Neutron-acceptance diagram shading (NADS) is a beam simulation technique. Unlike Monte-Carlo simulation codes like McStas, NADS does not trace individual neutrons but traces linearly-related bunches in a reduced-dimensionality phase space. Bunches are subdivided where necessary to follow accurately a simplified surface reflectivity model. This makes jnads results equivalent to Monte-Carlo simulations but about 5 orders of magnitude faster for difficult modelling tasks.
