  A spin-singlet ground state with a spin gap has been discovered in
antiferromagnetic spin chain substances when the spin value is 1/2, 1 or 2. To
find spin gap (singlet-triplet) excitations in spin-3/2 chain substances, we
performed inelastic neutron scattering and magnetization measurements on {\it
R}CrGeO$_5$ ({\it R} = Y or Sm) powders. As expected, we observed spin gap
excitations and the dispersion relation of the lowest magnetic excitations. We
proved that the spin system of Cr$^{3+}$ was an antiferromagnetic alternating
spin-3/2 chain.
