1) For Secular Equillibruim, the source age is taken as
approximately 10 half-lives of the longest-lived daughter in the decay chain to reach equilibrium. In case of Th-232, the longest lived isotope is Ra-228(whose half life is 5.75 years),
so the source age is taken as 10 times of 5.75 years.
2)To get S1/S2 signal, I have updated the LZ code and Shaun can use it from
/home/gomber/LUXSim_G4.9.4.p04-earlier
3)Regarding PMT_Vaccum volume, I find out in one of the paper that it is the back of the PMT, including base, "behind" the quartz window and photo-cathode. In real life, it is not 100% vacuum space because there are the dynodes for e- gain, etc., but those intricate and tiny little electronics and circuitry boards would be WAY too hard to simulate within Geant4 in great detail.
