Daniel S. Hussey, David L. Jacobson, Elias M. Baltic
During reactor operation, the neutron flux distribution is modified by the reactor control mechanisms, 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 regulation rod. The changing flux distribution results in a change in the optical axis of neutron beams which view 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 seen as a change in the position of a sharp edge and a change in the shape of the flatfield 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 flatfield shape change demonstrates a clear correlation with both shim arm angle and regulation rod position.