Saturday, July 15, 2006

Plausible deniability

In the interest of transparency and self-preservation, I should mention that the purpose for which I am trying to procure some gallium is NOT the one explained here. So, if the guy with the earpiece camped outside my back fence is reading this, relax, dude. All of my nuclear weapons use uranium; the gallium is just for checking for oxide interferences on rubidium.

Speaking of uranium, back when I was in grad school, my officemate discovered the hard way that the elemental U in standards made by the US government is depleted. As every geochronologist knows, the natural 238U/235U ratio is 137.88/1. Mr. Mouse* was trying to measure U isotopes in something- corals or fish ears or calcite veins, I suspect- and all his natural samples were coming out enriched in 235U - almost to the point where they could (if it wasn’t for the ppm concentrations) be used for fuel.

Obviously that made no sense. The problem, as it turned out, was that he was assuming the natural ratio for the standard, when the actual, depleted, ratio was about 450/1. so the observed enrichment was just the result of propagating the assumption of the standard into his unknowns. Somewhere out there is a paper that actually lists the depletion factors for a variety of synthetic geologic standards. But it hadn’t even occurred to him that the US military-industrial complex would have screwed with the normally immutable isotopic compositions on which geochemistry can usually rely. So he didn’t look for it. Fortunately, gentle readers, none of you have to repeat Mr. Mouse’s mistake. The first person who can post the reference to that paper here wins a clod of soggy permafrost.

In the meantime, spare a thought for the isotope that we under-measured, because it had been removed from our sample. Somewhere in a dark Montana bunker, or silent undersea vessel, those missing 235U atoms are carefully stowed away. Aside from the occasional alpha decay, they are completely passive. They are merely waiting; waiting for a fast neutron to break their hearts, so that they can destroy civilization with the energy released by that fragmentation.

*Not his real name.

No comments: