Pha Peter Mutsaers

Performance predictions of a focused ion beam from a laser cooled and compressed atomic beam

Focused ion beams are indispensable tools in the semiconductor industry because of their ability to image and modify structures at the nanometer length scale. Here we report on performance predictions of a new type of focused ion beam based on photo-ionization of a laser cooled and compressed atomic beam. Particle tracing simulations are performed to investigate the effects of disorder-induced heating after ionization in a large electric field. They lead to a constraint on this electric field strength which is used as input for an analytical model which predicts the minimum attainable spot size as a function of amongst others the flux density of the atomic beam, the temperature of this beam and the total current. At low currents (I<10 pA) the spot size will be limited by a combination of spherical aberration and brightness, while at higher currents this is a combination of chromatic aberration and brightness. It is expected that a nanometer size spot is possible at a current of 1 pA. The analytical model was verified with particle tracing simulations of a complete focused ion beam setup. A genetic algorithm was used to find the optimum acceleration electric field as a function of the current. At low currents the result agrees well with the analytical model while at higher currents the spot sizes found are even lower due to effects that are not taken into account in the analytical model.

Accurate evaluation of μ-PIXE and μ-XRF spectral data through iterative least squares fitting

Abstract The integration of the nonlinear least squares X-ray spectrum evaluation progam AXIL into a μ-PIXE and a μ-XRF setup is discussed. The use of the software when procesing data sets derived from biological and geological samples is described.

A system for on-line monitoring of light element concentration distributions in thin samples

At the Cyclotron Laboratory, a scanning proton microprobe is used to determine concentration distributions in biomedical samples. The data acquired in these measurements used to be analysed in a time consuming off-line analysis. To avoid the loss of valuable measurement and analysis time, DYANA was developed. DYANA is an on-line method for the analysis of data from biomedical measurements. By using a database of background shapes, light elements such as Na and Mg, can be fitted even more precisely than in conventional fitting procedures. The entire analysis takes only several seconds and is performed while the acquisition system is gathering a new subset of data. Data acquisition must be guaranteed and may not be interfered by other parallel processes. Therefore, the analysis, the data acquisition and the experiment control is performed on a PCI-based Pentium personal computer (PC), running a real-time operating system. A second PC is added to run a graphical user interface for interaction with the experimenter and the monitoring of the analysed results. The system is here illustrated using atherosclerotic tissue but is applicable to all kinds of thin samples.

Co(III)EDTA as extra-cellular marker in μPIXE-analysis of rat cardiomyocytes

In previous studies no clear difference was found between the intra- and extra-cellular compartment in nuclear microprobe elemental distribution maps of freeze-dried cryo sections of heart tissue. Probably due to artefacts during the preparation of these samples, the intra-cellular and the extra-cellular content of elements are mixed up. In this article a method, using NaCo(III)EDTA as an extra-cellular marker, was applied to deconvolute the total ion content in an extra- and intra-cellular contribution. This method was both applied to normoxic heart tissue and low-flow ischemic heart tissue. Intra-cellular ion concentrations calculated from the corrected ion contents of the normoxic tissue agrees well with literature values. Moreover a clear elevation of the intra-cellular sodium and chlorine concentration was found in low-flow ischemic tissue.

Micro-PIXE and FT–IR microscopic studies of mineralized nodules formed in rat bone marrow stromal cell cultures

Abstract A proton microprobe in combination with Proton Induced X-ray Emission (micro-PIXE) and Fourier transform–infrared (FT–IR) microscopy are used for examination of the elemental composition and structure of inorganic deposits formed in cell cultures of rat bone marrow. The results show that micro-PIXE is well suited for prior recognition of the localization and for the determination of the elemental composition of the deposits while FT–IR microscopy may be used to determine the structure of deposits in situ with a spatial resolution of about 20 μm. It is demonstrated that nodules in the cell culture are composed of different Ca compounds. In contrast to physiological mineralization, Ca–P compounds do not dominate the mineral phase. Discrepancies are observed between histomorphometric and micro-PIXE data.

On-line elemental mapping in biomedical research

In biomedical research, elemental maps give valuable information about the role of elements in the aetiology of many diseases. For creating such elemental maps we use a scanning proton microprobe set-up to perform Particle Induced X-ray Emission (PIXE), Nuclear Backscattering Spectroscopy (NBS) and Nuclear Forward Scattering Spectroscopy (NFS) experiments simultaneously, preceded by a Scanning Transmission Ion Microscopy (STIM) experiment to localise the area of interest. PIXE is used to detect trace elements and NBS/NFS or STIM is used to determine local areal mass densities. Transferring experimental list-mode data into the thick target corrected concentration distributions is a time-consuming process. However, it would be advantageous to display continuously updated elemental maps as the data accumulate. This paper introduces a data processing method for use in biomedical research, based on dynamic analysis. For calculating concentrations, dynamic analysis uses a matrix transform technique instead of the nonlinear least squares fitting procedures in conventional methods. Applying dynamic analysis reduces the processing time from several hours to less than 1 min. The matrix correction factors for use in thick target corrections, are calculated using the local areal mass densities. The over all time reduction of this method is sufficient to calculate the intermediate, peak overlap resolved, background subtracted and thick target corrected concentration distributions while the data acquisition system is gathering a new subset of data.

Optimisation of Pt sensitivity in PIXE analysis of thin biological samples

Abstract To obtain Pt concentration distributions in tumor biopsies, micro-PIXE is used. The Limit of Detection (LOD) for Pt is determined by the background production cross section together with the production cross section for the Pt–L lines. Both are functions of the type and energy of the projectile. In order to optimise the experimental conditions, the proton energy is varied from 2.5 to 5 MeV and the corresponding X-ray spectra are analysed. The optimum LOD is about 1 μg/g for a sample thickness of 1 mg/cm2 and a total charge of 100 μC. A study is performed to investigate the possibility of elemental analysis by means of α excitation. The corresponding LOD is 6 μg/g with 200 μC charge (He++ was used). For α excitation, sample damage limits the beam current and, thus, causes a tremendous increase in the measurement time.

PIXE microanalysis of trace elements of the normoxic, ischemic and reperfused rat heart

With a 3 x 3 mu m(2) proton microbeam spatial distributions of Na, Mg, P, S, K, Ca and Fe were measured via PIXE in 50 x 50 mu m(2) areas of rat heart, sliced into 10-15 mu m thick cryosections. The isolated rat hearts were subjected to normal perfusion, ischemia and reperfusion. Substantial changes in the elemental distribution were found in tissue after 40 min. of reperfusion, particularly indicated by locally elevated Ca and decreased K levels. Electron microscopic examination was used for assessment of artefacts due to sample preparation and handling. Results of stained cryosections analyzed via STIM demonstrated that this latter technique can be employed prior to PIXE analysis to localize individual cells in freeze-dried cryosections.

State and history of heart tissue preparation for proton microprobe elemental analysis at the Eindhoven Cyclotron Laboratory

Proton microprobe techniques are useful in investigating ischemia-reperfusion induced ion shifts between cardiac muscle cells and interstitial fluid. Preliminary results have shown that proper analysis of ion concentrations in various cardiac compartments is hampered by dislocation of elements during sample preparation. In this study the different stages of the preparation were evaluated as possible sources of artifacts. After improvements of the sample preparation procedure, sharp ion concentration gradients within heart tissue preparations were obtained, indicating no noticeable ion dislocation at the scale of the measurements.

PROTON AND X-RAY MICROPROBES IN THE STUDIES OF THE MINERALIZATION PROCESS

Abstract A proton microprobe in combination with proton induced X-ray emission and a X-ray microprobe are used in the study of the mineralization process. It is demonstrated that the application of both methods enables the determination of the crystallographic structure of the inorganic phase in biological samples with a spatial resolution of about 20 μm.