Radioactive materials are widely scattered in all the rocks of the earth. Pick up any volcanic rock from the ground and it is likely to contain atoms of radioactive elements.

Radioactive elements send out radiation all the time, although gradually the level of radiatin decreases.  However, whatever the level of radiation, all radioactive materials lose half of their remaining surplus energy at a fixed rate.  Thus,the time taken for a piece of an element to lose half of its remaining energy is always the same, whether the radioactivity is strong or weak.   The rate of change is described by the term half-life, meaning the time it takes for the radiation of an element to decrease by half.

The decay of some substances, such as Uranium-238 and Thorium-232, appears to continue almost without us detecting any difference in the size or quantity of these substances. Thorium-232, for example, has a half-life of 14 billion years.

Other radioactive substances show a marked decrease in their size according to their respective decay rates. Among these is the isotope Thorium-234 which, after isolation from uranium, decays to half its original radioactive intensity within 25 days. Another substance, such as Polonium, has a half-life of less than 10^-6 years (0,00001 years).

The half-life can be explained by taking a practical example. Let's take Carbon-11, which has a decay rate of 3,5% per minute. When this atom decays, we will find that only 96,5% of the original amount is left after one minute. After 2 minutes, 93,12% will be left and after 10 minutes, 70% of the original amount is left. After 20 minutes ± 50% of the original amount is left; therefore we can state that the half-life of Carbon-11 is 20 minutes.

The time it takes for the radiation coming from a sample of radioactive element to decrease by half

This number line indicates the percentage left, after the decay rate, per minute of Carbon-11.

To find the decay rate of a substance, you can leave a lump of Radium, which will gradually decay into various smaller particles. This rate of decay is measured by how long it would take for half the Radium to decay.

Nuclear fission is the process of splitting atoms. When a nucleus of an (e.g. Uranium 235) atom is hit by a neutron bullet and splits into two parts, it is called fission.  The total mass of the two parts will be less than the initial mass.  The "lost" mass will be converted into energy according to Einstein's equation, E = mc² (E = energy | m = mass | c = speed of light ~ 3 x 10⁸m s^-1.)

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halflife n.

the time taken for the radioactivity or some other property of a substance to fall to half its original value.

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