K.K. Loh

The National University of Singapore nuclear microscope facility

Abstract The National University of Singapore nuclear microscope facility is based around a HVEC AN2500 single ended Van de Graaff accelerator and an Oxford Microbeams coupled quadrupole triplet focusing system. Particle induced X-ray emission (PIXE), nuclear or Rutherford backscattering spectrometry (RBS) and scanning transmission ion microscopy (STIM) can be carried out simultaneously. Data acquisition is carried out using a simple but flexible PC based system (Oxford Microbeams DAQ) and the data is analysed using a combined RUMP and GUPIX PC based interactive package (NUSDAN) acting under WINDOWS. Resolution tests using a calibration grid and a multi layer integrated circuit have shown the facility to be capable of 600 nm spot sizes for 2 MeV protons at currents suitable for microanalysis.

Parametrization of the total photon mass attenuation coefficients in the energy range 0.1–1000 keV

Abstract It is convenient to generate mass attenuation coefficients using semi-empirical schemes. The validity of most of the existing schemes is limited to a relatively narrow energy interval (1–40 keV) and their accuracies are poor in some energy regions. In this work, a semi-empirical scheme flexible enough to give a good fit to data in a very wide photon energy range (0.1–1000 keV) was employed. Fitting coefficients for the entire range were obtained by utilizing mass attenuation data from two sources: (1) semi-empirical data of Henke et al. in the low photon energy region, and (2) theoretical values generated with the XCOM code for fitting in the high energy region. The root mean square of the fit is generally less than 0.2% except for energies below 1 keV where the available data are scattered. A computer code for generating mass attenuation coefficients based on the proposed scheme has been developed.

New parameters for the calculation of L subshell ionization cross sections

Abstract New parameters for the calculation of L subshell ionization cross sections using a semiempirical expression are presented. The parameters were obtained by fitting an analytical function to the experimentally obtained values of L subshell ionization cross sections. The experimental data included compilation of the published data from 1975 to 1982 by Sokhi and Crumpton as well as our newly compiled data published in 1982–1991. The number of data employed for the fitting was 2295, 3 1 2 times more than that used in Miyagawas work. The data were divided into six groups according to the Z values: 14 ≤ Z ≤ 42, 43 ≤ Z ≤ 50, 51 ≤ Z ≤ 60, 61 ≤ Z ≤70, 71 ≤ Z ≤ 80 and 81 ≤ Z ≤ 92. Each was fitted by the analytical function to obtain the parameters for the calculation of L1, L2 and L3 subshell ionization cross sections. For data in the group with Z ≤ 42, only parameters for the calculation of L total ionization cross sections were determined. A comparison of the various theoretical predictions with the experimental data and the fits is also presented.

Computer simulation of PIXE and μ-PIXE spectra for inhomogeneous thick target analysis

Abstract A computational algorithm capable of generating 2D elemental maps in μ-PIXE analysis of inhomogeneous thick targets has been developed and implemented in a program written in C language. The program allows users to define 3D inhomogeneous thick targets in terms of slices and inclusions. The use of this program to generate 2D elemental maps and line scan intensity distributions for several simple targets is demonstrated. One of the targets is a glass block with a thin dip on its surface. The line scan intensity distribution generated agrees reasonably well with that obtained from an experiment.

A PIXE MICRO-TOMOGRAPHY EXPERIMENT USING MLEM ALGORITHM

Abstract A new approach for PIXE tomography has recently been introduced by our group. The reconstruction program is based on the maximum likelihood expectation maximization (MLEM) algorithm. Since the algorithm is built upon the actual stochastic model of X-ray production and propagation, attenuation correction as well as ion energy loss are readily incorporated into the reconstruction. The resultant procedures have been previously tested by using simulated X-ray yields and the results were given in an earlier paper. In this work, we present the results of the tomographic reconstruction of a realistic sample done in order to study the effectiveness of the algorithm.

Application of an iterative maximum-likelihood algorithm in PIXE depth profiling of trace elements

Abstract We have applied an iterative maximum-likelihood algorithm to extact trace element depth profiles from PIXE yields data for various incident proton energies. This algorithm is based on the exact stochastic model of X-ray yield measurements. We have tested the algorithm using calculated X-ray yields generated from a PIXE simulation program. Several profiles of phosphorus with depth extensions of a few micrometers in a silicon matrix are used to generate the X-ray yields. The maximum-likelihood algorithm is able to reconstruct the depth profiles from these yields successfully even when Poisson noise is added to the yield data. The reconstructed profiles are smooth and do not suffer from erratic fluctuations normally encountered in other conventional deconvolution methods.

Nuclear microscopy of single aerosol particle

Abstract The Nuclear Microscope at the National University of Singapore is being used for the analysis of single aerosol particles. The methodology of utilizing off-axis Scanning Transmission Ion Microscopy (STIM) to identify and characterize the size and shape of the particles, Rutherford backscattering (RBS) to determine the matrix composition and effective thickness, and Particle Induced X-ray Emission (PIXE) to determine the minor and trace elements to the ppm level, is presented. Thin film pioloform substrates are found to be the most suitable backing material for the single particle analysis.

Stoichiometric analysis of YBaCuO superconductors using deuterons

In principle, deuteron irradiation can be used for a complete Stoichiometric analysis of YBaCuO superconductors. The contents of all the four chemical constituents can be determined by simultaneous detection of the 0.871 MeV prompt gamma rays from the 16O(d, pγ)17O∗ reaction and of the characteristic X-rays produced by DIXE (deuteron induced X-ray emission). In this paper we present the approach taken and the results obtained in exploring the applicability of this technique for accurate quantitative determinations of the chemical constituents of bulk YBaCuO superconductors.

TTPIXAN — the 4th generation

Abstract During the past six years the computer code TTPIXAN [I. Orlic et al., NIMB 49 (1990) 166] went through many changes and developments. From its infant stage when it was developed for a simple quantitative broad beam PIXE analysis, through a more sophisticated version [K.K. Loh et al., NIMB 77 (1993) 132] suited for applications in Nuclear Microscopy (NM). At present, the program is finally reaching its mature stage or its 4th metamorphosis [S.C. Liew et al., NIMB 104 (1995) 222]. It is now capable of not only simulating NM elemental images but also performing quantitative analysis and reconstruction of a complex 3D elemental composition encountered in NM applications. From a given initial 3D elemental distributions the program calculates exact elemental maps, compares them with the corresponding measured maps and by using a very efficient iterative Maximum-Likelihood Expectation-Maximisation (MLEM) algorithm, it calculates a new 3D elemental distributions. Energy loss of the incident particles and attenuation of the X-ray photons are incorporated into the reconstruction algorithm by using the most up-to date data base. The reconstruction algorithm has been successfully tested on several samples such as thin tungsten wire coated with paint, integrated circuits, and single aerosol particles. The computer code is briefly described and future plans outlined.

RECONSTRUCTION OF AR DEPTH PROFILES FROM PIXE MEASUREMENTS

Abstract We report on the application of an iterative maximum likelihood algorithm [1] to the reconstruction of depth profiles from PIXE measurements. PIXE spectra of 450 and 800 keV Ar implanted Al samples were taken at 1.5 and 1.3 MeV He + energy and at angles ranging from 18 to 83°. The measured Ar yields are in good agreement with those calculated by a PIXE simulation program. The reconstructed depth profiles are compared to those predicted by an ion implantation simulation code (TRIM).