An instrument for assaying radiation includes a radiation sensor and a collimator that covers at least a portion of the radiation sensor. The collimator defines a first field of view to the radiation sensor. An insert in the collimator defines a second field of view to the radiation sensor that is less than the first field of view.
FEDERAL RESEARCH STATEMENT
 This invention was made with Government support under Contract No. DE-AC09-08SR22470, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.