Question #2 - Some atomic nuclei including those of isotopes of uranium, carbon-14 and strontium-90 are not stable. These atoms spontaneously disintegrate over varying periods of time. Some take billions of years to disintegrate and some only a few seconds. Scientists can measure the rate of decay of such unstable nuclei but because individual nuclei of a particular element decay at slightly different rates the measurements involve the overall rate of decay of the nuclei in a collection of atoms of an element. The measurement is defined as the time it takes for a specific proportion of the nuclei in a sample to decay.

Question: When measuring the rate of decay of an unstable atomic nucleus, what is the specific proportion (of a sample of the atoms) used by scientists - is it one quarter, one third or some other fraction of the sample?