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Monday, December 18, 2006

Polonium 210

I've been meaning to post about the fascinating poisoning of former Russian spy Alexander Litvinenko.

The political ramifications are a bit beyond my understanding, other than the obvious consideration that someone was bold enough to assassinate a political enemy on foreign soil.

Why instead of radioactive poison didn't they just shoot him, disappear him, or run him over? Curious stuff...

What I find fascinating about this case is the technical challenges and genius at the nature of the poisoning, as well as the choice of the radionuclide, Polonium 210 (or Po-210 for those in the nuclear biz).

I've done a bit of reading on Po-210 since the news of Litvinenko's poisoning hit the wires. It's quite an interesting and quirky nuclide.

Let me start by sharing a little about radioactive decay. There are four basic decay mechanisms for radioactive nuclei, each with distinct mechanisms for releasing the energy of decay. As the mechanisms are distinct, so must be the means for detecting them.

Gamma rays are ubiquitous. These are elecromagnetic waves with very short wavelengths, somewhat shorter than hard x-rays. Gamma rays are usually associated with a neutron decaying to a proton and an electron, or just being fired out of an unstable nucleus. 90-95% of all decays release a gamma ray, and it's probably responsible for most of the background radiation everyone is exposed to, either indirectly from cosmic rays, or naturally occurring radioactive elements.

Biological damage due to gamma rays is not bad, as these things go. They create ions by photoelectric effect, pair production, or by compton scattering. The upshot is that charged electrons (or positrons) are knocked loose and are available to ionize (damage) internal tissues. I think of these as little BB gun shots. A few are not so bad, but a shotgun blast of them is quite harmful. Gamma rays have a biological damage factor of x 1

Sheilding: High density metals are best at absorbing gamma rays. The greater density of the metal increases the likelihood of the above interactions attenuating the gamma ray before it reaches you. The amount of sheilding required depends on the intensity (decays per second), and incident energy of the gamma rays.

Detection: Exept for the weakest gamma rays, which are rare, gamma is easily detected. A simple geiger detector can identify an energetic gamma emitter 10-20 feet away.

Beta particles (also known as electron emission) are also ubiquitous. Most of the known decay mechanisms are beta decays, and are usually accompanied by a gamma ray. Beta emissions are mostly associated with the two highly unstable nuclei remaining after a heavy nucleus is split (fissioned).

ASIDE: The highly radioactive spent fuel from nuclear plants undergoes a phenomenal amount of beta/gamma activity. In fact these unstable fission products generate about 7% of the heat of a reactor at full power immediately after it has been shut down. This heat tapers off over the course of several weeks, but without cooling, this decay heat can melt the a reactor core long after it's been shut down.

Back to beta particles. They are basically exceedingly fast-moving electrons, and because gamma rays cause damage by creating these particles, their biological damage mechanism and severity are the same, x 1.

Sheilding: Beta particles have quite a bit of penetrating power, though nowhere near the power of gamma rays. A 1/4 inch sheet of aluminum is the textbook example of what will stop beta particles from reaching you. Obviously this approach will not work if the beta emitter has entered your body.

Detection: Beta particles are easier to detect than gamma. A gamma ray can pass right through a geiger detector without interacting with a molecule of the gas. A charged beta particle is quite likely to have several interactions as it passes through the gas.

Neutrons: These particles are rare, except within the confines of a nuclear reactor, so I won't go into them in depth. I did want to mention them, as they are what cause atoms to become radioactive, (e.g. emit alpha, beta, and gamma radiation) and cause them to fission. This is important, because this part of what makes the Polonium 210 story so interesting.

Alpha particles are most often associated with heavy nuclei (think Radium, Thorium, Uranium, Radon). An unstable heavy nucleus usually kicks out an Alpha particle and a gamma ray as it seeks a lower (more stable) energy level. The alpha particle is actually just a helium nucleus (two neutrons and two protons), that dropped the electrons as it was blasted out of the nucleus.

Here's what's bad about alpha particles biologically: They have a large +2 electrical charge, and compared to the other decay particles, they are massive. Due to their mass and charge, they ionize tissue quite readily, doing a great deal of damage. In many respects, they are like little nuclear hollow-point bullets. They do not penetrate very far in skin tissue or in air, but are lethal when taken internally.

Sheilding: In general an alpha particle can be blocked by a piece of paper. Taken internally, there is nothing to prevent an alpha-emitter from doing truly massive damage. Alpha particles have a biological damage factor of x 20.

Detection of alpha emitters is not normally difficult. Since most nuclides emit a high energy gamma ray at the same time as the alpha particle, the gamma ray gives away the presence of a radionuclide. Interetingly Po-210 is one of the few alpha-emitters that does not emit a gamma ray during decay. Direct detection of alpha particles is more difficult; due to their large charge, they tend to come to have very short flight paths, even in air. Detectors must have thin windows (mylar) and be very close to the source. Geiger detectors and scintillation counters in close proximity to the source are your best bet for detection of alpha particles

Now that we have the primer on radiation, sheilding, biological damage, and detection taken care of, lets look at how Litvinenko was poisoned:

1) Polonium-210 is an alpha emitter that has a half-life of about 138 days. That means that 138 days after you take it from a reactor, only half will remain, after another 138 days you'll only have 1/4 of it, then 1/8, 1/16, etc, etc. There is no gamma ray associated with this decay.

2) Polonium is exceedingly chemically toxic. Weight-for-weight, polonium is around 5 million times more toxic than hydrogen cyanide, the gas used in state's gas chambers. 50 nanograms will kill 50% of those exposed to it. It accumulates in the liver and spleen where it chemically attacks them.

So what can we deduce about the poisoners' technological infrastructure from the above information?

They have a nuclear reactor (or access to freshly spent nuclear fuel) to even produce Po-210.

They have access to a hot cell which gives them the ability to remotely handle and chemically process highly radioactive spent nuclear fuel to separate the Polonium.

They realize that once the Polonium 210 is ingested, the victim's body will act as a shield to prevent alpha particles from reaching external radiation detectors. No gamma rays will give away the existence of the Polonium.

They understand that the dose of Polonium 210 will kill Litvinenko either chemically or due to the the alpha radiation effects.

Which leaves us with the question of who did it... all I can say for certain is that they have highly developed nuclear technology: Nuclear reactor, fuel handling, and isotope separation. There are a handful of countries and companies that could do this, but only one that had all of that plus a grudge ;)

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