A position detector arranged to be mounted at a radiation detector of a radiotherapy treatment apparatus, which includes a gantry rotatable about a gantry rotation axis, and a collimator rotatable about a collimator rotation axis. The radiation detector is mounted at the collimator. The position detector comprises:- an accelerometer device, which is arranged to detect at least gravitational acceleration in at least one dimension；- a gyro device arranged to detect at least angular velocity in at least one dimension；wherein the accelerometer device and the gyro device in common are arranged to be operative in three dimensions, and- a controller connected with the accelerometer and the gyro；wherein the controller is arranged t o receive first input data from the accelerometer device and second input data from the gyro device, and to determine at least a collimator angle and a gantry angle by means of the first and second input data.
