The timing where a radioactive material gives off radiation and which changes it to another nucleus is various by each nucleus.
But for example, if thinks by many human beings, probability will bring a meaning so that a life expectancy may have a meaning because the number of atoms is too much.
Time until the probability that a radioactive material will give off radiation will be 50% is called "half-life."
When this is put in another way, it is the time which it will take before the strength of radioactivity becomes half.,br. The strength of the radiation of the first state set to 1.
If the length of half-life changes with 1 time, twice, 3 times…10 times,
the strength of radiation is started Monotone decreasing with 1/2, 1/4, 1/8... 1/1024.

Now, the things that we doubt is "When is it set to 0?"
As a graph, the strength of radiation is halved for every half-life,
but finally, it does not become zero.

For example, the physical half life of radioactive iodine 131 is made into about eight days.
But the physical half life of the cesium 137 which be a problem now is about 30 years.
Thus, the length of half-life is decided for every radioactive material.
It is various from the short period of time of several day unit to what 100 million years and a semipermanent thing.

Using this,　There is "Radiometric age determination".（examine in detail →『artificial radiation』）