Sixto Bajo

Determination of 228ra, 226Ra and 224Ra in natural water via adsorption on MnO2-coated discs

A fast procedure based on sorption of Ra on MnO2 coated polyamide discs is presented for determination of radium isotopes (i.e. 228Ra, 226Ra, 224Ra) in aqueous samples. The sample discs can be used directly for low-level alpha-spectrometry without the need for further separation and preparation methods to produce planar sample sources. While the activity of alpha-emitting 224Ra and 226Ra can be determined during a first measurement, beta-emitting 228Ra is obtained via ingrowth of the progeny 228Th on the same sample disc after a standing time of about six months. Calculations are presented for optimizing the analytical accuracy as well as for predicting the sorption yield or chemical recovery of radium on the sample disc as a function of exposure time because the sorption uptake proceeds with first-order kinetics. The analyses can be carried out on small samples of 0.5-11 and, for long counting times of one week and use of high-purity silicon surface barrier detectors, a detection limit of 0.15 mBq l-1 is obtained for 226Ra. Since the half-life of 224Ra is only 3.7 d and since 228Th (as a measure for 228Ra) is built up only partially on the sample disc, a slightly higher detection limit of 0.24 mBq l-1 results for the latter isotopes. The procedure is therefore sufficiently sensitive to allow the investigation of Ra isotope relationships in aquifers at typical environmental levels.

Deposition of 134,137Cs from Chernobyl fallout on Norway spruce and forest soil and its incorporation into spruce twigs

Abstract The aims of the present research are to describe the amounts, and the variation with time, of 134 Cs and 137 Cs in spruce-twigs ( P. abies karst. ) and in the soil of a spruce forest in Switzerland following deposition of the Chernobyl fallout. The activity of the twigs was subdivided into 3 compartments: the activity on their surfaces (i.e. the activity which can be removed from the twigs along with their natural wax coating), the activity incorporated into the needles and, finally, the activity incorporated into the wood. These compartments were separately sampled 6 times over a period from 54 to 233 days after the Chernobyl incident. Twigs which sprouted in two successive years (1985, 1986) were sampled and were found to show different behaviours. The activities associated with the 1986 twigs were roughly constant with time, while those of the 1985 twigs decreased exponentially, with half-lives around 150 days. The mean activity associated with 1 g (dry) of 1985 twigs is 724 mBq 137 Cs g −1 , of which 58% is incorporated into the twig wood, 17% into the needles and 25% associated with the adhering aerosol. 137 Cs on the surface of the needles was found to be water-insoluble. It is believed to be strongly adsorbed on to the soil-derived fraction of the aerosol residing on the needle surface and thus provides a tracer for studying the behaviour of natural aerosols on such surfaces. The same soil profile was measured before and after the Chernobyl incident, allowing direct comparison between nuclear weapons and Chernobyl fallout. The latter is mainly (56%) stored in the litter layer, with only 4% below a depth of 13 cm; it has penetrated into the soil to a much lesser extent than weapons fallout. The forest soil inventory of 137 Cs showed 2600 Bq m −2 from nuclear weapons fallout and 6200 Bq m −2 from Chernobyl. The 134 Cs/ 137 Cs activity ratio of the Chernobyl fallout was found to be 0·58 ± 0·01; the activity ratios in the different compartments investigated prove that incorporation of Cs into spruce occurred exclusively by uptake through the needles. A rough estimate indicates that in a spruce forest the activity stored in the twigs is half that stored in the soil.

Precise two chronometer dating of Pleistocene travertine: The 230Th/234U and 226Raex/226Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230 Th/ 234 U ingrowth system. Additional analyses of further radionuclides within the 238 U decay chain, i.e. 226 Ra and 210 Pb, showed that the Th/ U chronometer started with insignificant inherited 230 Th over the entire formation period of the travertine setting (i.e. 230 Th(0)=0). A contribution from detritalimpurities with 230 Th/ 234 U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230 Th/ 234 U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238 U-decay series, based on decay of excess 226 Ra normalized to the initial, i.e. 226 Raex/ 226 Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230 Th/ 234 U chronometer. Analytical solutions of the coupled 234 U/ 230 Th/ 226 Ra radionuclide system predicted that the 226 Raex/ 226 Ra(0) chronometer is independent of the actual 230 Th activity build up from decay of 234 U, if the systems starts with zero inherited 230 Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226 Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226 Ra. This is concluded because (i) the 226 Raex/ 226 Ra(0) ages agreed well with those derived from 230 Th/ 234 U, (ii) all data plot within uncertainty on the 226 Raex/ 226 Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine materialindependent of the sample ages. Provided that such boundary conditions hold, 226 Raex/ 226 Ra(0) should be applicable to materials which are suitable for 230 Th/ 234 U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230 Th/ 234 U for samples that have been formed a few thousand years ago. # 2001 Elsevier Science Ltd. All rights reserved.

228Ra/226Ra/224Ra and 87Sr/86Sr isotope relationships for determining interactions between ground and river water in the upper Rhine valley

Ground and river waters of the upper Rhine valley (Alsace, France) were investigated for chemical composition of the major elements, Sr isotopes and radionuclides from the U and Th series. In particular, the isotope ratios and concentrations of Ra and Sr were used as geochemical tracers to distinguish between different types of water and their interactions. The bulk chemical analyses suggest that the surface waters in the Rhine valley can be described as mixtures between Ca-Na-HCO3-rich ground water and less mineralized slightly acidic river waters which have migrated through crystalline (mainly granitic) basement rocks of the Vosges mountains. Mixing of these waters yields positive correlation between bulk Sr, U, Ca and HCO3, indicating that carbonate-rich sediments are the main source of U and (non-radiogenic) Sr in the Rhine valley aquifers. The combination of the Ra and Sr isotope systems (228Ra/226Ra, 87Sr/86Sr) shows, however, that probably three sources contribute to the surface river waters in the upper Rhine valley, i.e. (i) a highly radiogenic crystalline component, (ii) a ground water source and, (iii) a third component from infiltrating Rhine water along the flow path of the parallel running river Ill in the northerly direction. The Sr and Ra isotope systems were also used to calculate small-scale mixing fractions of tributaries along the flow path of the Ill. Mixing ratios of non-pure end-member waters were determined using three isotope diagrams (i.e. 224Ra/226Ra vs. 228Ra/226Ra) and the results obtained with the Ra isotope system were found to be consistent with the data using Sr isotope relationships (i.e. 87Sr/86Sr vs. 1/Sr).

Major and trace elements in spruce needles by NAA.

Concentrations of 23 elements in needles of Norway spruce (P. abies) have been determined at 47 sites. It is shown that a thorough removal of the aerosols sitting on the needles surface is necessary in order to get the inherent needle concentrations. Neutron activation was used to determine concentrations from 10−9 to 10−2 g/g. Irradiation and counting conditions are given. The essential elements, Ca, Cl, Cu, Fe, K, Mg, Mn, P, Zn, and the nonessential elements, Al, As, Ba, Br, Co, Cs, Hg, La, Na, Rb, Sb, Sc, Sr, and V, could be determined. The concentrations of most elements are about a factor of 6 smaller than the mean concentrations in land plants. Analytical reproducibility was much better than the variation among individual trees, and the variation within sites is smaller than among sites. In general, essential elements have smaller variations than nonessential elements. For some elements, variations between sites are owing to differences in the soil pH or the emission situation.

Nadelinhaltstoffe und Ablagerungen auf Nadeloberflächen von Fichten (Picea abies Karst.)

ZusammenfassungEs wird das experimentelle vorgehen zur Befreiung von Nadeln von epikutikulärem Wachs und von anhaftendem Aerosol beschrieben. Das Verfahren arbeitet mit Tetrahydrofuran/Toluol und liefert einerseits sauber gewaschene Nadeln und andererseits das abgelöste Material. In beiden Probenmaterialien werden anschließend durch instrumentelle Neutronenaktivierungsanalyse 20 Makro- und Mikroelemente quantitativ bestimmt.Als Untersuchungsmaterial dienten 1jährige Nadeln der Fichte (P. abies) von 30 Standorten aus der Gegend von Winterthur (Schweiz). Das abgelöste Material entspricht in seiner Zusammensetzung weitgehend dem lokalen Boden. Die von der Oberfläche von 1 g Nadeln abgelösten Aerosolmengen sind am Waldrand wegen des Randeffektes 2,6mal größer als im Waldesinnern. Die an den Nadeln eines Fichtenwaldes angelagerte Menge von Bodenaerosol wird auf etwa 10 kg/ha geschätzt.Der Vergleich zwischen inhärentem Nadelgehalt und abgelöstem Material erlaubt eine quantitative Aussage darüber, wieweit ungewaschene Nadeln zur messung der Inhaltsstoffe verwendet werden können. Es ergibt sich, daß zwar einige Elemente, worunter die meisten Nährelemente, an ungewaschenen Nadeln mit kleinem Fehler bestimmt werden können, daß jedoch eine ganze Reihe von Elementen nur an in geeigneter Weise gewaschenen Nadeln zuverlässig gemessen werden können. Für Fe und Al werden die Resultate eingehender dargestellt. Für Fe ergibt sich eine Normalverteilung mit einem Mittelwert von 26 μg/g, beim Al eine logarithmisch-normale Verteilung mit einem Mittelwert von 16 μg/g. Die Al-Gehalte zeigen eine hochsignifikante negative Korrelation mit dem pH des Standortbodens, wogegen die Fe-Gehalte keine Abhängigkeit zeigen.SummaryThe procedure which is used to free needles from epicuticular wax and adhering aerosol is described. The needles are washed with tetrahydrofuran and toluene. Both the clean needles and the material removed were analysed for 20 macro- and microelements, using instrumental neutron activation analysis.One-year-old needles of Norway spruce (P. abies) from 30 sites in the vicinity of Winterthur (Switzerland) have been investigated. The material removed from the surface of the needles has a composition similar to that of the local soil. The quantity of aerosols removed from 1 g of needles is 2.6 times greater at the edge of the forest than in the interior. The amount of soil aerosols retained on the needles of a spruce forest is estimated to be approximately 10 kg/ha.The comparison of inherent needle concentration and amount of the removed material gives a quantitative indication as to what extent unwashed needles can be used to measure the true needle concentration. It follows that some elements, among them most plant nutrient elements, can be determined with unwashed needles; however, analysis of washed needles is mandatory for reliable determination of many other elements. As an example, results for Fe and Al are given in more detail. Fe is normally distributed (mean 26 μg/g), whereas Al has a lognormal distribution (mean 16μg/g). The Al-concentrations at the different sites show a highly significant negative correlation with the pH of the soil. There is no significant correlation for Fe.

Spruce needles: standards and real samples

SummarySpruce needles were sampled from stations over 6 different bedrocks and analyzed for 24 elements by neutron activation. The results were then compared to conifer needle Standard Reference Materials (SRMs), NIST SRM 1575 pine needles and BCR CRM 101 spruce needles. Values for Al, As, Br, Cr, Fe, La, Na, Sb, Sc, and Th are 1 to 2 orders of magnitude higher in the SRMs than in the actual samples, whereas the other investigated elements do not show this effect. The most probable reason for the discrepancy is the inclusion of external material with the SRMs. Some properties of this external material and the consequences of its presence on the investigation of spruce needles are discussed.

Na, Cl and Br in needles of norway spruce (P.Abies) and in the aerosol adhering to the needles∗

One year old needles of Norway spruce have been washed free of adhering wax and aerosol. Na, Cl and Br concentrations were determined in both fractions by neutron activation analysis. The range of the needle concentrations among the 47 sites investigated was found to be extremely large for Na, large for Br and moderate for Cl. The highest values were found near motorways. Mean concentrations for relatively clean sites are 3.7 ppm Na, 420 ppm Cl and 407 ppb Br. The quantities of these elements in the needle and in the adhering aerosol correlate. The main souce of Na in the aerosol in winter and spring is de‐icing salt, whose influence on the needle concentrations can be seen at a distance of many hundreds of meters from the nearest road. It is probable that the Na and Br needle concentrations are predominantly determined by foliar uptake of the ions from the aerosol and this may partially be trae for Cl. The concentrations of all three elements in the needles correlate with each other in the complete set o...

General method for the liquid-liquid extraction of molybdenum and its application to neutron activation analysis

SummaryThe liquid-liquid extraction of Mo(VI) from different acid solutions by zinc diethyldithiocarbamate in chloroform is described. This extractant allows the separation of nanogram to milligram quantities of Mo from even gram quantities of many other elements. The separation of Mo can be made selective by appropriate choice of the composition of the aqueous phase, by back-extraction of Mo and/or the coextracted elements, and by masking Mo with hydrogen peroxide. These techniques were adapted for the determination of Mo in several reference materials by radiochemical neutron activation analysis via 99Mo.

Some properties of the ash from spruce needles

Abstract The weight of the ash resulting from dry ashing spruce needles at 500° C can be predicted with good precision from the concentrations of calcium (Ca), potassium (K), magnesium (Mg), manganese (Mn), and silicon (Si) in the needles. The empirical factors associated with the individual elements are given. There is evidence that they can also be applied to leaves from other plants. The colour of the ash can be related to the Mn concentration in the needles. Dry ashing entails no losses for Ca, cobalt (Co), barium (Ba), K, rubidium (Rb), antimony (Sb), scandium (Sc), and zinc (Zn), whereas a very small loss was observed for cesium (Cs), and a very large for bromine (Br).