This part of IEC 61577 provides basic data and technical information in order to support the design of instruments and their practical application for the measurement. The document covers ²²²Rn as well as ²²⁰Rn and the short-lived decay products of both. It is an accompanying document for the application of the technical standards series IEC 61577, and provides physical and technical fundamentals of the measurements methods. For more information, reference is made to the Bibliography.