( It ( ( ( decays ( by beta-decay ) ) ( with ( ( a half-life ) ( of ( 87.51 days ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ decays) (PP (IN by) (NP (NN beta-decay))) (PP (IN with) (NP (NP (DT a) (NN half-life)) (PP (IN of) (NP (CD 87.51) (NNS days)))))) (. .)))
It decays by beta-decay with a half-life of 87.51 days.
(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))))))) (. .)))