Despite its dangerous reputation among non-physicists, the typical uranium atom is only weakly radioactive. More than 99 percent of the uranium found in nature consists of the isotope U-238, whose atomic nucleus contains 92 protons and 146 neutrons. The laws of physics make this a very stable configuration, with a half-life of 4.46 billion years. In other words, it takes four and a half billion years for half of the sample of U238 you hold in your hand to decay. U-235, with 143 neutrons, is far more radioactive: it has a half-life of just 704 million years. Anti-nuclear activists, among whose number I count myself, will often view these long half-life statistics with alarm, as they imply the material being discussed will be radioactive for a mind-bogglingly long time. And that’s true. For your handful of U-238 to become completely non-radioactive, you’d need to wait maybe eleven times longer than the universe has existed so far. But very long half-lives mean relatively low radioactivity. Stable isotopes have the longest half-lives of all, at infinity and change. It’s the stuff with the short half-lives you have to watch out for. Out in the desert, the naturally occurring uranium mixed in with minerals such as zircons and apatites and such is approximately as dangerous as lead. You wouldn’t want to refine it and pour it on your cornflakes, but if you did the heavy metal poisoning would get you long before the radiation would.

U-235 is a different matter: it’s what they make the bombs and nuclear power plants out of. U-235 spits out around seven times as much radiation as its heavier sibling, and is thus radioactive enough to support a self-sustaining chain reaction. U-238 isn’t. Of course, if you really want radioactive danger you turn to something like plutonium-239, which decays something like 200,000 times faster than U-238.

But U-238 does decay, and it does so at a known and predictable rate. Each decaying nucleus emits an alpha particle — a clump of two neutrons and two protons, a.k.a. a helium atom without its shell of electrons — to begin a cascade of decay, becoming one new unstable isotope after another as the nucleus tries to reach equilibrium. The first alpha emission transmutes the atom into thorium-234, which has a half-life of 24 days. The thorium-234 nucleus emits a beta particle, turning a neutron into a proton and raising its atomic number by one, and thus becomes protactinium-234. Protactinium-234 emits another beta particle to become the highly radioactive uranium-234, which emits an alpha particle to become thorium-230. Thorium begets radium; radium begets radon. Fourteen transmutations, in a cascade that can take a minute or millions of years, bring the decayed U-238 atom at long last to stable lead, 32 nucleons lighter. Eight alpha particles lighter.

When that decaying atom is part of a larger hunk of rock, each of those departing alpha particles tears through the surrounding rock. The particle ionizes many of the molecules it passes, slowing down with each encounter until it finally comes to rest a few micrometers from its parent nucleus. That parent nucleus recoils like a rifle, doing a bit of ionizing of its neighbors as well. The result is a tiny ionized tunnel, sometimes as long as a thousandth of an inch, through the surrounding rock. Each ionized molecule is repelled from its newly ionized, like-charged neighbors, and the tunnel widens a bit. If that tunnel, or fission track, is in a piece of rock of more or less uniform characteristics — a crystal, say, or a bit of volcanic glass, to provide a little bit of uniform background — you could see the tracks with a cheap microscope.

Zircon often has a significant amount of U-238 in it. So do obsidian and mica and titanite. Apatite, a class of phosphate minerals that together make up one of the most common substances in the earth’s crust, is another mineral that reliably contains uranium. Each of these minerals displays fission tracks rather nicely. Researchers will polish and etch a surface, train a microscope on it, and count the tracks.

Apatites are interesting for a number of reasons. Our bones, it turns out, are a sophisticated composite material consisting of organic fibers and apatite nanocrystals. Bone apatite less resistant to acid than fluoroapatites, which is why dentists encourage us all to substitute fluorine atoms for the hydroxyl or carbonate ions in the apatites in our teeth. (Fluoroapatites also have less tensile strength than do the apatites in our bones, which is why people with lots of fluorine in their groundwater have more hip fractures later in life.) Apatite is a source of phosphorus for industrial fertilizer, and those of you who are alive when we run short on available phosphorus, probably in about thirty years, will probably see the price of apatite skyrocket as lots of people starve. (Peak Phosphorus will mandate organic agriculture the way Peak Oil will mandate bicycling.)

Apatite also does something interesting when its temperature begins to approach the boiling point of water: its crystalline structure relaxes ever so slightly. It begins at around 70°C (158°F): small imperfections, scratches, and gaps in the mineral begin to smooth over. Tiny little flaws in the rock, fission tracks included, are annealed. Eventually, they vanish. The higher the temperature, the faster the annealing: at about 400°C, some apatites anneal their fission tracks in the time it might take you to eat a leisurely lunch. At 70-100°C, the time scales needed tend more toward the geological than culinary. But rocks do have time in abundance.

All this means that if you have a sample of apatite of which you know the uranium content, you can count the number of unannealed fission tracks and divide by the rate at which fission tracks would be produced by the sample’s uranium content. The result: a chronometer of time elapsed since that material was heated past 100°C or so. Usually, that means burial under a whole lot of rock. The rule of thumb is that each kilometer below the surface adds about 25°C to the ambient temperature, so if you determine that the apatite crystals in your sandstone show six million years worth of fission tracks, and you’ve determined by, say, the fossil content of the rock that the rock was first formed forty million years ago, then you know that the rock was more than a mile beneath the surface when it was 33 million years old or so. It’s one thing to simply know the age of the rock you’re hefting, another to gain some insight into the evolution of the landscape from which the rock came. The layers that were above your sample may have eroded away completely, no vestige of them remaining anywhere on the earth, but you have tangible proof that they existed, and evidence of their magnitude.

The few kilograms of apatite I hope to leave behind as a minor mineral deposit in the desert when I die are unlikely to be of much use to future researchers: the crystals are too nano-, and besides in my lifetime the desert has had its share of fission tracks significantly enhanced. But the phosphorus in my bones does have an infinite half-life. It isn’t going anywhere. The patience of rocks is also infinite, and I have been pleased these last few months to imagine, some few dozens of millions of years hence, some of that phosphorus deep-buried, refined and metamorphosed. That part of what was me becomes fine green crystals. The crystals await their chance to cool. Uncovered, they begin to count out the age of the earth again.

1. 
   Rana Ravens 2008 12 15 at 2:24:59 pm

   Uncovered, they begin to count out the age of the earth again.

   Nice.