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Are decay constants actually constant?



The question commonly arises whether the decay constants used in the isotopic dating of geological materials are actually constant, or do they vary in response to some external force?

The answer is that the decay constants used in the dating of geological materials are effectively constant and invariant to external forces.

The behavior of radioactive isotopes has been the focus of international scientific study since they were first recognized by Henri Becquerel in the late Nineteenth Century, and that behavior is now well understood.

The primary isotopes used to date rocks and minerals are given in the following table (Dalrymple, 1991, p. 80; Faure, 1986):

Parent IsotopeDaughter IsotopeDecay ConstantDecay Mechanism(s)
40K40Ar5.81x10-11 per yearelectron capture
87Rb87Sr1.42x10-11 per yearbeta decay
147Sm143Nd6.54x10-12 per yearalpha decay
176Lu176Hf1.93x10-11 per yearbeta decay
187Re187Os1.612x10-11 per yearbeta decay
232Th208Pb4.948x10-11 per yearalpha and beta decay in series
235U207Pb9.8485x10-10 per yearalpha and beta decay in series
238U206Pb1.55125x10-10 per yearalpha and beta decay in series

(K=potassium, Ar=argon, Rb=rubidium, Sr=strontium, Sm=samarium, Nd=neodymium, Lu=lutetium, Hf=hafnium, Re=rhenium, Os=osmium, Th=thorium, Pb=lead, U=uranium)

The mechanisms of radioactive decay that are relevant to the dating of geological materials include beta decay, electron capture and alpha decay. The effect of beta decay is that a neutron is converted to a proton within an atom's nucleus, accompanied by the ejection of an electron and an antineutrino from the atom. For a given atom, beta decay leads to an increase in atomic number by 1, and no change in the atomic mass number. Electron capture has the opposite effect, and occurs when an electron from the innermost orbital of an atom is captured by the nucleus, leading to the conversion of a proton into a neutron. For a given atom, electron capture leads to a decrease in atomic number by 1, and no change in the atomic mass number. Heavier radiogenic elements may undergo alpha decay, in which two protons and two neutrons are ejected from the nucleus, reducing the atomic number by 2 and the atomic mass number by 4.

The possible effects of changing temperature, pressure, chemical state, and electric or magnetic field strength on the three decay mechanisms relevant to geologic dating have been intensively studied, both theoretically and experimentally. These studies have shown that changing environmental conditions have either no measurable effect or a negligible effect (less than 1%, and that only for 7Be, which decays through electron capture) on the rate at which the decay processes occur (Dalrymple, 1991, p. 86-90). "There is no evidence that decay constants have changed as a function of time during the history of the solar system" (Faure, 1986, p. 41).


References and suggested reading

Dalrymple, G.B., 1991, The age of the Earth: Stanford, California, Stanford University Press, 474 p., ISBN 0-8047-2331-1.

Dalrymple, G.B., 2004, Ancient Earth, ancient skies -- the age of Earth and its cosmic surroundings: Stanford, California, Stanford University Press, ISBN 0-8047-4933-7.

Emery, G.T., 1972, Perturbation of nuclear decay rates: Annual Reviews of Nuclear Science, v. 22, p. 165-202.

Faure, G., 1986, Principles of isotope geology [2nd edition]: New York, John Wiley & Sons, 589 p., ISBN 0-471-86412-9.

Hensley, W.K., Bassett, W.A., and Huizenta, J.R., 1973, Pressure dependence of the radioactive decay constant of beryllium-7: Science, v. 181, p. 1164-1165.

Hopke, P.K., 1974, Extranuclear effects on nuclear decay rates: Journal of Chemical Education, v. 51, p. 517-519.

Richardson, S.M., and McSween, H.Y., Jr., 1989, Geochemistry -- pathways and processes: Englewood Cliffs, New Jersey, Prentice-Hall, 488 p., ISBN 0-13-351073-5.


The information on this page was written and approved by the faculty of the Geology Department at Baylor University.