Application of a mercuric iodide detector for the exploration and exploitation of niobium deposits in remote areas

The comparison of an ambient temperature mercuric iodide spectrometer (HgI₂) with a conventional cooled Si(Li) spectrometer is presented for the determination of niobium in Venezuelan laterites in the laboratory. The energy resolution of the HgI₂ spectrometer was only about 590 eV (FWHM) at 5.9 keV compared to about 150 eV for the conventional Si(Li) spectrometer. The efficiency was about 5 times lower in the region of the Nb_K peak as compared to the Si(Li) detector. Even though its energy resolution and efficiency is much poorer than the conventional Si(Li), the results it produced for the determination of niobium were acceptable for use in the exploration and exploitation of a niobium-rich deposit such as Cerro Impacto, Estado Bolívar, Venezuela.     

Distribution of137Cs,40K,238U and232Th in soils from Northern Venezuela

More than one hundred undisturbed soil samples from Northern Venezuela and the islands of Margarita and Los Roques have been analyzed for¹³⁷Cs,⁴⁰K,²³⁸U and²³²Th by -ray spectroscopy. The specimens were taken from between 5–10 cm below the earth's surface. Thus, they are valid not only for the¹³⁷Cs deposition studies but also for the estimation of the natural -ray dose from primordial radionuclides that form the terrestrial component. The concentration of⁴⁰K was directly determined from its 1461 keV -ray, while those of¹³⁷Cs,²³⁸U and²³²Th were performed using a -ray from one of their daughter radionuclides: the 661 keV -ray of^137mBa for¹³⁷Cs, the 1760 keV -ray of²¹⁴Bi for²³⁸U and the 2620 keV -ray of²⁰⁸Tl for²³²Th. Finally, the concentration values were compared with those of global estimates.     

Uncertainty limits for certified values of standard reference materials

Summary Uncertainty limits usually employ the 95% confidence limits and should include allowances for the uncertainty of known sources of systematic errors of the measurements. But in many cases, when an interlaboratory comparison study is used for certification purposes of standard reference materials, only the 95% confidence limits are considered and given as the uncertainty limit. In this work, we shall show the results as the number of mean values (n) reported vary and only the 95% confidence limits are considered, using as an example the intercomparison study on IAEA/SL-3/lake sediment sample. And finally, we shall compare the results if only the 95% confidence limits are considered with those of the standard deviation and propose when n is large that the standard deviation should be employed instead of the 95% confidence limits as the uncertainty limits, if the other systematic and random errors are not included.     

A semi-empirical approach to determine gamma activities in environmental cylindrical samples

A semi-empirical method to determine radionuclide concentrations in large environmental samples without the use of reference material and avoiding the typical complexity of Monte-Carlo codes is proposed. The calculation of full-energy peak efficiencies was carried out from a relative efficiency curve (obtained from the gamma spectra data), and the geometric (simulated by Monte-Carlo), absorption, sample and intrinsic efficiencies for energies between 130 and 3000 keV. The absorption and sample efficiencies were determined from the mass absorption coefficients, whereas the intrinsic efficiency was approximated by an empirical function. The deviations between calculated and experimental efficiencies for a reference material in most cases are less than 10%. Radionuclide activities in marine sediment samples calculated by the proposed method and by the experimental comparative method were not significantly different. This new method can be used for routine environmental monitoring when uncertainties up to 10% are acceptable.     

Application of an americium-241 source for the determination of barium,lanthanum and cerium in lateritic material by X-ray fluorescence

Barium, lanthanum and cerium were determined by excitation using a 100 mCi²⁴¹Am radioisotope source with the utilization of the K peaks. This work forms part of the UNESCO Project IGGP-129. This work includes data from the MS Thesis of D. ADAMES.     

Application of the relative uranium-series disequilibrium in soil to locate and/or confirm precisely active fault traces: A new technique

Fault traces have been previously located from measurements of ²²²Rn in soils taken a constant soil-depth across the fault trace. In this paper, we have studied the uranium-series disequilibria of the ²²⁶Ra, ²²²Rn (gas) and ²¹⁴Bi radionuclides, not only for their horizontal spatial patterns across the fault trace, but also for their vertical spatial patterns near and at the fault trace itself. Radon-222 activities in the soil-gas were measured on-site with a radiation monitor and a Lucas cell. Radium-226 and ²¹⁴Bi were determined in soil samples in the laboratory by gamma-ray spectroscopy. A new technique employing the measurement of ²²²Rn versus soil-depth shows a decrease in ²²²Rn activity at the fault trace due to the much higher soil-gas permeability as a result of the fractured soil, as well as relative larger uranium-series disequilibria, in respect to an increase in ²²²Rn activity at normal sites, where the soil is not fractured. Finally, it is suggested that fault trace detection could possibly also be performed by measuring ²¹⁴Bi in surface soils (0-100 cm) along a transect.     

The spatial distribution of cesium-137 in surface soils in the Gran Sabana and Sierra de Pacarima (Venezuela)

More than fifty surface (2-10 cm) soil samples were collected in the Gran Sabana and Sierra de Pacarima, State of Bolivar (Venezuela) and analyzed for their ¹³⁷Cs activity. The measurements were performed by high resolution gamma-ray spectrometry employing a hyperpure germanium detector with an energy resolution of about 1.8 keV for the 1.33 MeV line of ⁶⁰Co and a counting efficiency greater than 25%. About 500 ml portions of dried soil samples with less than 85 mm particle size were counted for 20,000 seconds and the data were collected, stored and analyzed with an IBM-compatible computer. The concentrations of ¹³⁷Cs in most samples were below the detection limit of 1.2 Bq/kg, but many of these soil samples could have been disturbed not only by man but the nature itself, for example, by the large amount of rainfall in the wet season. Only 15% of the soil samples were determined to have values equal or greater than 2 Bq/kg and only two of these were greater than 10 Bq/kg. The study area was divided into five zones, which had different environmental characteristics: climate, annual average rainfall, temperature ranges, elevations (m.a.s.l.), soil types and vegetation. Finally, it was concluded that the high ¹³⁷Cs activities could be associated with the areas of higher annual rainfall and with very humid pre-mountainous climate rather than with humid tropical climate. This revised version was published online in July 2006 with corrections to the Cover Date.     

A simple and rapid method to locate fault traces by measurements of 214Bi in soils

A simple and rapid technique is presented to determine the relative counts of ²¹⁴Bi in surface soils to locate active fault traces of the El Pilar Fault in the state of Sucre, Venezuela. The method employes a portable differential gamma-ray spectrometer on site with 300 seconds of measuring time. Three transects across the El Pilar fault of very different geological aspects were studied. One of the advantages of this technique in respect to determining ²²²Rn in soil-gas is that no soil-gas probes are required to be inserted in the soil and it is not necessary to know the appropriate depth. Finally, it has been suggested that measurements of 1000 seconds would be preferred rather than 300 seconds for future studies even though this would limit the number of measurements to about 20 per day.This revised version was published online in November 2005 with corrections to the Cover Date.     

Anomalously high values of cesium-137 in soils on the Península de Paraguana (Venezuela)

The activity of ¹³⁷ Cs in surface soils (2-5 cm) was determined at twenty-one sampling sites along the northwestern and eastern coast of the Paraguana peninsula (Venezuela), as well as, at nine locations, between 95 and 535 m.a.s.l. on Cerro Santa Ana. The measurements were performed by high-resolution gamma-ray spectroscopy employing a compatible IBM computer. Most of the values were much higher than those found along the coastline of the mainland; four sites were found to be anomalously high, with ¹³⁷ Cs values greater than 10 Bq/kg. It is difficult to explain these anomalous ¹³⁷ Cs values by geographical or climatological factors since there is little rainfall here and the clouds and fog are rarely if never present along the coast of the peninsula. Possibly, some mechanism of the mist that is blown ashore could explain these anomalies. The values of the ¹³⁷ Cs versus altitude on the Cerro Santa Ana show an increase of two or three times at 500 m.a.s.l. level, thus we have concluded that the base of the clouds was at this height when the fallout was directly deposited by condensation in this cloud forest. These results in the Cerro Santa Ana cloud forest are similar to those of other cloud forests along the Venezuelan coast, but the altitude (m.a.s.l.) of the base of the clouds here are much lower.     

Distribution of <Superscript>137</Superscript>Cs, <Superscript>40</Superscript>K, <Superscript>232</Superscript>Th and <Superscript>238</Superscript>U in coastal marine sediments of Margarita Island,Venezuela

This work presents the results of ¹³⁷Cs, ⁴⁰K, ²³²Th and ²³⁸U concentration (Bq kg⁻¹) values in coastal marine sediments collected from 38 sites along the coastline of the island of Margarita, Venezuela. The purpose was to determine baseline values for these radionuclides in surface marine sediments and to detect if there were any anomalously high concentration values. Only three of the 38 sediments analyzed had measurable values above the detection limit of 0.9 Bq kg⁻¹ for ¹³⁷Cs and the highest only being 1.4 Bq kg⁻¹. While, the concentration (Bq kg⁻¹) ranges for the primordial radionuclides, ⁴⁰K, ²³²Th and ²³⁸U were as follows: 12.2–211.7, <1.5–9.8 and <4.4–20.7, respectively. These concentration ranges for the primordial radionuclides can be considered as baseline values for surface marine sediments for areas that are considered not polluted by man or contaminated by nature. Finally, the concentration range of ¹³⁷Cs can also be employed as baseline values, which only seem to have been the result of the atmospheric testing of nuclear weapons in the past.