( ( The ( half life ) ) ( ( is ( ( the ( time period ) ) ( for ( half ( of ( ( the material ) ( to decay ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN half) (NN life)) (VP (VBZ is) (NP (NP (DT the) (NN time) (NN period)) (PP (IN for) (NP (NP (NN half)) (PP (IN of) (NP (NP (DT the) (NN material)) (PP (TO to) (NP (NN decay))))))))) (. .)))
The half life is the time period for half of the material to decay.
(ROOT (S (NP (DT The) (NN term) (NN half-life)) (VP (VBZ decribes) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NP (NN time)) (VP (VBN required) (PP (IN for) (NP (NP (NN half)) (PP (IN of) (NP (DT the) (JJ original) (NN material))))) (PP (TO to) (NP (NP (NN decay)) (PP (IN in) (NP (DT an) (NN isotope)))))))))) (. .)))