Answer to Question #7843 Submitted to "Ask the Experts"

Do you know of any papers or other sources which describe what factors (if any) affect radionuclide half-life? What I am interested in is whether any normal physical events (not in a particle smasher) can increase or decrease the likelihood of a nucleus emitting a particle or photon. My specific interest is whether changing the distance between quarks in a proton or neutron in the nucleus of a radionuclide will increase/decrease the probability of the nucleus decaying.

While it had been thought that the half-life for radioactive decay of a radionuclide was not subject to change by external factors, over the past few decades there have been demonstrated a number of instances in which the half-lives of selected radionuclides have been altered through external manipulation. This has been true particularly for radionuclides that decay by the process of electron capture or by the process of beta-particle emission. In the electron-capture mode, an inner-shell electron is captured by the nucleus, leading to the conversion of a proton into a neutron plus a neutrino. There is a weak dependence of the likelihood of capture on the external electronic environment. Thus, by changing the electron density and/or configuration in the immediate vicinity of the atom, small changes in half-life have been detected. The effect has been most noticeable in beryllium-7, a radionuclide that decays 100% of the time by electron capture and has only four electrons. Enclosing it in a buckyball kind of structure (C₆₀) has produced a nearly 1% decrease in the half-life. This is described in a paper by Ohtsuki et al. (2004).

For beta emitters, the beta particle is produced by the conversion of a nuclear neutron into a proton and an electron (beta particle), along with an antineutrino. The beta particle must penetrate the electron cloud around the nucleus in order to be detected. It has been found that if the electrons are all stripped from some radioactive atoms that decay by beta emission (what is referred to as bound state beta decay because the emitted beta particle becomes bound in the ionized atom), the decay half-life of the atoms has been noticeably reduced. I believe the most dramatic radionuclide in this regard has been rhenium-187, for which a remarkable reduction in the half-life from 4.1 x 10¹⁰ years to about 33 years has been observed. This is discussed in a paper in Physical Review Letters by Bosch and others.

Alteration in the electronic environment of alpha emitters may also produce some change in half-life, but such changes are not expected to be as great as those that might result for electron capture and beta decay radionuclides. Here is a link to an abstract to a paper by Raiola and others that discusses evaluation of half-life reduction for the well-known radionuclide polonium-210 by a specialized technique of cooling the radionuclide in a copper enclosure.

There is also an older, but good, review paper by Emery that might well be worth your perusal. You can access the abstract and links to obtain the full paper through Annual Review of Nuclear Science. 

To the best of my knowledge, none of the references provide any specific discussion regarding effects on nuclear structure that would involve changes among quarks that make up the nuclear constituents.

I hope the above is helpful to you. Good luck in your continuing research.

George Chabot, PhD, CHP

Reference
Ohtsuki T, Yuki H, Muto M, Kasagi J, Ohno K. Enhanced electron-capture decay rate of 7Be encapsulated in C60 cages. Phys Rev Lett 93:112501; 2004.