Measurements and analysis demonstrate that self-powered fixed in-core detectors can be used to reliably measure the spatial distribution of the gamma flux in a large nuclear reactor in the shutdown state. Measurements were made at the Hanford N Reactor, a large graphite-moderated dual-purpose nuclear reactor that has been equipped with a state-of-the-art fixed in-core detector system consisting of rhodium and hafnium self-powered detectors. Although the system was designed for neutron flux measurements at power, the detectors can be used to measure the gamma flux distribution in the reactor while the reactor is shut down. Except for some flux peaks caused by known radiation sources, the spatial distribution can be related to the fuel burnup surrounding each detector. Subsequent measurements were made after the fuel was removed and it was found that the gamma flux decreased by an amount that was predicted from the first measurement. These measurements demonstrate that well-designed self-powered fixed in-core detectors can be used to extract meaningful information at power levels much smaller than previously thought. A fixed in-core detector system, in conjunction with commercially available signal collection equipment, can be used to accurately and reliably measure neutron and gamma flux distributions in nuclear reactors at power levels on the order of 0.1%.