Changing the Half-Life of Radioactive Substances
Do rates of nuclear decay depend on environmental factors?
There are two known environmental effects that can matter:
(1) The first one has been scientifically well established for a long time. In the process of electron capture, a proton in the nucleus combines with an inner-shell electron to produce a neutron and a neutrino. This effect does depend on the electronic environment, and in particular, the process cannot happen if the atom is completely ionized.
(2) In some exceptional examples, such as 187Re, there are beta decays with extremely low energies (in the keV range, rather than the usual MeV range). In these cases, there are significant effects due to the Pauli exclusion principle and the surrounding electron cloud. See Ionizing a beta decay nucleus causes faster decay?
Other claims of environmental effects on decay rates are crank science, often quoted by creationists in their attempts to discredit evolutionary and geological time scales.
He et al. (He 2007) claim to have detected a change in rates of beta decay of as much as 11% when samples are rotated in a centrifuge, and say that the effect varies asymmetrically with clockwise and counterclockwise rotation. He believes that there is a mysterious energy field that has both biological and nuclear effects, and that it relates to circadian rhythms. The nuclear effects were not observed when the experimental conditions were reproduced by Ding et al. [Ding 2009]
Jenkins and Fischbach (2008) claim to have observed effects on alpha decay rates at the 10^-3 level, correlated with an influence from the sun. They proposed that their results could be tested more dramatically by looking for changes in the rate of alpha decay in radioisotope thermoelectric generators aboard space probes. Such an effect turned out not to exist (Cooper 2009). Undeterred by their theory's failure to pass their own proposed test, they have gone on to publish even kookier ideas, such as a neutrino-mediated effect from solar flares, even though solar flares are a surface phenomenon, whereas neutrinos come from the sun's core. An independent study found no such link between flares and decay rates (Parkhomov 2010a). Laboratory experiments[Lindstrom 2010] have also placed limits on the sensitivity of radioactive decay to neutrino flux that rule out a neutrino-mediated effect at a level orders of magnitude less than what would be required in order to explain the variations claimed in [Jenkins 2008]. Despite this, Jenkins and Fischbach continue to speculate about a neutrino effect in [Sturrock 2012]; refusal to deal with contrary evidence is a hallmark of kook science. They admit that variations shown in their 2012 work "may be due in part to environmental influences," but don't seem to want to acknowledge that if the strength of these influences in unknown, they may explain the entire claimed effect, not just part of it.
Jenkins and Fischbach made further claims in 2010 based on experiments done decades ago by other people, so that Jenkins and Fischbach have no first-hand way of investigating possible sources of systematic error. Other attempts to reproduce the result are also plagued by systematic errors of the same size as the claimed effect. For example, an experiment by Parkhomov (2010b) shows a Fourier power spectrum in which a dozen other peaks are nearly as prominent as the claimed yearly variation.
Cardone et al. claim to have observed variations in the rate of alpha decay of thorium induced by 20 kHz ultrasound, and claim that this alpha decay occurs without the emission of gamma rays. Ericsson et al. have pointed out multiple severe problems with Cardone's experiments.
In agreement with theory, high-precision experimental tests show no detectable temperature-dependence in the rates of electron capture[Goodwin 2009] and alpha decay.[Gurevich 2008] Goodwin's results debunk a series of results from a group led by Rolfs, e.g., [Limata 2006], which used an inferior technique.
He YuJian et al., Science China 50 (2007) 170.
YouQian Ding et al., Science China 52 (2009) 690.
Jenkins and Fischbach (2008), http://arxiv.org/abs/0808.3283v1, Astropart.Phys.32:42-46,2009
Jenkins and Fischbach (2009), http://arxiv.org/abs/0808.3156, Astropart.Phys.31:407-411,2009
Jenkins and Fischbach (2010), http://arxiv.org/abs/1007.3318
Parkhomov 2010a, http://arxiv.org/abs/1006.2295
Parkhomov 2010b, http://arxiv.org/abs/1012.4174
Cooper (2009), http://arxiv.org/abs/0809.4248, Astropart.Phys.31:267-269,2009
Lindstrom et al. (2010), http://arxiv.org/abs/1006.5071 , Nuclear Instruments and Methods in Physics Research A, 622 (2010) 93-96
Sturrock 2012, http://arxiv.org/abs/1205.0205
F. Cardone, R. Mignani, A. Petrucci, Phys. Lett. A 373 (2009) 1956
Ericsson et al., Comment on "Piezonuclear decay of thorium," Phys. Lett. A 373 (2009) 1956, http://arxiv4.library.cornell.edu/abs/0907.0623
Ericsson et al., http://arxiv.org/abs/0909.2141
Goodwin, Golovko, Iacob and Hardy, "Half-life of the electron-capture decay of 97Ru: Precision measurement shows no temperature dependence" in Physical Review C (2009), 80, 045501, http://arxiv.org/abs/0910.4338
Gurevich et al., "The effect of metallic environment and low temperature on the 253Es α decay rate," Bull. Russ. Acad. Sci. 72 (2008) 315.
Limata et al., "First hints on a change of the 22Na βdecay half-life in the metal Pd," European Physical Journal A - Hadrons and Nuclei May 2006, Volume 28, Issue 2, pp 251, http://link.springer.com/article/10.1140%2Fepja%2Fi2006-10057-1
Have a look at the paragraph "radioactive decay" .
The half life is characteristic of each radioactive nucleus and depends on the basic interactions holding the nucleus together.
It depends on the quantum mechanical probabilities of transition from one energy level to another, sometimes changing element in the periodic table.
Thus, to affect the half life, one would have to affect the basic interactions of the decay mechanism. There have been speculations on what would happen if the QFT vacuum is different, as in the Casimir effect, (a simpler explanation here), but I have not been able to find an experiment.
The simple answer is, no, the half life cannot change.
Short answer: yes, the decay rates could be changed considerably by environment. However, generally the energies required for this are comparable to the energy output of the nuclear reaction in question and so are (usually) not achievable in laboratory, but the relevant processes are of great importance in astrophysics for stellar nucleosynthesis (particularly in supernovae).
Long answer: The main problem with such effects is the discrepancy between energy of typical nuclear reaction (from several keV's to dozens of MeV's) and energies per reactant particle achievable in laboratory. Temperature 300K correspond only to energy 0.025 eV so temperature effects would be extremely small correction to nuclear reaction energy levels. Chemical bonds have energies around several eV which again is several orders of magnitude smaller than energies needed to affect typical nuclear reaction.
Let us consider the beta decay (ordinary one, not electron capture). It results in emission of electron of a certain energy distribution (and neutrino which we will ignore). But electrons obey Fermi-Dirac statistics so if prior to the decay event there is already an electron with a certain momentum and spin quantum numbers then another electron with exact same quantum numbers cannot be produced by the decay process. So, if we have electron degenerate gas around the unstable nucleus then the distribution of electrons modified to exclude the already occupied electron energies and overall decay rate would be changed.
In everyday environment the Fermi energy of electron gas is of the order of eV, so in phase space the prohibited zone for beta decay is only a small spot near the origin inside the large ball of allowed electron states. However, if we compress the matter, then the density of electrons increases along with the Fermi energy. Ultimately, if the Fermi energy of degenerate electron gas is greater than total energy released by beta decay then this beta decay would not occur at all -- we thus have stabilized the nucleus.
Let us do some calculations. Take for instance beta decay of tritium. The electron spectrum has a maximum electron energy of 18.6 keV with an average energy of 5.7 keV. So in order to suppress the decay fully we need to have Fermi energy of electron gas equal to 18.6 keV. From wikipedia page on Fermi energy we have $$E_F = \frac{\hbar^2}{2m} \left( \frac{3 \pi^2 N}{V} \right)^{2/3}, $$ which gives us electron number density $N/V = 1.152\times 10^{28} \text{cm}^{-3}$, which in turn corresponds to mass density needed of about $58 \text{kg}/\text{cm}^3$ if we are assuming only tritium is present or ~19 kg/cm$^3$ if we have small percentage of tritium together with normal hydrogen. If we only try to measurably slower the decay rate by having Fermi energy of 6keV we will need density of about 3 kg/cm$^3$.
These are, of course, enormous densities, however, much greater densities of degenerate electrons do exist inside white dwarfs, and therefore beta decays (at least with relatively low energies) would be greatly suppressed there. Another environment where this suppression effect is of great importance is the core collapse supernovae where the r-process is responsible for production of considerable number of heavy nuclei that exist in the universe. One of the things that enable it is the suppression of beta decay.
As to having this effect observable locally around us, one possibility is ultradense deuterium and protium. This hypothesized and possibly observed (the number of peer reviewed papers is quite high) states of matter are supposed to have densities of around 100kg/cm$^3$, so potentially this could provide the electron densities to stabilize or slow the beta decay rate.
Another similar effect is bound state beta decay since bound electrons around nucleus could be interpreted as the degenerate electron gas and so ionization would increase the possible allowable states of electrons.
All above concerns the $\beta^-$ decay, but for gamma decay we have Induced gamma emission which could potentially be exploitable (as in hafnium bomb -- so far purely theoretical construct).