talkorigins.org proposed article on mechanisms of radioactive decay

[Loren Petrich]

Radiometric dating uses radioactive decay for time measurement, and one may ask how confident can one be that one can be that radioactive-decay rates do not change over geological time. If fundamental constants change in value, that can make these rates change over time. But from how decay rates are functions of these constants, these rates will vary relative to each other, and such relative variations will be observable as different radionuclides systematically giving different ages, like U-238 ages vs. U-235 ages vs. K-40 ages.

All radioactive decay shares an odd quantum-mechanical effect, the "collapse of the wavefunction", illustrated by Erwin Schroedinger's famous thought experiment featuring a certain ill-fated cat. A nucleus becomes a mixture of its initial and final states, and gets frozen into that final state by that collapse. But wavefunction collapse does not affect decay rates.

There are three main kinds of radioactive decay, classes that include each of the three kinds of radioactivity originally discovered.

The first kind is disintegration of the nucleus. It includes alpha decay, emission of a helium-4 nucleus, spontaneous fission, splitting into two nearly equal halves, and proton and neutron emission. All but neutron emission take place by quantum tunneling. To illustrate this, run the decay in reverse. The products come close but do not touch, because of their electric repulsion. But in quantum mechanics, everything is both particle and wave, though macroscopically we see only one or the other of these aspects. This wave nature enables the decay products to penetrate the electric-potential -barrier, though the penetration rate becomes exponentially weaker with increasing barrier height.

The second kind is from weak elementary-particle interactions. A nucleus does beta decay, emitting an electron (beta-) or a positron (beta+), or else captures an election, in all three cases also emitting a neutrino. A neutron becomes a proton and a W particle, with the W then quickly making an electron and an antineutrino. A proton becomes a neutron and a W particle, with the W either making a positron and an ordinary neutrino, or else grabbing an electron and making an ordinary neutrino.

The third kind is electromagnetic, with the nucleus acting like a miniature radio antenna, as an atom or a molecule does, but emitting gamma rays, highly-energetic photons, units of electromagnetic wave. Remember that everything is both particle and wave.

Of these, only the first and the second are important in radiometric dating, but all three have rates that are functions of available energy and interaction strengths. Available energy is, in turn, a function of the strong elementary-particle interaction and the electromagnetic interaction.

[Loren Petrich]

Revision for collapse of the wavefunction: A nucleus becomes a mixture of its initial and final states, but outwardly, it stays in its initial state then jumps into its final state with its decay.