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The Lane-Lynn model for direct radiative capture (DRC) is extended to isotopes where no experimental information on the low-lying levels (spectroscopic factors etc.) is available. To this end we assumed that all the partial transitions to the final levels are lumped in the Ssub(1/2)->Psub(3/2), Ssub(1/2)->Psub(1/2) transitions only (where the p-wave final states are single particle states with complete strength). Within these assumptions the direct capture cross sections for calcium isotopes between A = 40 and A = 48 as well as for exp 136 Xe were calculated. We showed that the uncertainties of these calculations are < or approx. 30% if we compare the results to the calculations with the Lane-Lynn model for exp 42 , exp 48 Ca and exp 136 Xe where the experimental informations required are available. The new approach allows for an evaluation of thermal cross sections and, more important, for the calculation of DRC cross sections of extremely neutron rich nuclei which are significant for nucleo-synthesis of the heavy elements in the rapid neutron capture process. (ERA citation 08:048121)