( ( A half-life ) ( ( is ( ( the amount ) ( of ( time ( ( needed ( for ( ( ( one half ) ( the atoms ) ) ( of ( a ( radioactive isotope ) ) ) ) ) ) ( to decay ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN half-life)) (VP (VBZ is) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NP (NN time)) (VP (VBN needed) (PP (IN for) (NP (NP (QP (CD one) (NN half)) (DT the) (NNS atoms)) (PP (IN of) (NP (DT a) (JJ radioactive) (NN isotope))))) (PP (TO to) (NP (NN decay)))))))) (. .)))
A half-life is the amount of time needed for one half the atoms of a radioactive isotope 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)))))))))) (. .)))