One of the pitfalls of the drop deposition method is that radon permeability strongly depends on environmental parameters like temperature, humidity and pressure . Therefore, it was proposed by the project partners to quantify the relation of the radon permeability to the environmental conditions. JRC did not have a radon chamber with controlled atmosphere during this work. Therefore, the relation between radon emanation and environmental parameters was not quantified in our study. It was noted that temperature and pressure sometimes changed during the experiments which might result in unstable emanation. The following environmental parameters were recorded during an emanation experiment: t = 19.9–24.5 oC, p = 975–1035 mbar and 24–40% of relative humidity. To overcome the environmental dependence on the emanation through foils, chemisorption sources were prepared using radium specific discs.
The emanation coefficients of the JRC radon emanation sources by the two methods are compared to radon emanation sources from the literature based on different preparation approaches in Table 1.
One of the deposition sources, (RnDep3), was sealed in a 200 µm thick plastic film using a laminating machine. This laminating foil was proved too thick resulting in a significant drop (from 74 to 10 %) in emanation power.
Another shortcoming of our sources is the slow kinetics of radon emanation. It was observed that 2–3 days are needed to reach the saturation point in our small volume (2 L) setup from an 588 Bq 226Ra activity source with a maximum activity concentration of 44.3 (26) kBq/m3 measured by AlphaGuard.
The emanation sources were already tested in radon-in-water measurements where elevated radon activity concentration had to be produced in a low volume (0.2–1 L) container . The emanation sources were placed in an LDPE foil and immersed in a 1-L heat shock resistant borosilicate glass bottle with a gas-tight but flexible screw cap designed for volatile organic material storage. A more detailed description on the bottle and radon-in water setup is presented elsewhere [6, 7]. After 5 days of exposure approximately 275 Bq/kg 222Rn massic activity was measured. The maximum (equilibrium) of 525 Bq/kg 222Rn was reached after 25 days of exposure.
The homogeneity of the adsorbed radium over the disc surface of two chemisorption sources was tested by autoradiography as seen in Fig. 2.
Concentric circular region-of-interests are defined with the centre corresponding to the the physical centre of the source, the zero distance point (“0”) in the graphs of Fig. 2. The radial activity distribution is presented as digital light unit density over a surface area (DLU mm− 2). It was recorded along a line through the centre of the sources in 85 µm/pixels steps and 30 minutes approximate exposure time. is taken at the centre of the source. The relative weights shown in the lower part of Fig. 2, are the DLU of a ring over the DLU of the net sum of all the area divided by the surface area of that ring over the surface area of the nearest inner ring. The 226Ra activities of the sources were between 60 and 80 Bq.
As seen in Fig. 2, the deposited activity along the surface of the MnO2 coated source (Metro-Rn02) can be considered homogeneous. This source preparation method is a very good candidate for preparing homogeneous emanation sources. On the downside, we still observe very low 226Ra adsorption yield from the initial 226Ra stock solution (< 10–30%) and low emanation power that suggests the need for further research.
The environment related goals to reduce waste, energy consumption and chemicals can be achieved by applying the MnO2 disk source preparation approach. The procedure needs limited resources: few basic lab wares, MnO2 disk, stirrer, about 100 mL water and depending on the required activity maximum few mL of standardised 226Ra solution.
226Ra is fixed on the MnO2 coated disk and stays there unless this upper layer is dissolved or physically removed. Therefore, the risk of contaminating the experimental setup is low comparing to cases when powder type radon emanation sources or radon emanation sources are not fixed to a carrier or the material used for encapsulation is damaged.
MnO2 coated disks can be handled freely and placed directly into the experimental setup without extra precautions. The adsorbed 226Ra activity could be directly determined accurately by placing the disk in a defined solid angle alpha-particle spectrometry setup.
On the other hand, there are still difficulties to determine the total 226Ra activity with the desired accuracy (usually by gamma-ray spectrometry) and the emanation power correctly in powders or solid materials.
There is another drawback of inorganic porous gels based on heavy-alkali-earth metal hydroxides. Hydrated silicic acid and silica gel based emanation sources are sensitive to humidity . They adsorb moisture from the air in their pores that affects the emanation power and can fluctuate in time.
One of the aims of the MetroRadon project was to develop radon and thoron reference sources with stable and known radon emanation. Stable emanation power means it is as much independent as possible from the environmental parameters. Since humidity, pressure and temperature can influence the radon emanation property of many of the historical porous materials, they did not meet the requirements of the MetroRadon project. Furthermore, due to aging of these solid porous materials their emanation power degrades in time. It was observed that after 16 years radon emanation changed from few% up to 40% . Long-term chemical and physical stability of MnO2 coated disks have to be evaluated at JRC in the future.
JRC recently purchased a small size standardised radon emanation chamber with a well-defined geometry that will be used for future experiments. Using this chamber will enable us to reduce the uncertainty in the air volume and determine the emanation coefficient more accurately. We further plan to investigate the source preparation methods by changing some experimental parameters as listed in Table 2.