Radiation Measurements 38 (2004) 843–846www.elsevier.com/locate/radmeas

Comparative measurements of soil gas radon concentrationusing thermoluminescent and track detectors

Karel Tureka ;∗, Metody Gelevb, Iancho DimovbaNuclear Physics Institute, Department of Radiation Dosimetry, Na Truhl�a�rce 39/64, Prague 180 86, Czech Republic

bInstitute for Nuclear Research and Nuclear Energy, 72, Tzarigradsko chaussee Blvd., So/a 1784, Bulgaria

Received 26 November 2003; received in revised form 26 November 2003; accepted 7 February 2004

Abstract

Track etch detectors are extensively used for Rn measurement in the environment because of their excellent ability to detectheavy charged particles. However, sometimes their application requires additional equipment and evaluation without automaticimage analyzer can be time consuming. Moreover, due to track overlapping, track detectors are best suited for relatively lowRn concentrations. Thermoluminescent detectors (TLDs) are routinely used for detection of � and � radiation, and in somecases they can be successfully used to measure other kinds of radiation, e.g. neutrons and �. In this study we compare theperformance of TLDs and track detectors during 4eld measurement of soil gas Rn concentration. A specially designed detectorholder determines the irradiation geometry and contains either CR-39 track detectors or two sets of several (6–14) TLDs. The4rst set of TLDs (bare) detects both � and � and the second set is hermetically sealed and detects � radiation only. The responseof two kinds of Bulgarian thermoluminescent materials, CaSO4: Dy and a CaSO4: Dy/graphite mixture was compared to theresponse of reference track detectors. Measurements were carried out at three distinct sites with di=erent Rn concentrationand two types of detector sealing were tested. The obtained results suggest that TLDs can successfully be used to measuremedium or higher (¿ 50 kBq m−3) soil gas Rn concentrations. Compared to track detectors TLDs extend the upper limit ofdetection by as much as two orders of magnitude.c© 2004 Elsevier Ltd. All rights reserved.

Keywords: Soil gas; Radon concentration; Thermoluminescent detectors; Track-etch detectors

1. Introduction

A method for measurement of soil gas radon concentra-tion applying electrochemically etched solid state track de-tectors (SSTDs) was developed at NPI/DRD (Turek et al.,1997) and is being successfully used in 4eld measurementsand for study of time variation (Neznal et al., 2004). Themethod is based on two parallel track detectors with nar-row air gap between them. Limitations of this design arisemainly due to track overlapping and therefore it is not suit-able for applications in cases where high radon concentra-tions are expected, e.g. anomalous natural areas or uraniummill tailings. The aim of this work was to modify the method

∗ Corresponding author. Tel.: +420-283-842-791;fax: +420-283-842-788.

E-mail address: turek@ujf.cas.cz (K. Turek).

using thermoluminescent detectors (TLDs instead of trackdetectors). The idea was 4rst demonstrated in practice byPressyanov et al. (1996), who used CaSO4:Dy-based TLDand SSNTD-type Kodak LR115 placed in the so-called “4l-tered cups”. Here, we test the TLD/SSNTD combinationin a rigorously de4ned geometry and long exposure timesunder various 4eld conditions. This allows us to study thee=ect of environmental factors and assess the potential scopeof application of this technique.

2. Experimental

The response of two kinds of Bulgarian TLD materials,CaSO4: Dy and a mixture CaSO4: Dy with graphite (Leviet al., 1983; Gelev et al., 1991, 1994) were determined andrelated to the response of reference CR-39 track detectors(Page Mouldings Ltd., UK). Both materials were used in the

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844 K. Turek et al. / Radiation Measurements 38 (2004) 843–846

α + γα + γ γ

γ

α + γα + γ

holder body

TLDmetalizedsealed bag

Fig. 1. Detector arrangements: A pair of standard Al-holders (left)and single holder with two compartments (right).

Fig. 2. Open standard Al-holder with track detectors (left) andsingle holder with two compartments (right).

Fig. 3. Standard Al-holder with track detectors (below) and singleholder with two compartments (above) 4tted together.

form of powder 4xed by clean lacquer in open Al-capsules 5and 4:5 mm in diameter, respectively, and 1 mm thick. Theannealing proceeded at 380◦C, for 10 min, as recommended.Evaluation was carried out using TOLEDO 654 (PitmanInstruments, UK), preheating at 150◦C for 15 s and readingat 280◦C for 25 s.

In the 4rst set of measurements, a pair of standardAl-holders intended initially for track detectors (Figs. 1–3)each containing six TLDs of both types were exposed. Oneof the holders was open and the detectors registered both �and � radiation. The other holder was sealed in a metalizedplastic foil and the detectors inside registered only � radi-ation at the same geometry. The TLDs were 4xed to theholder body with double-sided scotch tape.

soil surface 10 cm

80 cm

track detector

TLD

10 cm

Fig. 4. Scheme of detector arrangement inside the tube (left) andphoto of upper closed end of the tube buried into a soil (right).

In the second set of measurements only single holderswith two compartments were exposed because of problemswith hermetical sealing of the whole holder. Detectors for �radiation were sealed separately (Figs. 1–3). Measurementstook place at three di=erent experimental sites used primar-ily for studies of long-term time variation, with soil gasRn concentrations ∼ 5; 30 and 50 kBq m−3 inside metallictubes 1 m long and 2:54 cm in diameter. The tubes wereburied 90 cm into a soil with closed upper end. The detec-tors were hung ∼ 80 cm under the soil surface, as shown inFig. 4 (Neznal et al., 2004).

Depending on the chosen site the exposure time t of eachpair or single holder with TLDs took from 3 to 18 weeks.In the same tube and at the same time, n track detectorswere exposed consecutively for shorter time intervals ti ; i=1; 2; : : : ; n (for 1 week as a rule, because of higher sensitivity)covering the exposure of TLDs, i.e. t = ti. The responseR of TLD (average value of six detectors of a given group)was expressed as (N�+� − N�)=Aint , where N�+� and N� arethe reading values in counts for open and sealed detectors,respectively, and Aint = Av(ti) ∗ ti is the integral meanvolume Rn activity calculated using mean volume activityAv(ti) measured by track detectors during time ti.

3. Results

Typical readings (in counts) obtained during 505 h ex-posure of a single holder at the site with Av ∼ 50 kBq m−3

are presented in Table 1. The response R values obtainedfrom both sets of measurements described above are shownin Table 2. They can be used to estimate the limits for us-ing of soil gas Rn measurements. The readings of TLDs ex-posed only to � radiation (sealed) were taken into accountas a background. Their mean values obtained from all mea-suring sites are about 2±1 and 1±0:5 counts=h of exposurefor 5 and 4:5 mm detectors, respectively. The volume radonactivity AL estimated from the condition

AL ∗ R= 3 ∗ �BG (1)

K. Turek et al. / Radiation Measurements 38 (2004) 843–846 845

Table 1Reading of single holder

Detector no. N�+� N�

5 mm 4:5 mm 5 mm 4:5 mm

1 5688 3111 966 4252 4489 2649 989 4183 4294 3510 931 4054 4932 3152 1006 4135 4553 2268 949 4156 5434 2945 945 402mean 4898 2939 964 413rms 511 395 26 8

Table 2Response R of Bulgarian TLDs in counts per kBq m−3 h

Detector Arrangement R(counts= AL tmin

size kBq m−3 h) (kBq m−3) (h)(mm)

mean rms

5 mm Pair ofholders 0.238a 0.086 13 133

4:5 mm 0.103b 0.049 15 2675 mm Single

holder 0.141c 0.019 21 1334:5 mm 0.082c 0.013 18 267

a10 exposures.b11 exposures.c7 exposures.

Table 3Practical range of application TLDs and track detectors

Rn concentration [Bq m−3] Applicable exposure time

Track detectors TLDs, single holder

gap 7 mm gap 1 mm 5 mm 4:5 mm

1 k ¿ 1:5 day ¡ 5:7 years — —10 k ¿ 3:6 h ¡ 7 months ¿ 5:2 daysa ¿ 10:2 daysb

100 k ¿ 21 min ¡ 3 weeks ¿ 1:2 day ¿ 2 days1 M — ¡ 2 days ¿ 2:8 h ¿ 4:9 h10 M — ¡ 5 h ¿ 17 min ¿ 29 min

Exposure time Measurable Rn concentration [Bq m−3]

Track detectors TLD, single holder

gap 7 mm gap 1 mm 5 mm 4:5 mm

1 h ¿ 36 k — ¿ 2:8 M ¿ 4:9 M1 day ¿ 1:5 k ¡ 2:1 M ¿ 118 k ¿ 203 k1 week ¿ 200 ¡ 298 k ¿ 21 k ¿ 29 k1 month ¿ 50 ¡ 69 k ¿ 21 k ¿ 18 k1 year ¿ 4 ¡ 5:7 k ¿ 21 k ¿ 18 k

aAt Av = 21 kBq m−3.bAt Av = 18 kBq m−3.

can thus be considered as a low limit for use of TLDsin practice. From the condition that the statistical errorof net reading does not exceed 5% (i.e. 400 counts atleast):

R ∗ AL ∗ tmin = 400; (2)

the value of minimum exposure time tmin can be calculatedusing (1) and (2):

tmin = 400=(3 ∗ �BG): (3)

The calculated values of tmin and AL for all situations aresummarized in Table 2. Using these values, the exposuretime ranges for di=erent Rn concentrations and the mea-surable Rn concentration for 4xed exposure times can bederived. A comparison of these values for TLDs and trackdetectors is presented in Table 3 and Figs. 5 and 6.

4. Conclusions

• Both types of studied TLDs are suitable for soil gas Rnactivity measurements, however, the greater ones (with-out graphite) are better because of lower limits of appli-cability.

• As shown in Table 1, a typical reading of six TLDs givesan rms ¡ 15%. However, the mean value from repeatedmeasurements using a pair of holders (Table 2) has anrms of 30–50%, most likely due to unreliable hermeticsealing in that arrangement. Therefore, the arrangement

846 K. Turek et al. / Radiation Measurements 38 (2004) 843–846

0

1

10

100

1000

10000

1 10 100 1000 10000

concentration, kBqm-3

exp

osu

re t

ime,

ho

urs

TLD 5 mm, min

TLD 4.5 mm, min

SSTD min(gap 7 mm)

SSTD max(gap 1 mm)

Fig. 5. Applicable exposure times for TLDs (above the thick lines)and track detectors (between the thin lines).

1

10

100

1000

10000

1 10 100 1000 10000

exposure time, hours

con

cen

trat

ion

, kB

qm

-3

TLD 5 mm, min

TLD 4.5 mm, min

SSTD min (gap 7 mm)

SSTD max (gap 1 mm)

Fig. 6. Measurable Rn concentrations for TLDs (above the thicklines) and track detectors (between the thin lines).

where the detectors for � are sealed separately and 4xed ina single holder in an extra compartment is more reliable.

• These results indicate that TLDs can be successfully ap-plied for medium or higher (above ∼ 20 kBq m−3) soilgas Rn concentration measurements. The absence of ef-fects similar to track overlapping in TLDs extends the up-per limit of measurable Rn concentration by two ordersof magnitude.

References

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Gelev, M.G., Mishev, I.T., Burgkhardt, B., Piesch, E., 1994.A two-element CaSO4:Dy dosemeter for environmentalmonitoring. Radiat. Protect. Dosim. 51, 35–40.

Levi, S.M., Radicheva, M.A., Gelev, M.G., 1983. Synthesis andfactors inPuencing the properties of phosphor CaSO4:Dy(Tm).Nucl. Energy 20, 57–65 (in Russian).

Neznal, M., MatolMQn, M., Just, G., Turek, K., 2004. Short-termtemporal variations of soil gas radon concentration andcomparison of measurement techniques. Radiat. Protect. Dosim.108, 55–63.

Pressyanov, D.S., Gelev, M.G., Klein, D., Kritidis, P.P., 1996.Measurement of 222Rn in soil gas by combination ofthermoluminescent and solid-state nuclear track detectors.Environ. Int. 22 (Suppl. 1), 491–493.

Turek, K., BednMaOr, J., Neznal, M., 1997. Parallel track-etch detectorarrangement for radon measurement in soil. Radiat. Meas. 28,751–754.