the stuff used in the diffusion plants, too,

They called it "Hex." And because of its chemically nasty nature, the name was more than just a contraction.

Several years ago I was involved in a very minor portion of a design project for a gamma-ray detector intended for orbit. I designed the circuit board on which the detector module is mounted. The detector did not fly because the Challenger explosion set back the whole launch schedule. Subsequently the detector was redesigned with more sophisticated electronics and better 'focus'.

The detector's objective, according to the people I worked with, was to detect gamma emissions from the center of our galaxy. If gamma emissions were detected then that would imply a black hole at the center. Gamma is supposedly emitted near the event horizon of a black hole.

However, considering that the technology of such extremely sensitive detection existed fifteeen years ago it makes sense that such technology, probably even further refined in the past decade, might also be orbited and focused on the planet.

BTW the sought after black hole was detected in our galaxy center as well as some other nearby galazies. How's that for sensitivity?