INTERCOMPARISON BETWEEN RADON PASSIVE AND ACTIVE
MEASUREMENTS AND PROBLEMS RELATED TO THORON
MEASUREMENTS*
B. BURGHELE, C. COSMA
Faculty of Environmental Science and Engineering, “Babes-Bolyai” University, Cluj-Napoca,
Romania, E-mail: [email protected]
Received November 15, 2012
Researchers around the world had already established that radon and thoron are a
hazard to the human health. Different techniques and methods are applied in order to
measure them as accurately as possible. In Romania, most measurements earlier
conducted were focused on using grab-sampling measurements to establish the two
gasses decay progeny activity concentration; however, recently, there has been an
increased interest in using passive devices together with active devices for long-term
measurements. In order to get more accurate knowledge about the real radon and thoron
indoor concentration in dwellings as well as in public buildings and work places, a new
survey was carried out in 15 locations within and surrounding Birlad Town, Vaslui
County. Passive methods including two types of discriminative detectors based on CR
39 chips as well as active measurements, such as RAD7 were used for this survey.
During this survey, it was also pointed out the continuous difficulty in measuring
thoron, due to its short half-life and faults in the measuring system.
Key words: radon, thoron, passive devices, active measurements.
1. INTRODUCTION
It goes without saying that radon (222Rn) represents a hazard to the human
health [1] and that along the years researchers around the world have developed
numberless methods and devices to measure it to estimate its contribution to the
total natural dose received by the population and to find remediation possibilities
[2]. Shortly thereafter was discovered that along with radon came thoron (220Rn), a
decay product of thorium (232Th); thoron concentration in some cases exceeded [3]
that of radon and therefore, could cause overestimations of the radon contribution
to the natural dose. Moreover, thoron interferes with some of the radon detection
devices [4]; consequently providing an overestimation of the effective dose
received by the general population. Due to its short half-life (56 seconds) thoron
*
Paper presented at the First East European Radon Symposium – FERAS 2012, September 2–5,
2012, Cluj-Napoca, Romania.
Rom. Journ. Phys., Vol. 58, Supplement, P. S56–S61, Bucharest, 2013
2
Intercomparison between radon passive and active measurements
S57
measurements often encounter certain difficulties whether if there are used passive
or active methods, i.e. due to the distance from the wall and floor, time of
measurement, diurnal and seasonal variations. The present survey included
dwellings, schools and public institutions; for each location a standard
questionnaire was filled in order to collect data regarding building materials, year
of construction, occupational habit, geological background and the dweller’s status
(smoker or non-smoker). It was also taken into consideration that all measurements
were taken during spring-summer period [5].
2. MATERIALS AND METHODS
In order to measure radon activity concentration more thoroughly, two types
of passive detectors and one active device were used in the present survey. Also,
there were used one passive detector and one active device to investigate the thoron
activity. All passive devices are represented by nuclear track detectors based on
CR-39 chips, provided by Radosys Ltd. Budapest, namely RSK, used to measure
radon and RADUET, used to measure both radon and thoron activity
concentration. A Durridge RAD7 device was used in both cases for active
measurements. In case of passive detectors the protocol was to deploy each device
in a high occupancy location (more than 4 hours spent inside) during a period of 90
to 100 days; each pair of detectors were placed onto present cupboards, tables or
other furniture within 30 to 50 cm from the wall. After the exposure, all devices
were collected and brought to the laboratory where were chemically etched in 6.25
M solution for 4 ½ hours at 90°C. Thusly prepared the CR-39 plates are ‘read’
using a Radosys microscope which provides the track density number on the chips.
According to the exposure period and the track density found on the chips the
exposure and the activity concentration of radon and thoron can be calculated. The
RAD7 system used for active measurements is based on spectral analysis of alpha
emitters such as radon and thoron but also their decay progenies. In the present
study it was used to measure the emission from the building material, floors, indoor
activities at different distances from the walls and indoor diurnal measurements.
The protocol used for emission was 5 measurements in Sniff mode, with 10
minutes cycle time.
3. RESULTS
During the survey, 15 locations within and surrounding Birlad Town of
Vaslui County were investigated with passive detectors, 6 of which were also
submitted to active measurements. The selected communes and the specified town
are marked in Fig. 1b, they represent 8% of the total area and approximately 20%
S58
B. Burghele, C. Cosma
3
population living in Vaslui County, Fig. 1a. Two communes had only one set of
detectors while the rest had two detectors each; three sets of detectors were used
for the city.
(a)
(b)
Fig. 1 – The investigated area represented by: (a) Vaslui County, Romania
and (b) 7 communes located in the south-west of the county.
The plot of all data obtained for radon measurements can be seen in Figure 2.
It can be observed that in case of radon, the activities obtained with RADUET are
higher compared to RSK and RAD7 diurnal measurements. It is to be specified that
both types of detectors were places in the same location, hence, large differences
between the two gas concentrations of radon were not expected. Standard deviation
of repetitive measurements with our microscope concluded in a range for track
densities of 0.01–0.23 (RSK) and 0.07–0.46 (RADUET) for radon and 0.1–0.34 for
thoron. However, there is yet no explanation for the high differences between
indoor concentrations (from 1 to 4 times higher values for RADUET compared to
RSK) obtained during this test. Upon further investigation it was pointed out that
among the reasons for these differences most relevant proved to be the age of the
detectors [6, 7] since the RSKS detectors were older than 2 years compared to
RADUET detectors or misplacement of detectors by the owners.
The thoron activity concentration was found to be slightly higher than radon
in two of the investigated dwellings, location 5 and 9, as plotted in Fig. 3. Due to
the fact that thoron has a half-life of 55.6 second it cannot travel as far from its
source as radon can before decaying. Thoron measurements performed with the
help of RAD7 indicated that there is a high activity in the vicinity of the walls and
low activity towards the centre of the room, Fig. 4. Moreover, emission
measurements in one adobe house pointed out that the walls (building material)
represent the source of thoron and the floor (filling) of radon with average activity
concentrations from walls of 395±96 Bq m-3 for thoron and 181±45 Bq m-3 for
radon while from the floor were obtain 66±43 Bq m-3 and 435±80 Bq m-3
respectively. Further investigation is needed in order to identify why the activity of
thoron is higher in some adobe houses and low in other buildings constructed with
the same material. It is commonly observed that compared to that of radon gas, a
much smaller fraction of the thoron gas in soil ever reaches the interior of a
4
Intercomparison between radon passive and active measurements
S59
building [8]. At a distance of 1m from the wall the thoron activity concentration
appears to be fairly constant and independent of the distance from the floor.
However, thoron gas can still be a hazard since its progeny 212Pb (half-life of 10.6
hours) has more that enough time to accumulate within the indoor air.
Fig. 2 – The different activity concentrations of radon obtained with different measuring systems.
Fig. 3 – The different activity concentrations of thoron obtained with different measuring systems.
S60
B. Burghele, C. Cosma
5
400
50 cm above the floor
on the floor
50 cm
from the wall
1m from the wall
Concentration (Bqm-3)
300
Radon
Thoron
350
1m from
the wall
1.5m from the wall
250
2m from
the wall
1.5m from the wall
200
150
100
50 cm from the wall
50
0
2012.04.07 7:12
2012.04.07 8:24
2012.04.07 9:36
2012.04.07 10:48
2012.04.07 12:00
2012.04.07 13:12
2012.04.07 14:24
Time
Fig. 4 – Variations of thoron concentration with the distance from the walls
and floor measured with RAD7.
For this text were taken into study 11 dwellings, 2 schools and 2 public
buildings. The construction age of the buildings varies between 150 to 7 years old,
80% being built in the second part of the XXth century. Adobe as specified in Table
1, represents the building material used for 60% of the locations, including those
with higher thoron activity.
In order to calculate the equilibrium equivalent concentration (EECRn) of
radon and thoron (EECTn) two different equilibrium factors [9] between the gas
and its short-life progeny were used, 0.4 for radon EEC and 0.02 for thoron EEC
and the average activity concentration for each location and gas.
The resulted arithmetic mean per gas shows an activity concentration of
129 Bqm-3 for radon and 53 Bq m-3 for thoron. It is to be enhanced that the AM
obtained for radon is quite close to the average value of 126 Bqm-3, calculated for
Romania [2].
Table 1
Measurements of radon and thoron with different investigating methods
and for different types of building materials
Location index
and building
materials
1–adobe
2–adobe
3–adobe
4–red bricks
5 – adobe and
Radon concentration
(Bq m-3)
RADUET
RSK
RAD7
218±1
289±1
125±1
226±4
177±4
85±3
126±0.4
37±2
93±3
46±1
186
EECRn
(Bq m-3)
65.2
83
32.4
63.8
44.6
Thoron concentration
(Bq m-3)
RADUET
RAD7
132±4
76±2
59±7
111±4
227±9
29
EECTn
(Bq m-3)
1.6
1.5
1.1
2.2
4.5
6
Intercomparison between radon passive and active measurements
mortar
6 – red bricks
7 – red bricks
8 – red bricks
and gypsum
board
9 – adobe
10 – adobe
11 – red bricks
12 – adobe
13 – adobe
14 – adobe and
gypsum board
15 – BCA
(cellularexpanded
concrete)
AM+SD
S61
208±6
430±8
63± 0.6
88±1
54.2
103.6
93±8
69±6
1.8
1.3
265±4
106±1
74.2
95±9
1.9
126±3
123±1
255±3
180±3
235±2
48±2
41±1
68± 0.3
62±1
80±1
89
70
34.8
25.2
64.6
44.1
51.3
166±7
31±6
143±6
24±8
32±5
189±1
107±3
146
58.9
232±2
74± 0.6
53
25
219 ± 78 75 ± 26 95 ± 60
7
25
3.3
0.4
2.8
0.3
0.5
32±2
4
0.3
47.8
76±2
10
0.8
56.5 ± 20.4
91 ± 57
15 ± 10
1.6 ± 1.1
12
4. CONCLUSIONS
After this survey, only one location, namely a school built 150 years ago with
red bricks presented a radon activity concentration higher than the reference level
for radon indoor [10], respectively 430 Bq m-3 measured with RADUET. An
average concentration between the two types of passive detectors decreases this
activity under the action limit. The rest of the data were bellow this reference level
making this region, until further research, a rather safe place to live when it comes
to radon and thoron. These measurements represent the first survey using passive
methods for investigating both radon and thoron gas within this region of Romania.
Acknowledgements. This paper was realised with the support of POSDRU CUANTUMDOC
“DOCTORAL STUDIES FOR EUROPEAN PERFORMANCES IN RESEARCH AND
INOVATION” ID79407 project funded by the European Social Found and Romanian Government.
REFERENCES
1. Stiefer P.S., Weir B.R., Health risk attributable to environmental exposure, Radon. Journal of
Hazardous Materials 39, 211-223, 1994.
2. A. Cucoş (Dinu), C. Cosma, T. Dicu, R. Begy, M. Moldovan, B. Papp, D. Niţă, B. Burghele, C. Sainz,
Thorough investigations on indoor radon in Băiţa radon-prone area (Romania), Science of the
Total Environment. Vol. 431, pp. 78–83, 1 August 2012.
3. J. Chen, D. Moir, T. Pronk, T. Goodwin, M. Janik and S. Tokonami, An update on thoron exposure
in Canada with simultaneous 222Rn and 220Rn measurements in Fredericton and Halifax.
Rad. Prot. Dosim., Vol. 147, No. 4, pp. 541–547, 2011.
S62
B. Burghele, C. Cosma
7
4. Y. Yasuoka, A. Sorimachi, T. Ishikawa, M. Hosoda, S. Tokonami, N. Fukuhori and M. Janik,
Separately measuring radon and thoron concentrations exhaled from soil using AlphaGuard
and liquid scintillation counter methods. Rad. Prot. Dosim., Vol. 141, No. 4, pp. 412–415, 2010.
5. V. Cuculeanu, F. Simion, E. Simion, A. Geicu, Dynamics, deterministic nature and correlations of
outdoor 222Rn and 220Rn progeny concentrations measured at Bacau, Romania, Journal of
Environmental Radioactivity 102, 703-712, 2011.
6. I. Csige, I. Hunyadi, J. Charvát, Environmental effects on induction time and sensitivity of different
types of CR-39, International Journal of Radiation Applications and Instrumentation. Part D.
Nuclear Tracks and Radiation Measurements, Volume 19, Issues 1–4, Pages 151-154, 1991.
7. Calamosca M., Penzo S., A new CR-39 nuclear track passive thoron measuring device, Radiation
Measurements 44, 1013–1018, 2009.
8. V. Urosevic, D. Nikezic, S. Vulovic, A theoretical approach to indoor radon and thoron
distribution, Journal of Environmental Radioactivity 99, 1829–1833, 2008.
9. The United Nations Scientific Committee on the Effects of Atomic Radiation. Annex E: Sources-to
effects assessment form radon in homes and workplaces. Volume II, UNSCEAR 2006 Report.
United Nations Publication.
10. ICRP 115 (International Commission on Radiological Protection). Lung Cancer Risk from Radon
and Progeny and Statement on Radon. ICRP Publication 115 Ann. ICRP 40. 2010.