Before the measurements the specimens were dried in a temperature controlled furnace (oven) at 1101C for more than 24 h to ensure complete removal of moisture. Later, specimens were placed one at a time in the 150 dm3 accumulation (measurement) chamber under a controlled atmosphere. The big volume of the chamber (150 dm3) was required for stabilization of the air atmosphere inside. Humidity was generated by a bubbling method which utilized a hot water bath. Before starting the measurement, each specimen was kept at the desired measurement conditions for 3 days to reach a state of equilibrium between it and the chamber environment. Thirty minutes before the start of the measurement the chamber was flushed with room air using a high volume pump (10 L min-1) to remove accumulated radon and thoron. The environment conditions inside the chamber were changed a few percent from the initial state but equilibrium was reached again within one to a few hours (depending on thefinal conditions) after starting measurements (Fig. 2). The chamber tightness was checked by an appropriate test utilizing CO2.