Half-Life The time required for one half of the nuciei in a sample to decay is called the half-life of the isotope. Half life is a common way of expressing the rate of radioactive Figure 11.17 illustrates what occurs when a radioactive parent decays directly into its stable daughter producL When the quantities of parent and daughter are equal(ratio 1 l), we know that one half-life has transpired. When one quarter of the original par ent atoms remain and three-quarters have decayed to the