The present invention relates to determining changes in the X-ray emission yield of an X-ray tube, in particular determining dose degradation. In order to provide determination of such changes, an X-ray source is provided comprising a cathode, an anode； and at least one X-ray sensor (16). The cathode emits electrons towards the anode and the anode comprises a target area on which the electrons impinge, generating X-ray radiation. An X-ray barrier (24) is provided with an aperture (26) for forming an emitting X-ray beam from the X-ray radiation, wherein the emitting X-ray beam has a beam formation (30) with a central axis. The at least one X-ray sensor is arranged within the beam formation and measures the X-ray intensity for a specific direction of X-ray emission with an angle with respect to the central axis. The at least one X-ray sensor can be positioned inside the beam formation (30), but outside the &ldquo；actual field of view&rdquo； (40) as determined by a diaphragm (36).
