The correlated emission of two electrons from a surface excited by a low energy primary electron (coincidence spectroscopy) involves elastic back scattering of the primary electron in the sample. In the present experiments we also used an internal electron source: photoelectrons from the valence band created by synchrotron radiation. Within the experimental error we found the same threshold energy as with an external electron source. We studied the angular dependence of the number of coincidence events following 25-45 eV primary electron scattering on W(100). The most probable angle between two scattered electrons was found to be about 60-75 degrees, regardless of the primary electron incidence angle. The best agreement between calculated (in the kinematic approximation) and measured angular distributions of the coincidence events was achieved when the effective mass of the valence electrons was taken to be 2-3 times that of the free electron mass. This may be due to the predominant scattering on the d-electrons of the tungsten valence band.