( ( ( ( ( These rates ) ( ( are usually ) ( expressed ( as ( ( the ( isotope 's ) ) half-life ) ) ) ) ) -- ) ( ( that is ) ( , ( ( ( the time ) ( it ( takes ( for ( one-half ( of ( the parent ) ) ) ) ) ) ) ( isotopes ( to decay ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT These) (NNS rates)) (VP (VBP are) (ADVP (RB usually)) (VP (VBN expressed) (PP (IN as) (NP (NP (DT the) (NN isotope) (POS 's)) (NN half-life)))))) (: --) (S (ADVP (DT that) (VBZ is)) (, ,) (NP (NP (DT the) (NN time)) (SBAR (S (NP (PRP it)) (VP (VBZ takes) (PP (IN for) (NP (NP (NN one-half)) (PP (IN of) (NP (DT the) (NN parent))))))))) (VP (VBZ isotopes) (PP (TO to) (NP (NN decay))))) (. .)))
These rates are usually expressed as the isotope's half-life --that is, the time it takes for one-half of the parent isotopes 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)))))))))) (. .)))