  We report an inelastic neutron scattering investigation of phonons with
energies up to 159 meV in the conventional superconductor YNi$_2$B$_2$C. Using
the SWEEP mode, a newly developed time-of-flight technique involving the
continuous rotation of a single crystal specimen, allowed us to measure a four
dimensional volume in (Q,E) space and, thus, determine the dispersion surface
and linewidths of the $A_{1g}$ (~ 102 meV) and $A_u$ (~ 159 meV) type phonon
modes for the whole Brillouin zone. Despite of having linewidths of $\Gamma =
10 meV$, $A_{1g}$ modes do not strongly contribute to the total electron-phonon
coupling constant $\lambda$. However, experimental linewidths show a remarkable
agreement with ab-initio calculations over the complete phonon energy range
demonstrating the accuracy of such calculations in a rare comparison to a
comprehensive experimental data set.

Superconductivity