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Measurements of time-averaged radon-daughter concentrations with passive dosemeters
389
The Science of the Total Environment, 45 (1985) 389--395 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
OF TIME-AVERAGED R A D O N - D A ~
CflkL~ZfRATIONS WITH PASSIVE
DOS~T~S
L.W. PUT and R.J. DE MEIJER Kernfysisch Versneller Instituut, Rijksuniversiteit Gronip~en, Zernikelaan 25, 9747 AA
Groningen, The Netherlands
ABSTRACT With standard dosemeters of the Karlsruhe type it is possible to measure simultaneously time-integrated radon and radon-daughter concentrations. For this purpose the do6emeters were equiped with a lid and a bottom foil. Pre-etching conditions for either type of foil were optimized; under these conditions no effects of plate-out were observed. The method seems to be sensitive to neutral radon daughters only. Possible improvements of the dosemeter geometry are indicated.
INTRODUCTION Measurements of time-integrated radon concentrations have been used as reliable tools in various national surveys (see e.g. refs.l-4). Radicbiological hazards related to 222Rn do not directly arise from radon itself but primarily from its daughters, which are ccmmonly attached to aerosols. It would therefore be more meaningful if the radon-daughter concentrations could be measured directly. Similarly to the radon concentration the radon-daughter density in air shows large variations over short periods and hence time-integrated measuring devices would be required. The development of a method to measure time-averaged radon-daughter concentrations in air by means of a passive detector has been a longstanding proble~ due to the contributions of plate-out on parts of the detector (see refs.5-7 and references therein). For this reason the radiobiological effects are usually calculated from the radon concentrations by assuming or estimating a timeaveraged equilibri~n factor F. Recently a method was presented (ref.7) in which the F value could be measured with a diffusion box via the working level ratio (WLR) defined as the daughter u-decay energy in air divided by the energy if the prc~pt daughters would be in full equilibri~n. This method still needs the apprcximation:
CRD = F x CR %
0048-9697/85/$03.30
F × CR
© 1985 Elsevier Science Publishers B.V.
(i)
390 where C R and %
are the time-averaged radon and radon-daughter concentrations,
respectively. Moreover it is not clear whether this method developed for the use in mines, would be sensitive enough for dwellings. In this paper we demonstrate that by placing an additional track-etch foil in the unfiltered part (the lid in this case) of an existing passive radon dosemeter, radon and radon-daughter ( 2 1 ~ o and 214po) concentrations can be measured simultaneously. The problem of plate-out can be avoided by choosing the appropriate etching conditions.
EXPERIMenTAL TECHNIQUE Fig. 1 shows the cross section of a passive dosemeter of the Karlsruhe type (ref. 8) used in the present investigation. On the bottGm and in the lid a MAKROFOL track-etch foil is mounted; the bottom one is exposed to radon only, the one in the lid is exposed to the ambient air. The use of these dosemeters to measure time-integrated radon concentrations has been described in refs. 3, 8-10. We will refer to the pre-etching (PE) and electro-chemical etching (ECE) of the bottom foils as the standard procedure. In the PE a layer of the foil is removed, if sufficient thickness is removed ECE will attack the high ionization part of the track, being its last part (Bragg effect). By this procedure the diameter of the track is enlarged and will became visible. In this way only tracks of a-particles that are stopped at a particular depth, i.e. with energies in a certain range, are made visible. The standard procedure makes the bottom foil sensitive to e-particles emitted from radon or radon daughters present in a layer positioned just under the filter. For a review on track-etch foil techniques we refer to an article by Fisher and Spohr (ref.ll). Since in the present configuration the distance between the filter and the foil in the lid is much shorter than in the bottom part of the device (see fig. i), the average incident e-energy will be higher and hence they will penetrate deeper into the foil. This means that a PE time much longer than standard has to be used; the ECE time was kept as in the standard procedure. To optimize the PE time several dose~neters were exposed in the same roQm during the same period of
MAKROFOL I
Kw3561 Fig. i. Cross sections of a dosemeter of the Karlsruhe type: the filter and the track-etch foils in the lid and at the bottGm are indicated. The arrows show the way the ambient air enters the dosemeter.
MAKROFOL
~/
i
L~
391 ~vl 356o
12001
etching of lid-foils
400
{
0
I0
I
'I I
20 Pre-etching time (h)
40
30
Fig. 2. Tne track density for a nLmlber of foils, exposed to the same concentration of radon daughters, as function of the pre-etching time. The arrow indicates the optimal PE-time (see text).
0
b
Fig. 3. Examples of foils after PE and ECE with a) a PE time of 20 hrs and b) a PE time of ii hrs.
392 time (total radon-daughter exposure approximately 5 kBq.day/m 3) . If one ass~nes that they were all exposed to the same concentrations of radon and radon daughters all foils should have the same track density. Fig. 2 shows the n ~ b e r of visible tracks/cm 2 as function of the PE time: one observes first a rise, followed by a plateau and a slow descent. The error in each data point reflects the standard deviation of the values obtained for about four foils. From the figure one may conclude that the region of interest is between 10-30 hrs of PE time. (The time scales refer to an PE solution with a lower KOH concentration than in the standard procedure, resulting in a 1.5 times higher etching rate). A closer examination of the etched foils indicated that for PE times longer than 13 hrs the track pattern on the foils showed clear clustering effects (see fig. 3a). These clusters, thought to arise from localization of ~-emlitters on the foil itself (plate-out), cause a large standard deviation in the nLmlber of tracks for PE-times larger than 13 hours. Gecmetrically one sees that the sensitivity for these tracks decreases with decreasing depth in the foil and hence with decreasing PE time. As indicated in fig. 2 the optimalization procedure has resulted in a PE time of ii hrs, which corresponds to the removal of about 18 ~m foil material. With this procedure a high sensitivity for ~particles produced in the air between the foil and the filter is cc~bined with a low sensitivity for ~-particles from plate-out. Fig. 3b presents a picture of a foil etched under this condition. The origin of the plate-out is thought to be related to areas of high electric charge which will attract charged particles. Since these charged spots should also be present on the device itself (also made of a synthetical material (ref.8)) most charged radon daughters (attached or unattached) will be res*~ved either by their approach to the cup or by plate-out on the foil. We therefore believe that with this method mainly neutral radon daughters are detected. It should be noted that bottGm foils etched with a PE time of 19 hrs also show clusters, indicating that plate-out on the foil also Occurs in the bottom part of the dosemeter.
RESULTS Several cups equiped with a lid and a bottom foil were exposed to known concentrations of radon and radon daughters at NIPR (Stockholm), KFA (Karlsruhe), ~4L (New York) and NRPB (Chilton). After exposure the foils were sealed and mailed back to the KVI, etched according to the appropriate procedures, and the tracks counted over an area of 2 ~ . A special exposure was made in air with a high concentration of radon and in comparison a negligible concentration of radon daughters. After etching under the appropriate conditions one obtains the relative sensitivity of the lid and bottom foil to radon: the nlmlber of tracks in the lid foil, N~, was 1.07 ± 0.04
393
times the ntmlber of tracks on the bottom foil, ~ ,
for the same counting area.
With this result we define the numlber of tracks on the lid foil due to radon daughters NRD as
N~
= Nx -
(2)
(1.o7 + 0 . 0 4 )
Fig. 4 shows NRD as function of the radon-daughter exposure; the line represents a least-squares fit to the data. The slope is smaller than 1.0, a result which was also found of the calibration of our radon track density. Tne scattering of the data points in case of the radon daughters is larger than in the case of radon calibration. This results in a larger uncertainty in the deduction of radon-daughter concentrations. This scattering and hence the uncertainty, partly arises from the larger statistical error due to correction for radon in eq.(2), the unknown sensitivity of this dosemeter for each of the radon daughters separately, and a similar uncertainty in the measured "radon-daughter" concentration and in the aerosol conditions in the different exposures.
IOOC -
-
K~ ,
,
,
~
,
,
3s6z
,
NRD
Ioo
Fig. 4. Track density, NRD, calculated from the track densities in lid and bottom foils, versus the radon-daughter exposure at various laboratories:
o
IOO
h
~
, i
I000
,
,
~
,I LO000
NIPR (A), KFA (D) and ~
(0).
Radon daughler exposure (Bq.day/m 3)
It should be pointed out that in a series of exposures at NRPB aerosols were generated by means of a hot wire and the ratio of radon-daughter to radon concentrations was controlled with an electrostatic precipitator. Under these circus%stances the radcn-daughter concentrations deduced from the foils were practically cc~stant. This result was attributed to the insensitivity of our method for charged radon daughter(carrier)s and/or to the small diameter of the hot wire aerosols; the corresponding data points have therefore not been included in fig. 4. The method has also been tested in an OECD interccmparison of passive
394
detectors held at NRPB in 1984. Nine dosemeters were exposed in a dwelling to both radon and radon daughters, of which the concentration was measured every hour with active devices. The radon and radon-daughter exposures, deduced frown the weighted averages of the track densities on the foils, are 6.5 ± 0.2 and 2.4 ± 0.4 kBq.day/m 3, respectively. The uncertainties are the external errors. These values for the exposure agree well with the values quoted by NRPB: 6.13 and 2.45 kBq.day/m 3, respectively.
St~MARY AND CONCLUSIONS Measurements with standard dose~eters of the K a r l s r ~ e type, which are equiped with a lid and a bottom foil indicate that time-integrated radondaughter concentrations can be derived frcm the track densities on both foils. A prerequisite for a reliable result is that each type of foil should be preetched appropriately. Under these circ~nstances no effects due to plate-out on the foils have been observed. The success of this method lays in the distinction between ~-particles originating from plate-out on the film and those originating frcm airborne radon and radon daughters. This distinction is possible due to the ccr~bination of the pre-etching and electro-chemical etching techniques. In the PE-phase a distinct layer of material is rem~Jved, in the ECE-phase only those tracks will be etched which correspond to ~-particles that just are stopped. Hence the method selects a certain range of ~-energies at the incidence of the foil and therefore a certain vol~ae of the dose~aeter. Such a selection is not possible with films that are treated with a single etching technique. A possible i~prove~ent might be obtained by increasing the distance between the position of the lid foil and the filter. A larger distance implies the ~particles emitted from the region close to the filter will have a lower energy at the incidence of the foil end hence the PE time will be reduced. Moreover the angle of incidence of the e-particles entering the foil will be better defined and the shorter PE time reduces the sensitivity for plate-out. Especially frGm the measurements at NRPB with the hot-wire aerosol generator we have the impression that the present device is predcminantly sensitive to neutral radon daughters. Plate-out either on the device itself or on the foil caused by highly charged spots has apparently the effect that charged radon daughters in the air in the cup are continuously removed. The agreement between the quoted radon-daughter concentrations and the nLmlber of radon-daughter tracks indicates that this resKgval is a general phencmena that also takes place in the dwelling itself. Moreover it seems that differences in aerosol conditions at various places have hardly any influence on the results. With the method described above radon-daughter concentrations will be deduced from the foils exposed in about i000 dwellings and 250 locations in the
395 outside air during the survey being part of the Dutch National Research program SAWORA. It is intended to establish radon-daughter concentrations distributions and correlations between these concentrations and various parameters similar to the case of radon (see ref.12).
ACKNOWLEDGEmenT This investigation is part of the Dutch SAWORA program financed by the Ministries of Econcmic Affairs and of Housing, Physical Planning and Environment. The KVI is a joint venture of the University of Groningen and the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) which is financially supported by the "Nederlandse Stichting voor Zuiver Wetenschappelijk Onderzcek" (~O).
~ E S 1 2 3 4 5 6 7 8 9 I0 ii 12
G.A. Swedjemark a ~ L.Mjbnes, Proc. ~ d Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 37-44. G. Sciocchetti, F. Scacco, P. G. Baldassini, L. Monte and R. Sarao, Rad. Prot. Dos. 7 (1984) 347-351 L.W. Put and R.J. de Meijer, Proo. ~ d Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 49-54. L. Brown, B.M.R. Green, J.C.H. Miles and A.D. Wrixon, Proc. ~rd Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 61-66. T. Dzma~ski, A. Wojda, W. Chru~ciela~ski and A. ZOrawski, Nucl. Tracks, 9 (1984) 1-14. T. D ~ s k i , W. Chru~cielewski and A. Z~rawski, Pad. Prot. Dos., 8 (1984) 231-238. R.L. Fleischer, L.G. Turner and A.C. George, Health Physics, 47 (1984) 9-19. M. Urban and E. Piesch, Rad. Prot. Dos. 1 (1981)_97-109. W. Burkart, Chr. Wernli and H. Brunner, Proo. 3r~ Int. Conf. on Indoor Air Quality and Climate, Stockholm, 1984 vol. 2, 67-72. J. Uyttenhove, R. Marijns, A. Janssens, H. Vsamnarcke and R. Jacobs, Pad. Prot. Dos., 7 (1983) 275-278. B.E. Fischer, R. Spohr, Rev. Mod. Phys. 55 (1983) 907-948. L.W. Put, R.J. de Meijer and B. Hogeweg, Survey of radon concentrations in Dutch dwellings, this conference.
The Science of the Total Environment, 45 (1985) 389--395 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
OF TIME-AVERAGED R A D O N - D A ~
CflkL~ZfRATIONS WITH PASSIVE
DOS~T~S
L.W. PUT and R.J. DE MEIJER Kernfysisch Versneller Instituut, Rijksuniversiteit Gronip~en, Zernikelaan 25, 9747 AA
Groningen, The Netherlands
ABSTRACT With standard dosemeters of the Karlsruhe type it is possible to measure simultaneously time-integrated radon and radon-daughter concentrations. For this purpose the do6emeters were equiped with a lid and a bottom foil. Pre-etching conditions for either type of foil were optimized; under these conditions no effects of plate-out were observed. The method seems to be sensitive to neutral radon daughters only. Possible improvements of the dosemeter geometry are indicated.
INTRODUCTION Measurements of time-integrated radon concentrations have been used as reliable tools in various national surveys (see e.g. refs.l-4). Radicbiological hazards related to 222Rn do not directly arise from radon itself but primarily from its daughters, which are ccmmonly attached to aerosols. It would therefore be more meaningful if the radon-daughter concentrations could be measured directly. Similarly to the radon concentration the radon-daughter density in air shows large variations over short periods and hence time-integrated measuring devices would be required. The development of a method to measure time-averaged radon-daughter concentrations in air by means of a passive detector has been a longstanding proble~ due to the contributions of plate-out on parts of the detector (see refs.5-7 and references therein). For this reason the radiobiological effects are usually calculated from the radon concentrations by assuming or estimating a timeaveraged equilibri~n factor F. Recently a method was presented (ref.7) in which the F value could be measured with a diffusion box via the working level ratio (WLR) defined as the daughter u-decay energy in air divided by the energy if the prc~pt daughters would be in full equilibri~n. This method still needs the apprcximation:
CRD = F x CR %
0048-9697/85/$03.30
F × CR
© 1985 Elsevier Science Publishers B.V.
(i)
390 where C R and %
are the time-averaged radon and radon-daughter concentrations,
respectively. Moreover it is not clear whether this method developed for the use in mines, would be sensitive enough for dwellings. In this paper we demonstrate that by placing an additional track-etch foil in the unfiltered part (the lid in this case) of an existing passive radon dosemeter, radon and radon-daughter ( 2 1 ~ o and 214po) concentrations can be measured simultaneously. The problem of plate-out can be avoided by choosing the appropriate etching conditions.
EXPERIMenTAL TECHNIQUE Fig. 1 shows the cross section of a passive dosemeter of the Karlsruhe type (ref. 8) used in the present investigation. On the bottGm and in the lid a MAKROFOL track-etch foil is mounted; the bottom one is exposed to radon only, the one in the lid is exposed to the ambient air. The use of these dosemeters to measure time-integrated radon concentrations has been described in refs. 3, 8-10. We will refer to the pre-etching (PE) and electro-chemical etching (ECE) of the bottom foils as the standard procedure. In the PE a layer of the foil is removed, if sufficient thickness is removed ECE will attack the high ionization part of the track, being its last part (Bragg effect). By this procedure the diameter of the track is enlarged and will became visible. In this way only tracks of a-particles that are stopped at a particular depth, i.e. with energies in a certain range, are made visible. The standard procedure makes the bottom foil sensitive to e-particles emitted from radon or radon daughters present in a layer positioned just under the filter. For a review on track-etch foil techniques we refer to an article by Fisher and Spohr (ref.ll). Since in the present configuration the distance between the filter and the foil in the lid is much shorter than in the bottom part of the device (see fig. i), the average incident e-energy will be higher and hence they will penetrate deeper into the foil. This means that a PE time much longer than standard has to be used; the ECE time was kept as in the standard procedure. To optimize the PE time several dose~neters were exposed in the same roQm during the same period of
MAKROFOL I
Kw3561 Fig. i. Cross sections of a dosemeter of the Karlsruhe type: the filter and the track-etch foils in the lid and at the bottGm are indicated. The arrows show the way the ambient air enters the dosemeter.
MAKROFOL
~/
i
L~
391 ~vl 356o
12001
etching of lid-foils
400
{
0
I0
I
'I I
20 Pre-etching time (h)
40
30
Fig. 2. Tne track density for a nLmlber of foils, exposed to the same concentration of radon daughters, as function of the pre-etching time. The arrow indicates the optimal PE-time (see text).
0
b
Fig. 3. Examples of foils after PE and ECE with a) a PE time of 20 hrs and b) a PE time of ii hrs.
392 time (total radon-daughter exposure approximately 5 kBq.day/m 3) . If one ass~nes that they were all exposed to the same concentrations of radon and radon daughters all foils should have the same track density. Fig. 2 shows the n ~ b e r of visible tracks/cm 2 as function of the PE time: one observes first a rise, followed by a plateau and a slow descent. The error in each data point reflects the standard deviation of the values obtained for about four foils. From the figure one may conclude that the region of interest is between 10-30 hrs of PE time. (The time scales refer to an PE solution with a lower KOH concentration than in the standard procedure, resulting in a 1.5 times higher etching rate). A closer examination of the etched foils indicated that for PE times longer than 13 hrs the track pattern on the foils showed clear clustering effects (see fig. 3a). These clusters, thought to arise from localization of ~-emlitters on the foil itself (plate-out), cause a large standard deviation in the nLmlber of tracks for PE-times larger than 13 hours. Gecmetrically one sees that the sensitivity for these tracks decreases with decreasing depth in the foil and hence with decreasing PE time. As indicated in fig. 2 the optimalization procedure has resulted in a PE time of ii hrs, which corresponds to the removal of about 18 ~m foil material. With this procedure a high sensitivity for ~particles produced in the air between the foil and the filter is cc~bined with a low sensitivity for ~-particles from plate-out. Fig. 3b presents a picture of a foil etched under this condition. The origin of the plate-out is thought to be related to areas of high electric charge which will attract charged particles. Since these charged spots should also be present on the device itself (also made of a synthetical material (ref.8)) most charged radon daughters (attached or unattached) will be res*~ved either by their approach to the cup or by plate-out on the foil. We therefore believe that with this method mainly neutral radon daughters are detected. It should be noted that bottGm foils etched with a PE time of 19 hrs also show clusters, indicating that plate-out on the foil also Occurs in the bottom part of the dosemeter.
RESULTS Several cups equiped with a lid and a bottom foil were exposed to known concentrations of radon and radon daughters at NIPR (Stockholm), KFA (Karlsruhe), ~4L (New York) and NRPB (Chilton). After exposure the foils were sealed and mailed back to the KVI, etched according to the appropriate procedures, and the tracks counted over an area of 2 ~ . A special exposure was made in air with a high concentration of radon and in comparison a negligible concentration of radon daughters. After etching under the appropriate conditions one obtains the relative sensitivity of the lid and bottom foil to radon: the nlmlber of tracks in the lid foil, N~, was 1.07 ± 0.04
393
times the ntmlber of tracks on the bottom foil, ~ ,
for the same counting area.
With this result we define the numlber of tracks on the lid foil due to radon daughters NRD as
N~
= Nx -
(2)
(1.o7 + 0 . 0 4 )
Fig. 4 shows NRD as function of the radon-daughter exposure; the line represents a least-squares fit to the data. The slope is smaller than 1.0, a result which was also found of the calibration of our radon track density. Tne scattering of the data points in case of the radon daughters is larger than in the case of radon calibration. This results in a larger uncertainty in the deduction of radon-daughter concentrations. This scattering and hence the uncertainty, partly arises from the larger statistical error due to correction for radon in eq.(2), the unknown sensitivity of this dosemeter for each of the radon daughters separately, and a similar uncertainty in the measured "radon-daughter" concentration and in the aerosol conditions in the different exposures.
IOOC -
-
K~ ,
,
,
~
,
,
3s6z
,
NRD
Ioo
Fig. 4. Track density, NRD, calculated from the track densities in lid and bottom foils, versus the radon-daughter exposure at various laboratories:
o
IOO
h
~
, i
I000
,
,
~
,I LO000
NIPR (A), KFA (D) and ~
(0).
Radon daughler exposure (Bq.day/m 3)
It should be pointed out that in a series of exposures at NRPB aerosols were generated by means of a hot wire and the ratio of radon-daughter to radon concentrations was controlled with an electrostatic precipitator. Under these circus%stances the radcn-daughter concentrations deduced from the foils were practically cc~stant. This result was attributed to the insensitivity of our method for charged radon daughter(carrier)s and/or to the small diameter of the hot wire aerosols; the corresponding data points have therefore not been included in fig. 4. The method has also been tested in an OECD interccmparison of passive
394
detectors held at NRPB in 1984. Nine dosemeters were exposed in a dwelling to both radon and radon daughters, of which the concentration was measured every hour with active devices. The radon and radon-daughter exposures, deduced frown the weighted averages of the track densities on the foils, are 6.5 ± 0.2 and 2.4 ± 0.4 kBq.day/m 3, respectively. The uncertainties are the external errors. These values for the exposure agree well with the values quoted by NRPB: 6.13 and 2.45 kBq.day/m 3, respectively.
St~MARY AND CONCLUSIONS Measurements with standard dose~eters of the K a r l s r ~ e type, which are equiped with a lid and a bottom foil indicate that time-integrated radondaughter concentrations can be derived frcm the track densities on both foils. A prerequisite for a reliable result is that each type of foil should be preetched appropriately. Under these circ~nstances no effects due to plate-out on the foils have been observed. The success of this method lays in the distinction between ~-particles originating from plate-out on the film and those originating frcm airborne radon and radon daughters. This distinction is possible due to the ccr~bination of the pre-etching and electro-chemical etching techniques. In the PE-phase a distinct layer of material is rem~Jved, in the ECE-phase only those tracks will be etched which correspond to ~-particles that just are stopped. Hence the method selects a certain range of ~-energies at the incidence of the foil and therefore a certain vol~ae of the dose~aeter. Such a selection is not possible with films that are treated with a single etching technique. A possible i~prove~ent might be obtained by increasing the distance between the position of the lid foil and the filter. A larger distance implies the ~particles emitted from the region close to the filter will have a lower energy at the incidence of the foil end hence the PE time will be reduced. Moreover the angle of incidence of the e-particles entering the foil will be better defined and the shorter PE time reduces the sensitivity for plate-out. Especially frGm the measurements at NRPB with the hot-wire aerosol generator we have the impression that the present device is predcminantly sensitive to neutral radon daughters. Plate-out either on the device itself or on the foil caused by highly charged spots has apparently the effect that charged radon daughters in the air in the cup are continuously removed. The agreement between the quoted radon-daughter concentrations and the nLmlber of radon-daughter tracks indicates that this resKgval is a general phencmena that also takes place in the dwelling itself. Moreover it seems that differences in aerosol conditions at various places have hardly any influence on the results. With the method described above radon-daughter concentrations will be deduced from the foils exposed in about i000 dwellings and 250 locations in the
395 outside air during the survey being part of the Dutch National Research program SAWORA. It is intended to establish radon-daughter concentrations distributions and correlations between these concentrations and various parameters similar to the case of radon (see ref.12).
ACKNOWLEDGEmenT This investigation is part of the Dutch SAWORA program financed by the Ministries of Econcmic Affairs and of Housing, Physical Planning and Environment. The KVI is a joint venture of the University of Groningen and the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) which is financially supported by the "Nederlandse Stichting voor Zuiver Wetenschappelijk Onderzcek" (~O).
~ E S 1 2 3 4 5 6 7 8 9 I0 ii 12
G.A. Swedjemark a ~ L.Mjbnes, Proc. ~ d Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 37-44. G. Sciocchetti, F. Scacco, P. G. Baldassini, L. Monte and R. Sarao, Rad. Prot. Dos. 7 (1984) 347-351 L.W. Put and R.J. de Meijer, Proo. ~ d Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 49-54. L. Brown, B.M.R. Green, J.C.H. Miles and A.D. Wrixon, Proc. ~rd Int. Conf. on Indoor Air Quality and Climate, Stockholm 1984, vol. 2, 61-66. T. Dzma~ski, A. Wojda, W. Chru~ciela~ski and A. ZOrawski, Nucl. Tracks, 9 (1984) 1-14. T. D ~ s k i , W. Chru~cielewski and A. Z~rawski, Pad. Prot. Dos., 8 (1984) 231-238. R.L. Fleischer, L.G. Turner and A.C. George, Health Physics, 47 (1984) 9-19. M. Urban and E. Piesch, Rad. Prot. Dos. 1 (1981)_97-109. W. Burkart, Chr. Wernli and H. Brunner, Proo. 3r~ Int. Conf. on Indoor Air Quality and Climate, Stockholm, 1984 vol. 2, 67-72. J. Uyttenhove, R. Marijns, A. Janssens, H. Vsamnarcke and R. Jacobs, Pad. Prot. Dos., 7 (1983) 275-278. B.E. Fischer, R. Spohr, Rev. Mod. Phys. 55 (1983) 907-948. L.W. Put, R.J. de Meijer and B. Hogeweg, Survey of radon concentrations in Dutch dwellings, this conference.