Results of special 220Rn measurements

Environment

International,

Vol. 22,

Suppl.1,pp.S1135-S1138,1996

Copyright8319%Elsevier

Science Ltd Printed in the USA. All rights reserved 01604120/96 $15.00+.00

PIISO160-4120(96)00229-2

RESULTS OF SPECIAL 220Rn MEASUREMENTS G. Keller and M. Schiitz Institute of Biophysics, Saar University, 66421 Homburg, Germany

EI 9512-412 M (Received 9 December 1995; accepted 12 July 1996)

Measurements of the activity concentration of the 220Rn progeny were performed inside German dwellings (n=148) and in the open air (n=lOO) by air sampling. The median value Cindoor= 0.25 Bq/m3 of the **’ Rn progeny concentration indoors was found four times higher than outdoors C 0utdoor = 0.05 Bq/m3. The indoor **%I progeny concentrations were compared to the ventilation rate of the rooms. There was no clear correlation. Due to the changing ventilation behaviour throughout a year, a wave-like variation of the indoor **%I progeny concentration was observed, with its minimum in July and its maximum during the winter months. These measured data were compared to model calculations that derived indoor %n concentrations from **%I exhalation out of the building materials. Measurements of a large sample of commercial building materials showed **%I exhalation rates ranging from 0.01 Bq/m*s up to 1 Bq/m* s. A mean **%I exhalation rate of 50 mBq/m* s results in an indoor **‘%I concentration of 8 Bq/m 3. That means that, for inhalation of airborne radioactivity in dwellings, the contribution of the **%I progeny to the effective dose may not be neglected. Copyright 01996 Elsevier ScienceLtd

INTRODUCTION The inhalation of 222Rn- and 220Rn-progeny inside dwellings represents the greatest contribution to absorbed dose in tissue from natural radiation for the average individual. The increased activity concentration of 220Rn in dwellings may be caused by the 220Rn exhalation from the building materials. During the last two decades, the 22% exhalation rates from typical building materials and the resulting indoor 22%n concentrations were studied. To separate 22%n from 222Rn for an arbitrary concentration mixture of both, a special measuring device was developed. The decay chains of the uranium-radium or the thorium-series show that the energies of the emitted alpha particles of the “‘Rn (5.49 MeV) and the 220Rn (6.29 MeV) differ little. The same is true for their progeny: 6.00 MeV and 7.69 MeV (222Rn progeny) and 6.78 MeV and 8.79 MeV (220Rn progeny). The differentiation of the single components in a gas mixture was achieved using a high resolution alpha spectroscopy.

By that, “%I exhalation rates of some typical building materials were yielded, from which resulting indoor 220Rn concentrations were estimated. These model calculations fit many measured 22~ progeny concentrations inside dwellings. EXPERIMENTAL METHODS The activity concentration of 22% may be measured by the electrostatic deposition of its freshly released first decay product 216Poonto the surface of a semiconductor detector. The electrodes that produce that electric field consist of the metallic hemisphere at positive potential and of a surface-barrier-detector at earth potential (Fig. 1). From any adjacent surface, the “‘Rn gas may freely enter the measuring chamber. The 220Rn exhalation rates out of the probes are related to the increase in time of the 22%n concentrations inside that measuring chamber.

G. Keller and M. Schtitz

Sl136

Fig. 1. Measuring device for **‘Rn - ***Rn exhalation rates.

Table 1. **‘Rn exhalation rates of some typical building materials. Building material

pumice gypsum lime stone heavy concrete areated concrete slag stone brick porphyr sandstone marble

Exhalation rate (mBq/m* s) min 27 10 24 10 15

mean 112 123 62 43 24 33 10 82 51 10

max 565 277 120 107 44

_

To reach a minimal peak width of the alpha radiation, the first 22cRn/222Rnprogeny are deposited directly onto the surface of a silicon charged particle detector. There, they decay again by emitting alpha particles with the above mentioned energies. The equilibrium with the preceding nuclides is reached after 30 min for *18Po and after 2 min for 216Po, according to their half-life periods. Since the alpha particles directly enter the detector, the full width at half maximum of their alpha peaks is lower than 20 keV and allows a distinct separation. The electrostatic field inside the 14 L measuring chamber, produced by applying a high voltage of 20 kV, gave an absolute counting efficiency of 34%. The temporal course of the 2’8Po and *r6Po impulse rates directly reflect the increase of the “%n and 222Rn concentrations inside the measuring chamber. These are proportional to

the **‘Rn and 222Rn exhalation rates. The 220Rn exhalation rates of some concretes, gypsum, bricks, and stones were measured under laboratory conditions. In principle, the described measuring technique can be applied to determine indoor 22% concentrations as well. However, due to its short half-life, the lower detection limit of indoor or outdoor 220Rn concentration is unsatisfactory. It is more suitable to calculate the 220Rn concentration by sampling the air on a filter and measuring the 220Rn decay products (Keller et al. 1982). In 120 dwellings in the south west of Germany, the concentrations of the decay products “‘Pb - 212Bi were measured under normal ventilation conditions. Some circumstances of the 22c’Rnprogeny measurements were determined by a questionnaire for a later evaluation of the data. The time expenditure of one measurement was about 2.5 h. The dwellings were randomly selected and, in eight different dwellings, repetitive measurements were carried out over the year. RESULTS AND DISCUSSION The mean 22c’Rnexhalation rate of building materials amounted to 50 mBq/m2 s, 95% of the exhalation rates ranging between 14 mBq/m2 s and 140 mBq/m2 s (Table 1). From that and from the ratio between the volume V of a room and its entire 220Rn exhaling surface F, the maximum reachable indoor 22’?Knconcentration can be estimated. With a quotient FN = 0.5, the estimation yields an indoor 22cRn concentration of 2 Bq/m3. Ventilation rates of more than 10 h-’ should reduce the indoor 22cRn concentrations significantly. According to the variation of the 220Rn exhalation rates, average indoor 22%n con-

s1137

Results of special *“Rn measurements

counts 50 45--

-

40--

_

35 -_

_

30--

_

N=148

’

25 -20 -1

15 -. 10 -5 -01

[ 0 0.5

1 1.5

2 2.5

3 3.5

4 4.5

b 5 5.5

;,:

:

6 6.5

7 7.5

;

; 6 8.5

;

:

;

,

9 9.5 10

m W.L. Fig. 2. Frequency distribution of the 220Rnprogeny concentrations in dwellings.

equilibrium equivalent concentration C of ‘“Rn 1.4

in dwellings

IV

- Bq/ma

-5 -- 4.5

ventilation index Iv (high index means low ventilation rate)

.-

-- 4 .=

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Ott

Nov

I”

-- 3,5

Dee month

Fig. 3. Seasonal variation of indoor **%I progeny concentration.

centrations from less than 1 Bq/m3 up to 10 Bq/m3 can be expected. Figure 2 shows the frequency distribution of the **!Rn progeny concentrations in dwellings. The median equivalent **‘?Rnconcentration amounted to 0.25 Bq/m 3and 90% of all values ranged between 0.08 Bq/m3 and 0.58 Bq/m3. With an estimated equilibrium factor F, = 0.05, a median equivalent **oRn concentration of 5 Bq/m was obtained with a l-sigma confidence interval from 2 Bq/m3 to 12 Bq/m . These values agree with the estimated 8 Bq/m3 of “h concentration based on the **ORnexhalation rates.

To find a possible coherence between building material and 22c’Rnconcentration, the examined buildings were classified by the main construction material. There was no significant difference between 22cRnprogeny concentrations in houses built with native stone or with bricks. Figure 3 shows the z20Rn concentration and an arbitrary ventilation index I, in one single dwelling during different months. A ventilation index I, = 0 means open windows or windows that were closed a few minutes before the measurement. The index I, = 5 means that the windows were closed for more than 6 h. Maximum **‘Rn concentrations were measured in January. As a

S1138

G. Keller and M. Schtttz

counts 100 90 80 70 60 50 -

4

I

;I j, 0

0.28 0.58

0.84 1.12

1.4

1.68

1.96 2.24

Bqlm'

Fig. 4. Frequency distribution of equilibrium zzcRn concentration in the open air.

consequence of higher ventilation rates during the warmer season from May to September, there is a difference of about a factor of 2 in the **‘Rn concentrations. Significant influences of meteorological parameters such as differences in barometric pressure, in air temperature or. wind velocity, on the indoor **‘Rn concentrations could not be found. The outdoor equilibrium equivalent 220Rnprogeny concentration showed a median value of about 1 Bq/m3 (Fig. 4). This is 25% of the corresponding indoor “(kn concentration. Although there is a great statistical uncertainty in these data due to the lower limit of detection, they compare favorably with the results of other authors (Mark et al. 1969). CONCLUSIONS

The **‘Rn exhalation rates of typical building materials range between 1 mBq/m* s and 600 mBq/m* s with a median value of 50 mBq/m* s. From these data,

resulting indoor **‘Rn concentrations up to 10 Bq/rr? could be expected and were measured in many dwellings. The activity concentrations of the **Qn progeny in dwellings merely depended on the building materials and on the ventilation rates. The seasonal variation of the activity concentrations of the **a progeny showed its maximum during winter. The concentrations of the short-lived **(&I progeny were found to be a factor of 4 higher inside dwellings than in the open air. Dose estimations showed that the contribution of the **a progeny to the mean annual lung dose may not be neglected. REFERENCES Keller, G.; Folkerts, K.-H.; Muth, H. Method for the determination of “*Rn (radon) and u”Rn (thoron) exhalation rates using alpha spectroscopy. Radiat. Prot. Dosim. 3(1/2): 83-89; 1982. Mar& D.E.; Holleman, D.F.; McCurdy, D.E.; Schiager, K.I. Analysis of atmospheric concentrations of RaA, RaR, and RaC by alpha spectroscopy. Health Phys. 17: 131-138; 1969.