Combined measurements of outdoor gamma background and radon in populated urban areas have not previously been conducted in Bulgaria. Such outdoor measurements can provide information on natural background concentrations, allowing the identification and quantification of anthropogenic contributions. In addition, local outdoor radon levels influence indoor radon concentrations; therefore, these data could supplement other parameters used to identify radon priority areas. Guided by these goals, an outdoor radiation background survey is planned under project КП-06-Н87/3 of the National Science Fund, Bulgaria. The purpose of this paper is to present the prepared research design. The preparatory activities include: (1) identification of the target population; (2) selection of a representative sample using an appropriate sampling method; and (3) determination of the types of detectors and the integrated system for combined measurements. The article presents the results of the statistical analyses performed for the sampling design using the Complex Samples procedures in IBM SPSS. The target population consists of all urban sites in Bulgaria (218 in total). Simple Random Sampling without replacement was applied, with each unit selected with equal probability. Population size was used for stratifying the urban sites into non-overlapping subgroups. The subgroups are defined as follows: fewer than 5,000 people, 5,001–30,000 people, and more than 30,000 people. The randomly selected sample consists of 96 towns (excluding Sofia city), each of which will be investigated using more than one detector. An integral system will be installed at each selected location, comprising passive radon detectors and TLD dosimeters for gamma background measurement, encased in a protective enclosure.

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