One salient property of long polymer chains in a melt is a constraint motion caused by a virtual tube along the chain profile as it has been proposed by deGennes in his famous Reptation model. This model proposes - next to the segment length - a second length scale in polymer melts, namely the tube diameter or entanglement distance. Neutron scattering can provide the single chain dynamic structure factor on the one hand and the time dependent segment mean squared displacement on the other hand. In the first case a protonated chain is measured in a deuterated matrix by neutron spin echo (NSE) - spectroscopy in the second the same method is applied to a fully protonated melt of polymer chains thereby providing information on the segment self correlation function. The different approaches, coherent and incoherent NSE, lead to the same tube diameter if the curvilinear segment motion along the contorted tube is accounted for.