Measurement of B(t -> Wb)/B(t -> Wq) at the Collider Detector at Fermilab

We present a measurement of the ratio of top-quark branching fractions R=B(t -> Wb)/B(t -> Wq), where q can be a b, s, or a d quark, using lepton-plus-jets and dilepton data sets with an integrated luminosity of similar to 162 pb(-1) collected with the Collider Detector at Fermilab during Run II of the Tevatron. The measurement is derived from the relative numbers of t (t) over bar events with different multiplicity of identified secondary vertices. We set a lower limit of R > 0.61 at 95% confidence level.