Sydney’s first terrestrial gamma-radiation map

Natural background radiation originates mainly from the decay of long-lived isotopes in rocks and soils, and the distribution of these radionuclides is reflected in terrestrial radiation maps. Such maps are essential for understanding environmental radioactivity, guiding urban planning, and establishing radiological baselines. For Sydney, no such map has existed until now. In this work, we produced the first terrestrial gamma-radiation map of Sydney on a 10 × 10 km 2 area using portable, low-cost gamma spectrometers (Radiacodes) for in-situ measurements and high-purity germanium gamma spectroscopy of soil samples to quantify radionuclide activities. Open-water measurements isolated the cosmic component of the total gamma dose rate. The mean terrestrial and cosmic contributions were (0.24 ± 0.27) mSv/yr and (0.173 ± 0.035) mSv/yr, respectively—both in line with global background levels. Regression–kriging combining lithology and mean radionuclide concentrations as predictors explained 98% of the variation in the data, confirming that geological composition largely determines the spatial pattern of natural radioactivity in Sydney. This reproducible low-cost approach addresses a relevant data gap and establishes a foundation for citizen-science radiation mapping in urban environments across Australia and worldwide.