An instrument based on a small and inexpensive silicon diode was developed and used to measure, for the first time, the gamma radiation exposure rate within a start-up instrumentation guide tube in a shut down CANDU® reactor undergoing refurbishment. The shape of the measured profile agreed with the expectation; however, the maximum measured exposure rate was about four times higher than the calculation. Two adjuster rods located above the core and adjacent to the guide tube, could be clearly identified from the features of the radiation profile measurement. These measurements provide confirmatory information for the creation of safe and effective radiation work plans and selection of appropriate in-core instrumentation. This technology can be used to accurately measure the positions of irradiated reactor components, such as adjuster rods, pressure tubes, calandria tubes, reactivity control absorbers and other sources of gamma radiation inside a shut down reactor.Copyright © 2010 by ASME and Atomic Energy of Canada Ltd.