A structure for supporting a detector head of a radiation detector which comprises an arm having a distal end supporting the head, a base, a joint mounted on the base and moveably supporting the arm and a slotted member rotatably mounted to the base in a predetermined distance from the joint. The slotted member includes a guidance slot. It also contains a mounting mechanism for mounting the arm in the guidance slot at a desired distance from the rotational axis in a manner that the arm is freely slidable along its longitudinal arm axis, and is non-rotatably fixed with respect to the guidance slot. A motor drive rotates the slotted member about its rotational axis.