We confirmed the X-ray emission characteristics using an X-ray tube that generates soft X-rays by emitting a 5keV electron beam from a carbon nanotube emitter (CNT). In the low-energy region below 5keV, the X-ray emission yield is primarily attributed to bremsstrahlung than characteristic x-ray, and we established Geant4 monte-carlo simulations and cold cathode field emission experiments to verify this phenomenon. Furthermore, based on the electron beam data, we experimentally confirmed that the emission yield reaches its maximum at an optimal thickness, as theoretically predicted based on the anode's X-ray self-shielding effect or the range of the electron beam in the anode.