  Background: Decay constants of unstable nuclei and particles are quantum
constants. Recent controversy on the existence (versus non-existence) of
variability in the observation of decay rate can be settled by considering
mixing in decay configuration. Purpose: Variability in decay rate was
investigated based on the available information of beta decay rate data, solar
neutrino flux, and energy distribution. Method: Full systematic analysis of the
oscillatory behavior was carried out. Based on the zero threshold energy for
neutrino absorption in beta emitters, a model for configuration mixing between
two distinct beta disintegration modes $\beta^\nu$-disintegration (electron
from neutrino interaction) and the $\beta^-$-disintegration (electron from
natural decay) was proposed. Results: The phenomenon of variability in beta
decay rate was related to the possible exothermic neutrino absorption by
unstable nuclei which, in principle, should include the whole range of flux
energies involving flux with energy below the $^{71}$Ga threshold at 0.23 MeV.
These two disintegration modes occur independently and model for their apparent
mixing rate was proposed. Conclusions: The configuration mixing between the two
modes causes depletion of radioactive nuclei which is subject to change with
seasonal solar neutrino variability. Ability to detect this variability was
found to be dependent on the Q-value of the $\beta^\nu$ disintegration and
detection instrument setup. Value of neutrino cross section weighted by the
ratio between $\beta^\nu$ and $\beta^-$ detection efficiencies was found to be
in the range $10^{-44}$ to $10^{-36}$ cm$^{2}$. For experiments that uses the
end point to determine the neutrino mass, interference due to mixing should be
taken into account.
