# How does radioactive decay relate to half-life?

##### 1 Answer

The half-life of a radioactive nucleus is the time required for one-half of the material to decay into a more stable substance.

For example, Sr-90 has a half-life of about 25 years. It will have an intensity of 100% when new. After one half-life (25 years), its intensity will be cut to 50% of the original. After two half-lives

(50 years), it will have an intensity of 25% of the original. After ten half-lives (250 years), less than one-thousandth of the original activity will remain.

The equation for the graph is

where

*t*,

λ is the *decay constant* for the nucleus and is calculated by the formula

If the half-life is 25 years, the decay constant is

That is, 2.8 % of the material decays every year.