Context : Knowing accurate lead abundances of metal-poor stars provides constraints on the Pb production mechanisms in the early Galaxy .
Accurately deriving thorium abundances permits a nucleo-chronometric age determination of the star .

Aims : We aim to improve the calculation of the Pb i and Th ii lines in stellar atmospheres based on non-local thermodynamic equilibrium ( non-LTE ) line formation , and to evaluate the influence of departures from LTE on Pb and Th abundance determinations through a range of stellar parameters with variations of the metallicity from the solar value down to [ Fe/H ] = -3 .

Methods : Comprehensive model atoms for Pb i and Th ii are presented .
We describe calculations of the Pb i energy levels and oscillator strengths .

Results : The main non-LTE mechanism for Pb i is the ultraviolet overionization .
Non-LTE leads to systematically depleted total absorption in the Pb i lines and accordingly , positive abundance corrections .
The departures from LTE grow with decreasing metallicity .
Non-LTE removes the discrepancy between the solar photosphere and the meteoritic lead abundance .
Using the semi-empirical Holweger & Müller ( 1974 ) model atmosphere , we determined the lead non-LTE abundance for the Sun to be \log \varepsilon _ { Pb, \odot } = 2.09 .
We revised the Pb and Eu abundances of the two strongly r-process enhanced stars CS 31082-001 and HE 1523-0901 and the Roederer et al .
( 2010 ) metal-poor stellar sample .
Our new results provide strong evidence for universal Pb-to-Eu relative r-process yields during course of the evolution of the Galaxy .
The stars in the -2.3 < [ Fe/H ] < -1.4 metallicity range have , on average , 0.51 dex higher Pb/Eu abundance ratios compared with that of the strongly r-process enhanced stars .
We conclude that the s-process production of lead started as early as the time when Galactic metallicity had grown to { [ Fe / H ] } = -2.3 .
The average Pb/Eu abundance ratio of the mildly metal-poor stars , with -1.4 \leq [ Fe/H ] \leq - 0.59 , is very close to the corresponding Solar System value , in line with the theoretical predictions of Travaglio et al .
( 2001 ) that AGB stars with [ Fe/H ] \simeq - 1 provided the largest contribution to the solar abundance of s-nuclei of lead .
The departures from LTE for Th ii are caused by the pumping transitions from the low-excitation levels , with E _ { exc } < 1 eV .
Non-LTE leads to weakened Th ii lines and positive abundance corrections .
Overall , the abundance correction does not exceed 0.2 dex when collisions with H i atoms are taken into account in statistical equilibrium calculations .

Conclusions :