H.W. Lefevre

Energy-loss radiography with a scanning MeV-ion microprobe

Abstract Beams of 3 MeV H+, He+ and Ne+ ions are focused to dimensions of the order of 5 μm in a scanning ion microprobe and are rastered across a specimen. Residual energies of individual ions transmitted through the specimen are measured with a Si(SB) detector at each point of a 256×256 grid. These data are reconstructed as microradiographs. Examples are shown which illustrate contrast ranges, spatial resolution, sensitivity and noise reduction techniques.

Energy-loss image formation in scanning transmission ion microscopy

Abstract One contrast parameter which can be used to form images in scanning transmission ion microscopy (STIM) is the energy loss of individual transmitted ions. This paper explores three techniques which can be used if energies of several ions are measured at each incident beam location. The techniques are energy averaging, summing of events within a preset energy window, and median filtering of energy values. Median filtering is the method of choice to reduce noise and to sharpen certain features. Examples of line scans of a sharp edge and of a ramp-like knife edge are presented. With median filtering localization of features to within 10% of beam size is demonstrated. A mechanism for identifying and mapping unresolved spatial structure is proposed. The other techniques are shown to be better tools for beam-profile diagnostics.

PIXE-STIM microtomography: Zinc and manganese concentrations in a scorpion stinger

Abstract Microtomography using PIXE (proton induced X-ray emission) is developed and applied. Iterative algorithms for emission tomography (Donner algorithms) are modified for this purpose. STIM (scanning transmission ion microscopy) microtomography provides the required local X-ray attenuation factors and X-ray production cross sections (σ x s). The variation in σ x is treated along with X-ray attenuation in an effective attenuation factor. Several criteria for the optimal number of iterations are discussed. Element density values obtained from PIXE tomography are normalized with density values from STIM tomography to provide local concentration values. Three transverse sections of the sting of a scorpion, Hadrurus arizonensis , are reconstructed tomographically. Local zinc concentrations reaching as high as 25(± 3)% of dry mass are found. Manganese concentrations reaching 3.8(± 0.5)% of dry mass are also found. In the section manifesting high concentrations of both manganese and zinc, the zinc is found mainly in a band beneath the manganese-rich surface layer. Specimen damage during PIXE is found not to have significantly affected the results.

Explosives detection through fast-neutron time-of-flight attenuation measurements☆

Abstract Computer simulations have been used to devise an algorithm for detection of explosives in luggage which is based upon measured projected number densities of H, C, N, and O. Other elements are lumped together as projection X. Dependence on luggage-thickness is reduced by normalizing the projection for each element by the total. Normalization constrains projections to a 4-dimensional space. Distributions of nonexplosive (N) and explosive (E) situations are generated by sorting results of simulations into bins in that 4-space. A detection matrix element, given by the ratio E (N + E) for each bin, is addressed by a measurement. For a realistic distribution of the numbers and types of luggage materials, the plastic explosive RDX, at 10% of suitcase thickness, can be detected in a single pixel with 85% reliability and a false alarm rate of 3%.

STIM with energy loss contrast: an imaging modality unique to MeV ions

Abstract Scanning transmission ion microscopy (STIM) through measurement of energy loss of individual ions is a quantitative imaging technique with several unique capabilities. The uniqueness derives conjointly from the large penetration with small scattering of MeV ions in low-Z specimens, from the simple relationship between energy loss and projected or areal density, and from the almost 100% efficiency with which one obtains pixel data from individual ions. Since contrast is in energy loss and not in numbers of events, the statistics of energy loss straggling affects the image but the statistics of counting does not. Small scattering makes it possible to observe details within transparent specimens. High efficiency makes it possible to collect large data sets for computed tomography, stereo, or high-definition imaging with a small radiation dose. High efficiency allows one to minimize aberrations by use of small apertures, to achieve good precision in the determination of areal density, or even to image live biological specimens in air since only one or a few ions per pixel are required. This paper includes a bibliography on STIM with MeV ions, it discusses the accuracy that one can achieve in the areal density coloring of a pixel with data from one or a few ions, and it supplements that review with recent examples from the Melbourne and the Eugene microprobes.

X-ray microanalytical surveys of minor element concentrations in unsectioned biological samples

Abstract Approximate concentration maps of small unsectioned biological samples are made using the pixel by pixel ratio of PIXE images to areal density images. Areal density images are derived from scanning transmission ion microscopy (STIM) proton energy-loss images. Corrections for X-ray production cross section variations, X-ray attenuation, and depth averaging are approximated or ignored. Estimates of the magnitude of the resulting error are made. Approximate calcium concentrations within the head of a fruit fly are reported. Concentrations in the retinula cell region of the eye average about 1 mg/g dry weight. Concentrations of zinc in the mandible of several ant species average about 40 mg/g. Zinc concentrations in the stomachs of these ants are at least 1 mg/g.

Scanning MeV-ion microprobe for light and heavy ions

Abstract The University of Oregon scanning ion microprobe uses a 65 cm focal length plasma lens to form 8.65 × demagnified image of an object aperture. The plasma lens focuses a positive ion beam using the self-electric field of a trapped cylindrical column of electrons of density 3−9 × 10 9 cm −3 and length 13–18 cm. Since the focusing field is electric, the focal length depends only on ion accelerating voltage and not on ion mass or charge state. Our 5 MV Van de Graaff accelerator illuminates the object aperture with a current density of ∼ 0.5 pA/mu;m 2 . The lens aperture is defined by a set of slits 3.35 m beyond the object aperture slits and 2.79 m from the lens. Four pairs of deflection plates are located between the intermediate aperture and the lens. Two pairs of plates are used for each scanning direction so the beam always passes through the lens center during rastering. The 1 kV operational amplifiers that drive these plates combine three sets of signals. Computer generated voltages raster the beam. Individual dc offset voltages align the beam with the lens axis. A small 60 Hz signal cancels the effects of background 60 Hz magnetic fields along the beam line. With 1 kV rastering voltage the rastered field at the focal plane is 3 mm square for 3 MeV ions. Focal spot size is now 10 μm with a 2 mm diameter lens aperture and 5 μm with a 0.5 mm lens aperture.

Thin Si3N4 windows for energy loss STIM in air

Abstract The use of thin (102 nm) windows of Si 3 N 4 to transmit an electronically rastered microbeam of MeV protons into ambient air with small scattering is reported. The windows were made by anisotropic etching of Si, exposing about 1 mm 2 of silicon nitride film integrally attached to a silicon supporting frame. Stress measurements on films of several thicknesses yielded a value of Youngs modulus of 350 GPa. Energy loss scanning transmission ion micrographs of several small living animals are presented. Focused beams of 100 pA have been passed through the films without failure.

Analysis of unsectioned specimens: 2D and tomographic PIXE with STIM

Abstract Two techniques for quantitative analysis of heterogeneous thick specimens are reviewed. First, a measurement of the total zinc content in the abdomen of a fly is reported. The calculation used to determine the uncertainty in this measurement is detailed. STIM measurements at two different angles were used to determine the possible range of the X-ray production cross section and the X-ray attenuation factors. The abdomen content of zinc was determined to be 0.022( + 0.009, −0.006) (μg. Second, a PIXE-STIM mutomographic determination of metal concentrations in the sting of a scorpion is reviewed. STIM tomography was used to determine the X-ray production cross sections and the X-ray attenuation factors for points inside of the specimen. Details of how this information was then used for reconstructions of PIXE tomographic data are given. Finally, 2D PIXE measurements on a thick section of this sting are reported. On this section the measured concentration of zinc reached 22(± 3)% of dry mass, in satisfactory agreement with the results from tomography, 25(± 3)%.

Average atomic number of heterogeneous mixtures from the ratio of gamma to fast-neutron attenuation

Abstract The attenuation of a continuous spectrum of fast neutrons by each pixel in a luggage image can be used to detect plastic explosives in the presence of other materials. The method involves deconvolution of the attenuations into elemental compositions of H, C, N, O and X, where X includes everything other than these elements. To improve discrimination, one can also measure the average atomic number, Z, of the mixture of materials in each pixel. We have measured attenuations, by single-element samples, of prompt γ-rays produced from bombardment of a thick Be target with 4.2 MeV deuterons. We report, in this regard, measured prompt γ-ray spectra, whose median energies indicate that associated γ attenuations are dominated by Compton scattering. Attenuations were measured subsequently for several thicknesses each of C, Al, Cu, Cd, Sn and Pb. We have also investigated the attenuation of the fast-neutron spectrum between 8.2 MeV and 5.5 MeV. We found that the ratio of γ-ray to neutron attenuation generally increases, albeit not linearly, with Z. We apply these results to examine the effective Z of heterogeneous mixtures of H 2 OC and CAl, and have begun to incorporate effective Z into explosives-detection algorithms.