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Abstract
In this study, 14 rock samples were collected from different sites in Yemen (Ma’rib, Al Jawf, Sana’a, Dhamar, Amran, Ibb, Al Bayda and Al Dalea). The samples were prepared for two gamma-ray techniques (passive and active techniques), natural radioactivity and neutron activation analysis.
For natural radioactivity, samples were prepared to be measured by HPGe. Samples were crushed. A 100 cc of each sample was put in a plastic container and sealed well. Sealed containers were kept for at least one month for secular equilibrium. HPGe detector with efficiency of 35% and 1.9 keV resolution was used. Each sample was counted over night (24 hrs). Spectra were collected and analyzed using Genie2k software. Counting repeated three times for each sample and the average was taken. Activity concentrations were calculated for 226Ra, 232Th series, and 40K isotope. Results show that samples 13 and 14 (Gneissose Granite-highly weathered) have the highest activity concentrations, whereas sample 7 (Albitite) has the lowest.
In general, results show that activity concentration of 226Ra (238U) ranges from 1.9±0.3 Bq/kg [sample7 (Albitite)] to 8797.2±21.8 Bq/kg [sample14 (Gneissose Granite-highly weathered)] with an average of 1008.6 Bq/kg. Also, activity concentration of 232Th ranges from 0.8 ± 0.2 Bq/kg [sample 7 (Albitite)] to 24984.1±40.1 Bq/kg [sample 14 (Gneissose Granite-highly weathered)] with an average of 2720.4 Bq/kg. In addition, activity concentration of 40K ranges from 2.2 ± 0.1 Bq/kg [sample 7 (Albitite)] to 2064.1 ± 39.4 Bq/kg [sample 14 (Gneissose Granite-highly weathered)] with an average of 912 Bq/kg.
Classifying samples to main groups according to their geological origin, the average activity concentrations of 226Ra, 232Th, and 40K, were 14.1±1.0, 19.1±1.3, and 490.6±6.8 Bq/kg in volcanic rocks; 34.0±1.9, 67.9±2.1, and 1153.7±12.5 in sedimentary rocks; 3468.8±13.8, 9398.1±21.8, and 958.9 ±24.8 Bq/kg in igneous rocks.
Consequently, the absorbed dose rate, radium equivalent activity, external hazard index and internal hazard index, are the highest for samples 13 and 14 (Gneissose Granite-highly weathered) and the lowest for sample 7 (Albitite).
Estimated absorbed dose rate ranges from 1.44 nGy/hr [sample 7 (Albitite)] to 20500.10 nGy/hr [sample 14 (Gneissose Granite-highly weathered)] with an average of 2282.84 nGy/hr. Radium equivalent activity ranges from 3.2 Bq/kg [sample 7 (Albitite)] to 44683.4 Bq/kg [sample 14 (Gneissose Granite-highly weathered)] with an average of 4969.12 Bq/kg. Internal hazard ranges from 0.01[sample 7 (Albitite)] to 144.45[sample 14 (Gneissose Granite-highly weathered)] with an average of 16.15. External hazard ranges from .01[sample 7 (Albitite)] to 120.67 [sample 14 (Gneissose Granite-highly weathered)] with an average of 13.42.
For neutron activation analyses which was carried out in Dubna Nuclear Research Reactor, Moscow, Russia, samples of about 0.2 g were packed in sealed polyethylene packs for short-lived elements which were irradiated for 3 min and cooled for 2-3 min. Spectra of induced ?-activity then measured for 5 and 10 min, respectively. Long-lived radionuclides were determined using activation with the epithermal neutrons. Primarily packed in aluminum cups, rock samples were repacked after irradiation for 4 days, and then were measured for the first time for 45 min since 4 days of cooling and for the second time for 90 minutes since 20 days of cooling.
The elements Sc, Cr, Fe, Co, Ni , Zn, As, Se, Br, Rb, Ag, Sb, Cs, Ba, La, Ce, Sm, Tb, Yb, Hf, Ta, W, Au, Th and U, were determined. Samples were irradiated for 4 d. After 4-5 days of decay the samples were repacked and then measured twice for medium and long-lived isotopes twice respectively.
Measuring time varied from 1 to 2 hrs. To determine the short-lived isotopes of Mg, Al, Cl, V, Mn, Cu, Al, In, and I. Samples were irradiated for 30 s and measured twice after 3-5 min and 20 min of decay for 5-8 and 20 min, respectively. To determine Cu, K, Na the same samples were re-irradiated for additional 30 min, and after 12-15 hrs of decay measured for 30 min.
Gamma spectra were measured using Ge-(Li) detectors with a resolution of 2.5 keV for the 60Co 1332.5 keV line, with an efficiency of about 6% relative to a 3x3” NaI detector for the same line. Data processing and element concentration determinations were performed. For long-term irradiation, single comparators of Au (1 ?g) and Zr (10 ?g) were used. For short-term irradiation a comparator of Au (10 ?g) was used.
Results show that the highest elemental concentrations are Zr (2261.1 ppm), then Mn (1113 ppm) in trace elements group. For marine origin elements group Mg (40000 ppm) is the highest, then Na (6980 ppm). For oil origin elements group V (133.5 ppm) is the highest, then Ni (85 ppm). And, for major elements group, Ca (309100 ppm) is the highest, then Al (56580 ppm), and Fe (20670 ppm).