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Collection and behaviour of radon progenies on thin Mylar foils
Radiation Measurements 46 (2011) 631e634
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Technical Report
Collection and behaviour of radon progenies on thin Mylar foils Jani Turunen a, *, Sakari Ihantola a, Kari Peräjärvi a, Roy Pöllänen a, Harri Toivonen a, Erich Hrnecek b a b
STUK e Radiation and Nuclear Safety Authority, P.O. Box 14, FI-00881 Helsinki, Finland European Commission Joint Research Centre Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2010 Received in revised form 4 April 2011 Accepted 7 April 2011
Thin Mylar foils are often used to protect detectors from contamination. However, these foils can be electrostatically charged, possibly leading to their contamination with airborne radon progenies. In the present work, the collection and behaviour of radon progenies on Mylar foils was investigated in detail using alpha spectrometry. The radon progenies collection rate of a small Mylar foil (3 cm2) is equivalent to an air sampler with a flow rate of approximately 0.1 m3/h. It was demonstrated that such contamination may jeopardise the validity of the entire analysis if not interpreted correctly. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Radon progenies Mylar foil
1. Introduction Alpha particle emission is a typical decay mode of heavy elements. Due to the large stopping power of alpha particles in air, alpha spectrometric measurements are most often carried out in a vacuum to achieve good energy resolution. For samples containing nuclides such as 229Th, 228Th or 226Ra, which produce shortlived alpha-particle-emitting daughters within their decay chains, recoil contamination of the detector is a well-known problem (Sill and Olson, 1970). This effect can be reduced by slightly increasing the pressure in the measurement chamber and applying a negative potential to the alpha source (Sill and Olson, 1970) or by using thin foils that do not significantly influence the energy resolution of a measurement to physically protect the detector (Vainblat et al., 2004). Commercially available biaxially-oriented polyethylene terephthalate (Mylar) foils are often used for this purpose. Besides alpha spectrometry, Mylar foils are also widely used for protection in other radiation measurement applications. The concentration of radon decay products in air is to a large extent influenced by the plateout parameter of unattached radon daughters and the aerosol concentration (Porstendörfer, 1984). Enhanced deposition of radon progeny on polyethylene surfaces has been observed by Knutson et al. (1992), and the enhancement of deposition on electrostatically charged surfaces under typical office conditions has been reported by Batkin et al. (1998). Finally, the electrostatically enhanced collection of 218Po and 214Pb on aluminised Mylar foils kept at a negative potential has been demonstrated by Maiello and Harley (1989). * Corresponding author. Tel.: þ358 9 759 88 440; fax: þ358 9 759 88 433. E-mail address: jani.turunen@stuk.fi (J. Turunen). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.04.009
It is generally known that plastic films may be charged and are thus able to collect aerosol particles from air. Radioactive radon progenies attached to an aerosol particle offer an efficient means for continuous re-ionisation of the particle. If attached aerosol particles remain on the foils during alpha spectrometric measurements in a vacuum, they may lead to extra counts or peaks in the resulting alpha spectra. This contamination would be highly unwanted, especially in studies of samples containing radon progenies or when their true existence cannot be completely excluded. In the present work, the behaviour of such contamination in a vacuum was investigated using plain Mylar foils and Mylar foils covering real samples. The collection of radon progenies on plain Mylar foils was also measured under ambient air pressure. The Mylar used in the measurements was obtained from Goodfellow Cambridge Ltd., England, and its thickness was 0.5 mm. The measurements were performed using a single silicon detector connected to a multi-channel analyser and with the PANDA (Particles And Non-Destructive Analysis) device (Turunen et al., 2010). In PANDA, thin Mylar foils are always applied to protect the instrument from contamination. 2. Experimental The PANDA setup has two measurement positions holding various detectors inside a vacuum chamber. In this work only its double-sided silicon strip detector (DSSSD) was used. The positionsensitive DSSSD is used for alpha particle detection. It has 32 horizontal and 32 vertical strips, making a grid of 1024 pixels. The pixel size is 2 mm 2 mm. The data are collected in event mode and the events are time stamped, thus giving the possibility to follow the location, energy and time of registration of every detected alpha particle.
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J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
2.1. Impactor sample measurement with PANDA This study started with the measurement of an impactor sample with multiple active spots (spot size approximately 1 mm) in a spiral-shaped formation (diameter 23e28 mm, distance between consecutive spots approximately 3 mm, see Fig. 1a). The sample was prepared by collecting aerosol particles originating from tungsten inert gas (TIG) welding using a Berner low-pressure impactor. Note that the tungsten electrodes used in welding contain 4% thorium and the spots in the sample therefore also include thorium and its progenies. There was a 13 mm circular hole in the centre of the sample foil. This sample was measured twice. Between the measurements, the Mylar foil between the sample and the DSSSD, as well as its location, was changed. Before each measurement was started the sample holders with the attached foils were left in the laboratory for some hours, i.e., the Mylar foils were directly exposed to the laboratory air. In the first measurement, the impactor sample was installed between two 1-mm-thick aluminium sample holder plates. These plates had a circular hole in the middle with a diameter of 35 mm (see Fig. 1a; the hole is marked with a yellow dashed line). The Mylar foil was attached to the front surface of the holder plate facing the DSSSD. It was not in contact with the actual sample. The source-to-detector (DSSSD) distance was 4 mm. The total measurement time was 14 days. In the second measurement of the same sample, the Mylar foil was installed in direct contact with the sample. Only the Mylar covering the area with the 13 mm diameter
hole was untouched. The second measurement was carried out in a tighter measurement geometry, with a source-to-detector distance of 3 mm, and this time the measurement lasted 7 days. A second similar impactor sample was also measured with PANDA using the same configuration as in the first measurement of the first sample. The purpose of this measurement was to determine whether the results obtained from the measurements of the first sample could be reproduced with a different sample. 2.2. Plain Mylar foil measurement with PANDA Measurements with plain Mylar foils, i.e., without an actual sample, were carried out with PANDA. In these measurements, the plain foil was attached to the front of the sample holder with a 35 mm hole in the centre. The sample holder with the Mylar foil was taken to an underground parking garage that has a radon progenies concentration of approximately 100 Bq/m3. The garage was chosen to get better counting statistics due to the higher radon progenies concentration than in the laboratory. After 2 h of collection, the foil was measured with PANDA. The delay between finishing the sample collection and starting the measurement was 30 min. This measurement lasted 18 h. After this, the same foil in its holder was taken back to the garage and again left there for 2 h. The foil was then measured again (delay 25 min, measurement time 21 h). Finally, a background measurement was made to ensure that no radon progenies or interfering contamination were present in the PANDA equipment.
Fig. 1. First measurement of the impactor sample. The total measurement time was 14 d. (a) Photograph of the sample and the Mylar foil in front of it. (b) Alpha particle hitmap for the first 3 h of measurement. (c) Alpha particle hitmap for the measurement period excluding the first 3 h. (d) Alpha spectrum recorded during the first 3 h of measurement. (e) Alpha spectrum recorded during the measurement period excluding the first 3 h.
J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
2.3. In-situ measurement of plain Mylar foil with alpha spectrometer Besides the measurements carried out with the PANDA equipment, plain Mylar foil was measured with a silicon detector (CAM 450 AM, Canberra, USA) on-site in the same parking garage. This time the Mylar foil was attached on-site to a 1 mm thick aluminium plate that had a hole with a diameter of 20 mm. The Mylar foil was only exposed to the air flow inside the garage during the measurement. The silicon detector was looking at the Mylar through the hole from the other side of the aluminium plate. The detector mounting was in contact with the plate. The distance between the foil and the detector surface was 2 mm. Both sides of the foil were exposed to the garage air. However, the flow at the detector side was negligible. The data acquisition was started after office hours and left to run overnight. The spectral data were saved in 1 min sequences. The radon concentration in the air was measured simultaneously with a portable radon monitoring device (AlphaGUARD, Genitron Instruments GmbH, Germany). Two other similar measurements with fresh plain Mylar foils were also made. In these two additional measurements the radon monitor was not available. 3. Results 3.1. Impactor sample measurement with PANDA Fig. 1 presents the data from the first impactor sample measurement. The alpha particle hitmap from the first 3 h of measurement (Fig. 1b) clearly does not correspond to the spiral shape of the source (Fig.1a). The alpha particle hits can be seen in the area where the Mylar foil was not in contact with anything. Most of these hits did not come from the actual sample, but from the radon progenies collected on the Mylar foil. Notice that the distribution of alpha particle hits is influenced by the used measurement geometry (Mylar-to-detector distance 3 mm). In Fig. 1c, the hitmap for the rest of the measurement period, i.e., two weeks of data, clearly retains the shape of the source. Note that it is a mirror image of the source, as the source is directly facing the DSSSD. The two alpha spectra in Fig. 1 were obtained using six y-strips located at the centre of the DSSSD, giving an intersection through the middle of the sample. The spectrum from the first 3 h of measurement (Fig. 1d) is completely different from that for the rest of the measurement period (Fig. 1e). The large peak at 7.7 MeV in Fig. 1d belongs to 214Po (a member of the 238U decay chain). Since its half-life is only 164.3 ms, the decay rate is primarily controlled by its precursors, 214Bi (T1/2 ¼ 19.9 min) and 214Pb (T1/2 ¼ 26.8 min). Both
633
of these nuclides disintegrate via beta decay. Thorium and its progenies are mainly responsible for the spectrum in Fig. 1e. The second measurement of the first impactor sample showed similar disturbance from radon progenies attached to the Mylar foil in front of the sample. This time, the extra counts in the alpha particle hitmap (not coming from the actual sample) were seen in a smaller area than in the previous measurement. The reason for this was the smaller untouched area (13 mm) of the foil than in the previous case (35 mm). These hits were again mostly recorded at the beginning of the measurement period. Radon progenies collected on the Mylar foil were also seen in the measurement of the second impactor sample. 3.2. Plain Mylar foil measurement with PANDA Fig. 2a and b presents the data recorded with PANDA from a Mylar foil that had been exposed to indoor air in a parking garage for 2 h. In the alpha spectrum shown in Fig. 2a, three peaks are visible. Besides the 7.7 MeV peak belonging to 214Po, the presence of 220Rn daughters 212 Bi (6.1 MeV) and 212Po (8.8 MeV) on the Mylar foil can be confirmed. The time behaviour of 214Po and 212Po is shown in Fig. 2b. The time behaviour graphs were made by gating the timestamp data with alpha gates matching the energy regions of each peak in Fig. 2a. These 18-h-long measurement data were binned to 700-s-long bits. The decay of 212Po (and 212Bi, results not shown in Fig. 2) closely follows the calculated decay curve. This agreement demonstrates that the studied nuclides remain on the Mylar foil, even in vacuum conditions. The few extra counts at the end of the experimental 214Po curve are due to the tailing of the 212Po peak. After the first measurement, the sample holder with the same Mylar foil still attached was taken back to the parking garage for 2 h and then measured again with PANDA. This time, no clear traces of 214Po from the collection were seen in the Mylar foil, indicating that the foil was no longer able to collect new aerosols. Some 212Bi and 212Po were still present on the foil from the previous collection. 3.3. In-situ measurement of plain Mylar foil with alpha spectrometer Fig. 3 presents the in-situ measurement of a plain Mylar foil with the CAM detector. The area of the 214Po alpha peak was determined for each 1 min measurement. The absolute alpha detection efficiency of the measurement setup was approximately 0.38. The 214Po activity on the foil during the measurement was determined using the peak areas and the known alpha detection efficiency. The activity of 214Po on the foil grows in very quickly.
Fig. 2. Measurement of plain Mylar foil with the PANDA setup. (a) Alpha spectrum of the measurement. (b) Time behaviour of 214Po and 212Po on the Mylar foil. The dashed line is a theoretical calculation for the shape of the 212Po curve, assuming only radioactive decay. The shape of 212Bi time behaviour was similar to 212Po (results not shown).
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J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
Fig. 3. The collection and decay of 214Po on a Mylar foil measured with a CAM detector and the prevailing radon concentration measured with an AlphaGUARD monitor.
After a few hours, the activity starts to decrease and goes down to a low level, since the foil can no longer collect new particles. The radon gas concentration level measured with the AlphaGUARD monitor in the parking garage is also illustrated in Fig. 3. Since the monitor gives a value for the total radon concentration in the air, it was not possible to accurately determine the relative amount of radon from the 238U decay chain alone. It has been assumed in the calculations that all detected radon is coming from the 238U decay chain and the decay chains of 232Th and 235U have been ignored. This way it is possible to determine a minimum value for the collection rate. The measured radon concentration during the first 3 h of the measurement was approximately 86 Bq/m3. This value was used as a fixed parameter in a fit made to the 214Po activity during the first 3 h of the measurement (Toivonen et al., 1988; Zhang and Luo, 1983). The result of this fit was the collection rate on Mylar. As compared with air samplers, the equivalent collection rate would be 110 l/h. The corresponding deposition velocity is approximately 10 cm/s. In the two additional measurements with plain Mylar foils similar behaviour of the 214Po count rate was observed. 4. Conclusions It is known that Mylar can act as an electrostatic collector of charged aerosol particles containing radon progenies. Presently, we
have investigated in detail both the collection and survival of radon progenies on thin Mylar foils. It was demonstrated that radon progenies remain well on Mylar foil, even in vacuum conditions, if the foil is not manipulated. As shown with the measurement of the impactor sample the use of contaminated Mylar foil can lead to false results and wrong conclusions. This is the case especially when studying samples with low activity. The studied phenomenon is obviously strongly dependent on the radon concentration in the air of the laboratory and the electrostatic charge of the Mylar foil. One way to reduce this effect would be to uncharge the foils before use. In conclusion, this effect should not be forgotten, especially while operating in areas where the radon concentrations in the air may be high. Looking at the effect from the opposite side, it can be concluded that the Mylar foils used are efficient radon progenies collectors. Therefore, the use of Mylar foils as costeffective electrostatic samplers is an option that deserves further attention.
Acknowledgements The authors would like to thank R. Engelbrecht, Seibersdorf Labor GmbH, for providing the impactor samples.
References Batkin, I., Brun del Re, R., Boutin, J.-G., Armitage, J., 1998. g-spectroscopy investigation of radon daughter deposition on electrostatically charged surfaces. Phys. Med. Biol. 43, 487e499. Knutson, E.O., Gogolak, C.V., Scofield, P., Klemic, G., 1992. Measurements of radon progeny activity on typical indoor surfaces. Radiat. Prot. Dosim 45 (1e4), 313e317. Maiello, M.L., Harley, N.H., 1989. Determining the charged fractions of 218Po and 214 Pb using an environmental g-ray and Rn detector. Health Phys. 57 (1), 51e59. Porstendörfer, J., 1984. Behaviour of radon daughter products in indoor air. Radiat. Prot. Dosim 7 (1e4), 107e113. Sill, C.W., Olson, D.G., 1970. Sources and prevention of recoil contamination of solidstate alpha detectors. Anal. Chem. 42 (13), 1596e1607. Toivonen, H., Rytömaa, T., Vuorinen, A., 1988. Säteily ja turvallisuus. Valtion painatuskeskus, Helsinki. Turunen, J., Peräjärvi, K., Pöllänen, R., Toivonen, H., 2010. PANDA e a novel instrument for non-destructive sample analysis. Nucl. Instrum. Methods Phys. Res. A 613, 177e183. Vainblat, N., Pelled, O., German, U., Haquin, G., Tshuva, A., Alfassi, Z.B., 2004. Determination of parameters relevant to alpha spectrometry when employing source coating. Appl. Radiat. Isot 61, 307e311. Zhang, C., Luo, D., 1983. Measurement of mixed radon and thoron daughter concentrations in air. Nucl. Instrum. Methods 215, 481e488.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Technical Report
Collection and behaviour of radon progenies on thin Mylar foils Jani Turunen a, *, Sakari Ihantola a, Kari Peräjärvi a, Roy Pöllänen a, Harri Toivonen a, Erich Hrnecek b a b
STUK e Radiation and Nuclear Safety Authority, P.O. Box 14, FI-00881 Helsinki, Finland European Commission Joint Research Centre Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2010 Received in revised form 4 April 2011 Accepted 7 April 2011
Thin Mylar foils are often used to protect detectors from contamination. However, these foils can be electrostatically charged, possibly leading to their contamination with airborne radon progenies. In the present work, the collection and behaviour of radon progenies on Mylar foils was investigated in detail using alpha spectrometry. The radon progenies collection rate of a small Mylar foil (3 cm2) is equivalent to an air sampler with a flow rate of approximately 0.1 m3/h. It was demonstrated that such contamination may jeopardise the validity of the entire analysis if not interpreted correctly. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Radon progenies Mylar foil
1. Introduction Alpha particle emission is a typical decay mode of heavy elements. Due to the large stopping power of alpha particles in air, alpha spectrometric measurements are most often carried out in a vacuum to achieve good energy resolution. For samples containing nuclides such as 229Th, 228Th or 226Ra, which produce shortlived alpha-particle-emitting daughters within their decay chains, recoil contamination of the detector is a well-known problem (Sill and Olson, 1970). This effect can be reduced by slightly increasing the pressure in the measurement chamber and applying a negative potential to the alpha source (Sill and Olson, 1970) or by using thin foils that do not significantly influence the energy resolution of a measurement to physically protect the detector (Vainblat et al., 2004). Commercially available biaxially-oriented polyethylene terephthalate (Mylar) foils are often used for this purpose. Besides alpha spectrometry, Mylar foils are also widely used for protection in other radiation measurement applications. The concentration of radon decay products in air is to a large extent influenced by the plateout parameter of unattached radon daughters and the aerosol concentration (Porstendörfer, 1984). Enhanced deposition of radon progeny on polyethylene surfaces has been observed by Knutson et al. (1992), and the enhancement of deposition on electrostatically charged surfaces under typical office conditions has been reported by Batkin et al. (1998). Finally, the electrostatically enhanced collection of 218Po and 214Pb on aluminised Mylar foils kept at a negative potential has been demonstrated by Maiello and Harley (1989). * Corresponding author. Tel.: þ358 9 759 88 440; fax: þ358 9 759 88 433. E-mail address: jani.turunen@stuk.fi (J. Turunen). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.04.009
It is generally known that plastic films may be charged and are thus able to collect aerosol particles from air. Radioactive radon progenies attached to an aerosol particle offer an efficient means for continuous re-ionisation of the particle. If attached aerosol particles remain on the foils during alpha spectrometric measurements in a vacuum, they may lead to extra counts or peaks in the resulting alpha spectra. This contamination would be highly unwanted, especially in studies of samples containing radon progenies or when their true existence cannot be completely excluded. In the present work, the behaviour of such contamination in a vacuum was investigated using plain Mylar foils and Mylar foils covering real samples. The collection of radon progenies on plain Mylar foils was also measured under ambient air pressure. The Mylar used in the measurements was obtained from Goodfellow Cambridge Ltd., England, and its thickness was 0.5 mm. The measurements were performed using a single silicon detector connected to a multi-channel analyser and with the PANDA (Particles And Non-Destructive Analysis) device (Turunen et al., 2010). In PANDA, thin Mylar foils are always applied to protect the instrument from contamination. 2. Experimental The PANDA setup has two measurement positions holding various detectors inside a vacuum chamber. In this work only its double-sided silicon strip detector (DSSSD) was used. The positionsensitive DSSSD is used for alpha particle detection. It has 32 horizontal and 32 vertical strips, making a grid of 1024 pixels. The pixel size is 2 mm 2 mm. The data are collected in event mode and the events are time stamped, thus giving the possibility to follow the location, energy and time of registration of every detected alpha particle.
632
J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
2.1. Impactor sample measurement with PANDA This study started with the measurement of an impactor sample with multiple active spots (spot size approximately 1 mm) in a spiral-shaped formation (diameter 23e28 mm, distance between consecutive spots approximately 3 mm, see Fig. 1a). The sample was prepared by collecting aerosol particles originating from tungsten inert gas (TIG) welding using a Berner low-pressure impactor. Note that the tungsten electrodes used in welding contain 4% thorium and the spots in the sample therefore also include thorium and its progenies. There was a 13 mm circular hole in the centre of the sample foil. This sample was measured twice. Between the measurements, the Mylar foil between the sample and the DSSSD, as well as its location, was changed. Before each measurement was started the sample holders with the attached foils were left in the laboratory for some hours, i.e., the Mylar foils were directly exposed to the laboratory air. In the first measurement, the impactor sample was installed between two 1-mm-thick aluminium sample holder plates. These plates had a circular hole in the middle with a diameter of 35 mm (see Fig. 1a; the hole is marked with a yellow dashed line). The Mylar foil was attached to the front surface of the holder plate facing the DSSSD. It was not in contact with the actual sample. The source-to-detector (DSSSD) distance was 4 mm. The total measurement time was 14 days. In the second measurement of the same sample, the Mylar foil was installed in direct contact with the sample. Only the Mylar covering the area with the 13 mm diameter
hole was untouched. The second measurement was carried out in a tighter measurement geometry, with a source-to-detector distance of 3 mm, and this time the measurement lasted 7 days. A second similar impactor sample was also measured with PANDA using the same configuration as in the first measurement of the first sample. The purpose of this measurement was to determine whether the results obtained from the measurements of the first sample could be reproduced with a different sample. 2.2. Plain Mylar foil measurement with PANDA Measurements with plain Mylar foils, i.e., without an actual sample, were carried out with PANDA. In these measurements, the plain foil was attached to the front of the sample holder with a 35 mm hole in the centre. The sample holder with the Mylar foil was taken to an underground parking garage that has a radon progenies concentration of approximately 100 Bq/m3. The garage was chosen to get better counting statistics due to the higher radon progenies concentration than in the laboratory. After 2 h of collection, the foil was measured with PANDA. The delay between finishing the sample collection and starting the measurement was 30 min. This measurement lasted 18 h. After this, the same foil in its holder was taken back to the garage and again left there for 2 h. The foil was then measured again (delay 25 min, measurement time 21 h). Finally, a background measurement was made to ensure that no radon progenies or interfering contamination were present in the PANDA equipment.
Fig. 1. First measurement of the impactor sample. The total measurement time was 14 d. (a) Photograph of the sample and the Mylar foil in front of it. (b) Alpha particle hitmap for the first 3 h of measurement. (c) Alpha particle hitmap for the measurement period excluding the first 3 h. (d) Alpha spectrum recorded during the first 3 h of measurement. (e) Alpha spectrum recorded during the measurement period excluding the first 3 h.
J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
2.3. In-situ measurement of plain Mylar foil with alpha spectrometer Besides the measurements carried out with the PANDA equipment, plain Mylar foil was measured with a silicon detector (CAM 450 AM, Canberra, USA) on-site in the same parking garage. This time the Mylar foil was attached on-site to a 1 mm thick aluminium plate that had a hole with a diameter of 20 mm. The Mylar foil was only exposed to the air flow inside the garage during the measurement. The silicon detector was looking at the Mylar through the hole from the other side of the aluminium plate. The detector mounting was in contact with the plate. The distance between the foil and the detector surface was 2 mm. Both sides of the foil were exposed to the garage air. However, the flow at the detector side was negligible. The data acquisition was started after office hours and left to run overnight. The spectral data were saved in 1 min sequences. The radon concentration in the air was measured simultaneously with a portable radon monitoring device (AlphaGUARD, Genitron Instruments GmbH, Germany). Two other similar measurements with fresh plain Mylar foils were also made. In these two additional measurements the radon monitor was not available. 3. Results 3.1. Impactor sample measurement with PANDA Fig. 1 presents the data from the first impactor sample measurement. The alpha particle hitmap from the first 3 h of measurement (Fig. 1b) clearly does not correspond to the spiral shape of the source (Fig.1a). The alpha particle hits can be seen in the area where the Mylar foil was not in contact with anything. Most of these hits did not come from the actual sample, but from the radon progenies collected on the Mylar foil. Notice that the distribution of alpha particle hits is influenced by the used measurement geometry (Mylar-to-detector distance 3 mm). In Fig. 1c, the hitmap for the rest of the measurement period, i.e., two weeks of data, clearly retains the shape of the source. Note that it is a mirror image of the source, as the source is directly facing the DSSSD. The two alpha spectra in Fig. 1 were obtained using six y-strips located at the centre of the DSSSD, giving an intersection through the middle of the sample. The spectrum from the first 3 h of measurement (Fig. 1d) is completely different from that for the rest of the measurement period (Fig. 1e). The large peak at 7.7 MeV in Fig. 1d belongs to 214Po (a member of the 238U decay chain). Since its half-life is only 164.3 ms, the decay rate is primarily controlled by its precursors, 214Bi (T1/2 ¼ 19.9 min) and 214Pb (T1/2 ¼ 26.8 min). Both
633
of these nuclides disintegrate via beta decay. Thorium and its progenies are mainly responsible for the spectrum in Fig. 1e. The second measurement of the first impactor sample showed similar disturbance from radon progenies attached to the Mylar foil in front of the sample. This time, the extra counts in the alpha particle hitmap (not coming from the actual sample) were seen in a smaller area than in the previous measurement. The reason for this was the smaller untouched area (13 mm) of the foil than in the previous case (35 mm). These hits were again mostly recorded at the beginning of the measurement period. Radon progenies collected on the Mylar foil were also seen in the measurement of the second impactor sample. 3.2. Plain Mylar foil measurement with PANDA Fig. 2a and b presents the data recorded with PANDA from a Mylar foil that had been exposed to indoor air in a parking garage for 2 h. In the alpha spectrum shown in Fig. 2a, three peaks are visible. Besides the 7.7 MeV peak belonging to 214Po, the presence of 220Rn daughters 212 Bi (6.1 MeV) and 212Po (8.8 MeV) on the Mylar foil can be confirmed. The time behaviour of 214Po and 212Po is shown in Fig. 2b. The time behaviour graphs were made by gating the timestamp data with alpha gates matching the energy regions of each peak in Fig. 2a. These 18-h-long measurement data were binned to 700-s-long bits. The decay of 212Po (and 212Bi, results not shown in Fig. 2) closely follows the calculated decay curve. This agreement demonstrates that the studied nuclides remain on the Mylar foil, even in vacuum conditions. The few extra counts at the end of the experimental 214Po curve are due to the tailing of the 212Po peak. After the first measurement, the sample holder with the same Mylar foil still attached was taken back to the parking garage for 2 h and then measured again with PANDA. This time, no clear traces of 214Po from the collection were seen in the Mylar foil, indicating that the foil was no longer able to collect new aerosols. Some 212Bi and 212Po were still present on the foil from the previous collection. 3.3. In-situ measurement of plain Mylar foil with alpha spectrometer Fig. 3 presents the in-situ measurement of a plain Mylar foil with the CAM detector. The area of the 214Po alpha peak was determined for each 1 min measurement. The absolute alpha detection efficiency of the measurement setup was approximately 0.38. The 214Po activity on the foil during the measurement was determined using the peak areas and the known alpha detection efficiency. The activity of 214Po on the foil grows in very quickly.
Fig. 2. Measurement of plain Mylar foil with the PANDA setup. (a) Alpha spectrum of the measurement. (b) Time behaviour of 214Po and 212Po on the Mylar foil. The dashed line is a theoretical calculation for the shape of the 212Po curve, assuming only radioactive decay. The shape of 212Bi time behaviour was similar to 212Po (results not shown).
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J. Turunen et al. / Radiation Measurements 46 (2011) 631e634
Fig. 3. The collection and decay of 214Po on a Mylar foil measured with a CAM detector and the prevailing radon concentration measured with an AlphaGUARD monitor.
After a few hours, the activity starts to decrease and goes down to a low level, since the foil can no longer collect new particles. The radon gas concentration level measured with the AlphaGUARD monitor in the parking garage is also illustrated in Fig. 3. Since the monitor gives a value for the total radon concentration in the air, it was not possible to accurately determine the relative amount of radon from the 238U decay chain alone. It has been assumed in the calculations that all detected radon is coming from the 238U decay chain and the decay chains of 232Th and 235U have been ignored. This way it is possible to determine a minimum value for the collection rate. The measured radon concentration during the first 3 h of the measurement was approximately 86 Bq/m3. This value was used as a fixed parameter in a fit made to the 214Po activity during the first 3 h of the measurement (Toivonen et al., 1988; Zhang and Luo, 1983). The result of this fit was the collection rate on Mylar. As compared with air samplers, the equivalent collection rate would be 110 l/h. The corresponding deposition velocity is approximately 10 cm/s. In the two additional measurements with plain Mylar foils similar behaviour of the 214Po count rate was observed. 4. Conclusions It is known that Mylar can act as an electrostatic collector of charged aerosol particles containing radon progenies. Presently, we
have investigated in detail both the collection and survival of radon progenies on thin Mylar foils. It was demonstrated that radon progenies remain well on Mylar foil, even in vacuum conditions, if the foil is not manipulated. As shown with the measurement of the impactor sample the use of contaminated Mylar foil can lead to false results and wrong conclusions. This is the case especially when studying samples with low activity. The studied phenomenon is obviously strongly dependent on the radon concentration in the air of the laboratory and the electrostatic charge of the Mylar foil. One way to reduce this effect would be to uncharge the foils before use. In conclusion, this effect should not be forgotten, especially while operating in areas where the radon concentrations in the air may be high. Looking at the effect from the opposite side, it can be concluded that the Mylar foils used are efficient radon progenies collectors. Therefore, the use of Mylar foils as costeffective electrostatic samplers is an option that deserves further attention.
Acknowledgements The authors would like to thank R. Engelbrecht, Seibersdorf Labor GmbH, for providing the impactor samples.
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