[en] Gamma-ray collimators in nuclear waste scanners are used for selecting a narrow vertical segment in activity measurements of waste vessels. The system that is used by NRG uses tapered slit collimators of both the single and double knife edge type. The properties of these collimators were investigated by means of Monte Carlo simulations. We found that single knife edge (SKE) collimators are highly preferable for a conservative estimate of the activity of the waste vessels. These collimators show much less dependence on the angle of incidence of the radiation than double knife edge (DKE) collimators. A disadvantage of the SKE collimator is that it shows considerable flux leakage outside of the angle of acceptance, especially for high γ-energies. This can be remedied to some extent by increasing the thickness of the collimator. The conclusion of this study also applies to cylindrical collimators of the single knife edge type, that are generally used in medical imaging spectroscopy