Terrestrial background gamma radiation in urban surroundings depends not only on the content of radionuclides in the soil and bedrock, but also on levels of radionuclides in building materials used for the construction of roads, pavements and buildings. The aim of this study was to characterize an outdoor absorbed dose rate in air in the city of Wroclaw and to indicate factors that affect the background gamma radiation in an urban space. Gamma spectrometric measurements of the radionuclide content and absorbed dose rate in air were performed by means of portable RS-230 gamma spectrometers and at sites with various density of buildings, in the city center and in more distant districts, over pavements and roads as well as in a park, a cemetery and on four bridges. Measurements were performed at a 1-meter height. The absorbed dose rate in air ranged from 19 to 145 nGy h-1, with the mean of 73 nGy h-1. This paper implicates that terrestrial background gamma radiation depends on the type of building material used for the construction of roads and pavements and on the density of buildings shaping the geometry of the radiation source. The highest background gamma radiation was observed in the center of the city, where buildings are situated close to each other (nearly enclosed geometry) and pavements are made of granite. The lowest background gamma radiation was noticed on bridges with nearly open field geometry. Additionally, three profiles at the heights of 0.0, 0.5 and 1.0 m were arranged between two opposite walls of the hall of the Main Railway Station in Wroclaw, where the floor is made of various stone slabs. The results indicated that the absorbed dose rate in air varied, depending on the type of building material, but became averaged along with the height.  

20(3)2015