Changing optical axis due to reactor operation

During reactor operation, the neutron flux distribution is modified by the reactor control mechanisms and in the case of the reactor at the National Institute of Standards and Technology, this is determined by the angular position of the Cd shim arms and the vertical position of an Al regulating rod. The changing flux distribution results in a change in the optical axis of neutron beams, whose view is a fixed position within the reactor core. The changing optical axis results in two noticeable image artifacts: poor registration between images of a static object taken at different times and a change in the shape of the flat field intensity. These two effects were measured during the first four days of reactor operation. Both measurements show correlation with the reactor control mechanisms, with combined correlation coefficients during the first two days after reactor startup approaching 1. The change in the edge position is well below the image spatial resolution, and has more uncertainty associated with it. However, the change in the flat-field shape demonstrates a clear correlation with both shim arm angle and regulating rod position.