11.3.1 Principle of the Measurements

Investigations executed with sealed radioactive sources are based on absorption or scattering of the radiation. Gamma radiation of a sealed radiation source installed

on one side of a piece of equipment penetrates into the equipment/contained mate­rial and generates signals in a detector placed on the opposite side of the equip­ment. This signal indicates the presence or lack of the material, while its intensity is proportional with the rate of absorption which depends on the physical features of the material in the equipment. In this way, some physical features (density, thickness) of the material inside the equipment can be determined from the inten­sity measured.