April 13, 2014  
Rare Elements

If you look up any chemical element at Wikipedia, the article will likely tell you how it was synthesized by stars, produced by supernovae, spallation, or radioactive decay. (Spallation is the splitting atoms by high energy cosmic rays). This reasoning generally explains the relative abundance of elements both here on Earth and in space. After the dawn of the nuclear age Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle came to realize that the abundance of elements and the life cycle of stars could be explained by stellar nucleosythesis. Their 1957 landmark work Synthesis of Elements in Stars stands to this day as a pillar of modern astronomy.

           

I first learned about nucleosynthesis and elemental abundance many years ago from Isaac Asmimov. His books taught me astronomy as far back as 1969. In one of his books he mused about life's bottleneck element, Phosphorous; and about two of the rarest naturally occurring elements -- Francium and Astatine. Francium's two naturally occurring isotopes are Fr-221 and Fr-223 with half lives of 4.8 minutes and 22 minutes respectively. A bulk collection of Francium atoms has never been observed thus it's chemical properties are mere speculation. It's estimated that about one ounce of this element exists in the entire Earth's crust as transient decay products of Uranium and Thorium. The most common, naturally occuring isotope of Astatine has a half life of only 32 milliseconds and about 100 milligrams is expected to exist in the Earth's crust at any given moment.

There are four primary radioactive decay series. These heaviest elements were created during supernova explosions and dispersed into the hydrogen cloud that eventually collapsed to form the sun and solar system. The first series really begins with Thorium-232 as it is the first element in the series with a half-life on the order of the age of the solar system. Plutonium 244 with an 80 million year half-life would have been reduced to 5 billionths of a billionth of its original amount after 57 half-lives since the solar system began. The Neptunium decay series has all decayed to Bismuth-209 as 2000 half-lives have passed for Neptunium-237, its longest lived element. The Radium series starts with Uranium-238, and the Actinium series starts with Uranium-235.

Thorium Series
Element Half-life Product
252 Cf2.645 a248 Cm
248 Cm3.4105 a244 Pu
244 Pu8107 a 240 U
240 U14.1 h 240 Np
240 Np1.032 h240 Pu
240 Pu6561 a236 U
236 U 2.3107 a232 Th
232 Th1.4051010 a228 Ra
228 Ra5.75 a228 Ac
228 Ac 6.25 h228 Th
228 Th1.9116 a224 Ra
224 Ra3.6319 d220 Rn
220 Rn55.6 s 216 Po
216 Po0.145 s212 Pb
212 Pb10.64 h212 Bi
212 Bi64.06% 60.55 min
35.94%
212 Po
208 Tl
212 Po299 ns208 Pb
208 Tl3.053 min208 Pb
        
Neptunium Series
Element Half-life Product
249 Cf351 a245 Cm
245 Cm8500 a241 Pu
241 Pu14.4 a241 Am
241 Am432.7 a 237 Np
237 Np2.14106 a233 Pa
233 Pa27.0 d233 U
233 U1.592105 a229 Th
229 Th7340 a 225 Ra
225 Ra14.9 d225 Ac
225 Ac10.0 d 221 Fr
221 Fr 4.8 min217 At
217 At 32 ms213 Bi
213 Bi97.80% 46.5 min
2.20%
213 Po
209 Tl
213 Po 3.72 s209 Pb
209 Tl2.2 min 209 Pb
209 Pb 3.25 h 209 Bi
209 Bi 1.91019 a205 Tl
        
Radium Series
Element Half-life Product
238 U4.468109 a234 Th
234 Th24.10 d234 Pa
234 Pa99.84% 1.16 min
0.16%
234 U
234 Pa
234 Pa6.70 h234 U
234 U245500 a230 Th
230 Th75380 a226 Ra
226 Ra1602 a222 Rn
222 Rn3.8235 d218 Po
218 Po99.98% 3.10 min
0.02%
214 Pb
218 At
218 At99.90% 1.5 s
0.10%
214 Bi
218 Rn
218 Rn35 ms214 Po
214 Pb26.8 min214 Bi
214 Bi99.98% 19.9 min
0.02%
214Po
210 Tl
214 Po0.1643 ms210 Pb
210 Tl1.30 min210 Pb
210 Pb22.3 a 210 Bi
210 Bi99.99987% 5.013 d
0.00013%
210 Po
206 Tl
210 Po 138.376 d206 Pb
206Tl4.199 min 206 Pb
        
Actinium Series
Element Half-life Product
239 Pu 2.41104 a235 U
235 U7.04108 a 231 Th
231 Th25.52 h231 Pa
231 Pa32760 a 227 Ac
227 Ac98.62% 21.772 a
1.38%
227Th
223 Fr
227 Th 18.68 d223Ra
223 Fr99.994% 22.00 min
0.006%
223 Ra
219 At
223 Ra11.43 d219Rn
219 At97.00% 56 s
3.00%
215 Bi
219 Rn
219 Rn3.96 s215 Po
215 Bi7.6 min 215 Po
215 Po99.99977% 1.781 ms
0.00023%
211 Pb
215 At
215 At0.1 ms211 Bi
211 Pb36.1 min211 Bi
211 Bi99.724% 2.14 min
0.276%
207 Tl
211 Po
211 Po516 ms207 Pb
207 Tl4.77 min207 Pb

Measuring the relative abundances of radioactive elements and their final decay products (Lead 206,207, and 208) allows us to determine the age of the Earth and solar system. By measuring the ratio of decay products from different decay series, scientists are able to construct multiple time-lines that intersect at their beginning. Clair Cameron Patterson of Mitchellville, Iowa and Grinell College, studied meteorites using a mass spectrometer at Argonne National Laboratory. In 1956 he deduced the age of the Earth to be 4.55 billion years and this value has stood the test of time. Incidentally, Clair campaigned to end the use of leaded gasoline which ultimately spared millions from low levels of lead poisoning thus ultimately raising the average IQ of the industrialized world. Dr. Patterson's work to ban tetraethyl lead from gasoline was strongly opposed by the Ethyl corporation at the time, but science won in the end. It's a prime example of how work in one field of science had a significant human impact in a seemingly unrelated field. Iowan Dr. James Hansen is currently on a strikingly similar journey which began with the study of the atmosphere of Venus and led to a campaign against fossil fuel corporations regarding climate change.

The neptunium decay series was not known at the time Dr. Patterson did his work, as it was only recently discovered. Although primordial Neptunium has disappeared from Earth, it might surprise you to learn you probably have a small amount of this stuff in your home. Your smoke detector is powered in part by Americium 241 which was created in nuclear reactors as a radioactive waste product. About 1 μg is used as a source of alpha particles which ionizes gas to create a small current which can be blocked by smoke, setting off the detector. A microgram of Am-241 (241/Avogadro's number) would contain about 2.5 X 1015 atoms with a half-life of 433 years. If you've had your smoke detector for 10 years, your smoke detector now contains 80 trillion atoms of Neptunium-237. And it may even contain up to 10 atoms of Bismuth-209, which means it also briefly contained Francium and Astatine.