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Radiation Protection Dosimetry (2017), pp. 1–5
doi:10.1093/rpd/ncx128
ASSESSMENT OF RADIUM ACTIVITY CONCENTRATION AND
RADON EXHALATION RATES IN IBERIAN PENINSULA
BUILDING MATERIALS
E. Andrade1,2,*, C. Miró3, M. Reis1,2, M. Santos1,2 and M. J. Madruga1,2
Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, Estrada
Nacional 10, ao km 139,7, 2695-066 Bobadela LRS, Portugal
2
Laboratório de Proteção e Segurança Radiológica, Instituto Superior Técnico, Universidade de Lisboa,
Estrada Nacional 10, ao km 139,7, 2695-066 Bobadela LRS, Portugal
3
Departamento de Física Aplicada, Universidad de Extremadura, Av. de la Universidad, s/n, 10005 Cáceres,
Spain
1
*Corresponding author: eva.andrade@ctn.tecnico.ulisboa.pt
Radium (226Ra) is a natural radioactive element of the uranium decay series, which could also be present in building materials. Radon (222Rn) is continuously produced by the decay of 226Ra and its presence inside buildings can contribute to the
increase of the population exposure to ionizing radiation. In this work, the amount of radium activity concentration and radon
exhalation rates in several types of building materials that are commonly used in the Iberian Peninsula have been tested. The
radium activity concentration was measured by gamma-ray spectrometry, whereas the radon exhalation rates were carried
out using a continuous radon monitor (active measuring technique) and a solid state nuclear track detectors (passive measuring technique). The 226Ra mean values range from 5.0 to 123.4 Bq kg−1. As expected, the results show that the radon exhalation rate is higher in granites samples relatively to others building materials analysed. A positive correlation was found
between radium activity concentration and radon exhalation rates in both techniques. The emanation fraction and alpha index
were also calculated.
INTRODUCTION
The knowledge of the natural radioactivity of building materials is important for the determination of
population exposure to radiations and for the assessment of the possible radiological hazards to human
health. Building materials contribute to natural radiation exposure, either by gamma radiation from 40K,
238
U and 232Th and their decay products to an external whole body dose exposure, or by radon exhalation to an internal dose exposure due to deposition
of radon decay products in the human lung tissue(1–3).
Radium (226Ra) is a natural radioactive element
of the uranium decay series, which could also be present in building materials. Radon (222Rn) is continuously produced by the decay of 226Ra and its
presence inside dwellings can contribute for the
increase of indoor radon levels as well as to a higher
risk of lung cancer(4, 5). Therefore, there is a considerable public awareness and a growing concern in
relation to radon exhalation from building materials,
in particular those that are used for interior decorations(1). Over the last years, the building industry has
developed new building materials that may contain
significant quantities of naturally or technologically
enhanced levels of radioactivity. For that reason, it
is important to evaluate the amount of natural
radioactivity present in the building materials, which
are used in the construction of dwellings(4, 6).
The main purpose of this study is to evaluate the
radium (226Ra) activity concentration in ornamental
rocks and composites, commonly used as building
materials in the Iberian Peninsula versus radon
(222Rn) exhalation rates by using passive and active
measuring techniques. For the assessment of the
potential radiological hazards, both the alpha index
and the emanation fraction are also determined.
MATERIALS AND METHODS
A total of 23 samples representing different types of
building materials were analysed for radium activity
concentration and radon exhalation rates. The sample set includes granites, limestone, slate, marbles,
ceramic tiles, clay brick, cements and concrete
mostly obtained at Extremadura (Spain), and
Portugal. The samples were reduced to powder by
grinding, afterwards were sieved (fraction grain size
<1 mm), dried, homogenised and placed inside different containers according to the measurement
techniques.
Radium activity concentrations were determined
by gamma-ray spectrometry and the radon
© The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com
E. ANDRADE ET AL.
exhalation rates were measured using a continuous
radon monitor and a solid state nuclear track
detectors.
Radon exhalation measurements using intact
building materials were also performed. However,
for the purpose of this work, those results are not
presented, since comparison were made with 226Ra
measurements obtained by gamma spectrometry
with processed samples.
Ci Vλ
E=
M [T +
1
λ
(e−λt − 1)]
(1)
where, Ci (Bq m−3 h−1) is the integrated radon concentration obtained by the LR115 track detectors, V
(m3) is the volume of the container, λ (h−1) is the
radon decay constant, T (h) is the detector exposure
time and M (kg) is the mass of the sample.
Regarding the active technique, the radon exhalation rate was measured by using a continuous
radon monitor RTM1688-2, from SARAD, in order
to follow the radon activity growth as a function of
time and determining the average equilibrium of the
gas concentration during the exposure period. The
radon emanated from the grains of the sample
migrates through the pores and is finally exhaled
from the surface. The equipment has an internal
pump working at a flow rate of 0.25 L min−1, and it
is connected to the sealed container in a closed loop.
The radon concentration inside the cylindrical sealed
container was measured for a period of ~8 days at
intervals of 2 h. The radon concentration growth as
a function of time is given by the following equation:
EXPERIMENTAL
Gamma-ray spectrometry
The 226Ra activity concentrations of the samples
were analysed by gamma-ray spectrometry using
HPGe detectors. The detectors are shielded from
natural radioactive background by lead shields with
cooper and tin lining. Genie 2000 software (version
3.0) was used for data acquisition and spectral analysis and GESPECOR software (version 4.2) was
used to correct for self-attenuation and coincidence
summing effects. The detection efficiency was determined using NIST-traceable multi-gamma radioactive standards with an energy range from 46.5 to
1836 keV and customised in a water-equivalent
epoxy resin matrix (1.15 g cm3 density) reproducing
exactly the geometry of the samples. The samples
containers were sealed and stored for a period no
lesser than one month in order to achieve the radioactive equilibrium between 222Rn and its short-lived
progeny. The 226Ra activity was calculated through
the 295.2, 351.9 and 609.3 keV photopeaks, by
assuming secular equilibrium. The stability of the
system (activity, FWHM, centroid) was checked
once a week with a 152Eu certified point source.
External QC was assured through the participation
in several international intercomparison exercises
(IAEA, EC, etc.).
C (t ) =
EM [1 − e−(λ + α ) t]
+ C 0 e−(λ + α ) t
(λ + α ) V
(2)
being C0 (Bq m−3) the radon concentration at t = 0,
and α (s−1) the leakage rate from the container.
Radon emanation fraction
The emanation fraction is defined as the fraction of
radon atoms generated that escape the solid phase in
which they are formed and become free to migrate
through the interstitial space between the grains(7)
and was determined through the following equation(2, 5, 8):
Radon exhalation rates
F=
The radon exhalation rates were determined by using
two different techniques, the active and passive
techniques(1).
The passive technique is based on integrated measurements by using solid state nuclear track detectors
(SSNTD) placed on the inner upper surface of cylindrical sealed containers. LR115 type II plastic track
detectors from Kodak were used for this purpose.
After 90 days of exposure, the detectors were chemically etched in 2.5 mol L−1 NaOH solution in a
water bath at 60 ± 1°C for 2 h. The resulting alpha
tracks were counted using Spark Counter model
UFC-2 equipment. After obtaining the radon activity concentration, the radon specific exhalation rate,
E (Bq kg−1 h−1), can be calculated through the following expression:
E
CRa λRn
(3)
where E (Bq kg−1 h−1) is the radon exhalation rate,
CRa (Bq kg−1) is the radium activity concentration
and λRn (h−1) is the decay constant of 222Rn.
Alpha index
Alpha index is an important index dealing with the
assessment of the excess alpha radiation due to
radon inhalation originating from building materials.
This index is defined as follows(2, 5, 9):
Iα =
2
CRa
200
(4)
ASSESSMENT OF RADIUM ACTIVITY CONCENTRATION
RESULTS AND DISCUSSION
since the detection limit for radon (active technique)
is ~4 Bq m−3.
As presented in Figure 1, the results show that in
all samples there are a common pattern between
226
Ra activity concentration and radon exhalation
rate, except to ceramic tiles and clay brick. As
expected, granites present the highest 226Ra content,
as well as higher radon exhalation rate relative to the
other building materials and are in the same range of
those reported for other countries(2, 9). Nonetheless, it
can also be seen that ceramic tiles show high radium
activity concentrations, but low or <DL radon exhalation rates.
Generally, the main opacifying constituent of
glazes, applied to ceramic tiles, is zircon and its presence should justify the high 226Ra activity concentration measured relative to the others composites
materials(10).
According to the results obtained for the different
types of building materials, a positive relationship
has been observed between radium activity concentration and radon exhalation rates calculated for
passive and active techniques (Figure 2).
However the correlation coefficients (r) obtained
are not statistically significant (p > 0.05). On the
other hand, the results obtained by both techniques
(passive and active) are strongly correlated (r =
0.95).
In order to highlight the results, the alpha index
and emanation fraction was also calculated
(Table 2).
The experimental results of radium activity concentration and radon exhalation rates, for the different
types of building materials considered in this study
are presented in Table 1. As it can be seen, in the different types of rocks analysed the granites show the
highest mean 226Ra values (123.4 ± 5.2 Bq kg−1),
whereas marbles present the lowest mean 226Ra
values (5.0 ± 0.7 Bq kg−1) measured. In addition, the
different types of granites sampled show a wide
range of 226Ra values (51.0–239.1 Bq kg−1), which
may be related with different amounts of uraniumbearing minerals (e.g. feldspar, biotite and zircon).
In turn, marbles show a more restricted range
(4.2–5.9 Bq kg−1), and the limestone and the slate
display 226Ra values of 40.7 ± 2.0 and 28.6 ± 2.2 Bq
kg−1, respectively.
Regarding the composites materials, the highest
value was found in ceramic tiles (92.9 ± 4.0 Bq
kg−1), while the lowest value was found in concrete
(10.8 ± 0.9 Bq kg−1). The clay brick presents 226Ra
values of 41.8 ± 2.0 Bq kg−1, and the cements show
a wide range of 226Ra values (8.3–62.2 Bq kg−1) with
a mean of 24.9 ± 1.4 Bq kg−1. The large variation of
226
Ra values in cements may be arisen due to the difference in the origin and composition of the samples.
The radon exhalation rate in terms of mass varies
from 0.9 ± 0.2 to 16.9 ± 1.8 mBq kg−1 h−1 for the
passive technique, and between 10.6 ± 2.6 and 70.3 ±
9.8 mBq kg−1 h−1 for the active technique. In some
samples, the radon exhalation rate is below the DL,
Table 1. The mean and range (minimum and maximum) of the radium activity concentration and radon exhalation rates
(passive and active techniques) measured in different building materials.
Samples type
No. of samples
226
Ra (Bq kg−1)
Mean ± σ (Range)
Rocks
Granite
9
Limestone
Slate
Marble
1*
1*
3
Composites
Ceramic tile
2
Clay brick
Cement
1*
5
Concrete
1*
1
E (mBq kg−1 h−1)
Mean ± σ (Range)
Passive technique
Active technique
123.4 ± 5.2
(51.0–239.1)
40.7 ± 2.0
28.6 ± 2.2
5.0 ± 0.7
(4.2–5.9)
16.9 ± 1.8
(6.0–44.5)
2.9 ± 0.1
8.4 ± 0.5
0.9 ± 0.2
(0.5–1.3)
70.3 ± 9.8
(20.6–221.3)
26.0 ± 9.0
21.5 ± 6.6
<DL1
92.9 ± 4.0
(84.5–101.3)
41.8 ± 2.0
24.9 ± 1.4
(8.3–62.2)
10.8 ± 0.9
1.2 ± 0.4
(0.7–1.8)
4.3 ± 1.3
4.5 ± 0.5
(2.6–8.5)
3.5 ± 1.0
<DL1
DL, detection limit; *Individual values (no. of samples=1), A ± U (k = 2).
3
<DL1
19.0 ± 1.9
(8.6–29.4)
10.6 ± 2.6
E. ANDRADE ET AL.
Table 2. Radon emanation fraction (passive and active
techniques) and alpha index in the different building
materials.
Samples type
Granite
Limestone
Slate
Marble
Ceramic tile
Clay brick
Cement
Concrete
Figure 1. Variations of the radium activity concentration
and the radon exhalation rates for the different types of
building materials using passive and active techniques.
Iα (Bq kg−1)
F (%)
Mean
Mean
0.62
0.20
0.14
0.02
0.46
0.21
0.12
0.05
Passive
Active
2.1
0.9
3.9
2.4
0.2
1.4
3.0
4.3
8.4
8.5
10.0
—
—
—
11.8
13.0
recoil range in solids is very small (typically
<0.05 μm) and most of the recoiling atoms remain
within the mineral grain. This means that if the
radium is mainly distributed on the grains surface,
radon could be more effectively released and become
available to migrate. Therefore, the emanation fraction depends not only on the radium content found
in the building materials, but also on the mineralogical aspects such as lattice structure, porosity,
grain size and shape, and elemental composition(7).
Moreover, UNSCEAR (2000) reported that the typical emanation coefficients for rocks and soils range
from 5 to 70%, and established 20% as a representative value. In the present study, all samples analysed
are below the representative value.
The alpha index was proposed based on the
assumption that if 226Ra activity concentration
exceeds 200 Bq kg−1, it is possible that the indoor
radon concentration will exceed 200 Bq m−3(11).
Hence, the recommended upper limit concentration
of 226Ra is 200 Bq kg−1 for an Iα equal to 1. The
results obtained for the alpha index show that the
samples analysed are below than the recommended
upper limit. These results ranged from 0.02 to 0.62,
which means that the building materials collected do
not present dangerous levels of indoor radon, except
one type of granite rock where Iα is 1.20.
Figure 2. Relationship between radium activity concentration and radon exhalation rates (E) by using passive and
active techniques.
The emanation fraction ranges from 0.2 to 4.3%
for the passive technique, and from 8.4 to 13.0% for
the active technique. It is interesting to note that the
highest emanation fraction (4.3 and 13.0%, for passive and active techniques, respectively) correspond to
the lowest 226Ra activity concentrations (10.8 Bq kg−1).
This could probably be explained by the nonuniform distribution of 226Ra on the minerals of the
different samples. In fact, alpha recoil plays an
important role in the emanation phenomenon, since
CONCLUSIONS
•
•
4
The granites presented the highest 226Ra activity
concentration, as well as higher radon exhalation
rates, when compared with the other studied
building materials.
The tiles can be considered good materials to be
used for interior decorative purpose since these
materials present very low radon exhalation
rates.
ASSESSMENT OF RADIUM ACTIVITY CONCENTRATION
•
In general, the building materials analysed did
not present dangerous levels of indoor radon
(Iα < 1), except one type of granite rock where Iα
is 1.20.
4.
Hence, it can be concluded that the use of the analysed building materials for the construction of
dwellings or workplaces is considered safe for inhabitants and for the public health hazard, except one
type of granite.
5.
6.
FUNDING
The “Centro de Ciências e Tecnologias Nucleares
(C2TN)/Instituto Superior Técnico (IST)” authors
gratefully acknowledge the “Fundação para a Ciência
e Tecnologia (FCT)” support through the UID/Multi/
04349/2013 Project, Portugal. Dr C. Miró acknowledges the “Junta Extremadura-FEDER” support
through the IB16114 Project, Spain.
7.
8.
9.
REFERENCES
1. Miró, C., Andrade, E., Reis, M. and Madruga, M. J.
Development of a couple of methods for measuring
radon exhalation from building materials commonly
used in the Iberian Peninsula. Radiat. Prot. Dosim. 160,
177–180 (2014).
2. Shoeib, M. Y. and Thabayneh, K. M. Assessment of
natural radiation exposure and radon exhalation rate in
various samples of Egyptian building materials.
J. Radiat. Res. Appl. Sci. 7, 174–181 (2014).
3. Stoulos, S., Manolopoulou, M. and Papastefanou, C.
Assessment of natural radiation exposure and radon
10.
11.
5
exhalation from building materials in Greece.
J. Environ. Radioact. 69, 225–240 (2003).
Cosma, C., Cucos-Dinua, A., Pappa, B., Begya, R.
and Sainz, C. Soil and building material as main sources
of indoor radon in Baita-Stei radon prone area
(Romania). J. Environ. Radioact. 116, 174–179 (2013).
Sharma, N., Singh, J., Esakki, S. C. and Tripathi, R. M.
A study of the natural radioactivity and radon exhalation
rate in some cements used in India and its radiological
significance. J. Radiat. Res. Appl. Sci. 9, 47–56 (2016).
Bavarnegin, E., Fathabadi, N., Moghaddam, M. V.,
Farahani, M. V., Moradi, M. and Babakhni, A. Radon
exhalation rate and natural radionuclide content in
building materials of high background areas of Ramsar,
Iran. J. Environ. Radioact. 117, 36–40 (2013).
Ishimori, Y., Lange, K., Martin, P., Mayya, Y. S. and
Phaneuf, M. Measurement and calculation of radon
releases from NORM residues. International Atomic
Energy Agency, Vienna (2013). (Technical reports series, ISSN 0074-1914; no. 474).
United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR). Sources and Effects
of Ionizing Radiation. Vol. 1, New York: United
Nations) (2000).
Amin, R. M. A study of radon emitted from building
materials using solid state nuclear track detectors.
J. Radiat. Res. Appl. Sci. 8, 516–522 (2015).
Senthilkumar, G., Raghu, Y., Sivakumar, S.,
Chandrasekaran, A., Prem Anand, D. and Ravisankar,
R. Natural radioactivity measurement and evaluation of
radiological hazards in some commercial flooring materials used in Thiruvannamalai, Tamilnadu, India.
J. Radiat. Res. Appl. Sci. 7, 116–122 (2014).
The Radiation Protection. Authorities in Denmark,
Finland, Iceland, Norway and Sweden. Naturally
occurring radioactivity in the Nordic countries—recommendations (2000). ISBN 91-89230-00-0.
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