A halflife usually describes the decay of discrete entities such as radioactive atoms in that case it does not work to use the definition that states halflife is the time required for exactly half of the entities to decay.
uranium235 is a naturally occurring isotope of uranium metalit is the only fissile uranium isotope being able to sustain nuclear fission uranium235 is the only fissile radioactive isotope which is a primordial nuclide existing in the nature in its present form since before the creation of earth.
the isochron method many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughtertype isotopes already exists.
To fully understand gentrys hypothesis a basic background in geology mineralogy and radiation physics is helpful the boxes on the next few pages present a brief tutorial in rocks minerals and radioactivity.
Radiometric dating he question of the ages of the earth and its rock formations and features has fascinated philosophers theologians and scientists for centuries primarily because the answers put our lives in temporal perspective.