J.C. Overley

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.

Element-sensitive computed tomography with fast neutrons

Pulsed-beam, time-of-flight techniques are used to measure the energy dependence of a collimated, fast-neutron continuum transmitted through an object. By comparing results to similar measurements on pure elements, number densities of elements projected along the neutron beam path are deduced. The method is applied to a translate-rotate scan of an object ∼15 cm in diameter containing hydrogen, carbon, nitrogen, oxygen, phosphorus, and calcium. Techniques of computed tomography are then used to determine the spatial distribution of the various elements in a plane slice of the object. Several factors affecting precision and accuracy are discussed.

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%.

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.

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.

Results of tests for explosives in luggage from fast-neutron time-of-flight transmission measurements

Energy-dependent neutron attenuations measured by time-of- flight techniques are fitted with measured cross sections to determine projected number densities of H, C, N, and O in each 3 by 3-cm2 pixel of a suitcase. Attenuations due to other elements are lumped together as element X. Number densities are normalize by the total to reduce effects of variable thickness. Normalized number densities are constrained to lie in a 4D space. That space is finely binned. A fifth dimension contains six bins of N + O density, as deduced from average suitcase thickness and measured projected number densities. Each bin is labeled with the probability of encountering an explosive, as determined from distributions of elements from simulated explosive and nonexplosive situations. Probabilities for individual pixels are combined to produce an overall probability of an explosive suitcase. About 130 measurements involving 50 suitcases have been made. For about half of those measurements, an explosive was placed in the suitcase. Ten different plastic explosives were used in varying amounts and configurations. When suitcases are ordered by explosive probability, and a threshold is set to minimize errors, the false alarm rate is about 2 percent and the explosive detection rate is about 88 percent. Potential improvements are discussed.

Using a fast-neutron spectrometer system to candle luggage for hidden explosives

A continuous spectrum of neutron switch energies up to 8.2 MeV is produced by a 4.2-MeV nanosecond-pulsed deuteron beam slowing down in a thick beryllium target. The spectrum form the locally shielded target is collimated to a horizontal fan-beam and delivered to a row of 16, 6-cm square plastic scintillators located 4 m from the neutron source. The scintillators are coupled to 12-stage photomultiplier tubes, constant-fraction discriminators, time-to-amplitude converters, analog-to-digital converters, and digital memories. Unattenuated neutron-source spectra and background spectra ar recorded. Luggage is stepped through the fan beam by an automated lift located 2 m from the neutron source. Transmission spectra are measured, and are transferred to a computer while the location is advanced one pixel width. As the next set of spectra is being measured, the computer calculates neutron attenuations for the previous set, deconvolutes attenuations into projected elemental number densities, and determines the explosive likelihood for each pixel. With a time-averaged deuteron beam current o 1(mu) A, a suitcase 60-cm long can be automatically imaged in 1600s. We will suggest that time can be reduced to 8s or less with straight-forward improvements. The following paper describes the explosives recognition algorithm and presents the results of teste with explosives.

Can an electron plasma lens produce sub-micrometer size focal spots of MeV ions?

Abstract The University of Oregon electron plasma lens has focused beams of light and heavy ions with energies up to 5 MeV to spots 5 μ in diameter within distances of 70 cm. The answer to the question of how much it can be improved is contained in the plasma physics of the device. This paper describes the lens and the diagnostic techniques used to study some of the many stable plasmas it will support. Measurements indicate that the spherical aberration limit of some plasmas is below 1 μ for a 1 mm lens aperture. A vacuum of 10 −8 Torr will be required to reduce a central aberration caused by positive ions.