Friday, 1 April 2016

There's radiation, then again, there's Radiation.

As I recall, what counts in radioactivity is:

  1. its chemistry
  2. how much
  3. half-life
  4. type of decay
  5. decay energy
  1. Chemistry is important because, for example, gases are easily dispersed in the atmosphere. E.g.2 Uranium is soluble, so may be taken in through the stomach to be easily absorbed (in theory). Any plutonium eaten tends to stay insoluble, so nearly all of it will pass right through your body. e.g. 3 Radioactive Group I and Group II elements such as Cesium and Strontium may substitute for naturally occurring Group I, II elements in your body (sodium, potassium, calcium). Strontium-90, in particular, may build up in your body if you absorb it (in bones in place of calcium).
  2. The amount of substances is puzzling because the quantities are often very low (with leaks). So low that radiation specialists use special units such as Becquerel, sievert, gray; fractions and multiples thereof, plus loads of obsolete units!
  3. Half-life is important. It measures the time for half the substance to decay to something else (usually a non-radioactive substance). Substances with short half-lives are more radioactive. Substances with long half-lives (thorium, depleted uranium) are pretty safe, not very radioactive at all.
  4. The four main types of radioactivity are neutron, alpha, beta, and gamma. We can ignore neutrons, they only really found in nuclear reactors, and free, they have a half live of only 10.2 minutes. Alpha particles are energetic helium nuclei, beta particles are energetic electrons, and gamma rays are energetic photons. Gamma rays are penetrating, so are dangerous in, or outside your body. Alpha and beta particles not very penetrating (your skin will stop them), so they are only really dangerous inside your body. Because Alphas are quite heavy a single alpha is considered 20 times more dangerous than a beta or gamma. Gammas are the least interactive, alphas the most interactive.
  5. Tritium decays by emitting an electron with a typical energy of 5.7 keV. Not very energetic. In contrast a radioactive isotope of potassium : potassium-40, decays 89% of the time, by beta emission, with a maximum energy of 1.33 MeV. Comparing these: K-40 (1,330,000 eV, maximum) and tritium (5700 eV, typical). The ratio of energies is 233 : 1

PS: This brings up the difference between Sieverts and Grays. A Gray measures the radiation dose. So does a Sievert. The difference is that a Sievert "weights" the radiation so alphas count 20 times more than betas or gammas. "Weight" :- as in count things in absolute importance. An astute reader may notice that if we're to weight radiation by effect rather than just amount, we should also count its energy (e.g. a K-40 beta being up to 20 times more energetic than a tritium beta).

PS: I didn't use the typical energy value for the K-40 beta because I don't know what it is! I am aware I'm not allowed to compare maximum with average values but I did it anyhow to show that there are gammas and gammas, and tritium is near the least of one's worries.

In terms the risks of radiation, I favour Mohan Doss submission to the NRC.

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