( ( rate ( of decay ) ) ( ( ( is usually ) ( expressed ( in ( terms ( of ( half life ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NN rate)) (PP (IN of) (NP (NN decay)))) (VP (VBZ is) (ADVP (RB usually)) (VP (VBN expressed) (PP (IN in) (NP (NP (NNS terms)) (PP (IN of) (NP (DT half) (NN life))))))) (. .)))
rate of decay is usually expressed in terms of half life.
(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))))))) (. .)))