i studied that xrays can be produced when a vacancy created ,by knocking of electrons by other high velocity electrons, is filled by other electrons of outer shell. but can electron knock a electron? in same mass collision as in above just velocity is exchanged. so the orbiting electron will gain velocity of striking electron and escape. but as the velocity is just exchanged the striking electron will recieve orbital velocity and start orbiting the atom. so no vacancy is created. so how is this "vacancy thing" possible?