The natural radioactivity of soil and rock samples is usually determined from the 226Ra, 232Th and 40 K contents  . A significant amount of man-made radionuclides 137Cs, may also present in the environment as a result of testing of nuclear weapons in the atmosphere, accidents and the routine discharge of radionuclides from nuclear installations  . The specific level radiation in the crust varies from one region to another as the concentrations of these natural radioactive elements vary due to their non-uniform nature in soils and the types of rock from which the soil originates  . The knowledge of radiation levels in the environment is an important for assessing the effects of radiation exposure. So, the aim of the present study is to measure the natural radioactivity levels for estimating the radiogecal hazard indices in soils and granite rocks. This work can be used as a baseline guideline for assessing the exposure of the natural radiation in the study region, especially, in this area, the rocks and soils are used as building materials. The studied regions located in southwestern Yemen (Figure 1).
2. Material and Methods
2.1. Samples Collection and Preparation
A total of twenty soil sample at depths (0 - 5, 5 - 50, 50 - 60 and 60 - 70 cm) underground and 6 granite rock samples were collected from different locations in Shabwah and Hadramout regions, Yemen, respectively. The sampling locations were chosen mostly in a wide area that we believe that it’s representative of the important region. All soil samples collected from undisturbed sites located away from buildings, trees and roads to ensure that there was no influence of man- made structures and anthropogenic activities.The collected samples (soil and
Figure 1. Location map of studying regions.
granite) were first sundried and then oven dried at 110˚C to constant weight. The samples were pulverized, homogenized and sieved through 1-mm mesh  . 500 gm of each meshed sample was replaced in Marinelli beaker and then was stored for four weeks to reach the equilibrium state between radium and its decay products  . Radioactivity measurements were performed by using High purity Germanium (HPGe) detector (25% relative efficiency). The measurement time of activity or background was 82800 seconds. The radioactivity concentration of 226Ra was determined from the photopeaks of 295.09, 351.87 Kev (214Pb), 609.31 Kev & 1120.27 keV & 1764.49 keV (214Bi). 232Th radioactivity was determined from 238.58 Kev (212Pb), 727.25 Kev (212Bi) and 911.16 keV & 968.97 Kev (228Ac) while the 40K radioactivity was determined from 1460.8 Kev and finally, 137Cs was determined from the decay of 40Ar and 137Ba  .
The activity concentrations (Bq/kg) for the natural radionuclides in the measured samples were calculated after decay correction using the equation:
where Ac is the activity concentration (Bq/kg) in the sample, Nc is the net area under the corresponding peak/second, m the dry weight sample mass (kg), β the branching ratio, and εabs is the detection absolute efficiency of a specific energy.
The radium equivalent activity is a very useful guideline in organization the safety standards in radiation protection for humans. The index calculated by the following relation  .
Raeq = ARa + (ATh × 1.43) + (AK × 0.077) (2)
where ARa, ATh and AK are the activities of 226Ra, 232Th and 40K, respectively, in Bq/kg, The formula is based on the assumption that 370 Bq/kg of 226Ra, 259 Bq/kg of 232Th and 481 Bq/kg of 40K produce the same gamma-ray dose rate  .
The representative level index, Iγ is used to estimate gamma radiation associated with the natural radionuclides in soil and calculated by the following equation  :
where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K, respectively in Bq/kg.
The external hazard index Hex can be calculated by the following equation  :
where ARa, ATh and AK are the activity concentrations of 226Ra, 232Th and 40K, respectively. The maximum value of Hex to be less than unity  .
The absorbed gamma dose rates D(nGy/h) in air at 1m above the Earth’s surface for the uniform distribution of radionuclides were calculated based on guidelines given by  :
where, ARa, ATh and AK are the activity concentrations (Bq/kg) of 226Ra, 232Th and 40K, respectively.
The annual effective dose rate was calculated from the absorbed dose through the application of dose conversion factor of 0.7 Sv/Gy with occupancy in the outdoor factor 0.2. 
where 8,760 h is the number of hours in 1 year. 10−6 is a conversion factor of nano and Milli.
3. Results and Discussion
3.1. Activity Concentrations
The results of the measured values of activity concentrations for 226Ra, 232Th, 40K and 137Cs obtained from the samples are presented in Table 1. From this table, the average values, concentrations of 226Ra, 232Th, in soil and are less than the world average values  . 40K activity concentrations are in the range between 510.92 in the sample (S3) to 600.75 Bq/kg in sample (S5) with an average value is
Table 1. Activity concentrations of natural radionuclides (226Ra, 232Th and 40K) and the fallout nuclide 137Cs (Bq/kg) in soil and granite samples.
higher than the world average value  . Higher concentration values of 40K were obtained from the soil of Shabwah region, probably because the use of inorganic fertilizer in some locations and could also be attributed to the wearing away from the weathered surfaces of the potassium rich, igneous rocks which dominate a part of the geology of this region. The artificial radionuclide of 137Cs concentrations in the soil samples are within the global atmospheric fallout and lower than the reported data for other countries. For granite samples, 226Ra, 232Th and 40K average concentrations are 45, 106, and 1235 Bq/kg respectively. The average values of 226Ra fall within the worldwide average values. While, 232Th and 40K average values are twice the worldwide average values (50, 50 and 500 Bq/kg) respectively  . The content of 40K in granite samples is the highest in all the sampling areas. So, it is the most abundant radionuclide of the total (238U + 232Th + 40K), it is abundant about 89% followed by 232Th 7.7%. And finally 226Ra is 3.3%. 137Cs is detected in all granite samples under investigation. The mean concentration of 137Cs is 0.67 Bq/kg. These values are not radiologically important. Table 2 shows the comparison between the activity concentrations of soil samples with that of other countries of the World. The variations in the activity concentrations in the soil of the various locations of the world, depend upon the geological and geographical conditions of the area and the extent of fertilizer applied to the agricultural lands  . Table 3, shows that, for granite samples results obtained in the present work are consistent with previous results and all results being within the range given in the report  . In general, the radioactivity in granite samples varied from one country to another  . Figure 2 shows a comparison between the obtained average values activity concentrations in soil and granite and the recommended values by UNSCEAR2000.
3.2. Radiological Hazard Indices in Soil and Granite Rock Samples
The computed results of the radiological hazard indices were listed in Table 4.
Table 2. Comparison of natural radioactivity concentrations (Bq/kg) in the present soil samples with the reported values from different countries of the world.
Table 3. Comparison of natural radioactivity concentration (Bq/kg) in the present granite samples with the reported values from different countries of the world.
Figure 2. The activity concentrations of 226Ra,232Th and 40K average values in soil and granite samples under investigation and UNSCEAR2000 values.
The values of Raeq Bq/kg for soil and granite rock samples are lower than the allowed value of 370 Bq/kg  . Representative level index (Iɣ) average value is less than one, this means that the external radiation dose within the soil samples is less than the recommended dose of 1 Bq/kg  . The Iɣ average values for granite rock is higher than 1 Bq/kg. External hazard index (Hex) for soil and granite samples are less than the limit value of unity. As can be seen in Table 4. The total absorbed dose rate calculated for Shabowah’s soil, it less than the world average value of 57 nGy/h. The presence of 40K in Shabwah’s soil contributes a maximum of 52% to the total absorbed dose rate, followed by 232Th of 35% and 226Ra of 13%, these contributions were illustrated in Figure 3.
Table 4. Shows values of radium equivalent (Raeq), level index (Iγ) and external hazard (Hex) for soil samples (Shabwah) and granite rock samples (Hadramout), Yemen.
Figure 3. Comparison between percentage contributions absorbed dose of 226Ra, 232Th and 40K to total absorbed dose of soil and granite rock samples, Yemen.
The average value of annual effective dose equivalent of the study soil less than the world average value effective dose of 0.07 mSv/y  . According to these results of radiogecal hazard indices, one can indicate that the soil samples are safe and can be used as a construction material without posing any significant radiological threat to the population. As shown in Table 4, the average value of the absorbed dose rate of Hadramout region’s granite samples is almost twice the world average value. So, the local people must avoid using these granite samples in the building construction, especially in the interior decorative materials of dwelling without radioactivity control. The average total effective dose rates (Deff) for granite samples is far lower than the 1.0 mSv/y recommended by  . This value is According to these results of radiogecal hazard indices, one can indicate that the soil samples are safe and can be used as a construction material without posing any significant radiological threat to the population. As shown in Table 4, the average value of the absorbed dose rate of Hadramout region’s granite samples is almost twice the world average value. So, the local people must avoid using these granite samples in the building construction, especially in the interior decorative materials of dwelling without radioactivity control. The average total effective dose rates (Deff) for granite samples is far lower than the 1.0 mSv/y recommended by  .
The natural radioactivity levels of 226Ra, 232Th and 40K have been measured in soil and granite rock samples of two regions in Yemen using gamma ray spectroscopy. The mean activity concentrations of 226Ra, 232Th and 40K in soil are 14.28, 25.78 and 566.05 Bq/kg, respectively, and for the granite rock samples the mean concentrations are 45,106 and 1235 Bq/kg, respectively. These values are compared with published values of other countries. The estimated hazard indices in soil samples are well lower than the recommended limit. Therefore, the present soil can be used as a construction or building materials without posing any radiological effect to the population. In granite rocks, the calculated dose rate was observed twice the world average value of 57 μSv/Y, which put the users of the granite rocks and people around the area on radiological hazard. This work has established background guideline on the natural radioactivity levels in the study area, which will provide a future reference for other studies.
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