Answer
I cannot
understand your units.
why do you have grams in there? why a unit of area (cm2)?
the thickness of lead will just be a distance, e.g. 1 cm.
most people use 2" (2 inches) as the tenth thickness in examples but it depends on the energy of the gamma emissions. you need to give some details. at the moment your questions makes no sense because your units and values are unintelligible.
if you just want a definition then the tvt (tenth value thickness) is the amount of lead shielding you would need to reduce the strength of a gamma emission to just one tenth of the value at source.
