Experimental determination of shielding factor for a Swedish farmhouse

Abstract: The protection that different types of buildings can provide against gamma radiation from radioactive fallout has been investigated since the late 1950s. The DCPA Standard Method has served as the basis to determine this protection, which is given by the so called “protection factor”. Several studies in the U.S. have utilized enclosed sources of radioactive nuclides to simulate fallout and test the Standard Method. Following the Chernobyl accident a set of measurements were performed in the Gävle region of Sweden to determine the protection that typical Swedish dwellings would provide against fallout gamma. The determined protection was quantized in the so called “Shielding factor”, a quantity similar to the protection factor. In recent years, a need has emerged for a feasible method to experimentally determine the protection that buildings may provide against fallout radiation. In this paper the possibility of determining the shielding factor with the help of in situ gamma spectrometry and point sources has been investigated. The aim of the study was to see whether the in situ spectrometric method of determining shielding factors would be possible to perform with point sources for a given building and, if so, the determined shielding factor would be similar to earlier determined values. A building, and associated land, was lent from the Swedish Armed Forces. An enclosed source of 137Cs was placed according to a predetermined pattern around the building and the resulting count rate was registered by a 123% HPGe detector positioned inside the building. With the obtained spectrum data, measured dimensions of the house and a calculated buildup factor, the shielding factor for the building could be assessed. The shielding factor for the farmhouse was estimated to be in the range of 0.093-0.10. Furthermore, the measurements were carried out within the planned time frame and without exposing the personnel to dose levels above guideline values. The in situ spectrometric method of determining shielding factors in combination with point sources seems promising. However, in order to fully assess whether or not the method can be used to determine the shielding factor for a given house further investigations are needed in order to establish the precision of the method and the uncertainties that the method imposes on the shielding factor.