The Primordial Radionuclides Activity Concentrations and Associated Minerals in Rocks from Selected Quarries in Northern Uganda

The primordial radionuclides (PR) activity concentrations and associated minerals in 20 rock samples from Gulu and Amuru district quarries were determined using Na (Tl) gamma ray spectrometer and Transmitted Light Microscope (TLM). These were carried out at Physics and Geology Departments of Makerere University, Kampala in Uganda. The PR activity concentrations, the gamma dose rates, and the radiological hazard indices were all determined to assess the radiological hazards and risks of the exposure on the quarry workers and on the inhabitants. The minerals were obtained to determine whether minerals containing PR were highly radioactive. The computed PR activity concentrations from the five quarries ranged from 4.1 ± 1.7 to 207.6 ± 6.5 for 238U, 7.0 ± 2.0 to 452.0 ± 2.3 for 232Th, and 8.6 ± 2.0 to 2055.4 ± 73.1 Bq kg-1 for 40K, respectively with averages of 57.1 ± 4.7, 122.3 ± 11.1, and 914.2 ± 30.3 Bq kg-1. These values were found to be above the world limit of 33.0, 45.0 and 420.0 Bq kg-1 by 1.7, 2.7, and 2.2 factors. The computed average values of absorbed gamma dose rates (D), annual effective dose equivalent outdoor (Eo), excess lifetime cancer risk (ELCR), radium equivalent activity (Raeq), external hazard index (Hex), internal hazard index (Hin) and gamma index (I) for the five quarries were 134.58 ± 10.13 nGy h-1, 0.34 ± 0.03 mSv yr-1, 1.07 ± 0.09 x 10-3, 294.08 ± 17.88 Bq kg-1, 0.80 ± 0.06, 0.93 ± 0.05 and 1.35 ± 0.09, respectively. The computed average values of D, Eo and ELCR for the five quarries were above the world limit by factors of 2.2, 4.9, and 3.7, respectively. Therefore, quarry workers and the inhabitants were exposed to PR hence significant health risks associated with induced cancer and gene mutation. However, average values of Raeq, Hex, and Hin were found to be below the world limit except for one of the quarry sites. Although the studied rocks could be used as building materials without causing external diseases and respiratory infections, the average value of, I, showed that studied rocks should be used in moderation to limit the radiological hazards except for Palukere rocks. Mineralogical analyses has also shown the presence of quartz, potassium feldspar, plagioclase, biotite, muscovite, and opaque minerals. Quartz was the major mineral while the rest were accessories. The results of this study could be used by different institutions such as Uganda Atomic Energy Council (AEC) that protects the environment from dangers resulting from ionizing radiation. This body could assist to advise quarry workers on good practices. The National Environmental Management Authority (NEMA) that controls environmental systems should ensure that Gulu and Amuru districts work in line with set principles on environment, and where necessary to enforce the quarry by-laws and ordinances. To urge the Directorate of Geological Survey and Mines (DGSM) to also conduct geological studies for valuable radioactive minerals in the study areas of Gulu and Amuru districts. The Uganda Ministry of Health (MoH) experts could also carry out additional studies to establish the correlation of cancer infections and quarry works. This would inform the government on corrective measures that protects the population of the region.

Natural Radioactivity Levels and Radiogenic Heat Production in River Sediments from Gulu and Amuru Districts, Northern Uganda

The activity concentrations (AC) of 238U, 232Th and 40K in sediments from Gulu and Amuru districts, Uganda were determined using NaI(Tl) detector at Physics Department, Makerere University, Kampala to obtain; radioelement concentrations (RC), radiogenic heat production (RHP) and associated heat flow (Hf). The AC varied from (36.1 ± 2.3 - 261.2 ± 15.3) for 238U, (97.4 ± 13.5 - 334.2 ± 36.6) for 232Th, and (47.5 ± 3.9 - 1442.3 ± 58.9) Bq kg-1 for 40K, with averages of 89.6 ± 6.3, 168.6 ± 17.9, and 275.4 ± 14.0 Bq kg-1, respectively, above the world limits of 35, 30, and 400 Bq kg-1, for 238U, 232Th and 40K. The RC varied from (2.9 ± 0.2 - 20.8 ± 1.2) ppm for 238U, (23.9 ± 3.3 - 82.0 ± 9.2) ppm for 232Th, and (0.2 ± 0.01 - 3.2 ± 0.1) % for 40K, with averages of 7.1 ± 0.5 ppm, 41.3 ± 4.3 ppm, and 1.1 ± 0.05%, respectively. The 238U and 232Th averages were above the Earth’s crust values of 3 ppm and 12 ppm, respectively. The 40K average is below the Earth’s crust value of 2.33%.  The RHP varied from (2.5 ± 0.6 - 10.2 ± 2.6) µW m-3 with an average of 4.5 ± 1.1 µW m-3, above the world average of 4µW m-3. High and moderate RHP, each account for 50% of the samples. The overall RHP mainly depended on 232Th amounts, with 60% contribution. However, an increase in the AC of 238U, 232Th and 40K reflected the integrated effect of RHP. The Hf varied from (19.6 ± 1.0 - 80.0 ± 4.0) mW m-2 with an average of 35.3 ± 1.8 mW m-2. The high RHP and Hf values indicates feasibility for geothermal exploration. This calls for further studies to validate these findings

The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE - Minimizing cyclotron line biases

We present an analysis of X-ray spectra of the high-mass X-ray binary 4U 0115+634 as observed with Suzaku and RXTE in 2011 July, during the fading phase of a giant X-ray outburst. We used a continuum model consisting of an absorbed cutoff power law and an ad hoc Gaussian emission feature centered around 8.5 keV, which we attribute to cyclotron emission. Our results are consistent with a fundamental cyclotron absorption line centered at ∼10.2 keV for all observed flux ranges. At the same time we rule out significant influence of the 8.5 kev Gaussian on the parameters of the cyclotron resonant scattering feature, which are not consistent with the cyclotron line energies or the depths of previously reported flux-dependent descriptions. We also show that some continuum models can lead to artificial line-like residuals in the analyzed spectra, which are then misinterpreted as unphysically strong cyclotron lines. Specifically, our results do not support the existence of a previously claimed additional cyclotron feature at ∼15 keV. Apart from these features, we find for the first time evidence for a He-like Fe XXV emission line at ∼6.7 keV and weak H-like Fe XXVI emission close to ∼7.0 keV.We acknowledge funding by the European Space Agency under contract number C4000115860/15/NL/IB, by the Bundesministerium für Wirtschaft und Technologie under Deutsches Zentrum für Luft- und Raumfahrt grants 50OR0808, 50OR0905, 50OR1113, and 50OR1207, and by the Deutscher Akademischer Austauschdienst. MTW is supported by the NASA Astrophysical Data Analysis Program and the Chief of Naval Research. VG is supported through the Margarethe von Wrangell fellowship by the ESF and the Ministry of Science, Research and the Arts Baden-Württemberg. SMN and JMT acknowledge Spanish Ministerio de Ciencia, Tecnología e Innovación (MICINN) through the grant ESP2016-76683-C3-1-R and ESP2017-85691-P, respectively

Physical and mechanical properties of porcelain tiles made from raw materials in Uganda

This study was intended to determine the physicomechanical properties of porcelain tiles made from raw materials in Uganda, hitherto not used for the same. The raw materials were mixed in proportions of 40–60% clays, 30–40% feldspar and 10–30% sand, the goal was to identify a mixture with properties that meet the ISO 13006 standards after firing. Test tile samples were formulated at a pressure of 40 MPa and fired from 1050 to 1250 °C in steps of 50 °C. A firing rate of 40 °C per minute and a dwell time of 2 h were applied. The microstructures and phase analysis of the fired samples were studied using a scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. The XRD analysis revealed mullite and quartz as crystalline phases present in the fired bodies. At 1150 °C, the SEM micrograph depicted few, short and thick mullite fibers. At 1200 °C, the mullite population increased. The nature of the fibers (long, fine and more interlocking) resulted in an increase in flexural strength. A further rise in temperature had no significant change on the nature of the fibers and flexural strength of the samples. The best properties; flexural strength (33 MPa) and water absorption (0.08%) were exhibited at 1200 °C by samples with 30–40% kaolin, 30–40% feldspar, 20% ball clay and 10% sand. The results indicate that these samples are comparable to the ISO 13006 standards for porcelain tiles which are ≥ 35±2 MPa flexural strength and <0.5% water absorption. The other mix proportions did not meet these standards, specifically in terms of flexural strength (<25 MPa)

Variability and accuracy of Zenith Total Delay over the East African tropical region

The Global Navigation Satellite System (GNSS) can be used to derive accurately the Zenith Tropospheric Delay (ZTD) under all-weather conditions. The derived ZTDs play a vital role in climate studies, weather forecasting and are operationally assimilated into numerical weather prediction models. In this study, variations and statistical analysis of GNSS-derived ZTD over the East African tropical region are analysed. The data is collected from 13 geodetic permanent stations for the period of 4 years from 2013 to 2016. The 13 stations consist of 5 International GNSS Service (IGS) stations plus 8 stations as follows: 4 Africa Array stations and 4 Malawi Rifting stations from Uganda, Kenya, Tanzania and Rwanda. The ZTD time series were processed using goGPS software version 1.0 beta1, a MATLAB based GNSS processing software, originally developed for kinematic applications but recently re-engineered for quasi static applications. The annual variation of the ZTD time series was investigated using Lomb Scargle periodograms. The semi-annual frequency has the dominant power in subregion 1 (latitudes 4 °S and 4 °N) and the annual frequency has the dominant power in subregion 2 (latitudes 12 °S to 4 °S). The highest ZTD estimates occur during the rainy seasons, at all stations, and the lowest estimates occur during the dry seasons. The results also show that the ZTD estimates are largest at stations located at low elevation (regions close to the Indian Ocean). The derived ZTDs are compared to the values derived from the GIPSY-OASIS via Jet Propulsion Laboratory (JPL) online Automatic Precise Positioning Service (APPS) and the Unified Environmental Modelling System (UEMS) numerical weather prediction (NWP) model. The comparison of goGPS and APPS ZTD at the 13 stations shows an overall average bias, Root Mean Square (RMS) and standard deviation (stdev) of −0.9 mm, 3.2 mm and 3.0 mm respectively, with correlation coefficients ranging from 0.974 to 0.999. The comparison of goGPS ZTD against UEMS NWP ZTD at 8 selected stations shows average bias, RMS and stdev of −12.4 mm, 22.0 mm and 17.6 mm respectively, with correlation coefficients ranging from 0.802 to 0.974. The agreement between the GPS ZTD and the NWP ZTD indicates that goGPS ZTD can be assimilated into NWP models in the East African region.</p

Variability and accuracy of Zenith Total Delay over the East African tropical region

The Global Navigation Satellite System (GNSS) can be used to derive accurately the Zenith Tropospheric Delay (ZTD) under all-weather conditions. The derived ZTDs play a vital role in climate studies, weather forecasting and are operationally assimilated into numerical weather prediction models. In this study, variations and statistical analysis of GNSS-derived ZTD over the East African tropical region are analysed. The data is collected from 13 geodetic permanent stations for the period of 4 years from 2013 to 2016. The 13 stations consist of 5 International GNSS Service (IGS) stations plus 8 stations as follows: 4 Africa Array stations and 4 Malawi Rifting stations from Uganda, Kenya, Tanzania and Rwanda. The ZTD time series were processed using goGPS software version 1.0 beta1, a MATLAB based GNSS processing software, originally developed for kinematic applications but recently re-engineered for quasi static applications. The annual variation of the ZTD time series was investigated using Lomb Scargle periodograms. The semi-annual frequency has the dominant power in subregion 1 (latitudes 4 °S and 4 °N) and the annual frequency has the dominant power in subregion 2 (latitudes 12 °S to 4 °S). The highest ZTD estimates occur during the rainy seasons, at all stations, and the lowest estimates occur during the dry seasons. The results also show that the ZTD estimates are largest at stations located at low elevation (regions close to the Indian Ocean). The derived ZTDs are compared to the values derived from the GIPSY-OASIS via Jet Propulsion Laboratory (JPL) online Automatic Precise Positioning Service (APPS) and the Unified Environmental Modelling System (UEMS) numerical weather prediction (NWP) model. The comparison of goGPS and APPS ZTD at the 13 stations shows an overall average bias, Root Mean Square (RMS) and standard deviation (stdev) of −0.9 mm, 3.2 mm and 3.0 mm respectively, with correlation coefficients ranging from 0.974 to 0.999. The comparison of goGPS ZTD against UEMS NWP ZTD at 8 selected stations shows average bias, RMS and stdev of −12.4 mm, 22.0 mm and 17.6 mm respectively, with correlation coefficients ranging from 0.802 to 0.974. The agreement between the GPS ZTD and the NWP ZTD indicates that goGPS ZTD can be assimilated into NWP models in the East African region.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Water Resource

Performance of ERA5 data in retrieving Precipitable Water Vapour over East African tropical region

The accuracy of surface meteorological measurements is vital to derive accurate Global Positioning System (GPS) Precipitable Water Vapour (PWV) data. However, in absence of surface meteorological data, data from Numerical Weather Prediction Models (NWP) are used. The accuracy of these models varies depending on the model, region, season and other climatic conditions. In this study, GPS data for derivation of PWV is collected from 13 geodetic permanent stations for the years 2013 to 2016. Five out of 13 GPS stations are equipped with meteorological sensors. The interpolated European Centre for Medium-Range Weather Forecasts (ECMWF) 5th Re-Analysis (ERA5) dataset at these locations is first validated using the meteorological data from these sensors. The assessment shows that the average root mean square errors (RMSE) of surface pressure and temperature values are about 0.72 hPa and 1.04 K respectively. PWV determined also requires information on the weighted mean temperature (Tm); the Tm at the 13 GPS stations was also evaluated using three methods: the temperature and humidity profiles from ERA5 (ERA5 Tm), as well as the Tm derived from Bevis and Yao Tm - Ts relationships. The Tm derived from ERA5 was used as a reference to assess the errors of the Bevis and Yao Tm. The comparisons show that the average RMSE values of Tm derived from the two methods are 2.05 K and 2.02 K respectively. The GPS PWVs calculated using the ERA5 interpolated data (GPS ERA5 PWV) were compared to GPS PWV calculated using the sensor meteorological data (GPS MET PWV) at the 5 stations. The GPS ERA5 PWV values were used as the reference. The RMSE values of the two PWV datasets are in the range of 0.19 mm to 0.90 mm with overall average value of 0.44 mm. Also the computed ERA5 PWV data are compared to GPS ERA5 PWV at the 13 stations. ERA5 PWV values were used as the reference and the RMS errors of the two PWV values are in the range 1.35 mm to 2.25 mm with the overall average value of 1.66 mm. Regarding the seasonal variations, the stations between latitudes 4 °S and 4 °N and latitudes 12 °S to 4 °S have the highest PWV values in spring and summer while the lowest values occur in summer and winter respectively.Water Resource