( ( ( The ( decay rate ) ) ( of ( a ( radioactive isotope ) ) ) ) ( ( is ( measured ( in half-lives ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN decay) (NN rate)) (PP (IN of) (NP (DT a) (JJ radioactive) (NN isotope)))) (VP (VBZ is) (VP (VBN measured) (PP (IN in) (NP (NNS half-lives))))) (. .)))
The decay rate of a radioactive isotope is measured in half-lives .
(ROOT (S (NP (DT The) (NN decay) (NN rate)) (VP (VBZ is) (VP (VBN measured) (PP (IN in) (NP (NP (DT a) (NN unit)) (VP (VBN called) (NP (DT the) (NN half-life))))))) (. .)))