Environment International, Vol. 14, pp. 345-348, 1988 0160-4120/88 $3.00 + .00 Printed in the USA. All rights reserved. Copyright © 1989 Pergamon Press plc

A METHOD FOR DETERMINATION OF RADON CONCENTRATIONS USING x-SClNTIL.LATION CELLS

INDOOR

I. Kobat, J. Vaupoti~, and J. Burger "J. Stefan" Institute, "E. Kardelj" University of Ljubljana, P. O. Box 100, 61111 Liubljana, Yugoslavia

(Revised 2 September 1988)

Radon air concentrations in dwellings are usually too low to be measured directly by alpha scintillation cells. Thus, a preconcentration of radon from a larger air volume is necessary. A method is described in which air is sampled into a 1.2 dm 3 glass vessel by a simple pump, transported to the laboratory where radon is isolated in a spiral glass tube filled with glass beads cooled with liquid nitrogen, and then transferred to a 180 cm 3 alpha scintillation cell and counted. Using this method radon was deter- mined in some selected homes in different regions of Slovenia, Yugoslavia.

Introduction

Alpha scintillation cells (Van Dilla and Taysum, 1955; Lucas, 1957) have been successfully used for radon gas (222Rn) determination for more than three decades. Cell efficiency and the lower limit of detection depend on the cell shape and volume, as well as on the construc- tion material and scintillator used (usually Ag activated ZnS).

With our 180 cm 3 scintillation cells (Kristan and Ko- bal, 1973) made of pyrex glass and coated with Lumilux Flu Blau scintillator (Riedel-de-Ha6n AG, Seeltze- Hanover, F. R. Germany), concentration of radon higher than about 100 Bq m -3 can be measured by direct sampling of air into the cell and subsequent counting in the laboratory (Kobal, Smodig and Skofljanec, 1986). But to measure indoor radon air concentrations in homes ranging from a few Bq m -3 up to a few hundred Bq m -3 (McGregor et al . , 1980; Swedjemark and Mj6nes, 1984; Pfister and Pauly, 1984; Battaglia, Bazzano, and Carioni, 1984; Wrixon et al. , 1984; Castren et al. , 1984; Abu-Jurad and AI-Jarallah, 1984; Cohen, 1986), scin- tillation cell cannot be used for direct air sampling. For this concentration range we thus propose a 1.2 dm 3 glass vessel for sampling air and a laboratory vacuum system for the isolation and transfer of radon from the sample into a scintillation cell.

Equipment and Procedure

Air in homes is sampled into 1.2 dm 3 glass vessels usually used in underground coal mines to sample air

for analysis of methane and carbon oxides. The air is either sucked into the vessel, if it has been previously evacuated, or filled by a foot-pump commonly used for inflation of pneumatic boats and air beds.

Ampoules are transported to the laboratory where radon is transferred to scintillation cells using the glass apparatus shown in Fig. 1. Air is sucked from the vessel through a trap cooled with liquid nitrogen by means of a vacuum pump. As the trap, a pyrex glass spiral is used of length about 30 cm, inner diameter 5 mm, and filled with 3 mm diameter glass beads. The gas flow rate through the trap is about 0.5 dm 3 min-1. At the end of this stage the spiral tube and the scintillation cell is evacuated and closed from the other parts of the ap- paratus. The spiral is then warmed up to room tem- perature and nitrogen gas added (up to a pressure of 1 bar) through the spiral, thus flushing all the isolated radon from the spiral into the scintillation cell. The alpha activity is measured after three hours.

345

Testing the Method

Various radon activities were expelled by aged ni- trogen gas into evacuated 1.2 dm 3 glass sampling vessels from 15 cm 3 radon bubblers containing known activities of standard radium chloride solution (National Bureau of Standards, U.S. Department of Commerce, Standard Reference Material 4953 D), which are usually used for calibration of scintillation cells. Nitrogen gas is added to the vessel up to a pressure of 1 bar. Radon is isolated from the vessel and transferred to the scintillation cell

346

A

V 1.2 dm 3

glass air sampler

N2

-(

180 cm 3 alpha sc in t i l la t ion ceil

--=,.- to vacuum pump

L - - 5

30 cm long spiral tube cooled with liquid nitrogen

Fig. 1. Apparatus for isolation of radon from air samples in 1.2 dm' glass vessels and its transfer to alpha scintillation cells.

according to the procedure described above. In Fig. 2, radon concentrations in the vessels as obtained by the proposed method are plotted versus corresponding true values (calculated from the radon activities introduced). Figure 2 confirms the satisfactory reliability of the pro- cedure.

The method has been already successfully used in a preliminary study of radon in air concentrations in about 150 homes in different regions of Slovenia, Yugoslavia. Some of the results are gathered in Table 1.

Ljubljana is the capital of the Republic of Slovenia with about 300,000 inhabitants. The heating of blocks of flats and public buildings and offices is mostly sup- plied from a coal-fired thermal plant, while individual houses use coal and wood, as well as natural gas to a lesser extent. As seen from the Table 1, the highest values of radon concentrations were found in houses built of fly-ash bricks.

In Titovo Velenje more than half of the houses were built of fly-ash bricks originating from lignite. Though expected, concentrations were not found to be distin- guishly higher than in other regions thanks to low rad- ium concentration in lignite.

Pohorje is a mountain of granite rock where in- creased gamma dose rate was measured. In homes made of this material no elevated radon concentrations were found, probably because of more intense ventilation or different way of living.

In Gorenjska, increased radon levels were found in houses built of volcanic material.

In Zasavje, in a small area are situated both a phos- phate ore processing plant and a coal-fired thermal plant

I. Kobal, J. Vaupoti6, and J. Burger

Table 1. Some results of the determination of radon in air concentrations in Slovene homes, Yugoslavia.

222Rn Concentration Sampling Place Bq m 3

Ljubljana concrete building

house of fly-ash bricks

old stone house

Titovo Velenje old stone house

house of fly-ash bricks

Pohorje old granite house

Gorenjska old volcanic stone house

old stone house

1 1 3 -+6 " 2 6 8 - + 11 3 46-+ 13 4 45 -+ 11 5 59 -+ 11 1 200 --- 19 2 360 -+ 20 3 860 -+ 40 1 80-+ 11 2 4 2 - + 8 3 71 -+ 33 4 3 7 - + 9

1 3 1 - + 8 2 25 -+9 3 140 _+ 13 1 4 4 - + 9 2 102 -+ 11 3 158 -+ 15

1 63 -+ 10 2 LLD = 13 b 3 90 -+ 10 4 16-+6 5 LLD = 13

1 139 -+ 18 2 610 -+ 37 1 6 1 - + 9 2 LLD = 7

Zasavje phosphate ore store 98 -+ 12 phosphate ore mill 80 -+ 12 coal store 44 _+ 8 old stone house 1 87 -+ 11

2 63 -+ 10 3 151 _+ 16

Krgko old stone house

concrete house Zirovski vrh

uranium ore deposit area old stone house

1 161 -+ 17 2 2 1 - + 7 3 110 -+ 14

288 + 24

1 1424 -+ 35 2 173 -+ 16 3 847 -+ 46 4 385 -+ 25 5 537 -+- 26 1 3 1 - + 8 2 19-+6 3 23 --- 12 4 LLD = 11

new brick house

astandard deviation defined in HASL as S = (ns/t~ + nb/t2) ~'2 with n number of counts during a time period of t counting a sample, s, and a background, b (HASL, 1983). blower limit of detection as defined in HASL as LLD = 4.66 (nb) "2 (HASL, 1983).

Indoor radon concentrations 347

1500

!

E

m 1000 v

. m

> ° ~

u

¢-

" 0 L,-

~ 500

E , / X" I I I

500 1000 1500 introduced Rn activity (Bq.m -3 )

Fig. 2. Radon concentrations in 1.2 dm 3 glass vessels as determined according the proposed procedure versus the corresponding true values calculated from the activity of radon introduced from standard radium chloride solutions.

with accompanying coal storage facilities and fly-ash pile. The highest concentration was found in a house built of stone•

Kr~ko is interesting due to its nearby lying 635 MW nuclear power plant, though no special sources of nat- ural radioactivity is expected. The house with the high- est radon concentration is built of concrete to which fly-ash instead of gravel was incorporated.

At Zirovski Vrh, situated on the uranium deposit, all houses made of local stone show elevated indoor radon concentrations with the highest value of almost 1.5 kBq m -3.

It is beyond the scope of this paper to make a detailed explanation of the data in Table 1, but we want to prove the method and propose it as a convenient one for use in the measurement of the concentration range of in- door radon. A skillful technician is able to carry out 4 to 5 analyses per hour.

Acknowledgemen t s - The authors are grateful to the people living in all.the selected houses for their cooperation and patience. The Research Council of Slovenia financially supported this work.

References

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Battaglia, A., Bazzano, E., and Carioni, T. (1984) Indoor dose in Milan, Radiat. Prot. Dosim. 7, 283.

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348 I. Kobal, J. Vaupoti~, and J. Burger

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