The Dreaded Sievert.

Amid the problems at Japan's Fukushima-Daiichi nuclear power station much is being made of a unit of measurement known as the Sievert (Sv) in attempts to explain any radiation. This can actually be a little misleading because rather then being strictly a measure of radiation the Sievert is an attempt to quantify the effect that radiation will have on a known element at the atomic level and is defined as the energy of the element in joules divided by the weight of the element in kilograms (j/kg). While this measure is very useful in the controlled environment of the laboratory or theoretical physics it is much less useful in the wider world where there are many different things made up of many different elements all with different atomic weights.

Regardless of the unit of measurement used beta radiation is measured the same way using something called a Geiger tube which is connected to a Geiger counter. A Geiger tube is simply a metal tube with a thin mica window at one end. The tube is filled with a gas, usually argon, of which the atomic weight is a known constant. When a radioactive particle enters the tube through the window it loses it's radioactivity by colliding with the gas particles and ionizes them giving some of them a positive electric charge and others a negative electric charge. Within the tube there is a positive electrical terminal and a negative electrical terminal and when the charged ions get attracted to the corresponding terminal it completes the circuit and creates an electrical pulse. Each one of these electrical pulses represents a radioactive particle entering the tube and in the Geiger counter a rate meter is used to calculate the number of pulses per second. As the atomic weight of the gas is a known constant the voltage of these electrical pulses is used to calculate the Sieverts per second of the radioactive gas.

As air and humans are made up of a variety of elements such as hydrogen, oxygen and carbon all of which have different atomic weights I think that generally it is more useful to think in terms of the electron voltage (eV) and particle per second rate of the radiation rather then it's Sievert value. This allows you to better calculate how far the radiation will travel and how long it will it will last which are more important factors in assessing how much damage it will do. However to put the Sievert counts into some sort of perspective at yesterday's (15/3/11) peak Tokyo was reporting levels of 0.8 microsieverts per hour which is around 0.0000002 radioactive particles per second. When I went to the pub last night because I drank a few beers, smoked a couple of cigarettes and ate a portion of red meat I was exposed to around 26 microsieverts per hour or 0.0000072 radioactive particles per second.

Therefore the health risk from radiation in Japan is only a possibly slightly increased risk of cancer at some point in the future rather then immediate death from radiation poisoning.

However this has not stopped France, Britain and Germany to either order the evacuation or suggest the evacuation of their passport holders from Tokyo. It is worth remembering that all of these countries have nuclear power programs of their own. By threatening to evacuate their passports holders these countries put pressure on Japan's superior nuclear power program to share the details and therefore trade secrets of their operations. So basically Britain, France and Germany are worried about money, not people.