The present specification relates to an additive manufacturing apparatus comprising an X-ray reference object (18) for calibrating an electron beam unit in the additive manufacturing apparatus by detecting X-rays generated by sweeping an electron beam from the electron beam unit over a reference surface (19) of the X-ray reference object (18) and processing the detected signals, the X-ray reference object (18) comprising a support body (20) that has a top surface (21) and comprises a plurality of holes (22) in the top surface (21), The X-ray reference object (18) comprises a plurality of target members (23) inserted into the plurality of holes (22) of the support body (20). The present specification also relates to an X-ray detector to be used in the additive manufacturing apparatus, and to a method for calibrating such an additive manufacturing apparatus.