Some nuclides which decay by double beta decay can have half-lives much longer than the present age of the Universe (being about 15×1010 yrs) like Tellurium-128, with a half-life of 5×1024 yrs. Determining halflives greater than 1016 years is very difficult, because these nuclides have very low disintegration rates, usually below background radiation levels; it is sometimes impossible to say whether some nuclides are completely stable or weakly radioactive.

Other nuclides have the shortest known halflives, like that of Li-5 with a halflife of 3×10-22 seconds, which is only 50 times as long as it takes light to traverse the nucleus. I have called these nuclides 'ephemeral', other examples include helium-5, beryllium-6, boron-8, beryllium-8 and boron-9, most have un-bound neutrons or protons.

Carbon-14, with a half-life of 5730 years, is useful in dating once-living organisms, because, once dead, the ratio of stable C-12 to radioactive C-14 changes exponentially with time. Several other longer lived nuclide pairs are useful in dating rocks and minerals, ground water and polar ice-cores, like O-16/O-18, Rb-87/Sr-86 and Sr-87/Sr-86 ratios. The Earth was created about 4600 million years ago and any nuclide with a halflife exceeding 80 million years will still have a detectable presence on Earth. Conversely, any nuclide with a halflife shorter than 80 million years will have all decayed by now unless it is still being created, either from being in the decay chain of another longer-lived isotope (see Secular equilibrium), or from being created by cosmic ray bombardment.

When the Earth was born, about 4.6×109 years ago, there was twice as much U-238 and 99 times as much U-235 as there is today, the U235/U-238 ratio then being 35%; it is 0.7% today.

As a general rule, the halflives for beta/inverse-beta decay are inversely proportional to the fifth power of the energy released by decay (shown in MeV on the RELATIVE MASS plot).

Shown are Tellurium-128 (double beta decay) and Lithium-5 (alpha + proton decay)