invention the alignment fixture 15 can also be an array of radio opaque markers positioned symmetrically at the gimbal assembly 39 s common rotation center or the like in other embodiments of this invention in this embodiment of the present invention the alignment fixture 15 is a metallic ball fig2 shows the present invention with the radiation beam detection apparatus 10 mounted on a gimbal assembly 40 a radiation detector 20 such as film is attached to the beam detector apparatus 10 the alignment mirror 18 is shown supported by the beam detector apparatus 10 such that it is parallel to the radiation detector 20 a laser light beam 5 which corresponds to the central axis is shown exiting the linac 50 and striking the alignment mirror the alignment mirror 18 is used to indicate when the radiation detector 20 is perpendicular to the beam central axis the beam detector apparatus 10 supports the alignment mirror 18 and the radiation detector 20 which are parallel to each other on either side of the alignment fixture 15 the alignment fixture 15 is located at an appropriate distance from the radiation detector 20 in the current embodiment approximately 2 cm the gimbal assembly 40.