Arlyn J. Antolak

Ion microbeam tomography

Abstract Proton beams with energies of 5 and 7 MeV are focused to 5 μm and used to produce tomograms of capillary tubes and low-density foams. In this energy range, proton energy loss is primarily due to interactions with electrons. Therefore, by measuring the residual energy of protons transmitted through samples in a manner similar to that used for Scanning Transmission Ion Microscopy (STIM), and reconstructing a cross-sectional image from multiple projections, we can map out spatial variations in electron density due to sample geometry and composition. In our experimental arrangement, the sample is translated and rotated in a stationary proton beam. Transmitted proton energies are measured using a silicon surface barrier detector. Tomographie reconstructions are produced from the calculated line-average densities using a procedure based on a filtered backprojection algorithm developed for X-ray computed tomography (CT) systems. The technique is especially useful in characterizing samples where large variations in Z or low total density limit the applicability of X-ray CT analysis.

PIXEF: the Livermore PIXE spectrum analysis package

Abstract PIXEF (for PIXE-fit) is a proton-induced X-ray emission (PIXE) data analysis program designed for analyzing medium to heavy element matrices while retaining the capability to treat lower atomic number targets. Using nonlinear least squares fitting techniques, algorithms have been developed or modified for both fitting the characteristic X-ray peaks and representing the associated bremsstrahlung and γ-ray background. Self-absorption and secondary fluorescence are explicitly determined for K-shell and L-subshell X-rays. Data bases have been created or improved, where necessary, from reliable current literature values or by direct measurement for element mass attenuation coefficients, photoionization and proton ionization cross sections, Coster-Kronig transition probabilities, fluorescence yields, and relative line intensities. The utility of the program is demonstrated with PIXE spectra obtained at Livermore.

Some practical considerations for ion microtomography

Abstract Spatial broadening and energy straggling of the analysis beam can dictate the choice of ion species and required fluence for given spatial and density resolution in ion microtomography (IMT). For extended objects, fine spatial resolution also implies huge data sets. Data acquisition, reduction, and reconstruction rates must be maximized to achieve optimal materials analysis times. In this paper, we discuss these topics and current efforts in this field, present examples of IMT, and consider future directions.

The stand-alone microprobe at Livermore

Lawrence Livermore National Laboratory (LLNL) and Sandia National Laboratories/California have jointly constructed a new stand-alone microprobe facility. Although the facility was built to develop a method to rapidly locate and determine elemental concentrations of micron scale particulates on various media using PIXE, the facility has found numerous applications in biology and materials science. The facility is located at LLNL and uses a General Ionex Corporation Model 358 duoplasmatron negative ion source, a National Electrostatics Corporation 5SDH-2 tandem accelerator, and an Oxford triplet lens. Features of the system include complete computer control of the beam transport using LabVIEW TM for Macintosh, computer controlled beam collimating and divergence limiting slits, automated sample positioning to micron resolution, and video optics for beam positioning and sample observation. Data collection is accomplished with the simultaneous use of as many as four EG&G Ortec IGLET-X TM X-Ray detectors, digital amplifiers made by X-Ray Instruments and Associates (XIA), and LabVIEW TM for Macintosh acquisition software.

PIXE tomography of samples with inhomogeneous elemental composition

Abstract Proton induced X-ray emission tomography (PIXET) can provide quantitative, three-dimensional maps of elemental composition in small (less than a few mm) samples with fine (in principle micron scale) spatial resolution. The concept of PIXET is similar to single photon emission tomography which produces cross-sectional concentration maps of photon emitting radioactive elements within a sample. In PIXET, the photon emitting “sources” are elements along the trajectories (rays) of the incident beam as it slows down in the sample. The number of X-rays detected from a particular element at a given location in a sample depends on the local proton energy dependent X-ray production cross-section and the attenuation of the X-rays from that location to the detector. X-ray mass attenuation coefficients and proton stopping powers are weighted by the local elemental fractions along the detected X-ray path or incident beam direction. Local areal densities needed to complete the calculation of ion energy loss or X-ray attenuation are determined using the complementary density map obtained from ion microtomography (IMT) data. A software package is described which reconstructs cross-sectional images of density and element concentration of data obtained from samples having inhomogeneous elemental composition. A code has also been developed which generates simulated IMT and PIXET sinograms from user-specified density and composition maps of test objects. Simulated data from test objects having inhomogeneous elemental composition have been used to study the quality of images produced by the reconstruction code. Limitations of the PIXET technique are addressed. PIXET and IMT reconstructions from measured experimental data are discussed.

Elastic scattering of electrons and positrons by bound phosphorus, indium, and antimony atoms

Elastic differential and total cross sections of electrons and positrons scattered by bound phosphorus, indium, and antimony atoms have been calculated using the method of partial waves. The interaction potential consists of an electrostatic potential, a Buckingham‐type polarization potential, and, in case of electron impact, the Mittleman–Watson exchange potential. The incident energies range from a few eV to 10 keV. A parameterization of the computed partial‐wave total cross sections in terms of the screened Rutherford total cross section is presented.

Minimum data set requirements for ion microtomography

Abstract Data acquisition and analysis rates and data set size seriously affect the practicality of conducting ion microtomography studies that map out material densities of extended objects in three dimensions. For the promise of todays fine spatial resolution capabilities to be met, huge data sets must be acquired, processed, and analyzed efficiently. In this paper, we present the results of an experimental parametric study exploring the limits of minimizing the amount of data required for tomographic reconstruction. We consider data acquisition system resolution and the effect of limited accuracy of stopping powers. We show graphically the specific results of variation of parameters affecting minimum data set size.

Mapping high-pressure Bridgman Cd/sub 0.8/Zn/sub 0.2/Te

Single crystals of Cd/sub 0.8/Zn/sub 0.2/Te grown at the Institute of Solid State Physics, Chernogolovka, Russia, by the high-pressure vertical Bridgman method (HPVB) were mapped using X-ray fluorescence (XRF), X-ray diffraction (XRD), photoluminescence (PL), and leakage current measurements, most of the Russian samples which we refer to as p-type CZT were more uniform in Zn composition than U.S. commercially produced material. The Russian material had a poorer crystallinity and, in the best case, could only count nuclear radiation. Differences in the material properties between Russian (p-type) and U.S. (n-type) material will be described.

Material inhomogeneities in Cd 1−x Zn x Te and their effects on large volume gamma-ray detectors

Cadmium zinc telluride (Cd1−x ZnxTe or CZT) has shown great promise as a material for room temperature x-ray and gamma-ray detectors. In particular, polycrystalline material grown by the high pressure Bridgman method with nominal Zn fraction (x) from 0.1 to 0.2 has been used to fabricate high resolution gamma-ray spectrometers with resolution approaching that of cooled high-purity Ge. For increased sensitivity, large areas (> 1 cm2) are required, and for good sensitivity to high energy gamma photons, thick detectors (on the order of 1 cm) are required. Thus, there has been a push for the development of CZT detectors with a volume greater than 1 cm3. However, nonuniformities in the material over this scale degrade the performance of the detectors. Variations in the zinc fraction, and thus the bandgap, and changes in the impurity distributions, both of which arise from the selective segregation of elements during crystal growth, result in spectral distortions. In this work, several materials characterization techniques were combined with detector evaluations to determine the materials properties limiting detector performance. Materials measurements were performed on detectors found to have differing performance. Measurements conducted include infrared transmission, particle induced x-ray emission, photoluminescence, and triaxial x-ray diffraction. To varying degrees, these measurements reveal that “poor-performance” detectors exhibit higher nonuniformities than “spectrometer-grade” detectors. This is reasonable, as regions of CZT material with different properties will give different localized spectral responses, which combine to result in a degraded spectrum for the total device.

Density and composition analysis using focused MeV ion mubeam techniques

Abstract Nuclear muscopy uses focused MeV ion mubeams to non-destructively characterize materials and components with mun scale spatial resolution. Although a number of accelerator-based mubeam methods are available for materials analysis, this paper centers on the techniques of Ion mutomography (IMT) and Particle-Induced X-ray Emission (PIXE). IMT provides quantitative three-dimensional density information with mun-scale spatial resolution and 1% density variation sensitivity. Recently, IMT has become more versatile because greater emphasis has been placed on understanding the effects of reconstruction artifacts, beam spatial broadening, and limited projection data sets. PIXE provides quantitative elemental information with detection sensitivities to 1 μg/g or below in some instances. By scanning the beam, two-dimensional maps of elemental concentration can also be recorded. However, since X-rays are produced along the entire path of the ion beam as it penetrates the sample, these measurements only give depth-averaged information in general. PIXE tomography (PIXET) is the natural extension from conventional PIXE analysis to the full three-dimensional measurement and forms the bridge linking the complementary techniques of PIXE and IMT. This paper presents recent developments and applications of these ion beam techniques in a diverse range of fields including characterizing metal-matrix composites, biological specimens and inertial confinement fusion targets.