Melquíades Casas

Universidad de Cádiz | UCA · Department of Applied Physics

This report presents a new methodology to isolate and measure ²¹⁰Po, as well as uranium and thorium isotopes. This new methodology reduces the standard time of operation, the minimum amount of chemical reagents and the quantity of resin used in comparison with other standard and well-established procedures for alpha spectrometry. Thus, the amount o...

This study was focused on the historical evolution of the waste management policy carried out by the Spanish industry devoted to titanium dioxide pigments manufacturing for minimising its environmental impact. This challenge was achieved by modifying the original chemical process and converting the originally dissolved sulphate and sulphuric acid p...

A new method for the determination of radium quartet (223,224,226,228Ra) in environmental samples by alpha-particle spectrometry with PIPS detectors is described. This uses ²²⁵Ra as yield tracer, in equilibrium at the beginning with ²²⁹Th. Thorium is removed from the sample by using AG1X8 anion-resin, and then radium isotopes are isolated and purif...

Numerous biogeochemical studies in the Southern Ocean have focused on the importance of trace metals in controlling primary production (e.g. Fe, Co) or as potentially toxic to the ecosystem (e.g. Ag, Cd). However, the reasons why Fe limitation is prevalent in the Southern Ocean or the concentrations of non-biogenic elements in surface waters are hi...

The Sancho Reservoir (SWSpain)was built in 1962, about the time of maximum137Cs fallout, and it has been affected by acid mine drainage (AMD) particularly since the mining cease in 2001. This is a unique scenario for studying the radiogeochronological fingerprints in AMD-affected sediments deposited over the former flood plain. A sediment core samp...

A generic theoretical methodology for the calculation of the efficiency of gamma spectrometry systems is introduced in this work. The procedure is valid for any type of source and detector and can be applied to determine the full energy peak and the total efficiency of any source-detector system. The methodology is based on the idea of underlying p...

This paper relates the calibration of a low background gas-flow proportional counter. This calibration has been used to determine low activity of ²³⁴Th in coastal water samples. Two methods were used to prepare calibration samples: Evaporation and Electrodeposition. First method was rejected due to the lack of reproducibility because the different...

In this study, we reconstruct the recent environmental evolution of the inner Cadiz Bay using sedimentary records reaching back as far as AD 1700. We report lithological descriptions of the sediments and extensive mineralogical and geochemical analyses. An extraction technique that identifies different Fe phases provides an assessment of diagenetic...

Results of ultrafiltration experiments performed in acid waters from the Río Tinto acid mine drainage system in order to evaluate the role of colloids under acid mine drainage conditions indicate that for U and Th, a constant permeation model is applicable and does produce results that are compatible with the predicted behavior of these elements un...

The activity ratios of (234)U/(238)U were used to investigate processes of subterranean groundwater discharge into coastal marine waters in a study location at Bay of Cádiz (southwest Spain). Marine waters in the bay and surrounding open ocean exhibited U concentrations of 3.4 ± 0.1 μg/L and activity ratios of 1.15 ± 0.01, in agreement with the exp...

Castañar cave is a subterranean site with an outstanding natural environmental radioactivity. The maintaining of high radon activity of cave air and the detection of spatially anomalies of this gas in some cave emplacements, suggests that some natural geochemical processes are involved on the mobilization of radioactivity sources to cave environmen...

a b s t r a c t A numerical model to simulate the dispersion of particle-reactive tracers in Cádiz Bay (SW Spain) has been developed. It includes a hydrodynamic submodel to provide water currents, a sediment transport submodel, which provides suspended matter concentrations and sedimentation rates and the pollutant dispersion model. Pollutant excha...

Activity concentrations of several man-made radionuclides (such as 131I, 132I, 132Te, 134Cs and 137Cs) were detected along the Iberian Peninsula from March 28th to April 7th 2011, including the sampling stations, “El Carmen”, located at the Campus of the University of Huelva (37.28 N, 6.91 W) (Southwest Spain). The highest activity concentrations o...

In order to investigate the radiological hazard of naturally occurring radioactive material (NORM) and man-made (137)Cs radionuclide in the Bay of Cádiz, 149 samples of sediments have been analysed. Activity concentration in all the samples was determined using a HPGe detection system. Activity concentrations values of (226)Ra, (232)Th, (40)K and (...

High activity concentrations of several man-made radionuclides (such as (131)I, (132)I, (132)Te, (134)Cs and (137)Cs) have been detected along the Iberian Peninsula from March 28th to April 7th 2011. The analysis of back-trajectories of air masses allowed us to demonstrate that the levels of manmade radionuclide activity concentrations in the south...

Radioactivity levels in aquatic environments can be assessed through the study of superficial sediments. Anthropogenic activities may alter radioactivity levels leading to the anomalous accumulation of natural radionuclides in coastal areas. In this work, marine sediments from a significant area subjected to severe industrial development were colle...

The techniques for the direct measurement of the sedimentation rate are reliable but slow and imprecise, given that the time intervals of measurement cannot be very long. Consequently it is an extremely laborious task to obtain a representative map of the sedimentation rates and such maps are available for very few zones. However, for most environm...

A new method using the inventory determined for the activity of the radionuclide (137)Cs, coming from global radioactive fallout has been utilised to calculate the sedimentation rates. The method has been applied in a wide intertidal region in the Bay of Cádiz Natural Park (SW Spain). The sedimentation rates estimated by the (137)Cs inventory metho...

There exist diverse radioactivity sources in the environment coming from anthropogenic activities that alter the natural levels of radiation. The detailed study of the environmental impact of these sources is of great interest, because it provides knowledge for subsequent decontamination works and environmental control. In this work, it is analyzed...

A previous study on seabed sediments of the Bay of Cadiz (SW of Spain) enabled us to identify several relations between sedimentological variables and activity concentrations of environmental radionuclides such as (137)Cs, (226)Ra, (232)Th and (40)K. In this paper the study has been extended to a large neighbouring inter-tidal area in order to esta...

The potential radiological impact of the increase of radioactive substances in the environment makes interesting the study of the migration of the contaminant radionuclides in soils and sediments, which are the last receiver system of these substances. By using a battery of sedimentary columns controlled in the laboratory, the diffusion of the (226...

This is the first of a two-part paper. This first part describes a detailed study conducted of the spatial and vertical distribution of the concentration of the radionuclide (137)Cs, utilising columns of sediment extracted from the seabed of the Bay of Cadiz (Spain) as specimens. The activity concentrations have been measured by means of gamma spec...

This second part of the study reports the development of a model to describe the vertical migration of the artificial radioisotope (137)Cs in the sediment column on the seabed of the Bay of Cadiz. The application of the model provides an overall picture of the process of sedimentation in the Inner Bay of Cadiz. The spatial distribution of the rate...

Given the strategic situation of the José León de Carranza bridge, which spans the Bay of Cádiz (in the SW of Spain) and carries very heavy motor traffic, together with knowledge of the currents and tidal flows in the zone, we have used a technique of radioactive dating of sediments to study the temporal evolution presented by contamination from le...

In this paper the time evolution of heavy metal concentration of Pb, Zn, Cd and Hg, in the sediments of the Bay of Cádiz (southwest of Spain) is studied during the past century, as a result of the industrial influence in the zone. The study has been performed using sedimentary profiles that have been extracted from the seabed. The measurement of 21...

Natural radionuclides (232Th, 226Ra, 40K) and 137Cs, coming from atmospheric radioactive fallout, have been measured in sea-bed sediments of the Bay of Cádiz (South Western Spain). In this report, multivariate analysis methods have been employed to study the relationships between the activities of the radionuclides and some sedimentological variabl...

A model was developed to determine the optimum height in cylindrical geometry samples, to measure by gamma spectrometry a given fraction of the maximum number of possible detections. The method was applied to samples of water and seabed sediment of the Bay of Cádiz (Southwestern Spain). The influence of sample density was also studied. An important...

In this paper, a new method for the gamma-efficiency calibration of voluminal sea-sediment samples in cylindrical geometry is proposed. The efficiency function can be written as the product of a water-matrix-sample efficiency in the same geometry and a correction factor, improved with regard to those used to elate, dependent on the energy and the a...

Commercial activities in the Bay of C�diz (south-western Spain), together with the erosion and transport of sediments through the Guadalete and San Pedro rivers and the natural contribution of the salt pans, suggest that this region is altered from a radiological point of view. Alterations in radioactivity levels produced by such processes are here...

Differential scanning calorimetty (DSC) is one of the most used indirect techniques in the study of thermal properties of solids. It is widely applied for computing the kinetic parameters describing the crystallisation processes in amorphous materials. There are plenty of methods developed for calculating such parameters, most of them called classi...

Coincidence-summing effects play an important role in HPGe spectrometry at low source-to-detector distances (usual arrangements when environmental samples have to be measured). Although these corrections are not important for environmental samples (less than 5%), they can be significant in the efficiency calibration with multi-gamma-ray radioisotop...

The crystallization kinetics of Fe-B-Si metallic glasses, characterized by the effective activation energy E and the Avrami exponent n was studied by non-isothermal DSC technique. The data show values both for E and n in good agreement with the theoretical JMA model for the first crystallization stage, but not for the second.

A sample of the alloy As47Se6Te47 has been subject to an aging process at room temperature for two years and to different annealings at intermediate temperatures between the glass transition and crystallization temperatures. By means of DSC calorimetric techniques the further crystallization reactions were studied, computing the parameters E, n, y,...

Cited By (since 1996):11, Export Date: 11 November 2013, Source: Scopus, CODEN: PCGLA, Language of Original Document: English, Correspondence Address: Ligero, R.A.; Universidad de Cadiz, Puerto Real, Spain

A procedure was considered for determining theoretical expressions of the area under the first radial distribution function (RDF) peak, and of the relative co-ordination numbers, n ii which depend on the co-ordination hypotheses and on the numbers of bonds between elements of a single type of pair. In this method, two important facts were taken int...

Summary The radial atomic distribution of the amorphous alloy As0.45Se0.05Te0.50 was studied through the X-ray diffraction data supplied by samples obtained through quenching. The short-range order was determined by interpreting the radial distribution function (RDF), using a theoretical expression which takes into account the variation in the atom...

The radial atomic distribution was studied in the amorphous alloy Cu0.08Ge0.18Te0.74, using X-ray diffraction data of samples obtained by quenching the molten material. The short-range order proposed was determined through the interpretation of the radial distribution function, using a theoretical expression which takes into account the variation i...

The crystallization reaction kinetics of three glassy alloys belonging to the Cu-Ge-Te system were studied by differential scanning calorimetry, using continuous heating experiments. In order to determine the kinetic parameters which describe these reactions, a calculating technique based on the Augis and Bennet method was used. The alloys studied...

The radial atomic distribution of the amorphous alloy Cu0.03Ge0.20Te0.77 was studied using the X-ray diffraction data supplied by samples obtained through quenching. The short-range order was determined by interpreting the radial distribution function (RDF), using a theoretical expression that takes into account the variation in the atomic scatteri...

Cited By (since 1996):8, Export Date: 11 November 2013, Source: Scopus, CODEN: PCGLA, Language of Original Document: English, Correspondence Address: Ligero, R.A.; Universidad de Cadiz, Cadiz, Spain

A procedure was developed for determining theoretical expressions for the area under the first radial distribution function (RDF) peak and for the relative coordination numbers (nij) which depend on the coordination hypotheses and on the number of bonds between atoms of a single element or a pair. In this method, two important facts were taken into...

The crystallization reaction of an As-Se-Te alloy was taken as a reference for determining the kinetic parameters which describe the said reaction by differential scanning calorimetry and using non-isothermal techniques.Three sets of kinetic parameters were obtained, using three calculation methods within the theoretical Johnson-Mehl-Avrami model....

The radial distribution function (RDF) of the semiconducting glassy alloy As0.20Se0.30Te0.50 was obtained by X-ray diffraction. Once the hypotheses on the local order of the alloy had been formulated, the RDF analysis made it possible to evaluate them, referring specifically to the coordination of the As. On this basis, and using the semi-random Me...

The radial atomic distribution was studied in the amorphous alloy CuO.OSGeO.,STe ,_, *,,, using X-ray diffraction data of samples obtained by quenching. The short-range order was determined through the interpretation of the radial distribution function (RDF), using a theoretical expression which takes into account the variation in the atomic scatte...