updated 9-DEC-1993 by SIC
Original by Bill Johnson

How to Change Nuclear Decay Rates

"I've had this idea for making radioactive nuclei decay faster/slower than they normally do. You do [this, that, and the other thing]. Will this work?"

Short Answer: Possibly, but probably not usefully.

Long Answer:

"One of the paradigms of nuclear science since the very early days of its study has been the general understanding that the half-life, or decay constant, of a radioactive substance is independent of extranuclear considerations." (Emery, cited below.) Like all paradigms, this one is subject to some interpretation. Normal decay of radioactive stuff proceeds via one of four mechanisms:

Emission of an alpha particle -- a helium-4 nucleus -- reducing the number of protons and neutrons present in the parent nucleus by two each
"Beta decay," encompassing several related phenomena in which a neutron in the nucleus turns into a proton, or a proton turns into a neutron -- along with some other things including emission of a neutrino. The "other things", as we shall see, are at the bottom of several questions involving perturbation of decay rates
Emission of one or more gamma rays -- energetic photons -- that take a nucleus from an excited state to some other (typically ground) state; some of these photons may be replaced by "conversion electrons," of which more shortly
Spontaneous fission, in which a sufficiently heavy nucleus simply breaks in half. Most of the discussion about alpha particles will also apply to spontaneous fission.

Gamma emission often occurs from the daughter of one of the other decay modes. We neglect very exotic processes like C-14 emission or double beta decay in this analysis.

"Beta decay" refers most often to a nucleus with a neutron excess, which decays by converting a neutron into a proton:

         n ----> p + e- + anti-nu(e),

where n means neutron, p means proton, e- means electron, and anti-nu(e) means an antineutrino of the electron type. The type of beta decay which involves destruction of a proton is not familiar to many people, so deserves a little elaboration. Either of two processes may occur when this kind of decay happens:

        p ----> n + e+ + nu(e),

where e+ means positron and nu(e) means electron neutrino; or

        p + e- ----> n + nu(e),

where e- means a negatively charged electron, which is captured from the neighborhood of the nucleus undergoing decay. These processes are called "positron emission" and "electron capture," respectively. A given nucleus which has too many protons for stability may undergo beta decay through either, and typically both, of these reactions.

"Conversion electrons" are produced by the process of "internal conversion," whereby the photon that would normally be emitted in gamma decay is virtual and its energy is absorbed by an atomic electron. The absorbed energy is sufficient to unbind the electron from the nucleus (ignoring a few exceptional cases), and it is ejected from the atom as a result.

Now for the tie-in to decay rates. Both the electron-capture and internal conversion phenomena require an electron somewhere close to the decaying nucleus. In any normal atom, this requirement is satisfied in spades: the innermost electrons are in states such that their probability of being close to the nucleus is both large and insensitive to things in the environment. The decay rate depends on the electronic wavefunctions, i.e, how much of their time the inner electrons spend very near the nucleus -- but only very weakly. For most nuclides that decay by electron capture or internal conversion, most of the time, the probability of grabbing or converting an electron is also insensitive to the environment, as the innermost electrons are the ones most likely to get grabbed/converted.

However, there are exceptions, the most notable being the the astrophysically important isotope beryllium-7. Be-7 decays purely by electron capture (positron emission being impossible because of inadequate decay energy) with a half-life of somewhat over 50 days. It has been shown that differences in chemical environment result in half-life variations of the order of 0.2%, and high pressures produce somewhat similar changes. Other cases where known changes in decay rate occur are Zr-89 and Sr-85, also electron capturers; Tc-99m ("m" implying an excited state), which decays by both beta and gamma emission; and various other "metastable" things that decay by gamma emission with internal conversion. With all of these other cases the magnitude of the effect is less than is typically the case with Be-7.

What makes these cases special? The answer is that one or more of the usual starting assumptions -- insensitivity of electron wave function near the nucleus to external forces, or availability of the innermost electrons for capture/conversion -- are not completely valid. Atomic beryllium only has 4 electrons to begin with, so that the "innermost electrons" are also practically the outermost ones and therefore much more sensitive to chemical effects than usual. With most of the other cases, there is so little energy available from the decay (as little as a few electron volts; compare most radioactive decays, where hundreds or thousands of kilovolts are released), courtesy of accidents of nuclear structure, that the innermost electrons can't undergo internal conversion. Remember that converting an electron requires dumping enough energy into it to expel it from the atom (more or less); "enough energy," in context, is typically some tens of keV, so they don't get converted at all in these cases. Conversion therefore works only on some of the outer electrons, which again are more sensitive to the environment.

A real anomaly is the beta emitter Re-187. Its decay energy is only about 2.6 keV, practically nothing by nuclear standards. "That this decay occurs at all is an example of the effects of the atomic environment on nuclear decay: the bare nucleus Re-187 [i.e., stripped of all orbital electrons -- MWJ] is stable against beta decay [but not to bound state beta decay, in which the outgoing electron is captured by the daughter nucleus into a tightly bound orbital -SIC] and it is the difference of 15 keV in the total electronic binding energy of osmium [to which it decays -- MWJ] and rhenium ... which makes the decay possible" (Emery). The practical significance of this little peculiarity, of course, is low, as Re-187 already has a half life of over 10^10 years.

Alpha decay and spontaneous fission might also be affected by changes in the electron density near the nucleus, for a different reason. These processes occur as a result of penetration of the "Coulomb barrier" that inhibits emission of charged particles from the nucleus, and their rate is very sensitive to the height of the barrier. Changes in the electron density could, in principle, affect the barrier by some tiny amount. However, the magnitude of the effect is very small, according to theoretical calculations; for a few alpha emitters, the change has been estimated to be of the order of 1 part in 10^7 (!) or less, which would be unmeasurable in view of the fact that the alpha emitters' half lives aren't known to that degree of accuracy to begin with.

All told, the existence of changes in radioactive decay rates due to the environment of the decaying nuclei is on solid grounds both experimentally and theoretically. But the magnitude of the changes is nothing to get very excited about.

Reference: The best review article on this subject is now 20 years old: G. T. Emery, "Perturbation of Nuclear Decay Rates," Annual Review of Nuclear Science vol. 22, p. 165 (1972). Papers describing specific experiments are cited in that article, which contains considerable arcane math but also gives a reasonable qualitative "feel" for what is involved.