  In simulations of a model with topological sectors, algorithms which proceed
in small update steps tend to get stuck in one sector, especially on fine
lattices. This distorts the numerical results; in particular it is not
straightforward to measure the topological susceptibility chi_t. We test a
method to measure chi_t even if configurations from only one sector are
available. It is based on the topological charges in sub-volumes, which we
denote as "slab". This enables the evaluation of chi_t, as we demonstrate with
numerical results for non-linear sigma-models and for 2-flavour QCD. In the
latter case, the gradient flow is applied for the smoothing of the gauge
configurations, and the slab method results for chi_t are stable over a broad
range of flow times.
