B. Gonsior

Determination of lateral trace element distributions with the bochum proton microprobe

Abstract For a spatially resolved analysis of element distributions in inhomogeneous samples by means of PIXE, we equipped our microprobe with a scanning and data-handling system based on a PDP 11/44 computer with CAMAC interface. This system is capable of program-controlled beam movement on the target surface, in order to follow the optimum scan path required for the analysis of a certain type of sample. X-ray events are processed with respect to the actual beam position. Element distributions can be determined on-line by selecting software single channels, or — in order not to lose any of the information supplied by the experiment — all X-ray events together with the actual beam position can be stored in list mode on magnetic tape. This enables us to perform off-line analysis of the data with more sophisticated programs. The most significant features of our scanning and data-handling system will be described; its advantages will be demonstrated by measurements on biological samples.

Trace element detection sensitivity in PIXE analysis by means of an external proton beam

A systematic investigation of the detection sensitivities achievable for PIXE analysis by means of an external proton beam is described. Six different metal foils were used as exit windows. The X-ray background spectra were measured for the bombardment of thin carbon foils with protons of four different energies. From these measurements we calculated the minimum detection limits using experimentally determined ionization cross sections. Simultaneously γ-ray spectra were used to elucidate the higher-energy background in the X-ray spectra. Two examples of application are given to demonstrate our results regarding detection sensitivity.

Trace element analysis by ion induced X-ray emission spectroscopy☆

Abstract K and L X-ray emission rates for 37 elements and continuous background radiation from a thin carbon matrix produced by bombardment with protons of 1.5, 2, 3, and 4 MeV have been investigated. K-shell ionization cross sections for 25 elements for 2 MeV proton bombardment, minimum detection limits through the periodic system in the ppm range and optimum proton energies for highest sensitivity for a given element have been extracted from these measurements. 16O-ions turn out to be much less sensitive than protons of the same velocity. Two examples of applications to mineralogical and environmental problems are given.

PIXE analysis by means of an external proton beam

Abstract To optimize PIXE analysis by means of an external proton beam we determined the minimum detection limits for 14 different exit foils. Influences of geometrical parameters were investigated. One important result of our measurements is that the continuous background yields are nearly independent of the matrix thickness. On that account analysis of thick samples is indispensable for obtaining highest relative detection sensitivity. To evaluate actual concentrations from measured X-ray spectra of thick samples we imagine the samples to consist of thin layers so that the X-ray yields can be computed numerically. The ionization cross-section data used in these calculations are theoretically as well as experimentally determined values. Examples of the application of this technique to the analysis of metals and to the analysis of water, making use of appropriate preconcentration methods are shown.

Analysis of the lateral distribution of trace elements in biological and other materials by proton-induced X-ray fluorescence using a microbeam scanner

Abstract A proton microbeam facility for trace multi-element analysis has been built at the Bochum Dynamitron Tandem Laboratory. A proton beam of 4 MeV is collimated by a diaphragm of 200 μm diameter and then focused by four magnetic quadrupole lenses; thus a beam spot of 60 × 40 μm 2 size and 120 nA intensity is produced at the target position. Using the technique of X-ray fluorescence analysis, investigation of cooperative cellular colonies in the biological microstructure has been started. First lateral distributions of several trace elements are presented.

PIXE analysis of environmental and biological samples using a proton microprobe

Abstract For the investigation of environmental and biological samples a proton microprobe has been built up at the Bochum Dynamitron Tandem Laboratory. By several improvements of the apparatus, e.g. optimization of the focal lengths and fine adjustment of the quadrupole lenses according to the results of a detailed beam transport calculation the beam diameter has been reduced to 6 μm fwhm. This device is used to measure lateral distributions of trace elements with Z > 15 in thin samples by means of PIXE. This technique has been applied to environmental and biological samples. An impactor sample has been investigated in order to test PIXE trace element measurements performed with a standard accelerator beam. In the field of biological samples our investigations concern the level of cooperative cellular colonies. Proceeding from a microphoto of a thin sample of human skin the element content in the region of interest has been determined by scanning.

Comparison of different excitation methods for X-ray spectral analysis

ZusammenfassungDie verschiedenen Anregungsmethoden der Röntgenspektralanalyse mit Protonen, Photonen und Elektronen (PIXE, XRFA bzw. EPMA) werden hinsichtlich ihres Nachweisvermögens, ihrer Präzision und Genauigkeit verglichen. Die Beurteilung der jeweils optimierten Anregungsmethoden basiert auf praktischen Problemen der Spurenanalyse: auf der Analyse von Aluminium, von Blei und von Spurenkonzentraten aus Wasser und Glas. Für dicke homogene Proben erwies sich die XRFA wegen der sehr guten Stabilität der heutigen Geräte als überlegen. PIXE ist vorteilhaft anzuwenden, wenn dünne Proben vorliegen oder wenn eine Probenvorbereitung, wie z.B. eine chemische Voranreicherung, erforderlich ist, die zu einer zusätzlichen Streuung von ⩾ 1% führt, so daβ die hohe Stabilität der XRFA-Geräte nicht mehr zum Tragen kommt. EPMA ist sowohl PIXE als auch XRFA unterlegen, solange nicht die lokale Verteilung von Elementen zu ermitteln ist.SummaryThe different excitation methods of X-ray spectral analysis with protons, photons and electrons (PIXE, XRFA and EPMA, respectively) are compared with regard to detection power, precision and accuracy. The evaluation of the independently optimized excitation methods is based on practical problems of trace-analysis: on the analysis of aluminium, lead and trace-concentrate targets of water and glass. For thick homogeneous samples XRFA turned out to be the best method, because of the very high stability of modern instruments. PIXE is advantageous for thin samples and if some kind of sample preparation like chemical preconcentration is necessary which introduces an additional scatter of ⩾ 1%, so that the high stability of XRFA instruments is no longer effective. EPMA is inferior to both, PIXE and XRFA, in all cases, where information on the lateral distribution of the elements is of no interest.

Measurements of mineralization process in the femur growth plate and rib cartilage of the mouse using pixe in combination with a proton microprobe

SummaryThe femoral bone from the 18-day pregnancy embryo and an rib cartilage of mature mice have been investigated using PIXE (proton induced X-ray emission) in combination with a proton microprobe on snap frozen cryosectioned material. The localization and the results of quantitative measurement of P, S, Cl, K, Ca, Fe and Zn have been correlated with the histochemical localization of inorganic deposits. It has been found that in calcifying and degenerating cartilage of the growth plate there is substantial loss of S; this element being indicative for sulphate groups of glycosaminoglycans. This change seems to be an important factor conditioning the process of mineralization. Zn is found in higher concentration in mineralized tissues, both in embryonal and mature cartilage as well as in the bone, and this suggests that Zn is also involved in the mineralization process. The mineralization of rib cartilage exceeds that of embryonal bone, and the Ca/P ratio is higher in the former than in the hydroxyapatite of the latter. The method described is a useful analytical tool especially for such types of studies in which elements are not easily redistributed by freezing, cutting and drying; e.g. in investigations of mineral deposits.

K-shell ionisation of heavy target elements (42 ⩽ ZT ⩽ 62) bombarded with protons in the energy range 0.5 MeV–6 MeV

Abstract Total K-shell ionisation cross sections were measured for the irradiation of thin solid targets ( μg cm 2 ) of 92 Mo, 104 Pd, 120 Sn, 138 Ba, 140 Ce, 1142 Nd and 144 Sm with protons of 0.5 MeV u ⩽ E Lab M Proj ⩽ 6.0 MeV u . Only isotopes were chosen as targets for which internal conversion of de-exciting transitions of nuclear excited states does not contribute to K-shell ionisation. The measured cross sections are compared with theoretical calculations within the framework of the perturbed-stationary state theory and with calculations based on the semiclassical approximation. Only for very low proton energies ( ξ ⩽ 0.25) are there systematic differences between the experimental data and the theoretical calculations; for ξ > 0.25 most of the measured ionisation cross sections agree with both calculations within the experimental errors (9–15%).

Calcification of the aortic wall in hypercalcemic rabbits.

The mineralization process was investigated in the aortic wall of hypercalcemic rabbits. The elevated calcium level in serum was induced by intramuscular injection of vitamin D3. The animals were killed at different times of the experiment (max. 246 d). The freeze-dried tissue homogenates were used for elemental composition studies by means of proton induced X-ray emission (PIXE) and atomic absorption spectroscopy. The structural information was obtained from infrared (IR) and X-ray diffraction (XRD) spectra. Moreover, the ascending part of the aortic arch was separated and used for micro-PIXE (PIXE in combination with proton microprobe) and histochemical examinations. It was found that hypercalcemia (blood serum Ca content elevated by about 20%) induced calcification of the aortic wall. The mineral phase within the aortic wall consisted of Ca-P salts. The Ca/P ratio continuously increased during the experiment and approached 2 after 246 d of the vitamin D3 treatment. The IR and XRD studies made possible the identification of the complex phase composition of the samples. The hydroxyapatite crystals were detected after 196 days, however, in earlier phases of the experiment, amorphous calcium phosphate, dicalcium phosphate dihydrate and octacalcium phosphate were also observed. On the basis of the data obtained, the mechanism of the precipitation and growth of inorganic deposits in the tunica media of the aortic wall was discussed.