Making
a Poisoner: Half-life
in a scientific detective story
Palestinian Liberation
Organization leader Yasser Arafat died in November
2004 in a French hospital, where he had been flown after
falling ill in October 2004. After suggestions were
made that Arafat
was poisoned with the radioisotope Polonium (210Po),
an autopsy was begun in February 2012.
The 2006 Litvinenko assassination has been reconstructed as a single oral dose of 109 Bq (= 1 GBq) of 210Po, which resulted in death in <30 days, from easily diagnosed acute radiation syndrome. Of this dose, 90% would have been excreted rapidly. The remaining 10% would have been absorbed into the blood system, and thence into the kidneys, where it would be excreted initially at a rate of 0.5% / day. Then, (0.5% / day)(10% x 109 Bq) = (5 x 10-3 x 10-1 x 109 Bq) = 5 x 105 Bq / day.
1) The evidence of an assassination attempt was the presence of 210Po in Arafat's urine-stained underwear. Suppose a typical urine stain in underwear arises from 1 ml urine. At the above excretion rate of (5 x 105 Bq / day) / (500 ml urine / day) = 103 Bq / ml, any one such stain would contain 103 Bq. As in the Litvinenko case, Arafat died in < 30 days after falling ill, during which time he would have excreted a maximum of (30)(0.5% / day) = 6% of the single initial dose. Alternatively, the biological half-life is 50 days, thus over 30 days, (1 - 0.005)30/50 = (0.995)0.6 = 99.7% would be retained and 0.3% lost. This is negligible.
2) 210Po has a physical half-life of 138.4 days, so there are 19 half-lives between death in October 2004 and testing in February 2012. The physical decay factor is then (0.5)19 = 2 x 10-6. Biological decay stops at death.
3) A 1 ml urine stain would therefore be expected to decay to (103 Bq)(2 x 10-6) = 2 x 10-3 Bq = 2 mBq. Authors of an autopsy report in The Lancet concluded “it is reasonable to expect an activity in the order of magnitude of 1 ~ 10 mBq….” in the observed stains in Arafat's underwear, under the assassination model above.
4) The background [210Po]
= ~3 mBq: suspicion of poisoning begins to arise at a
residual level of >10 mBq. The two most
highly contaminated stains in Arafat's underwear showed 60
~ 181 mBq, and his toothbrush 21 mBq)
contamination. A dose of 1 GBq is 100 times
the LD50 lethal oral dose of 10
MBq, if the poisoner wanted to be subtle.
5) 210Po is produced artificially by neutron bombardment of Bismuth (210Bi) in specialized reactors. It also bio-accmulates in nature, for example in tobacco and cotton. Arafat’s brand of underwear, tested new out of the package, had as much as 20 mBq.
Conclusions: A single 109 Bq (= 1 GBq) poisoning assassination would be expected to decay to below background levels, by physical processes, in the interval between death and autopsy. The reported residual levels are as much as 90 times that. The poisoning scenario is (very) unlikely to underestimate dose by as much as a factor of 10, and if it did expect classical acute radiation syndrome manifested, as with Litvinenko. News media took “contamination well above background” as proof of poisoning, however the higher the estimate above background, the poorer the fit to a poisoning model, as versus (deliberate) contamination. A dose of 1 GBq is 100 times the LD50 lethal oral dose of 10 MBq, if the poisoner wanted to be subtle.