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Sources of the systematic errors in measurements of Po-214 decay half-life time variations at the Baksan deep underground experiments

The design changes of the Baksan low-background TAU-1 and TAU-2 set-ups allowed to improve a sensitivity of Po-214 half-life (τ) measurements up to the 2.5 x 10^{-4} are described. Different possible sources of systematic errors influencing on the $τ$-value are studed. An annual variation of Po-214 half-life time measurements with an amplitude of A=(6.9 \pm 3) x 10^{-4} and a phase of (ϕ=93 \pm 10) days was found in a sequence of the week-collected τ-values obtained from the TAU-2 data sample with total duration of 480 days. 24 hours' variation of the τ-value measurements with an amplitude of A=(10.0 \pm 2.6) x 10^{-4} and phase of (ϕ=1 \pm 0.5) hours was found in a solar day 1 hour step τ-value sequence formed from the same data sample. It was found that the Po-214 half-life averaged at 480 days is equal to (163.45 \pm 0.04) mks.