2.1. Measurement system
A modified standard method utilized radon and thoron gas
monitor (type RAD7) was used to measure radon exhalation
(RnExh) and thoron exhalation (TnExh) rates from the materials
(Fig.1). More details about the standard method and its validation
can be found elsewhere (Hassan et al., 2011a).
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 dm
3
accumulation (measurement) chamber under a controlled atmosphere. The big volume of
the chamber (150 dm
3
) 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 wasflushed 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.
The accumulation chamber was connected to the RAD7 monitor using vinyl tubing, and a gas-drying unitfilled with a desiccant
was installed between them, to maintain the relative humidity at
o10%within the measurement system as recommended by the
manufacturer. The system was a closed loop in which the gas was
circulated continuously with the flow rate generated by an
external pump with the sameflow rate as the RAD7 pump. The
flow rate was measured for the correction factor calculation for
thoron concentration. The concentrations of radon and thoron
released from each specimen inside the chamber were allowed to build up with time and were measured by the RAD7 monitor in a
cycle time of 60 min. Simultaneously, the temperature and the
relative humidity inside the accumulation chamber were also
recorded. The measurements were performed in an airconditioned room with temperature controlled at 22–231C and
normal room atmospheric pressure. The RAD7 device was calibrated in the reference thoron atmosphere at the PTB (The
Physikalisch-Technische Bundesanstalt, Germany) (Röttger et al.,
2010) and in the NIRS radon chamber (Tokonami et al., 2008).
When a specimen is put into the accumulation chamber, the
radon exhaled from the specimen increases exponentially whereas
the thoron concentration can reach stable condition very quickly as
presented inFig. 3for granite, upper and lower graphs, respectively