Position Sensitive Detector System Research Articles

Segmented array reconstructions for tomographic imaging of large objects: Theory and an application to neutron computed tomography

many nondestructive testing applications, high resolution images are desired of large engineering superstructures, such as aircraft. Traditionally, this is done by sequentially imaging small segments of the objects using small area detectors. Recently, we have developed a position sensitive neutron detector system using a LiF-ZnS scintillator screen and a cooled charge coupled device (CCD) camera. Large arrays of these individual detectors can be assembled to image large areas simultaneously at high resolution. For tomography, radiographs are obtained from multiple viewing angles with the large area detector. Tomographic reconstruction of large arrays is prohibitively time consuming and has several associated practical problems. By using the inherent segmentation of the detector, the reconstruction problem can be reduced to one of several small area reconstructions performed in parallel. A parallel tomographic reconstruction scheme based on segmented arrays is presented. It is shown that the parallel algorithm produces images that are identical to images produced without segmentation, however, using smaller individual arrays. The intrinsically parallel segmented array reconstruction provides a means of rapidly reconstructing images of any dimension using small, tractable arrays. An N x N two-dimensional tomographic reconstruction can be performed with m individual detector segments and m2 processors each reconstructing 2N/m x 2 x N/m image segments. This approach is demonstrated for neutron tomography; however, it is applicable to any tomographic imaging problem.

Time-resolved X-ray scattering studies of rapid crystallization of amorphous metals

Time-resolved X-ray scattering has been used to study the kinetics of crystallization of amorphous metals on time scales varying from minutes to milliseconds. Measurements have been at the National Synchrotron Light Source, using a wide-bandpass monochomator and fast linear position-sensitive detector system at the IBM/MIT beamline X-20C. With this apparatus, scattering patterns from the transforming material can be acquired with 3-ms time resolution. A fast pyrometric temperature controller has been developed to change and regulate the sample temperature with microsecond response. In a typical measurement, two synchronized position-sensitive detectors provide complementaryin situ information about the transformation. The first is used for wide-angle scattering and allows us to determine which phases are present and to what extent. Changes in lattice parameter and particle size can also be deduced. The second detector measures small-angle scattering, yielding additional information about the microstructure, such as the spacing between lammellae in eutectic systems. At relatively slow transformation rates, quantitative measurements of crystal volume fractions as small as 10−4 are possible. In many systems, as the transformation rate is increased, the crystallization mechanism changes such that new metastable crystal phases are formed en route to the equilibrium structure.

Diffuse scattering in yttria-stabilized cubic zirconia

We have used a position-sensitive detector (PSD) system to make measurements of the diffuse X-ray scattering on a cubic Y 2O 3-stabilized zirconia, Zr 0.61Y 0.39O 1.805, in far greater detail than has hitherto been reported. In addition to the fairly prominent diffuse peaks visible in 〈1 1 0〉 sections that have been the center of discussion in many previous studies we see a number of other characteristic diffraction features. We report results of some Monte Carlo simulations in which we attempt to find the origins of these various features. We believe one feature, in the form of sets of dark planes normal to 〈1 1 0〉, occurs because of size-effect-like strains induced along 〈1 1 0〉 intermetal vectors, and is only visible because of the similarity of the scattering factors for Zr and Y. A second feature, in the form of bow-tie-shaped regions of scattering, originates from the same basic strains but the symmetry of these requires that displacements in the [1 1 0] direction are out of phase with those in the [1 – 1 0], implying a distortion of the basic metal coordination such that an expansion along [1 1 0] is accompanied by a contraction along [1 – 1 0].

Three-dimensional reconstruction of atomic-scale composition with the position-sensitive atom probe

The combination of a field ion microscope with a time-of-flight mass spectrometer provides the capability for chemical microanalysis at the single atom level. Such an instrument is termed an Atom Probe. Conventionally, the connection between the microscope and the mass spectrometer is made via a small aperture hole in the imaging screen. This defines a region on the specimen, typically about 2nm across, from which the analysis is obtained. The disadvantage of this arrangement is that other regions of the specimen cannot be examined, as ions from all but the selected area strike the image screen and therefore do not pass into the mass spectrometer. In order to overcome this problem, we have developed a version of the Atom Probe which incorporates a wide-angle position sensitive detector system. This instrument, which we have termed the POSAP, is shown schematically in figure 1. Typically, the field of view in this instrument is about 20nm across. The number of ions collected per atom layer removed from the specimen surface is therefore approximately 5,000.

A position-sensitive detector system for the measurement of diffuse X-ray scattering

The use of a one-dimensional position-sensitive detector for diffuse X-ray scattering measurements is described. Calibration procedures for scattering angle and intensity measurements are discussed. Some nonuniformities have been found in the counting efficiency as a function of distance along the detector. A procedure is described for measuring the diffuse scattering in a section of reciprocal space.

X-Ray Powder Diffraction Data of a New Compound As4Mo3015

AbstractArsenic molybdate, As4Mo3015, has been investigated by means of X-ray powder diffraction. The diffraction data were collected with a focussing (Guinier-type) transmission diffractometer equipped with a primary-beam monochromator (Ge 111) for CuKα radiation and a scanning position-sensitive detection system. A monoclinic unit cell was determined using an indexing program.

Instrumentation for millisecond-resolution scattering studies (invited)

Time-resolved x-ray scattering studies of phase transition kinetics have been carried out using the wide-bandpass monochromator and fast linear position-sensitive detector system at the IBM/MIT beamline X-20C at the National Synchrotron Light Source (NSLS). We report here on the instrumentation that has been developed for these studies, and in particular on the methods used to measure, change, and control sample temperature with millisecond resolution.

Development of a new type of self-scanned electron image sensing integrated circuit

The design and fabrication of an integrated circuit which is a position-sensitive detector for low-energy (typically up to 100 eV) charged particles are discussed. The device comprises a linear array of electron-sensing electrodes coupled to detection and readout circuitry in the form of amplifiers and counters all integrated onto a single CMOS chip. The chip is intended to replace existing position-sensitive detection systems in electron spectrometers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A position-sensitive X-ray detector for an X-ray/ion coincidence experiment

For an atomic lifetime experiment on slow, highly-charged recoil ions a low-cost, position-sensitive detector system has been developed. It consists of a Soller slit system of 10 cm length which may be surrounded by up to four position-sensitive proportional counters for maximum detection solid angle. The design considerations, efficiency and linearity test of the PSPC are described as well as the performance in the actual experiment.

Disordering of the (111) surface of germanium crystal near its bulk melting temperature

The reversible disordering transition of the Ge(111) surface near 1050 K (160 K below the bulk melting temperature) has been studied by low-energy electron diffraction (LEED). Measurements were made using a position-sensitive detection system with the following characteristics: maximum speed, ${10}^{5}$ electrons/sec; resolution, 256\ifmmode\times\else\texttimes\fi{}256 channels with up to ${2}^{16}$ counts per channel. LEED peak and total (integrated) intensities I were recorded for varying electron energy E [I(E) plots] or varying crystal temperature T [I(T) plots]. Angular intensity profiles and intensity contour plots were also recorded. The I(E) plots are interpreted to indicate that layerlike crystalline order is preserved in the transition, up to but possibly not including the outermost double layer. The angular intensity profiles are interpreted to rule out thermal roughening as a disordering mechanism. The ratios of nonspecular beam intensities to the specular beam intensity for T&gt;1050 K are found to be incompatible with a surface-melting mechanism like that described by molecular-dynamics (MD) simulations. The I(T) plots exhibit features, such as ranges of positive slope, which cannot be explained by any previously proposed mechanism of surface disordering. The intensity contour plots for T near 1050 K reveal satellite peaks which also have no conventional explanation. A domain-disordering mechanism is proposed to explain the observations qualitatively. In this mechanism, the domains are laterally strained to a depth of one double layer of crystalline Ge(111). The disordering is described as a loss of registry between the strained domains and the substrate. The possible role of intrinsic lateral compressive stress at the Ge(111) surface is discussed with reference to the observations, the disordering mechanism, and relevant MD simulations. Experimental results for Ge crystal films and a theoretical treatment of LEED from strained domains are presented in the Appendixes.

A position-sensitive detector system with subnanosecond timing and its utilization in the measurement of pion absorption

A complex position-sensitive, time-of-flight detector was designed at the SIN to be used in the kinematically complete measurements of pion absorption in light nuclei, for the detection of both neutrons and charged particles. Design parameters and TOF properties resulting from the computer calculations and suitable test measurements are given for the system consisting of 200 × 10 × 5 cm 3 NE 102A scintillator bars, XP 2230 photomultipliers and discriminator. Position dependence of the timing parameters of the 2 m long NE 102A bar viewed from the ends by photomultipliers has been clearly revealed in our model and experimentally verified.

Evidence for a resonance in positron-electron scattering at 810 keV centre-of-mass Energy

A monoenergetic positron (e+) beam (ΔT/T<5·10−4) from the Stuttgart pelletron accelerator and a 4.6 mg/cm2 Be target has been employed to investigatee+e−scattering in the MeV region in a large solid angle with high statistics by means of a novel positionsensitive detector system. Superimposed on the Bhabha scattering, a structure has been found at ane+ kinetic energy of 2263 keV (810 keV excitation energy in thee+e−rest frame). From the energy-integrated resonance cross-section of\(\sigma _{res} \cdot \Gamma _{res}^{e^ + e^ - } \) ≅30 b·eV (c.m.) and the standard resonance cross-section we estimate partial resonance widths\(\Gamma _{res}^{e^ + e^ - } \) ≅72 meV or 24 meV for total angular momenta J=0 or J=1, respectively. The structure, which has not been predicted within the framework of quantum electrodynamics, coincides with one of the sum energies of the correlatede+e−lines observed in heavy-ion collisions.

Characterisation of Ultrafine Microstructures Using a Position-Sensitive Atom Probe (POSAP)

ABSTRACTA new development in the experimental techniques of atom probe microanalysis is described, which involves the use of a position sensitive detector system. This detector subtends a large solid angle (∼20°) at the specimen, and therefore permits the collection of ions from a substantial fraction of the whole surface area of the emitter. Progressive pulsed field evaporation leads to the construction of a three-dimensional map of the atomic chemistry of the specimen. The new instrument is ideally suited to the investigation of complex, ultrafine microstructures. Applications to the study of age-hardened aluminium alloys and Alnico permanent magnet materials are described.

The use of oscillation on PSD-based instruments for X-ray measurement of residual stress

Measurements of surface residual stress by X-ray diffraction can be accomplished very rapidly using a position-sensitive detector (PSD) to record the diffraction profile. It is shown that when the sample and detector are fixed during data accumulation, the profile shape may be distorted by large grain size or preferred orientation, which results in substantial error in the calculation of residual stress. Sample oscillation decreases the sensitivity of the PSD system to such microstructural variables in many cases. Instruments employing a PSD to measure surface residual stress should be designed with the capability of oscillating either the sample or the measuring head (X-ray tube and detector).

Novel two-dimensional position-sensitive detection system

A novel two-dimensional position-sensitive detection system has been designed, built, and tested. It has a relatively short processing time (900 ns) which enables countrates up to 105–106/s. The detector has a ring-shaped geometry. Both polar coordinates can be determined with an accuracy better than 1%. Although intended for use in an ion scattering spectrometer, the system can also be used for detection of other atomic particles or photons.

A parallel electrode position sensitive detector system for high counting rates

The detector system has been developed for structural biology and protein crystallography study with synchrotron radiation. The detector consists of a parallel gap amplification configuration, consisting of meshes instead of thin anode wires as in MWPCs. The fast two-dimensional readout system (in ECL technology) has dead-times as short as 50 ns, and a positional resolution of 0.3 mm fwhm. A local counting rate capability of 2.5 × 10 5 s −1 mm −2 has been measured.

A position sensitive gas detector system for Coulomb excitation experiments

A position sensitive parallel-plate avalanche counter for detection of heavy ions, primarily from Coulomb excitation experiments, is presented. The angular region is 40° < θ < 140°, with a division in θ of 2°.

An apparatus for measuring collisional dissociation and electron capture of molecular ions

Dissociated fragments and neutral particles resulting from the collision of diatomic molecular ions with rare gas target atoms are measured using a microchannel plate position sensitive detector (MCP-PSD) system, while ionized target particles are monitored with another MCP detector. Coincidence and TOP techniques are applied to examine their timing relations. The yield fractions of the collisional products as a function of the target thickness are processed according to a growth rate method, thus the cross sections for electron capture and dissociation with and without target ionization are derived.

A detector system for discrete nuclide identification of ions with A ⩽ 60

An x− y position-sensitive detector system which provides simultaneous mass and charge identification with discrete nuclide resolution is described. This system is specially suitable for low-energy, medium-mass ( E 1 ≈ 1–10 MeV/mu; A ≈ 10–60) heavy tons. Charge identification is determined from a two-element gas-ionization counter coupled with a semiconductor stop detector. The gas-ionization counter employs a simple electron drift time technique to obtain two-dimensional position information. Mass identification is derived from measurement of the fragment time-of-flight with a pair of channel-plate time devices and the fragment energy, as provided by the ΔE y – ΔE x – E telescope. This system yields mass and charge resolution ⩽0.7 units in both A and Z and gives position resolutioon <0.5 mm for fragments with A ≲60, as determined from measurements performed with a 7.2 MeV/u 56Fe beam incident on 238U.

Position sensitive and Bragg curve spectroscopy detector system

A heavy ion gas detector system consisting of a Bragg-curve spectroscopy ionization chamber for particle identification and a multiwire proportional chamber as position sensitive fast trigger device is described. The Bragg IC has been tested with several beams up to Z = 36 to investigate some aspects of the BCS method. Results are reported on energy resolution and linearity, Z resolving power and mass sensitivity. The energy resolution is well below 1%. The Bragg-peak amplitude is fairly independent of the energy in a wide energy range and single elements are identified up to Z = 38 with a resolving power Z/ ΔZ − 50-80. Isotope identification by range measurement is limited by the straggling in the ionization process and the mass resolving power is M/ ΔM ∼ 20–26 for S and Si isotopes. The MWPC allows subnanosecond time resolution and position identification along the in-plane coordinate within ±0.5 mm.