Gamma Radiation Measurements in Tunisian Marbles

The radioactivity of 15 kinds of different granites collected in Tunisia was determined by gamma-ray spectrometry using hyper-pure germanium (HPGe) detector. The average activity concentrations for primordial radionuclides U, Th and K were respectively 33.24, 8.01 and 116.98 Bq/kg. The activity concentrations ranged from 3.59 to 87.37 Bq/kg for U, from 0.45 to 25.34 Bq/kg for Th and from 24.06 to 380.23 Bq/kg for K. The measured activity concentrations were used to assess of the radium equivalent activity ranged from 22.2 to 995.8 Bq/kg, the absorbed dose rate in air from 7 to 1209 nGy/h and the internal (0.1 to 2.8) and external (0.1 to 2.7) hazard indices. The data obtained in this study may be useful for natural radioactivity mapping.


Introduction
Natural radioactive materials (rocks, soil, sediment, water and aliment) contain low-level radioactivity.Marble used in building and decoration are also possible sources of radioactivity.Radioactivity in the marbles consists of the 238 U series [1][2][3] with a relative abundance of 99.284% and a half-life of 4.468 × 10 9 years, the thorium series 232 Th with a relative abundance of 100% and a half-life of 14.05 × 10 9 years and the actinium series 235 U with a relative abundance of 0.716% and a half-life of 9.47 × 10 10 years.There are also several singly occurring radionuclides, the most important one is 40 K with a relative abundance of 0.0117% and a half-life of 1.248 × 10 9 years.The emanation of radon ( 222 Rn) is associated with the presence of 226 Ra and its ultimate precursor 238 U in the ground.The inhalation of its short-lived daughter products is a major contributor to the total radiation dose.
The objective of the work is to estimate the radioactivity concentrations of primordial radionuclides; 238 U, 232 Th and 40 K in marble samples collected in Tunisia.The absorbed and effective dose rate, the external and internal hazard indices in marble samples have been determined.The measurements have been carried out using a HPGe spectrometer.

Experimental Setup
A total of 15 different kinds of marble in Tunisia have been collected.Figure 1 shows the Tunisian states where the commercial marble samples were collected.The samples were pulverised, dry-weighed [4,5], sealed in 1 l plastic Marinelli beakers and stored for 4 weeks in order to allow the reaching of equilibrium between 226 Ra and 222 Rn and its decay products [6,7].Spectra for different samples were measured with a hyper-pure germanium (HPGe) detector of high-resolution gamma-ray spectrometer.Figure 2 show a spectrum of Tunisian marble sample 1.The detector has a photopeak efficiency of 30% and an energy resolution of 1.85 keV (FWHM) for the 1.332 MeV reference transition of 60 Co.The measurements have been carried out at the Center for Nuclear Sciences and Technology, Sidi Thabet, Tunisia (CNSTN).
To obtain the gamma spectrum with good statistics, the accumulation time for each sample is set at 20 hours.Marble samples container were placed inside a shield, in order to minimize the background radiation.The background distribution in the environment around the detector was counted in the same manner and in the same geometry as the samples.It was determined using an empty sealed beaker.The background spectra were used to correct the net peak area of gamma rays of measured isotopes.

Calculation of Activity Concentrations
The specific activity i E A (Bq/kg) of a nuclide i and for a peak at energy E, is given by: where  From the 15 samples measured in this study, Sample (2) appears to present the highest concentrations for 238 U, Sample (11) presents the highest concentration for 232 Th and Sample (7) presents the highest concentration for 40 K.

Absorbed and Effective Dose Rate
Absorbed dose rate D (in nGy/h) can be calculated using the following formula [8,9]: where i E A is the activity concentration (in Bq/kg), and F C is the dose conversion factor (absorbed dose rate in air per unit activity per unit of soil mass in nGy/h per Bq/kg).
To estimate the annual effective dose, one has to take into account the conversion coefficient from absorbed dose in air to effective dose and the indoor occupancy factor [10].A value of 0.7 Sv/year was used for the conversion coefficient from absorbed dose in air to effective dose received by adults, and 0.8 for the indoor occupancy factor, implying that 20% of time is spent outdoors.The where D (nGy/h) is the calculated dose rate, 0.7 (Sv/Gy) is the conversion coefficient from the absorbed dose rate in air to the effective dose rate and 0.2 is the outdoor occupancy factor [11].
Table 1 shows the absorbed and effective dose rate.The average absorbed and effective dose rates are 25.7 nGy/h and 0.23 mSv/year respectively.The highest value of the dose rate is for Sample (15), 51.20 nGy/h and the minimum value is for Sample (1), 4.31 nGy/h.The maximum effective dose rate is 0.45 mSv/year and the minimum is 0.04 mSv/year.

Radium Equivalent Activity
The distribution of 238 U, 232 Th and 40 K in marbles is not uniform.Uniformity in respect of exposure to radiation   [12].A radium equivalent of 370 Bq/kg in marbles will produce an exposure of about 1.5 mSv/year to the inhabitants [11].The radium equivalent activity which can be calculated by the following formulae [13,14]: where A U is the 238 U activity concentration (Bq/kg), A Th is the 232 Th activity concentration (Bq/kg) and A K is the 40 K activity concentration (Bq/kg).The average calculated (Ra eq ) values for marble samples were ranging from 9.04 to 110.9 Bq/kg with the average value of 54.88 Bq/kg.

External and Internal Hazard Indices
To estimate the radiation dose expected to be delivered externally if a building is constructed using marbles, the external hazard index (H ex ), due to the emitted γ-rays, can be calculated using the following equation [8,15]: In addition to the external irradiation, radon and its daughter products are also hazardous to the respiratory organs.The internal exposure to radon and its short-lived products is proscribed by the internal hazard index (H in ).For the safe use of a marbles in the construction of dwellings H in should be less than unity.The Internal hazard index is calculated by the following formula [15][16][17]: The calculated external and internal hazard indices values (Table 1) are ranged from 0.05 to 0.3 and from 0.04 to 0.51 respectively with an average value of 0.15 and 0.24 respectively.

Figure 1 .
Figure 1.Localization of the Tunisian states where the commercial marble samples were collected.

N
is the net peak area, E  the detection efficiency at energy , E D  the gamma ray yield per disintegration of the specific nuclide for the transition at energy , E s M the mass of the measured sample (kg) and the counting live-time (s).If there is more than one peak in the energy range of analysis, then the result is the weighted average nuclide activity.t The gamma-ray transitions of 214 Pb, 214 Bi were used to determine the concentration of the 238 U series.The gamma-ray transitions 228 Ac, 208 Tl 228 Ac were used to calculate the concentration of the 232 Th series.The gama-ray

Figure 2 .
Figure 2. A spectrum of Tunisian marble sample 1. transition of 40 K was used to determine the concentration of 40 K in different samples.The activity concentrations of the natural radionuclides of the 15 kinds of marbles are shown in Figure 3.The activity concentrations ranged from 3.59 to 87.37 Bq/kg for 238 U, from 0.45 to 25.34 Bq/kg for 232 Th and from 24.06 to 380.23 Bq/kg for 40 K.The average activity concentrations are 33.24Bq/kg for 238 U, 8.01 Bq/kg for 232 Th, and 116.98 Bq/kg for 40 K.From the 15 samples measured in this study, Sample (2) appears to present the highest concentrations for 238 U, Sample(11) presents the highest concentration for 232 Th and Sample(7) presents the highest concentration for 40 K.
effective dose rate   eff Dindoors (in mSv/year), is calculated by the following formula:

Figure 3 .
Figure 3.The activity concentrations for primordial radionuclides 238 U, 232 Th and 40 K.

Table 1 . Absorbed dose rate (nGy/h), effective dose rate (mSv/an), H ex , H in and Ra eq (Bq/kg) for the marble samples.
Ra eq ) in Bq/kg to compare the specific activity of marbles containing different amounts of 238 U, 232 Th and 40 K.It is assumed that 370 Bq/kg of 238 U, 259 Bq/kg of 232 Th and 4810 Bq/kg of 40 K produce the same gamma dose rate