concentrações de radônio e taxas de dose gama · pdf...

Concentrações de radônio e taxas de dose gama ambiental: há

correlação?

Elisabeth M. Yoshimura, Henry J. Ccallata, Almy A. Rodrigues da Silva, Roberto M. dos Anjos, Nancy

K. Umisedo, Marcos Rizzotto, Hugo Velasco, Diego L. Valladares

Universidade de São Paulo, Universidade Federal Fluminense, Universidad San Luis (Arg.)

• Motivation

• Background

• Experimental

• Results

• Conclusions

Motivation

• Radon levels in dwellings (indoor) are difficult to measure – long integration period; difficulties in entering in the house and positioning a detector.

• Gamma dose rate can be monitored with a rapid and less invasive measurement; lower seasonal variation.

• Both quantities are important for the radiation protection of the population.

• Both are related to the radionuclide concentration in soil and building materials

Motivation

• Radon levels in dwellings (indoor) are difficult to measure – long integration period; difficulties in entering and positioning a detector.

• Gamma dose rate can be monitored with a rapid and less invasive measurement.

• Both quantities are important for the radiation protection of the population.

• Both are related to the radionuclide concentration in soil and building materials

• Radon gas diffuses from the soil and the building materials to the indoor air

• Rn atoms can migrate from relatively long distances till reach the inside air volume.

• The dynamics of the radon transport from the source to the air depends on porosity of the material, humidity, temperature, presence of fissures in soil or walls.

Background: Rn Diffusion

Background: Gamma Interaction

• Natural gamma-rays: tenths of keV to 2.6 MeV

• Multiple scattering – energy degradation: broad spectrum

• Exponential decrease in the number of photons with distance (monoenergetic)

• Large mean-free path in air

Both radon and gamma rays may interact with the

human body very far from their sources.

Background: Rn concentration × G dose rate

• Fujimoto (1998):

– Large survey of houses in Japan (~7000)

– Good linear correlations between log of radon concentration and gamma dose rate

– The correlation depends on the type of structure of the measured houses (wooden, concrete of prefabricate houses)

• Chougaonkar and coworkers 2004:

– Survey of 200 houses in Kerala, India

– Found a poor and negative correlation between radon concentration and gamma dose rate.

• Iimoto et al, 2001:

– Monitoring in two areas of Taiwan

– Found a weak positive relationship between the environmental gamma dose rate and the concentrations of radon and progeny.


• Recently we got a good correlation in La Carolina old gold mine (Argentina)

Anjos, Umisedo, Da Silva, Estellita, Rizzotto, Yoshimura, Velasco and Santos, Journal of Environmental Radioactivity 101 (2010) 153–158


Experimental

• Gamma dose rate: thermoluminescent dosimeters

• Radon concentrations: monitors with SSNTD

• Measurement places – 59 houses in São Paulo (6 months)

– 16 Workplaces in radiotherapies (São Paulo 5 mo)

– 15 Workplaces in metro stations (São Paulo 5 mo)

– 14 points in tunnels of a mine (La Carolina Mine - San Luis – Argentina, 3 mo)

together

Experimental Details

• Houses randomly distributed in the city; ground floor; bedroom or living room, one set per house ~1.5 m from the soil.

• Radiotherapies distributed in the city, calculation room or offices, far from radiation sources, 1.5 m from the soil.

• Metro stations in various regions of the city, workplaces located away from the public circulation in the stations, and also far from the train routes, situated in depths of 4.5 to 22 m from the street entrance of the station.

Experimental Details

• Tunnels in an old gold mine (San Luis – Argentina)

Results

0.5 1 2 3 4 5

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Annual Gamma Dose (mGy)

Results

Houses Metro Radiotherapies La Carolina Mine

0.5 1 2 3 4 5

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Results

Mean std dev (CV) median

0.142 0.11 (77%) 0.108

1.14 0.22 (19%) 1.15


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Results


4.84 1.06 (22%) 5.05

3.05 0.59 (20%) 3.15


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Results


0.805 0.039 (4.9%) 0.810

1.89 0.150 (8.0%) 1.90


0.658 0.082 (12%) 0.635

1.97 0.34 (18%) 2.04


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Results


Y = 2.1 × X – 1.7

Conclusions • We found no correlation between indoor radon or alpha emitter

concentrations and annual gamma doses in samples of houses and low-ventilation workplaces in São Paulo.

• The variations obtained in the alpha emitter concentration in houses were much higher than the ones in the workplaces in São Paulo. This is due to higher variation in the ventilation conditions of the houses, and to the type of construction of the workplaces in metro stations and radiotherapies.

• A nice correlation was established between the annual gamma dose and radon concentrations inside the La Carolina mine.

• This correlation can be due to the low ventilation conditions in this mine, and to the fact that the soil portion which contributes to the gamma irradiation is the same from which the radon atoms diffuse to the air inside the mine.

• Other explanation is that in the La Carolina mine there is a large uniformity of the radionuclide concentration in the soil around all the monitored region.

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