CR-39 SAMPLING OF INDOOR RADON IN SOUTHERN ROMANIA*
ANGELA VASILESCU
Horia Hulubei National Institute for Nuclear Physics and Engineering (IFIN),
P.O.Box MG-6, RO-077125 Bucharest-Magurele, Romania, E-mail: angela@.nipne.ro
Received November 15, 2012
The paper presents studies associated with the implementation of the indoor radon
measurement technique using CR-39 nuclear track detectors and an automatic Radosys
set-up in the Applied Nuclear Physics Department of the Horia Hulubei National
Institute for Physics and Nuclear Engineering and new, in-situ, indoor radon data in
various locations in South/South-East Romania.
Key words: indoor radon, solid-state nuclear track detectors, CR-39.
1. INTRODUCTION
The new Solid State Nuclear Track Detector (SSNTD) Indoor Radon Laboratory
of the Applied Nuclear Physics Department at the Horia Hulubei National Institute
of Physics and Nuclear Engineering (IFIN) employs CR-39 plastic detectors and a
Radosys set-up [1]. It is mainly aimed at the regional coverage of the necessities of
indoor radon measurements especially in the Southern part of Romania, but also to
become a reliable member of a National Radon network.
The implementation of this technique brought along a series of tests for the
better understanding of this already well established method, and was doubled by
indoor sampling measurements at various sites.
The geographical region covered by our study was expected to be of
reasonably low impact, compared to radon prone areas in the country (e.g. Crucea
in Moldova, Stei in Transylvania), however, as to our present knowledge, the statistic
of existing measurements for this region is low and any new data are welcome.
Radon originates from the 238U - 226Ra natural decay series and escapes into
air easily from the material in which it is formed (be it soil, rocks, water or building
materials). The air we inhale (outdoors or indoors) contains radon. Average radon
*
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. S311–S319, Bucharest, 2013
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levels are of 8 Bq/m3 for continental outdoors, at 1m from the soil, and only
0.04 Bq/m3 above the sea. Indoor radon concentrations reach orders of magnitude
higher values, typically 12-300 Bq/m3 up to maximum values of several thousands
of Bq/m3 in radon prone areas [2]. In temperate climate, heat conservation and
reduced ventilation habits increase the radon levels, especially in the cold months,
determining, e.g. for Romania, an increase of the measured levels by a factor of
about 2 [3]. Published data show e.g. values in the limits of 15-1005 Bq/m3, with
an arithmetic mean of (115±48) Bq/m3 for measurements performed in winter,
spring and summer for several sites in Transylvania [4-5].
2. EQUIPMENT AND PROCEDURE
The implementation of the CR-39 SSNTD technique in IFIN was started by
A. Danis with the development of a state-of-art radon probe and calibration
chamber [6], and continued in our new Radon lab [7]. Presently we use Radosys
equipment [1], 10x10 mm2 CR-39 detector chips in RSKS type chambers, with
4.5 h etching in 25% NaOH at 90ºC in the Radobath RSB4 unit. The reading of the
tracks and the calculation of track density is done automatically with Radometer
2000, employing routine or slightly modified counting programs for small round
tracks, provided by the manufacturer. The exposure and radon volume
concentration is evaluated using calibration factors provided by the manufacturer
of the chips.
In the routine field measurements, we used automatic reading, repeated three
times, with calibration for each measurement. In the intercomparison tests we read
the detector chips five to ten times and applied the Chauvenet exclusion test for
outliers. We also used a modified slower reading routine, with repeated
focalization on the surface of the chip, both in separate individual calibration at one
reading and in a version of repeated measurement three to five times at the same
focalization, in order to reduce the spread between readings. The final choice for
the routines adopted for the procedures to be used in our lab was to apply the fast
routine for the field measurements (repeated five times), while for intercomparison
and quality control exercises we decided for the modified “slow” method, and
check also all the other options. At each site (“point of measurement”) we installed
two Radosys detectors, and for each session we kept two blank background chips,
to monitor the CR-39 set used.
The conditions for the experimental tests performed during the implementation
of this radon measuring system will be described together with the discussion of
the results.
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3. RESULTS AND DISCUSSION
In the process of evaluation of our procedures and their standardization, we
performed several exercises, including comparative readings by human operator
with an optical microscope, comparisons of our previous “manual” scanning
technique [7] with the automatic Radosys system measurements, parallel counting
and measurement. We also included some “field measurements” in our comparisons.
The following evaluation tests were the most significant to define our
working procedures:
– Intercomparison exercises with CR-39 automatic and operator reading
– Repeatability and reproducibility tests, and
– Background survey of unexposed (blank) detectors.
Automatic reading is a sensitive issue in the very low density region (for
background studies in particular), where various program variations have been
tested, with operator supervision of the accuracy in the identification of α tracks. A
problem which cannot be neglected at very low densities is the misidentification of
various unavoidable defects (etching defects, impurities, scratches) as tracks, which
transfer mainly into higher final values. But also some real α tracks can be lost,
especially in the fast (standard) routine. If such a background correction is then
applied in the lower radon concentration domain, it can lead to underestimation of
the actual radon exposure.
Several trial tests in state-of-art calibration vessels both in the IFIN and in the
Cluj-Napoca labs and parallel reading of detector sets etched in both laboratories
were performed in order to compare our results with the Environmental
Radioactivity and Nuclear Dating group at Babes-Bolyai University (UBB). Both
laboratories used Radosys systems, but of different generations and, accordingly,
slightly different etching recipes (5h at UBB) and evaluation software versions. In
the most elaborate test, we used 20 detectors, and 10 background detectors (all
from one batch). The detectors were exposed in the UBB radon chamber, and then
divided between the two labs (two sets of 10), together with the background
detectors (5 blanks to each lab), for etching. The read-out was done for both (10+5)
detector sets in both laboratories, and the final results were compared.
The difference in the results obtained in the two laboratories was within
–3.6%. As a general rule, the IFIN results were somewhat lower, but with better
reproducibility of the track density for both etching conditions:
σn(IFIN)/σn(UBB)= 0.28/0.34 (etch IFIN);
σn(IFIN)/σn (UBB)=0.39/0.57 (etch UBB).
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The repeatability of the measurements at IFIN was tested for track density
evaluation on ten repeated measurements on a set of twelve chips in the 40-50
tracks/mm2 range, and was better than 99%.
The reproducibility of the measurements was tested on two sets of ten
detectors exposed in the same conditions, and it varied between 1.2 - 1.7%.
Our first participation at an international intercomparison exercise (2011
Intercomparison exercise in field conditions Saelices el Chico, Spain) yielded good
results [8]: of the three domains of exposure, our results were situated both in the
low (110 kBq/m3) and high (3800 kBq/m3) exposure region within 1σ of the
average, and within 2σ for the intermediate exposure (700 kBq/m3). The final
report [8] situated the IFIN results in class A (better than 10% from the overall
average of the participants) for the low and high exposure and class B (better than
15%) for the intermediate exposure. This exercise used three sets of 15 detectors
for the three exposure values and a set of 15 transit detectors (mounted chips),
which accompanied the devices sent to Spain and back, and were etched in the
same session. We also monitored this batch with four unexposed chips, for our
background evaluation. This was not imposed by the intercomparison exercise, but
is an internal checking routine of our detector batches.
3.1. BACKGROUND STUDIES
We performed a study of the background for the CR-39 chips from two
batches of detectors we used for measurements during the years 2007-2012. The
background detectors were etched in each measurement session together with the
“field” detectors, and were used in the evaluation of the background corrections. In
between the sessions, the detectors were stored packaged in a freezer, they were
extracted from the original packaging at the same time with the “field” detectors
and kept wrapped firmly and packaged, in the laboratory, at room temperature. We
used a minimum of two (unmounted) CR-39 chips as background detectors at each
etching session.
The timeline for the detectors can be reconstructed from the laboratory
logbook (from arrival in the lab to unpacking and etching).
During the monitored time, we used two different batches, series J (until
2010) and series T, manufactured by Radosys Ltd. Due to the fact that the lab did
not fulfill yet the necessary legal conditions for authorization for third party
measurements (e.g. intercomparison results, elaboration and approval of all quality
assurance procedures needed), during this time, only tests and field sampling
measurements were performed as research. (The number of detectors used was
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therefore also limited.) Figure 1 shows the track density values obtained on batches
J and T. The average Poisson uncertainty for background was ~19%.
A batch is delivered in several sheets, packaged individually in radon free
aluminized bags, with cut and marked chips glued on a foil, from which the chips
are extracted with tweezers and mounted in the radon (monitor) device (or wrapped
as blanks, as already mentioned). The background detectors were selected
randomly from the set of unexposed chips taken for a measurement session (field
or other test).
The time dependence (time is correlated with the number of the detector, as
the bags were opened in increasing order of the numbers) is not evident, and we see
rather random local non-uniformities in the batch (more evident for the already
used-up J series).
Fig. 1 – CR-39 background track density variation in two detector series (batches).
Differences in the background densities up to 50% have been seen even
within the set used in the same session. In the cases when only a pair of detector
was used, the decision is difficult: the chips had to be analyzed carefully, checked
by microscope, to see whether the discrepancy was not due to the program or some
material or etching defects. In most cases, the differences could not be explained
by the presence of an accidentally bad chip.
The background results on the T series are lower and less spread.
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Fig. 2 – Frequency distribution of background track density for the J and T CR-39 chips.
Fig. 2 presents the frequency distribution of background track densities
measured on both detector series.
The background correction can amount in the typical (2–3 months) exposures
up to 10–11 Bq/m3, which is definitely high, representing about 10% of the
warning level at 100 Bq/m3 recommended by the World Health Organization
(WHO) [9]. This aspect might need further investigation, and a reasonable decision
in the acceptance of a measurement especially in the low track density region,
where the background correction could be of significant impact, leading to the
underestimation of the radon risk.
3.2. FIELD MEASUREMENTS
As the final goal is actually the measurement in field conditions, we used
CR-39 RSKS detectors in the cold season (late autumn/winter/early spring) in pairs
of two, installed in various locations, in homes mainly, in Bucharest and in Ilfov
county (Magurele, Ciorogarla, Copaceni, Buftea, Corbeanca), in Giurgiu (Giurgiu),
Fetesti (Ialomita), Cernavoda (Constanta), Busteni (Prahova), Mogosesti, Stilpeni
and Corbsori (Arges). (The location of the sites is presented on an edited Google
map, see Fig. 3). Some of these sites were only temporarily inhabited, so, in those
cases, the results correspond to a “worst case scenario” as to ventilation conditions.
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Fig. 3 – Location of field measurements by CR-39 SSNTD (black squares on an edited Google map).
The measurements were performed in several campaigns, starting with 2007
to 2011. Some sites were measured several times, especially those where the home
owners responded to our first results with changes. We noticed an improvement in
a few cases after repairs or changing ventilation habits, but in some cases the new
measurement did not show any change or even an increase above the 200 Bq/m3
level shown in the plot in Fig. 4a.
Figure 4 summarizes the data on the field measurements: the plot (a) shows
the measured values for each location, and the histogram (b) presents the frequency
distribution of the radon concentration in Bq/m3.
All the experimental values in the plot are averages over two detectors put in
the same location with one exception, when a detector was broken by accident. The
time of exposure was typically 2-3 months, and the transit times were minimized
(from a few hours to a few days - less than one week). The volunteers participating
in the survey were instructed to keep the devices in “radon free” condition, as
much as possible (e.g. in open air), until they could be returned to the lab.
Action levels recommended by ICRP [10] for indoor radon levels are
between 200–600 Bq/m3, and the value accepted is a political decision for every
country, depending on the local geology and affordable costs. The latest
recommended (WHO) warning level of 100 Bq/m3 is also shown in Fig. 4a [9].
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a)
b)
Fig. 4 – CR-39 sampling in various locations in Southern Romania, between 2007
and 2011/2012, in the cold season (a); frequency distribution of radon concentration (b).
4. CONCLUSIONS
The IFIN Indoor radon lab can perform reliable measurements employing
CR-39 detectors and a Radosys system.
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Different sets of detectors can have variations of background track densities,
but differences can be seen within the same set as well. With careful handling,
storing and monitoring, their lifetime can be prolonged after the one stated by the
manufacturer.
Due to the automatic statistical counting routine, it is reasonable to evaluate
whether the background correction should be taken into account at each
measurement or not at all, or its impact on the result discussed separately,
depending on the character of the measurement (research, accuracy evaluation of
the method or routine mapping measurements).
The field measurements showed reasonably low values, with a few
exceptions, to confirm our expectations for the Southern part of Romania.
Acknowledgements. Financial support for this work was ensured by the National Agency for
Scientific Research ANCS, in part by the 2CEX06-10-78 and PN09-37-02-04 projects. The very good
collaboration and useful discussions with the members of the Environmental Radioactivity and
Nuclear Dating group led by Professor Constantin Cosma at the Babes-Bolyai University (Professor
Cosma and Ms. Kinga Szacsvai, in particular) during the implementation of the CR-39/Radosys
system at IFIN are acknowledged by the author. Thanks go also to all the volunteers participating in
the field survey.
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