Cesium-137 has a radioactive half-life of about 30 years, and traces of the isotope still persist from above-ground nuclear bomb tests in the 1950s and '60s.

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Once read they will self-destruct in your brain faster than the half-life of radioactive cobalt.

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Its half-life the time taken for half the radioactive atoms in a sample to decay is 109 minutes, compared with two minutes for oxygen-15, ten minutes for nitrogen-13 and 20 minutes for carbon-11.

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