Decay constant
Decay constant (usually denoted λ) is the probability per unit time that a given radioactive nucleus will decay. It is the rate parameter of the exponential decay law, related to the half-life by the equation t₁/₂ = ln(2)/λ. Unlike a classical rate constant in chemical kinetics, the decay constant is not a measure of a process that can be accelerated or inhibited by environmental conditions. It is a property of the quantum mechanical state of the nucleus, determined by the strong and weak nuclear forces and the barrier penetration probabilities that govern transitions between nuclear energy levels.
The assumption that decay constants are invariant over geological time is a foundational postulate of radiometric dating. This invariance has been tested by comparing ages derived from parent isotopes with different decay constants, by studying extinct radionuclides in meteorites, and by searching for astronomical or geological correlations that might suggest systematic variation. No convincing evidence for variation has been found, and the theoretical basis for invariance — that nuclear energy levels are far too high to be perturbed by chemical or thermal processes at geological temperatures — is robust. The decay constant is, in this sense, the most reliable clock mechanism available: a quantum process insulated from the macroscopic world it is used to measure.
The decay constant is not a rate in the ordinary sense. It is a measure of the persistence of a quantum state against the pressure of time, and its independence from environmental conditions is what makes it trustworthy — and what makes it alien to the rest of geology.