Physics

MEASUREMENT OF SOIL-GAS RADON CONCENTRATION AND GEOGENIC RADON POTENTIAL MODELLING FOR SOUTHWEST NIGERIA

2024-05-23T12:02:50Z

MEASUREMENT OF SOIL-GAS RADON CONCENTRATION AND GEOGENIC RADON POTENTIAL MODELLING FOR SOUTHWEST NIGERIA FAJEMIROYE, Joseph Adesoji Ademola Radon-222 is a radioactive gas in the natural decay series of Uranium-238. It easily emanates from the soil to constitute radiological hazard and is the leading cause of lung cancer apart from smoking. High indoor radon buildup could occur in buildings sited over high radon-bearing bedrocks. Radon hazard, expressed as Geogenic Radon Potential (GRP), is due to a combination of soil-gas radon concentration ( ) and soil-air permeability ( ), both of which depend on bedrocks. Data on these two quantities over different bedrock formations and soil types in Southwest (SW) of Nigeria are very scarce resulting in limited knowledge on radon hazard and lack of requisite radon control guidelines. This study was designed to measure , determine GRP and model the distribution of GRP over different bedrocks of SW Nigeria. Measurements of were made using a calibrated real-time semiconductor radon monitor at a depth of 0.80 – 1.00 m in 150 randomly selected locations across 20 bedrocks in the six states of SW Nigeria. Saturated hydraulic conductivities of undisturbed soil samples taken from these locations were measured with a constanthead permeameter in order to determine . The GRP for each location was calculated from and and categorised using Neznal classification for radon hazard ratings. A Levenberg-Marquardt feed-forward-back-propagation artificial neural network was employed to develop a predictive model for . Data was randomly split in 70:15:15 for training, testing and validation, respectively, for six different architectures and the best was chosen following standard procedure. Goodness-of-Prediction (G), Average Validation Error , Mean Bias Error and Root Mean Square Error were used to determine performance and validation of the model. The and GRP maps were generated on existing geological map for SW region. The measured ranged . The ranged , while ranged . The GRP ranged . Sedimentary formation had highest of , while granitic bedrocks had highest and GRP of and , respectively. Radon hazard classification showed that , and of the sites were of low, medium and high radon hazard rating, respectively. Out of the 13 sites with high radon hazard rating, granitic and metamorphic bedrocks presented more sites (84.6%). The best performing architecture was 2 x 8 x 1. Performance indices of the model, yielded G of 73.5%, of 0.073, of 0.42 and of 4.62 kBqm-3. Validation indices yielded G of 86 , of , of and of 1 , indicating good model performance. Values of measured and GRP were used to generate maps which showed spatial distribution of low, medium and high radon hazard ratings. The values of measured soil-gas radon concentration and determined geogenic radon potential were highest in granitic bedrocks. The performance indices of the developed neural network model showed good reliability in predicting geogenic radon potential for southwest Nigeria.

SOIL-TO-PLANT TRANSFER FACTORS OF NATURAL RADIONUCLIDES OF THREE COMMON FOOD CROPS GROWN ON A TIN-MINING IMPACTED SOIL

2022-03-02T14:50:43Z

SOIL-TO-PLANT TRANSFER FACTORS OF NATURAL RADIONUCLIDES OF THREE COMMON FOOD CROPS GROWN ON A TIN-MINING IMPACTED SOIL ADESIJI, NKIRUKA EUNICE Tailings produced from tin-mining operations containing elevated levels of Natural Radionuclides (NRs; ) need to be disposed properly to prevent environmental contamination. The knowledge of the mobility of NRs in contaminated farmlands is important because of possible accumulation and their radiological implication in crops grown on such farmlands. The Transfer Factor (TF) of NRs is an important parameter for predicting migration and accumulation of radionuclides through the food chain. However, there is dearth of information on the TFs of NRs in the tropics. This study determined the TFs of NRs in three widely consumed food crops grown on tin mining-impacted soil and Committed Effective Dose (CED) to assess radiological hazards in Nigeria. Tin-tailings were collected from an abandoned tin-mining site in Alheri, Jos. Soil samples were collected from uncultivated, non-tin-mining site at the Botanical Garden of Redeemer’s University, along Lagos-Ibadan Expressway. Three soil sample groups were purposively formulated; group-A (non-tin-mining soil), group-B (tin-tailings) and group-C (tin-tailing and non-tin-mining soil). Cowpea (Vigna unguiculata) and maize (Zea mays) seeds were obtained from the Institute of Agricultural Research and Training and cassava stems (Manihot escalenta) from International Institute of Tropical Agriculture Ibadan. Ten planting pots were prepared for each plant per soil group. The seeds and stems were planted and harvested at their maturity periods. The activity concentrations (AC) of NRs in the formulated samples and plant compartments (seeds, tubers, stems, leaves and roots) were determined using sodium-iodide thallium activated gamma-detector. The TFs of the NRs and CEDs of the edible parts were evaluated using standard methods. Data were descriptive statistics and The mean AC (Bqkg-1) of 40K, 238U and 232Th obtained for the soil groups ranged from 179.65 ± 2.88 (group-A) to 3421.51 ± 3.64 (group-B); 90.35 ± 3.37 (group-A) to 1992.61 ± 1.55 (group-B), and 273.06 ± 5.37 (group-A) to 25232.30 ± 1.33 (group-B), respectively. The mean AC (Bqkg-1) of the plant compartments were 39.39 ± 26.67 (tuber; group-B) to 2400.17 ± 1791.18 (cowpea-leaf; group-A) for 40K; Below Detection Limit (BDL) (maize stems; group-A-B-C) to 717.90 ± 404.86 (cowpea-leaf; group-B) for 238U, and 89.05 ± 110.86 (tuber; group-C) to 15972.92 ± 453.97 (cowpea-seed; group-B) for 232Th. The calculated geometric mean of the TFs of 40K, 238U and 232Th, in cowpea ranged from 1.40 (1.15) (seed) – 9.67 (2.49)(leaf), 0.39 (1.18)(stem) – 2.34(1.23)(root) and 0.77(3.01)(seed) - 9.73(1.45)(leaf) for group-A; 0.05(1.88)(seed) - 0.18(3.09)(leaf), 0.02(3.31)(seed) – 0.33(1.53)(leaf) and 0.05(1.70)(stem) – 0.12(1.53)(root) for group-B; 0.21(2.37)(seed) – 2.56(1.38)(root); BDL(stem, leaf) – 0.21(1.18)(root) and 0.07(1.37)(stem) – 0.31(1.78)(root) for group-C. Those of maize were 0.65(1.31)(seed) – 3.61(1.38)(stem), BDL(seed, stem) – 1.00(5.18)(root) and 0.58(2.79)(seed) – 2.68(2.43)(root) for group-A; 0.03(2,14)(seed) – 0.07(1.88)(stem) – BDL(stem, leaf) – 0.20(2.45)(root) and 0.02(1.97)(stem) – 0.13(1.72)(root) for group-B; 0.23(3.31)(root) – 0.43(2.88)(stem), BDL(seed, stem) – 0.11(2.37)(root) and 0.015(4.00)(seed) – 0.12(2.59)(root) for group-C. And those of cassava were 0.74(1.75)(stem) – 1.46(2.64)(leaf), BDL(tuber) – 0.90(2.21)(leaf) and 0.49(1.87)(tuber) – 1.54(4.26)(leaf) for group-A; 0.01 (1.40)(stem) – 0.12(2.04)(leaf). BDL(tuber, stem, leaf) and 0.03(1.57)(stem) – 0.04(2.23)(tuber) for group-B; 0.11(1.50)(tuber)– 2.91(1.79)(leaf), 0.01(1.11)(stem) – 0.07(2.04)(leaf) and 0.006(2.51)(tuber) – 0.21(1.91)(leaf) for group-C. The transfer factors of the natural radionuclides were observed to be in the order cowpea  maize  cassava. Significant differences occurred in the TFs of theNRs among the soil groups. Cowpea exhibited the highest potential for possible phytoremediation of the natural radionuclides.Cassava tubers had the highest mean CEDs (mSv.y-1) (2.19 - 35.7) while cowpea seeds had the least (0.002 - 0.07). The CEDs of tuber and maize seeds exceeded the reference value (0.12 mSv.y-1) recommended by the United Nations. The transfer factor of the natural radionuclides varied across the food crops and soil groups and the cassava and maize grown on tin-mining impacted soil were of radiological concerns.

ATTENUATION OF RADIO COMMUNICATION SIGNALS BY RAIN AT MILLIMETER WAVE BAND AT SOME LOCATIONS IN NIGERIA

2022-03-02T14:45:46Z

ATTENUATION OF RADIO COMMUNICATION SIGNALS BY RAIN AT MILLIMETER WAVE BAND AT SOME LOCATIONS IN NIGERIA IBE, OSITA Millimeter Wave (MMW) radio systems operating at 30 to 300GHz band provides higher bandwidth, frequency reuse and communications security but suffers greatly from attenuation by rain. The design of radio communication equipment has been based on predicted rain rate and attenuation from the International Telecommunication Union-Radio (ITU-R) model. However, such equipment fails in the tropics because rainfall in this region is more intense with larger drop sizes than those in the temperate regions on which the ITU-R model was based. Thus, Rain Rates (RR) and attenuation information from the tropics are needed for the design of radio communication equipment for the region. This study was therefore aimed at estimating rain induced attenuation of radio communication signals at some locations across Nigeria. The RR data was obtained from 14 automatic weather stations across eco-climatic zones of Nigeria.The equipment measures rainfall at 1- min (2 locations) and 5-min (12 locations) integration time. Lavergnat and Gole model was used to convert the 5- min to 1-min RR, while logarithmic scale was used to convert the RR to exceedance time percentages (0.001 to 1%). These RR were compared with those predicted by the ITU-R model using the MatLab RR statistics. The specific rain attenuation   R  for Horizontal ( RHP  ) and Vertical ( R p  v ) Polarisation at MW Frequencies (MWFs) were estimated at 0.01% RR (R0.01) using the power law     0.01 R  kR relationship where k and α are regression coefficients. These estimated values were then compared with the ITU-R predicted values. The Path Attenuation (PA) at 20 km (at 0.01%, A0.01) Path Length (PL) was computed using A0.01   Rdeff dB to determine the Clear Signal Bands (CSBs) at MWFs (deff is the effective path length). The mean annual 1-minute RR (mm/hr) ranged from 22.78 in Mid Altitude Savanna (MAS) to 116.67 mm/hr in Southern Guinea Savanna (SGS). Throughout the country, the ITU-R predicted RR ranged from 87.10 mm/hr in the MAS to 91.60 mm/hr in the SGS. The highest value of R  for all 14 locations occurred at 120 and 150 GHz MWFs and RHP  > RVP  . The estimated maximum RH p  vs ITU-R values at 120 GHz were: 35.05, 29.85; 26.96, 17.90; 30.94, 18.01; 11.85, 28.86; 29.78, 30.56; and 35.91, 33.52 dB/km at SGS, Sudan Savanna (SS), Northern Guinea Savanna (NGS), MAS, Derived Savanna (DS) and Humid Forest (HF), respectively. The corresponding RVp  vs ITU-R values were: 34.64, 29.52; 26.68, 17.74; 30.60, 17.85; 11.77, 28.55; 29.46, 20.00; and 35.48,33.14 dB/km, respectively at the 0.01%. At 150 GHz, maximum RH p  vs ITU-R were: 34.81, 29.74; 26.93, 18.03; 30.81, 18.14; 12.05, 28.78; 29.68, 30.43 and 35.63,33.32 dB/km, respectively. The corresponding maximum RVp  vs ITU-R were: 34.50, 29.50; 26.72, 17.93; 30.55, 18.04; 12.00, 28.56; 29.44, 30.18 and 35.32, 33.03 dB/km, respectively. The CSBs at 20 km PL were estimated to be 40 and 45 GHz across the zones, while the predicted value by ITU-R included 150 GHz and overlaps with 45 GHz band. The estimated PA at 40 and 45 GHz ranged from 64.65-206.85 dB and 71.40-219.45 dB, respectively; the ITU-R predicted range of PA at 40, 45 and 150 GHz were 90.01-192.18 dB, 107.79-204.43 dB and 135-150 dB, respectively. vii The rain rates across the eco-climatic zones have been determined. The estimated specific, path attenuation and clear signal bands varies from the International Telecommunication Union-Radio predictions for Nigeria.

OPTIMISATION OF A SEMI-EMPIRICAL MODEL FOR ACCURATE DETERMINATION OF EXCITATION ENERGIES AND ABSORPTION SPECTRA OF QUANTUM DOTS

2022-03-02T14:41:11Z

OPTIMISATION OF A SEMI-EMPIRICAL MODEL FOR ACCURATE DETERMINATION OF EXCITATION ENERGIES AND ABSORPTION SPECTRA OF QUANTUM DOTS OYENIYI, EZEKIEL Quantum dots are nanomaterials that have several potential applications including the production of efficient solar cells. Accurate theoretical studies of excitation energies and absorption spectra of quantum dots are essential for harnessing such potentials. The existing high-level ab-initio methods for obtaining excitation energies and absorption spectra are computationally expensive for quantum dots. However, the semi-empirical methods, including the Intermediate Neglect of Differential Overlap for spectroscopy (INDO/s) model, are computationally cheap but are generally less accurate. Unlike some ground-state semi-empirical methods, INDO/s has not attracted significant attention to improving its level of accuracy because of some difficulties associated with optimising its parameters. Therefore, this research was aimed at developing an improved INDO/s model that will be computationally cheap and capable of producing accurate excitation energies and absorption spectra for quantum dots. A semi-empirical Hamiltonian based on INDO/s was parameterised with benchmark excitation energies from Equation-Of-Motion Coupled-Cluster Singles Doubles (EOM CCSD) for Si, S, Cd and Zn diatomics at different interatomic separations. The Mean Absolute Errors (MAE) were calculated for different sets of parameters and the opti mised set of parameters were those with the least MAEs. The optimised model was called optimised for excitation Intermediate Neglect of Differential Overlap (oeINDO). The oeINDO was validated by computing the MAEs of the oeINDO and INDO/s excita tion energies and absorption spectra maxima for Sin, Sn, Znn ,Cdn, (ZnS)n and (CdS)n (n is the number of atoms) clusters. The validation was carried out relative to EOM-CCSD for small clusters (n<6) and Time-Dependent Density Functional Theory (TDDFT) for large clusters (n ≥ 6). All computation times were recorded. The oeINDO was then employed to predict the absorption spectra of Si, S, Zn, Cd, ZnS, and CdS quantum dots, and the optimal size of CdS and ZnS quantum dots for solar cell applications. The optimised parameters obtained for Si, S, Zn and Cd diatomics had MAEs 0.21, 0.19, 0.23,and 0.29 eV, respectively. The oeINDO produced excitation energies with MAEs 0.18, 0.56, 0.25, 0.22 eV for small Si, S, Zn, and Cd clusters, respectively, and MAEs 0.22, 0.36, 0.15, 0.24, 0.36 and 0.23 eV, for large Si, S, Zn, Cd, ZnS, and CdS clus i ters, respectively. The unoptimised INDO/s however, produced excitation energies with MAEs 1.23, 1.29, 0.70, and 1.23eV for small Si, S, Zn, Cd clusters, respectively, and MAEs 1.05, 2.51, 2.49, 0.63, 0.76 and 1.04eV for large Si, S, Zn, Cd, ZnS, and CdS clusters, respectively. Also, the MAEs of oeINDO and INDO/s absorption spectra max ima relative to those from TDDFT were 0.41eV and 1.49eV, respectively. The results showed that oeINDO agreed reasonably well with the benchmarks and it was more ac curate than INDO/s. The time of computing with oeINDO (0.08 minutes) was found to be less than a hundredth of the time utilised for EOM-CCSD (2946.51 minutes). The oeINDO predicted a red-shift in the quantum dots absorption spectra with an increase in dot size. It also predicted Si, Zn and Cd dots to be metallic. The 1.2 nm and 1.4 nm spherical-like CdS and ZnS quantum dots, respectively, were found to be theoretically optimal for solar cell applications. The improved INDO/s was computationally cheap and capable of producing more accu rate excitation energies and absorption spectra for quantum dots.