Objective: Decommissioning of nuclear facilities is a critical phase in the lifecycle of nuclear energy utilization, and the safety of the procedure directly impacts environmental and public health. As an increasing number of nuclear facilities worldwide near the end of their designed service life, the question of how to carry out safe and effective decommissioning has become an urgent issue to address. During this process, precisely evaluating the radioactive contamination of structural components is fundamental to formulating decommissioning plans and management measures. Although diverse source term survey methods exist, in practice, because of the inadequacy or inaccuracy of process data, it is frequently necessary to rely on radiation measurement techniques to obtain specific information on radioactive contamination. Therefore, this study aims to develop a measurement technology to enhance the accuracy of measuring the radioactivity distribution of structural components during the decommissioning of nuclear facilities, providing a scientific basis for the secure decommissioning of nuclear facilities. Methods: This research employs a collimated gamma detector, which primarily consists of a gamma detector and a collimator. The collimator is used to limit the direction of incident rays to enhance the spatial resolution and sensitivity of the measurement. The structural components involved in the decommissioning of nuclear facilities can be categorized as flat plates and pipelines based on their geometric features, each requiring different scanning and measurement strategies. The scanning measurement techniques appropriate for flat panel structural components can be selected based on the contamination type. A circular scanning measurement method is adopted for pipe structural components. During the measurement process, the collimated gamma detector traverses the structural component surface, recording gamma-ray signals from all potentially contaminated areas and conducting preliminary analysis of the collected data to assess data quality and integrity. To determine the radioactivity distribution of the structural components from the measurement data, both equal-resolution reconstruction and super-resolution reconstruction methods are proposed. Equal-resolution reconstruction employs grid sizes that are identical to the collimator's aperture size for gridding the area under test, while super-resolution reconstruction uses grid sizes smaller than the collimator's aperture size to achieve higher resolution. Equal-resolution reconstruction is suitable for tasks requiring faster reconstruction speeds, and super-resolution reconstruction is suitable for tasks demanding higher resolution. Both methods are implemented through iterative algorithms. Results: The results demonstrate that the methods proposed are effective in measuring the radioactivity distribution of structural components during the decommissioning of nuclear facilities, with good position and angular resolution. By conducting Monte Carlo simulation validation, the relative deviation of the radioactivity derived by both the reconstruction methods is within 10%, and the position resolution at a detection distance of 60 cm derived by equal-resolution reconstruction and super-resolution reconstruction methods reached 3.2 mm and 1.6 mm, with corresponding angular resolutions of 0.3° and 0.2°.The average reconstruction speed of the super-resolution method is slower than that of the equal-resolution method. However, in practical applications, the appropriate reconstruction method that is most suitable for specific needs may be selected. Conclusions: This study develops a measurement technology for radioactivity distribution based on a collimated gamma detector, providing a novel technical strategy for accurately measuring radioactivity in structural components during the decommissioning of nuclear facilities. The technology enhances the accuracy and resolution of measurements through systematic modeling and algorithm design, providing technical support for the secure decommissioning of nuclear facilities. Future research can further optimize the hardware parameters of the detector, which, when combined with this study's results, provide more comprehensive technical support for the secure decommissioning of nuclear facilities.