  For topological magnon spectra, determining and explaining the presence of a
gap at a magnon crossing point is a key to characterize the topological
properties of the system. An inelastic neutron scattering study of a single
crystal is a powerful experimental technique that is widely employed to probe
the magnetic excitation spectra of topological materials. Here, we show that
when the scattering intensity rapidly disperses in the vicinity of a crossing
point, such as a Dirac point, the apparent topological gap size is extremely
sensitive to experimental conditions including sample mosaic, resolution, and
momentum integration range. We demonstrate these effects using comprehensive
neutron-scattering measurements of CrCl$_3$. Our measurements confirm the
gapless nature of the Dirac magnon in CrCl$_3$, but also reveal an artificial,
i.e. extrinsic, magnon gap unless the momentum integration range is carefully
controlled. Our study provides an explanation of the apparent discrepancies
between spectroscopic and first-principles estimates of Dirac magnon gap sizes,
and provides guidelines for accurate spectroscopic measurement of topological
magnon gaps.
