Circumstantial Evidence - Evidence that proves a fact by means of an inference. For example, from the evidence that a person was seen running away from the scene of a crime, a judge or jury may infer that the person committed the crime.
While there appears to be no direct linkage between the death of former KGB then Russian FSB agent Alexander Litvinenko and his former masters, the rare substance that was used to bring about his death is strong circumstantial evidence that the resources of a nation/state were involved.
Or does it?
Polonium 120 is an alpha emitter and can be purchased legally without a license for $69 (plus $11.95 for S&H). Because alpha particles are the big kahunas of radiation (alpha particles are basically helium nuclei - two protons and two neutrons), alpha emitters are very destructive: imagine mini-deer slugs slamming through molecules within your body. But because of their size they do not penetrate far; alpha particles can be stopped by a sheet of paper.
And from the New Scientist: “Alpha particles are ionising. When they strike tissue they knock electrons out of molecules. Such damage can in serious cases wreck cellular machinery resulting in cancer, radiation sickness, or worse," said chemist Andrea Sella at University College London.
For more information on the effects of radiation on human tissue and why it is so devastating, read this Department of Energy article.
Radioactive materials aren't all that hard to procure. Read this story about a high school student who tried to build a nuclear reactor in his garage. He came a lot closer to accomplishing this than I care to think about. He nearly poisoned his whole neighborhood.
To be effective as a poison, polonium must be ingested (more on the effectiveness of polonium as a poison from Blogs of War). It is alleged that Litvinenko was given the poison at a London sushi restaurant on 1 NOV 06, possibly administered as a salt, polonium nitrate. He died last Thursday, 23 NOV 06.
From another source:
So it seems that a very small amount is needed to provide a lethal dose. Just how lethal is this stuff and what constitutes a lethal dose? Radioactivity in America is measured in curies (Ci) and internationally measured in becquerels. A becquerel is defined as "the activity of a quantity of radioactive material in which one nucleus decays per second ." A Curie is 37 billion becquerels.
One of the deadliest radioactive isotopes is 210Po (Polonium-210). It is a strong alpha emitter with a half-life of 128 days. Polonium metal is also rather volatile, with a melting point of 255 degrees C, making it a particularly deadly component of tobacco smoke (inhaling alpha emitters is not a very good idea). Fortunately, polonium is found only in minute amounts in Nature.
Polonium is so radioactive that a 0.50 gram sample will reach temperatures greater than 500 degrees all by itself.
One measure of lethality in radioactive material (decaying nuclei) is its half life. Plutonium (Pu-239) a deadly weapons grade metal, has a half life of over 24,000 years. Polonium, (Po-210) has a half life of 138 days.
For $81 ($69 plus $12 S&H) you can purchase a small amount of polonoium that generates 0.1uCi or one tenth of a microcurie (one millionth of a curie). A microcurie is the amount of radioactive material in which 37,000 nuclei decay per second, so one tenth of that is 3,700 nuclei decays per second. These small amounts are primarily used for certifying various kinds of equipment.
Here are some terms to help define what constitutes a lethal dose of polonium.
Curie (Ci) - Amount of radioactivity produced by a given amount of a substance.
1 Ci = 37 billion particle emissions per second
rad (radiation absorbed dose) - The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material.
1 rad = 100 erg/g
rem (radiation equivalent, man) - The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose.
1 rem = rad × Q (for alpha particles Q = 20).
Roentgen is not used as it is a measure of exposure to x-rays and gamma rays in air.
Becquerel (Bq) - One Becquerel is that quantity of a radioactive material that will have 1 transformation (decaying nucleus) in one second.
Gray (Gy) - Used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material.
1 Gy = 100 rads.
Sievert (Sv) - Used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose.
1 Sievert = 100 rem
The US Environmental Protection Agency lists a fatal dose as 1,000 rad. The US Nuclear Regulatory Agency states: The dose of radiation expected to cause death to 50 percent of an exposed population within 30 days (LD 50/30). Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5 sieverts) received over a very short period.
So pick your poison: either 450 rem or 1,000 rad. If you use the conversion from above where rem = rad x Q (where Q = 20 for alpha particles) then a fatal dose is over 20,000 rem.
It starts to get real murky here. I believe that 1,000 rad is a single exposure, while the 450 rem is a continuous exposure over a short period of time. But it still doesn't tell me how much polonium is needed to give us this amount of radiation exposure over a three week period? Should this material be watched at least as closely as, say, firearms?
Polonium has a specific activity (amount of radioactivity - or the decay rate - of a particular radionuclide per unit mass of the radionuclide) of 4.5E3, or 4500 curie (4.5 curie times 1,000)per gram of polonium. This means that a gram of polonium will undergo 167 thousand billion nuclei decays per second.
I'd appreciate it if someone checks my math and my methodology. I divided 1.67E+14 (the number of decaying nuclei per second in a gram of polonium) by 3,700 (the number of decays in a microcurie of polonium). This means that a sample of polonium that emits .1 microcurie weighs only 0.00000000002 grams. That is approximately 2 ten billionths of a gram.
This crap is incredibly dangerous. I go to great pains to avoid using the NY Times as a source for anything, but this article gives an indication as to how much polonium was needed to kill Alexander Litvinenko:
Polonium 210 is highly radioactive and very toxic. By weight, it is about 250 million times as toxic as cyanide, so a particle smaller than a dust mote could be fatal. It would also, presumably, be too small to taste.Anyone have a clue as to the weight of a dust mote? Will a dust mote of polonium generate 450 rad? The problem is that converting a curie to anything resembling a dosage measure is very imprecise. In fact, I can't find one. A curie is a measure of radiation activity, a rad is the measure of energy absorbed by tissue and a rem is the biologically effective dose in terms of the particle type. How many curies to generate 100 erg/g? Your guess is as good as mine.