Lamentations on Chemistry

One of the interesting parts of having a blog is that you get to see the search terms that people use to find your site.  We’ve been getting lots of hits lately from persons trying to squeeze polonium information out of the web.  I went through some venerable reference books on my shelf and collated some general fun factoids on that fashionable metal - polonium.

Polonium is actually a natural element found in thorium and uranium deposits.  In the Radiation Health Handbook I count 33 isotope entries for polonium, 7 of which are metastable, or isomeric, states. The known isotopes are Po-193 through Po-218. Po-209 has the longest half-life at 103 years.   Bismuth 209 is the heaviest stable nuclide. Nuclei heavier than bismuth often emit alpha particles, and do so exothermically or spontaneously.  Polonium, one atomic number above Bi, has no stable isotope. 

Loss of an alpha particle results in a drop of atomic weight of 4 and atomic number of 2.  You can think of an alpha particle, or helium nucleus, as a good leaving group.

Polonium is very scarce.  Its discovery, well known for being famous, was by Marie Curie and was accomplished by isolation from tons of ore. It was named after her home country of Poland. Today the the production of Po is effectively limited to Po-210  and is bred in nuclear reactiors via the transmutation of Bi-209 by neutron absorption to afford Bi-210.  Neutron rich nuclides can drop their neutron count through the emission of beta particles (electrons) with a subsequent uptick of atomic number by one. So the Bi-210 nucleus transforms to Po-210 by beta emission.  The polonium is isolated by fractional distillation from the remaining bismuth.

One gram of pure Po-210 is said to evolve 141 watts of heat. Consequently, one use of Po-210 has been for thermal electric power generation. It’s near exclusive emission of alpha’s minimizes shielding problems.  Another important use of alpha emitters is for the generation of neutrons. This interesting process uses alpha particles to interact with beryllium nuclei to afford the extrusion of neutrons.  In this way it is possible to have a compact neutron source.  Place the source in a tank of water or paraffin, arrange for an opening, and presto! You have a cheap neutron beam source- sometimes called a neutron Howitzer.  Plutonium-beryllium (PuBe) is more common than polonium because of the long half-life of available non-fissile plutonium sources. The neutron Howitzer is commonly used in neutron activation studies.

The chemistry of polonium is exotic by virtue of it rarity and the pragmatics relating to its high specific activity.  It’s high specific activity causes it to radiolyze the solvent that the reaction or other manipulation is occuring in.  This is especially problematic for organic solvents. The high activity will pose serious safety risks for the chemist in handling. Advances in organopolonium chemistry have been complicated by the pyrolysis of the organic fragments via radiolysis.  This also complicates the preparation of crystals for x-ray crystallography. A properly equipped facility night have a remote manipulation setup for handling high activity materials.  This is especially critical when the permissable body burdens are in the picogram range.