The effect of oxides on PIXE measurements

The presence of oxides on metallic samples used for PIXE analysis affects quantitative measurements. This effect has been calculated based on thick target analysis on layers of different compositions. The ratio of oxidized metal yield to clean metal yield is seen to depend on proton energy and on oxide thickness. Calculations are presented for oxides of Al, Si, Fe, and Cu, and applied to experimental data on Si. The method may be applied to measuring thicknesses of oxides of known stoichiometry.     

PIXE measurements of Renaissance silverpoint drawings at VERA

Silverpoint drawings from the Renaissance are among the most precious and rarest treasures of graphical art. Our research group is particularly interested in the analysis of silverpoint drawings by Albrecht Dürer (1471-1528). A very sensitive and non-destructive analytical method, either spatially resolved synchrotron-radiation induced X-ray fluorescence (SY-XRF) or proton-induced X-ray emission (PIXE), is needed to determine the chemical composition of the very faint silver marks on such drawings. Dürer drawings from the collection of the Albertina, Vienna, were analyzed to amend existing data on Dürer drawings. For this purpose an external-beam PIXE setup was installed at the Vienna Environmental Research Accelerator (VERA). It allows to analyze a spot of ∼0.15 mm on the object in air with 3 MeV protons, and to detect the emitted X-rays that are characteristic for the chemical composition with very good sensitivity and without harming the precious objects. After successful measurements on artificial test samples, four original silverpoint drawings were investigated: two portraits from Albrecht Dürer’s very early period (self-portrait and portrait of his father) and two drawings from Dürer’s sketch book of his travel to the Netherlands 1520/21.     

Chemical shift measurements of PIXE spectra using a position-sensitive crystal spectrometer

The in-air high-resolution PIXE system composed of a flat analyzing crystal and a position-sensitive proportional counter was applied to measure line shifts of Si K and P K X-rays from various samples induced by an external proton beam. Line shifts that reflect chemical environments of atoms in target samples were determined with the precision of 0.1 eV. The utility of the method for chemical state analysis of minor elements down to the order of 0.1% was demonstrated.     

Chemical shift measurements of chlorine K X-ray spectra using a high-resolution PIXE system

A high-efficiency high-resolution wavelength-dispersive spectrograph with a von-Hamos configuration was developed for chemical state identification of elements in environmental samples using PIXE analysis. To evaluate the performance of this system, chlorine K X-ray spectra for NaCl, NH₄Cl and polyvinylchloride (PVC) targets were measured and compared. Also, to study the applicability to environmental mixed samples, mixtures of NaCl and NH₄Cl with different mixing ratios were measured. Through observation of Cl Kα₁ X-ray from NaCl, the energy resolution of the system was determined to be 1.1 eV. For the NaCl sample, a Kβ_x line was observed at an energy, which is higher than that of the Kβ main peak by 2 eV, whereas no Kβ_x emission was observed for the NH₄Cl sample. The chemical shift of the Kβ main peak for PVC relative to that for NaCl was about 1.2 eV. For NaCl-NH₄Cl mixture targets, the relative intensity of Kβ_x satellite to the Kβ main line provided an indication of mixing ratio. Energies and relative intensity of Cl Kβ X-ray satellites for NaCl and NH₄Cl samples calculated by a simple molecular-orbital method agreed only qualitatively with the experimental results.     

Analysis of atmospheric aerosols by PIXE: the importance of real time and complementary measurements

Particle-Induced X-ray Emission (PIXE) has been used for more than 30 yr in many urban and background air pollution studies. The technique has certainly contributed to the understanding of source-receptor relationship for aerosol particles as well as to aerosol physics and chemistry. In the last few years, where aerosol issues were strongly linked to global climate change through the relationship between aerosol and atmospheric radiation points to new challenges in atmospheric sciences, where PIXE could play an important role. Also the recognition for the inter-relationship between aerosol and liquid and gas phases in the atmosphere makes important to integrate PIXE aerosol analysis with other complementary measurements. The use of Nephelometers and Aethalometers to measure scattering and absorption of radiation by aerosol particles can be done in parallel with particle filter collection for PIXE analysis. Parallel measurements of trace gases using traditional monitors as well as with new techniques such as Differential Optical Absorption Spectroscopy (DOAS) that can provide concentration of O₃, SO₂, NO₃, NO₂, HCHO, HNO₃, Benzene, Toluene, and Xylene, is also important for both urban and remote aerosol studies. They provide information that allows a much richer interpretation of PIXE data. Recently developed instruments that provide real time aerosol data such as the Tapered Element Oscillating Microbalance (TEOM) PM₁₀ monitor and automatic real time organic and elemental carbon analyzers provide extremely useful data to complement PIXE aerosol analysis. The concentrations of trace elements measured by PIXE comprise only 10–30% of the aerosol mass, leaving the organic aerosol characterization and measurement with an important role. The aerosol source apportionment provided by PIXE analysis can be extended with other aerosol measurements such as scattering and absorption, estimating for example, the radiative impact of each discriminated aerosol source. The aerosol bulk PIXE measurements can be complemented with soluble concentrations provided by Ion Chromatography (IC) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Recent developments in remote sensing techniques and products also enhance significantly regional aerosol studies. Three-dimensional air mass trajectories should be integrated in aerosol studies for urban and remote areas. The applications of these techniques to study urban aerosols from São Paulo and Santiago de Chile have broadened extensively the scientific scope of these studies.     

The role of the ionization and stopping cross sections in PIXE thin film thickness measurements

It is shown how the proton stopping power and ionization cross sections affect thin film thickness measurements with PIXE, for proton energies below 0.70 MeV. The examples used were silicon-aluminum and aluminum-silver for the K-shell ionization, and aluminum-tungsten for the L-shell ionization. It is found that there is no appreciable difference in thickness measurement with the different ways of calculating the stopping cross sections available today. However, the expression used to compute the ionization cross section may strongly affect the method's accuracy.     

Use of rutherford scattering on a secondary carbon target for correcting intermediate thickness sample PIXE measurements

A method is presented for correcting PIXE measurements on intermediate thickness targets. The correction deals with the X-ray attenuation and the incident proton energy loss. It is based on the simultaneous acquisition of the PIXE spectrum along with the energy distribution of protons elastically scattered by a thin secondary carbon foil located behind the PIXE target. These measurements allowed a linear correlation between the proton energy loss ΔE and the correcting factor for X-ray absorption XF to be demonstrated.The validity of these simplifying linear approximations is discussed. The method is applicable to inhomogeneous thickness samples like aerosol impactor stages.     

Preprocessing PIXE spectra

Proton induced X-ray emission (PIXE) spectra are normally represented by a model containing both linear and nonlinear parameters. Considerable improvement in calculation time can be made if initial values for these parameters are accurate. This paper discusses simple preprocessing of measured data to obtain estimates for such parameters as energy calibration, detector resolution, and possible elements. This information is useful as either input to a spectrum analysis code or a database for an expert system driving the analysis.     

PIXE研究的进展

本文介绍第四届国际PIXE会议上关于PIXE的应用开发和研究进展的情况,PIXE在环境科学、生物医学、材料、地质、考古以及其它领域得到广泛开发利用,与此同时,PIXE在方法学和技术方面也在不断改进。     

掠入射质子荧光分析技术研究

首次提出采用掠入射质子荧光分析(PIXE)方法对表面存在污染的铝片、注入铁离子的单晶硅片以及注入氧隔离SOI半导体材料进行了分析.实验结果表明,现有PIXE分析技术不需要对设备做任何重大的升级改造,只是简单的采用掠入射方法,就能有效提高其表面分析的灵敏度.通过研究不同角度时峰面积与探测限之比与穿透深度之间的关系,得出质子柬在样品中的穿透深度越浅,峰面积与探测限的比值就越大,可检测的表面污染的灵敏度就越高.     

用质子激发X荧光分析技术鉴别玉器种类承焕生

质子激发X荧光分析技术可以测定玉器的主量、次量及微量元素的成分和含量,因而可以将那些外形及颜色相近的玉器因其化学组分的不同而清楚地分开,并得知其真实的材料种类．对于主要化学组分一致的同类玉器,如果产于不同的地方,则由于其成矿地区地球化学环境的不同,它们的微量元素种类及含量也将有所不同,因而这一方法也可用来判定玉器的产地．     

External PIGE–PIXE measurements at the São Paulo 8UD tandem accelerator

An external PIGE–PIXE setup was installed on a beam line of the 8 MV tandem Pelletron accelerator of the Open Nuclear Physics Laboratory (LAFN). Proton beam energy was chosen in the 8–12 MeV range, sufficient to get an acceptable gamma ray yield but not so high as to prevent us from measuring X-rays. This also allowed the use of a thick aluminum exit window (0.5 mm) instead of the usual thin and sometimes fragile plastic windows. This external PIXE–PIGE system was used to analyze trace element concentrations in the enamel of human and animal teeth. The main interest was to find compatible human teeth substitutes for dentistry laboratory practice and chemical tests. In spite of their morpho-histological similarity, trace element concentrations in human and animal teeth have not yet been compared. Teeth from humans, cattle and swine collected primary at São Paulo region were analyzed. The elements Cu, K, Zn, Fe, Ti, Sr, V, Mn and Zr were detected by high energy external beam PIXE technique. Though preliminary, the results showed that the trace element concentrations observed in the enamel of human and swine are more similar to each other than to cattle teeth.     

用质子激发X荧光分析技术鉴别玉器种类

质子激发Ｘ荧光分析中以测定玉器的主量、次量及微量元素的成分和含量,因而可以将那些外形及颜色相近的玉器因其化学组分的不同而清楚地分开,并得知其真实的材料种类。对于主要化学组分一致的同类玉器,如果产于不同的地方,则由于其成矿地区地球化学环境的不同,它们的微量元素种类及含量也将有所不同,因而这一方法也可用来判定玉器的产地。     

PIXE analysis of Zn enzymes

Zinc is a necessary component in the action and structural stability of many enzymes. Some of them are well characterized, but in others, Zn stoichiometry and its association is not known. PIXE has been proven to be a suitable technique for analyzing metallic proteins embedded in electrophoresis gels. In this study, PIXE has been used to investigate the Zn content of enzymes that are known to carry Zn atoms. These include the carbonic anhydrase, an enzyme well characterized by other methods and the cytoplasmic pyrophosphatase of Rhodospirillum rubrum that is known to require Zn to be stable but not how many metal ions are involved or how they are bound to the enzyme. Native proteins have been purified by polyacrylamide gel electrophoresis and direct identification and quantification of Zn in the gel bands was performed with an external proton beam of 3.7 MeV energy.     

Bayesian PIXE background subtraction

The subtraction of the X-ray background in a PIXE spectrum has been the subject of many investigations and different techniques have been developed. These techniques vary from filtering to fitting polynomial functions. The promising Bayesian Statistics technique has been used in this study to eliminate the background from the spectrum in a rigorous and self-consistent manner. We compare the results of the Bayesian background subtraction method to that obtained by stripping and the “rolling ball”; method.     

Charge integration in external-beam PIXE

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.     

Evaluation of depth profiling with PIXE

The problem of depth profiling with PIXE is formulated as an explicit convolution integral, and the deconvolution algorithms previously used in the literature are reviewed. The best of them, up to now, which consists of an iterative procedure associated with data smoothing at each step of the iteration, is generalized and analyzed in detail. Starting from published cross sections for X-ray production, X-ray absorption coefficient and energy-range correspondence, it is shown that this procedure is able to reproduce nicely the flat profile of a homogeneous Ag-3 at.% Zn alloy as well as the erf-like profile of Zn depletion in the same alloy after an annealing treatment under vacuum. The emphasis is put on the sensitivity of the method and on how straining conditions can improve it.     

Virtual PIXE and RBS laboratory

An interactive software package for a complete simulation of Particle Induced X-ray Emission (PIXE) and Backscattering Spectra (BS) is being developed. The user is in a position to define all experimental parameters such as incident ions (protons, deuterons or He ions), their energy, excitation and detection geometry, detector parameters and filters as well as sample composition and thicknesses of a number of layers. This is all done through an easy to operate interactive user interface. Simulated PIXE and RBS spectra are immediately displayed on the screen and can be saved either as bitmaps and/or files suitable for further processing. Each output comes with a complete set of experimental parameters, absolute and relative line intensities (including all major K and L lines), peak to background ratio and detection limits for all elements of interest.

The program has also a number of utility routines to calculate various fundamental parameters such as photon cross sections, K and L shell ionization and production cross sections, energy loss, and detector efficiency. All these routines use the state-of-the art data base sources.

The program operates on a personal computer under the MS Windows operating system. The simulation is fast and the program is easy to operate. The package will be useful in many ways. Firstly, it will be an excellent teaching tool for researchers/students without experience with PIXE/RBS. Secondly, it will be of immense help in planning and optimizing PIXE and/or RBS experiments. The user can ‘run’ a simulated experiment for any sample/experimental parameters and test various settings/scenarios to achieve optimal results without actually carrying out the experiment, thereby saving a lot of expensive machine time that would otherwise have been spent on trial and error experiments.     

External-beam PIXE analysis of small sculptures

Non-destructive analysis of precious art objects is an important tool to solve provenance problems or to facilitate restoration. External beam PIXE analysis is one of the most powerful and popular methods used in this respect. The paper summarises the external beam PIXE set-up at the Accelerator Laboratory in Budapest, and two selected applications are described. Different parts of a small Cambodian metal sculpture probably made in the 11th century were analysed. It was observed that the sculpture was composed of an iron core and an outer bronze shell. This sculptural technique was well known in the Middle-East but no indication has been found about its use in the Far-East, yet. Before its restoration paint traces on a wooden relief of St. Jerome made by an unknown artist in about 1600 were analysed. Different white and red paints were distinguished at selected points and the presence of titanium on a certain part of the relief was attributed to a repainting in modern times.     

PIXE analysis of Trojan gold jewelry

Technological advancements in the production of gold jewelry from the Troad in northwest Anatolia in the third millennium BC are investigated by PIXE. Results indicate a higher percentage of Cu at the interface between joined pieces of gold than exists elsewhere on the jewelry, away from joinings. The results indicate the probable use of copper salts as a flux in the manufacture of jewelry with granulation.