J.-O. Lill

Environmental monitoring of trace elements in bark of Scots pine by thick-target PIXE

Bark samples were taken from Scots pines (Pinus sylvestris L.) from a polluted area near a metal plant and from a relatively non-polluted site. Thick-target particle-induced X-ray emission (PIXE) was used for the analyses after different types of prepreparation of the samples. The bark samples were analysed directly by radially scanning from inner to outer bark in order to study the variability of elemental concentrations in different layers. Some clear differences were found in the chemical composition of the inner and outer bark. The lowest detection limits for the analyses of heavy metal ions were obtained by combining dry ashing at 550 °C with the PIXE method. More than 100 times higher concentrations were found for the heavy metal ions Fe, Ni, Cu, Zn, Ga, As and Pb in the bark samples from a polluted area in comparison to samples from a non-polluted area. The work demonstrated that external-beam thick-target PIXE is a sensitive and reliable method for quantitative determination of heavy metals in tree bark samples.

Charge integration in external-beam PIXE

Abstract Direct measurements of the beam current in external-beam PIXE are difficult due to the ionization of air molecules. A method for indirect charge integration has been earlier presented by our research group. The method utilizes the light emission from air molecules excited by the particle beam. The light emission originates from the second positive band system in N2. The light was measured with a photomultiplier tube. The geometry and the electronics have been improved during the six years that have passed since the method was taken into use. The current from the PM-tube is today about 210 times higher than the particle beam current measured by the means of a Faraday cup. This amplification is useful in the monitoring of small beam currents. The linearity and accuracy of the improved system are discussed.

Study of seasonal variations of trace-element concentrations within tree rings by thick-target PIXE analyses

Abstract Thick-target PIXE has been used for the quantitative determination of trace elements in annual growth rings of different tree species. A scanning device was developed for the remote control of the sample and a video-camera system for the exact monitoring of the spot to be analyzed. The samples were analyzed in steps of 1 mm. The widths of the tree rings studied were in the range 2.5–8.0 mm. Samples of Norway spruce and Scots pine were collected from Harjavalta, a polluted area in southwestern Finland. The elements studied were S, Cl, Br, K, Ca, Mn, Fe, Zn, Cu, Ni, Pb, Rb and Sr. Large variations were observed in elemental concentrations within individual tree rings. The highest concentrations for most elements were obtained for earlywood in the beginning of the growth season and the lowest values for latewood thus reflecting the biological activity. The method was calibrated using international standard reference materials. For most elements the matrix effects were found to be negligible. The detection limits for most metals studied were in the range 1–5 ppm.

Preconcentration of trace elements in biological materials by dry ashing for TTPIXE analysis A study of matrix effects

Abstract Trace elements in biological materials were preconcentrated by dry ashing at 550°C in order to improve the detection limits for thick-target PIXE analyses. The analytical procedure was calibrated with equally prepared ashes of certified biological standard reference materials (SRMs). The matrix effects were studied both theoretically and experimentally. A drawback of the ashing procedure is that volatile elements such as halogenides and sulfur can be partly lost depending on the composition of the material studied. Thick-target PIXE combined with dry ashing is a sensitive and reliable technique for the determination of elements with atomic number > 20 in biological materials with a low ash content.

A novel method for charge integration in external beam TTPIXE. Application to analyses of biological materials

Abstract An irradiation facility was developed for the elemental analyses of thick-target samples with particle induced X-ray emission (PIXE) spectroscopy. The samples were placed in air outside the cyclotron vacuum. The analyses were carried out with a beam of 6.4 MeV H+2-ions giving rise to 3.2 MeV protons at impact on the vacuum foil. The integrated charge on the target was estimated utilizing measurements of light emission in air during irradiation of the target sample. The measured light intensity was used for normalizing the X-ray spectra obtained. Special attention was paid to the application of the thick-target PIXE (TTPIXE) technique to the analyses of biological materials. A commercial pelleting device was used for the preparation of composite target pellets with the biological sample pressed onto the surface of spectroscopically pure graphite. The TTPIXE method presented enables direct analyses of solid samples. The use of standard reference materials for calibration of TTPIXE analyses was tested. The accuracy and precision of the TTPIXE method are good for most elements.

Monitoring of chromium, copper and arsenic in contaminated soils using thick-target PIXE

Abstract Soil contamination by toxic metal ions is one of the most serious environmental problems today. In this work soil samples from an area where a wood preservation plant has been active were studied. Chromium, copper and arsenic (CCA) are the main metal ions used in preservation of lumber. For the monitoring of these metal ions and of other elements in soil, thick-target PIXE (TTPIXE) has been proved to be a successful method. About 20 elements were quantitatively determined in the soil samples from the contaminated area and from a reference area. The reliability of the results was evaluated by analysing geochemical certified reference materials (CRMs) of chemical compositions similar to the soil samples.

Excitation functions of proton-induced reactions in natCu in the energy range 7―17 MeV

Elemental production cross sections were measured for (p,x) reactions on natural Cu targets, leading to the formation of (62,63,65)Zn. These reactions are generally used for monitoring the proton beam intensity and energy e.g. in isotope production facilities. Cross sections were obtained by activation of stacked foils and subsequent gamma spectroscopy. The production data for (62,63,65)Zn between 7 and 16.5 MeV proton energy are presented as well as comparisons with literature values. Good agreement with the evaluated values was found for most of the cross-section values.

Thick-target PIXE analysis of chromium, copper and arsenic impregnated lumber

Chromium, copper and arsenic (CCA) have for decades been used for wood preservation. Of these elements especially arsenic is very toxic. As CCA impregnated wood is still today used for many construction purposes, a monitoring of these metal ions is of great environmental importance. Thick-target PIXE is a powerful method for the determination of trace metals in wood. The TTPIXE method enabled study of variations of the elemental concentrations in lumber treated with CCA impregnation solution. Distribution patterns were obtained for both naturally occurring elements and elements introduced in the treatment process. During the impregnation process a desorption of e.g. alkali metal ions takes place from the wood. The sensitivity of the method is improved by dry ashing of the samples prior to PIXE analysis. The TTPIXE method was calibrated and validated using international certified reference materials (CRM) based on wood material. ” 1999 Published by Elsevier Science B.V. All rights reserved.

On the decay of the Jπ = 11/2−, T1/2 = 38.9 h, isomer in 133Ba: search for the E5 transition and verification of the EC branch

This paper presents a search for the competing 11/2− → 1/2+ E5 branch to the main 11/2− → 3/2+ M4 transition from the T1/2 = 39 h, Jπ = 11/2− isomeric state in 133Ba. An upper limit of 1.8 × 10−5 could be established for the E5/M4 intensity ratio. In addition, a longstanding controversy in the EC decay of the same isomer was resolved: only one EC branch to the 11/2+ state in 133Cs was observed. The shell-model structures of the involved states in 133Ba and 133Cs were studied with the microscopic quasiparticle-phonon model. Good agreement with the decay rates B(λ) was obtained. In particular, the B(M4) value is well reproduced and further only one low-lying 11/2 state could be identified in the latter nuclide, supporting the experimental results.

Thick-target PIGE analysis of plant materials preconcentrated by dry ashing

Plant materials were dry ashed at 550 degrees C and analysed using particle-induced prompt gamma-ray emission (PIGE). The analyses were performed with an external beam of 3 MeV protons incident on the target. Seven biological certified reference materials were analysed and used for the evaluation of the method for Na, Mg, Al, P and Mn. The elemental concentration to detection limit ratios were greatly enhanced by dry ashing of the biological materials. The concentrations of the elements in ashes were clearly above the values at which reliable analyses can be made. The method was applied to samples of spruce and pine. Due to the low ash content of the wood samples, the sensitivity of the method was radically improved. The detection limits for the five elements studied in spruce wood were in the range 0.014-2.5 mug g(-1). The set-up and the beam current used enabled simultaneous particle-induced X-ray emission spectrometry (PIXE) analyses, with the sensitivity optimised for heavier trace elements.