Making a
Poisoner: Half-life
in a scientific detective story.
Former Palestinian
Liberation Organization leader Yasser Arafat died in
November 2004 in a French hospital, where he was flown
after falling ill in October 2004. After suggestions
that Arafat
was poisoned with the radioisotope Polonium (210Po),
an autopsy was begun in February 2012.
The previous Litvinenko
assassination has been reconstructed
as a single oral dose of 109 Bq
(= 1 GBq) of 210Po., which
resulted in death in <30 days, accompanied by
characteristic toxicity and acute radiation syndrome.
Of this dose, 10% would
have been absorbed into the blood system, and thence into the
kidneys and urine, where it would be excreted initially at a
rate of (0.5% / day)(108 Bq)
= 5 x 105 Bq / day. The
biological half-life is 30 ~ 70 days
could introduce a factor of no more than two prior to
death within <
30 days.
1) The evidence in the Arafat case was the
presence of
210Po in his urine-stained
underwear. Suppose a typical urine stain in underwear derives
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 during which time he would have excreted a
maximum of (30)(0.5% / day) = 6% of the single
initial dose.
2) 210Po has a physical half-life of 138.4 days, and there are 19 half-lives between death in October 2004 and testing in February 2012. The physical decay factor is (0.5)19 = 2 x 10-6
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 conditions and assumptions above.
4) The background = ~3 mBq, and suspicion of poisoning would arise at >10 mBq. The two most highly contaminated stains in Arafat's underwear showed 60 ~ 181 mBq, his toothbrush 21 mBq
Conclusions: A 109
Bq (= 1 GBq) oral dose would be
expected to decay to below background levels, by biological
and 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.
5) 210Po
is produced artificially by neutron bombardment of Bismuth
(210Bi)
in specialized reactors. However, it also occurs in
nature, for example in cigarette tobacco and cotton. Arafat’s
brand of underwear, tested new out of the package, had as much
as 20 mBq.