  Inelastic neutron scattering is used to identify single-particle and
continuum spin excitations in the quasi-one-dimensional $S=1/2$ antiferromagnet
\ba. In the data analysis, close attention is given to resolution effects. A
gap in the continuum spectrum at $\Delta_c=4.74.8$ meV is directly observed.
The gap energy is in excellent agreement with existing theoretical predictions.
Below the gap, the spectrum is dominated by sharp accoustic spin wave
excitations.
