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The specific radioactivity of several building materials used in France, which is considered a direct exposure to radiation, has been assessed by γ-ray spectrometry. Corrected for coincidence summing and self-absorption effects, the values for ^{226}Ra, ^{232}Th and ^{40}K were in the ranges 4 - 56, 3 - 72 and 9 - 1136 Bq·kg^{-1}, respectively. The samples were found to have radium-equivalent activities between 5 and 245 Bq·kg^{-1}. Values of 0.02 - 0.67 for the external and 0.03 - 0.82 for the internal hazard indexes were estimated. The calculated absorbed dose in air agrees closely with MCNPX simulations. The conversion of absorbed dose to annual effective dose gave values between 0.03 - 1.09 mSv·y^{-1}. All these values are below action limits recommended by the International Commission on Radiological Protection. The materials examined would not contribute a significant radiation exposure for an occupant and thus are acceptable for construction.

Knowing the natural radioactivity originating from building materials allows assessing radiological hazards to human health [1,2]. These materials usually contain radionuclides from both the uranium and thorium decay series, so the radiation exposure arises mainly from ^{226}Ra, ^{232}Th and ^{40}K [3,4]. One distinguishes an external hazard caused by exposure to radiations, while an internal hazard is the consequence of inhalation of ^{222}Rn and its decay products [

Some studies have reported measurement of radioactivity in concrete, gypsum, brick and sand [6-9]. In the present work, the estimation of ^{226}Ra, ^{232}Th and ^{40}K radioactivity has been carried out by HPGe γ-ray spectrometry. The building materials studies were samples offered by a large supplier in Strasbourg. For an accurate estimation of the absorbed dose (KERMA), coincidence summing and self-absorption corrections [10,11] have been taken into consideration.

From the specific radioactivity of the three abovementioned radioelements, the radium equivalent, the external hazard, the internal hazard, the absorbed dose and the annual effective dose were evaluated. The results are discussed according to criteria proposed by the UNISCEAR [

The samples were collected, crushed, dried and homogenized, and then were conditioned in cylindrical SG50 containers. When the equilibrium between ^{226}Ra and its daughters was reached (3 - 4 weeks), the radioactivity was measured for ^{226}Ra (186 keV), ^{232}Th via ^{228}Ac (911 and 969 keV) and ^{40}K (1460 keV).

The natural radioactivity was measured using a planar BE detector model 3830 and associated electronics supplied by Canberra [^{241}Am (60 keV), ^{109}Cd (88 keV), ^{57}Co (122, 136 keV), ^{139}Ce (165 keV), ^{51}Cr (320 keV), ^{113}Sn (391 keV), ^{85}Sr (514 keV), ^{137}Cs (661 keV), ^{88}Y (898, 1836 keV) and ^{60}Co (1173, 1332 keV), having the same dimensions and in the same conditions of measurement as the samples.

The specific radioactivities of ^{226}Ra, ^{232}Th and ^{40}K in the samples before and after correction for coincidence summing and self-absorption are reported in

The radioactivity ranges between 4 - 56, 3 - 72 and 9 - 1136 Bq∙kg^{−1 }for the ^{226}Ra, ^{232}Th and ^{40}K, respectively. The lowest activity found was in the gypsum; the highest activity arose from red brick. Similar values have been reported in numerous other countries [8,13].

The distribution of ^{226}Ra, ^{232}Th and ^{40}K activities in the samples is non-uniform and that is why the radiumequivalent Ra_{eq} has been defined to evaluate one of the hazards of the building materials. Ra_{eq} is calculated using the following formula [3,4,6].

where A_{c}(Ra), A_{c}(Th) and A_{c}(K) are the activities in Bq·kg^{−1} of ^{226}Ra, ^{232}Th and ^{40}K, respectively, found in ^{−1} of these radionuclides give the same γ-ray dose equivalent.

All the values in ^{−1} recommended by UNSCEAR [

H_{ex} is a radiation hazard index used to assess the indoor radiation dose rate resulting from the direct exposure to

γ-rays. To be within the norm, H_{ex} must be less than or equal to 1. Von Krieger [_{ex}. The equation is given by:

H_{in} deals with ^{222}Rn, a decay product of ^{226}Ra, and its short-lived daughters. The model used to calculate this index is given by [

One sees in _{ex} ranges from 0.02 to 0.66 while the internal hazard index H_{in} is from 0.03 to 0.82. Again the lowest values were found for gypsum and the highest values for red brick. All of these values are acceptable because they are below unity.

The absorbed dose is due to γ-rays arising from a building structure. The model consists of a room with internal dimensions of 4 × 5 × 2.8 m^{3} enclosed on all sides by 20 cm of material with density 2.350 g∙cm^{−3} (concrete) [

where 0.92, 1.10 and 0.08 nGy^{−1}·Bq^{−1}∙kg are the dose conversion coefficients given by UNSCEAR for ^{238}U, ^{232}Th and ^{40}K. A_{c}(Ra), A_{c}(Th) and A_{c}(K) are the activities of ^{226}Ra, ^{232}Th and ^{40}K. This relation permits calculating the absorbed dose in air at 1 m height.

From ^{−1} set by the European Commission.

To calculate the effective annual dose (mSv), the conversion factor of 0.7 Sv·Gy^{−1 }for an adult has been introduced (other values are adopted for infants and children) to convert the absorbed dose in air to the effective dose. The indoor occupancy factor is taken to be 0.8 and the allowed dose is 1 mSv·y^{−1} [

The following relation provides the annual effective dose.

These values are plotted in

Knowing the volume of a room and the apparent density (ρ) of the building materials, we can simulate the radioactivity within the confines of the room and the distribution of the absorbed dose. Simulated results are presented for red brick (ρ = 1.63 g·cm^{−3}) with a radium-equivalent activity of 246 ± 17 Bq·kg^{−1}. This activity corresponds to a γ-ray emission rate for ^{226}Ra and its daughters, which is taken into consideration in the simulation by the WGT card [

The dose rate varies as d^{−2} from any point of emission to any other point inside the room. The simulations were performed using a mesh tally as in ^{−2}. Multiplication of the fluence tally F5 (cm^{−2}) by these coefficients allows obtaining the absorbed dose of γ-ray photons (

One sees in

Comparison between the measured and simulated absorbed dose shows good agreement as one can see in

The natural radioactivity measured for ^{226}Ra, ^{232}Th and ^{40}K in nine building materials available in France is comparable with values found in other countries except for red brick. The radium-equivalent, the external and internal indexes, the absorbed dose rate in air and the annual effective dose were found to be within acceptable limits. Therefore, the materials do not represent a significant radiological health risk and can be used for dwelling construction.

MCNPX can be used to determine the absorbed dose in air for any dwelling. The radiation hazard due to the natural radioactivity can be estimated for the walls, floor and roof. The simulations take into consideration the attenuation factors and the chemical composition of the building material in the absorbed dose rate calculations for γ-rays. Simulated absorbed dose rates are in good agreement with calculated absorbed doses, which lends confidence to the model used.

We wish to thank all the RaMsEs colleagues for their help.