A neutron monitoring system uses a gamma thermometer to calibrate a nuclear instrument to ensure more accurate neutron flux measurements in a nuclear reactor core. The nuclear instrument generates a first signal proportional to a measured local neutron flux. The gamma thermometer generates a second signal proportional to a measured local gamma flux. A processor is used to compensate the second signal by filtering groups of delayed gamma sources to a reduced number of specific groups of delayed gamma sources. A third signal is calculated based upon yield fractions and time constants that are determined for the specific groups of delayed gamma sources. A gain of the nuclear instrument is calibrated based on a compensated second signal that is determined by subtracting the third signal from the second signal.
 This invention was made with Government support under contract number DE-FC07-07ID14778, awarded by the U.S. Department of Energy. The United States Government has certain rights in the invention.