Jieqing Zhu

The design of a versatile scanning proton microprobe of high resolution and efficiency

Abstract The designer of a scanning proton (or nuclear) microprobe must make many decisions, some of which may be compromises. There is a wide range of lens types and configurations. Microprobe performance will depend on performance of the accelerator and its ion source, on stability and control of the lens current supply, on the nature of the microprobe supports, on the vacuum system, on magnetic shielding and connection to the accelerator. There are many possible modes of observation and analysis to be considered when the specimen chamber is designed and a versatile chamber should make provision for most of them. They include optical microscopy of front and back surfaces of the specimen, secondary electron imaging, X-Ray imaging, channelling contrast microscopy, Rutherford backscattering and forward scattering, nuclear reaction analysis and scanning transmission ion microscopy in brightfield and darkfield modes. Microprobe performance will also inevitably depend on the ease of operation and the extent to which the operator has been considered in the overall design and layout of the microprobe. The equally important considerations involved in data collection and analysis are discussed in a second paper.

Characteristics of a hydrogen gas field ion source for muprobe application

Abstract A hydrogen gas field ion source has been built specifically for use with a scanning proton muprobe. Source parameters have been measured on a testbench and show that a current of 20 nA within an angular spread of ± 550 mrad can be achieved from an Ir emitter, with an estimated virtual source size of 0.001 μm. This is 10 5 times brighter than the beam available to the muprobe from standard accelerator sources.

Grid shadow pattern analysis of the Shanghai nuclear microprobe

Abstract The ion optics of the Shanghai nuclear microprobe system have been investigated with the grid shadow method. The level of parasitic aberration found in the magnetic quadrupole lenses of the probe forming lens system was found to be very low and typical of levels seen in other high-quality systems, such as the Melbourne system. It was found that the parasitic field components were mainly sextupole with a strength of between 0.070% and 0.28% of the main quadrupole field. The small level of parasitic field is not a significant limit to the resolution achievable by the system. A complication in the analysis of the grid shadow patterns was that some of the parasitic field was due to skew sextupole components (non-zero phase angle); however, these could be identified by comparing with simulated shadow patterns. Further experiments with the grid shadow method revealed an improvement by a factor of 4–6 in focused probe intensity, and could be achieved by installation of a beam steerer between the accelerator condensor lens and the beam switcher dipole magnet. The measurements show that the lens system is potentially able to focus probes with 1 μm resolution.

Study on the occurrence of platinum in Xinjie Cu-Ni sulfide deposits by a combination of SPM and NAA

Abstract A combination of neutron-activation analysis (NAA) and scanning proton microprobe (SPM) was used to study the distribution of platinum-group elements (PGEs) in rocks and ores from Xinjie CuNi deposit. The minimum detection limits of PGEs by NAA had been much improved by means of a nickel-sulfide fire-assay technique for pre-concentration of PGEs in the ore samples. A simple and effective method was developed for true element mapping in SPM experiments. A pair of moveable absorption filters was set up in the target chamber for high sensitivities of both major and trace elements. The bulk analysis results by NNA indicated that the PGE mineralization occurred at the base of Xinjie layered intrusion in clinopyroxenite rocks and the CuNi sulfide minerals disseminated within the rocks had high abundance level of PGEs. However, the micro-PIXE analysis of the CuNi sulfide mineral grains did not find PGEs above the MDL of 6–9 ppm for Rh, Ru and Pd, and 60 ppm for Pt. The search for platinum occurrence in sulfide minerals was followed by scanning analysis of SPM when some smaller platinum enriched grains were found in the sulfide minerals. The microscopic analysis results suggested that platinum occurred in the CuNi sulfide matrix as independent arsenide mineral grains. The chemical formula of the arsenide sperrylite was PtAs2. The information of the platinum occurrence was helpful to future mineralogical research and mineral processing and beneficiation of the CuNi deposit.

Precious metals assay by means of microbeam XRF technology

Abstract There have been increasing demands for a means whereby the transactors and dealers in jewelry can accurately and efficiently appraise the precious metal contents on the premises. These values of such articles cannot be obtained accurately or nondestructively by conventional methods, such as specific gravity measurement, touchstone method, fire assay method, atomic absorption spectrometry and the like. A new method of microbeam X-ray fluorescence analysis (Microbeam XRF) has been developed to analyze the type and quantity of precious metal elements. In this method a beam of X-ray generated from a low powered X-ray tube is collimated by a collimator with a diameter of 0.1–5 mm. An object to be analyzed is positioned by moving a three-dimensional sample stage till the sample can be observed clearly with optical microscope. The X-ray beam will coincide with the viewing point on the surface of the sample. The X-rays produced are detected in a high efficient proportional detector. Its spectrum, whose scattering background is lower than that with the broad beam method, is recorded and analyzed by a multichannel analyzer. The intensities of selected peaks corresponding to target elements are then determined and the relative compositions of the elements are also calculated by a fundamental parametric method. Two sets of standards (No. SYS-01) were measured by the instrument. It showed that the precision of gold measurement was 0.1% and the correlative coefficient between the measurement results and the given values of the standards was better than 99.998%. A piece of gold necklace was also checked step by step. The microbeam XRF is a reliable and accurate method for precious metals jewelry assay.

Scanning mu-analysis of SiCSi3N4 material by proton non-Rutherford elastic backscattering

Abstract By the use of a 3 MeV proton mubeam we succeeded in studying SiCSi 3 N 4 composite ceramics quantitatively. Proton backscattering in the MeV non-Rutherford energy region is more sensitive to detecting low-Z elements, such as C, N and O, than other ion beam methods when a heavier element is present in the matrix. The mubeam analysis was performed by scanning an inclined cross section of the sample for improving the depth resolution. The results confirm that a SiCSi 3 N 4 composite layer can be formed on the presintered SiC substrate under the condition of an isostatic pressure of 200 MPa in N 2 atmosphere at 1850°C. About 60% of the SiC was transformed into Si 3 N 4 in a surface layer of over 10 μm, when the reaction lasted one hour. The uncertainty in the concentration is not larger than ±10% in directions parallel to the target surface. The maps of nitrogen and carbon, in three dimensions, are also given in the interface region.

A very simple method for true elemental mapping using the scanning proton microprobe

Abstract An elemental map by means of SPM is traditionally obtained by total counts entering an energy window (TCEW) corresponding to a characteristic X-ray of that element. Besides the characteristic X-ray signals of the element, the total counts contain the contribution of continuum background and overlapping peaks of the interfering elements as well. A method called NCEW (net counts entering an energy window) has been developed for off-line true elemental mapping. The continuum background is calculated with a trapezoid area determined by the counts within border gates and is subtracted from the total counts of the peak. The branch ratios of the characteristic X-rays of the interfering elements are used to correct their overlapping peaks. Because the net counts eliminate the contribution of the background and the overlapping peaks from the total counts, the map obtained by the NCEW is free of artifacts due to the continuum background and the peak overlapping. This method was tested by scanning a microbeam over a multi-foil target. Applications of the NCEW method to some environmental and geological samples are described.

PIXE and SPM analysis of V, Cr, Cu and Bi in nodular cast iron

Abstract The nodular cast iron was prepared with the same nodularizing process in which the elements V, Cr, Cu and Bi were added respectively. The graphite phase was extracted from the nodular cast iron to measure the trace elements in it. It was made into a thick target and measured by proton induced X-ray emission (PIXE) analysis. The contents of V, Cr and Cu were 3, 40 and 14 ppm respectively in the graphite, as the element was separately increased to 2100, 5000 and 2500 ppm in the matrix. Therefore, V, Cr and Cu do not intrude the graphite as the contents of these elements are lower and have no obvious effect on the nodularization. Bi, which belongs to the antinodular elements, easier intrudes into the graphite than others. The content of Bi in graphite was 74 ppm and the nodularizing rate went down by 34% when it was 200 ppm in the matrix. The distributions of V, Cr, Cu and Bi in the sample were measured using a scanning proton microprobe (SPM) to research the properties of these elements further. The results showed that most of the V, Cu and Cr distributed in the matrix. Some of the Cu intruded into the graphite, some existed in the matrix yet as Bi was increased to 200 ppm. These results of SPM agree with that of the PIXE analysis.

Application of nuclear microprobe to the study of metasomatism in mantle peridotites

Abstract In recent years, some models have been proposed to explain the chemical heterogeneity of the earth mantle. Among them the metasomatism by the mantle fluid and the mantle melt is the most popular one. The trace elemental behavior of the mantle minerals from different areas in micron scales can provide important information on the understanding of the mantle metasomatism. This paper describes the study of the metasomatism in mantle peridotites from Raobazhai, Anhui Province of China by the nuclear microprobe. The concentrations and distributions of trace elements in the metasomatized mantle peridotite were determined quantitatively by the micro-PIXE software package TTSPM. The internal standard and the integrated iterative method were used for the calculation. The matrix elements in the minerals were analyzed by EPMA. The accuracy of the quantitative analysis was examined by three geochemical standards of rocks (GSR). The results were better than 15% for most elements. According to the elemental maps, the trace element distributions in the amphibole, which was crystallized from the mantle fluid, displayed obvious heterogeneity and zoning. The enrichment of Ni, Zn, Ga, Rb, Sr, Y, Zr and Pb in the mineral could be associated with the metasomatism of the mantle fluid. The local coexistence of Ni, Cu, Pb, Fe and S in the amphibole suggested that there were xenoliths trapped in the minerals during their crystallization. The characteristics of the trace element composition and distribution in the mantle peridotites demonstrated that the metasomatism by the mantle fluid and the mantle melt might change the chemical composition of the mantle peridoties and thus resulted in the chemical heterogeneity of the earth mantle.

Localized depth distribution in the bombarded surface of AgCu alloy

Combined with electron microbeam techniques such as scanning electron microscopy and electron-induced X-ray emission, a helium microbeam is used to study the effect of Ar+ ion bombardment on the surface of a binary alloy Ag37Cu63 by particle-induced X-ray emission and Rutherford backscattering. A pronounced change of the surface concentration and depth profile has been found at both Cu-enriched solid solution micro-region and Ag-enriched eutectic mixture micro-region on the alloy surface due to ion bombardment. The preferential sputtering of Ag and bombardment-induced surface segregation is suggested to be attributed to the composition change in the near-surface regions. A superposition of depth distributions determined from different micro-phase-regions is experimentally confirmed.