  We determine the excitations and $S$ matrix of an integrable isotropic
antiferromagnetic quantum spin chain of alternating spin 1/2 and spin 1. There
are two types of gapless one-particle excitations: the usual spin 1/2
(``spinor'') kink, and a new spin 0 (``scalar'') kink. Remarkably, the
scalar-spinor scattering is nontrivial, yet the spinor-spinor scattering is the
same as if the scalar kinks were absent. Moreover, there is no scalar-scalar
scattering.
