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This paper reports current developments of a tungsten-moderated positron gun, which has proven to be very practical. It is being used in a multifunctional manifold, designed for applications of slow positron spectroscopy to surfac...
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This paper reports current developments of a tungsten-moderated positron gun, which has proven to be very practical. It is being used in a multifunctional manifold, designed for applications of slow positron spectroscopy to surface analysis, and alternately allowing in situ characterizations, by other methods, of the surfaces being studied. The system includes a miniature differential spectrometer which has a large solid angle that accommodates the wide convergence angles usually required to focus slow positrons. A demonstration that microchannel plates can be used for position-sensitive detection of positrons, without background from gamma rays, has been made. Attempts to improve brightness with transmission moderators and fine particle moderators are reported. Preliminary measurements of positron energy loss spectra are reported. (ERA citation 07:046064)
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Slow-position beams produced from negative-work-function solid-state moderators have found numerous applications in condensed matter physics. There are potential advantages in using low-energy primary electron beams for positron p...
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Slow-position beams produced from negative-work-function solid-state moderators have found numerous applications in condensed matter physics. There are potential advantages in using low-energy primary electron beams for positron production, including reduced radiation damage to single-crystal moderators and reduced activation of nearby components. The authors present numerical calculations of positron yields and other beam parameters for various target-moderator configurations using the Argonne Wakefield Accelerator (AWA) (1) and Advanced Photon Source (APS), (2) electron linacs, (3) as examples of sources for the primary electron beams. The status of experiments at these facilities is reviewed.
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Positron spectroscopy has advanced to the point where it is in the best interest of DOE to assess past progress and to identify research needs/opportunities that can be exploited to advance the understanding of materials problems ...
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Positron spectroscopy has advanced to the point where it is in the best interest of DOE to assess past progress and to identify research needs/opportunities that can be exploited to advance the understanding of materials problems important to DOE. Purpose of the workshop is to identify areas of materials science where positron spectroscopy can serve to advance goals of DOE in energy research: problem areas for which positron spectroscopy can serve as a unique or complementary tool for materials characterization and analysis, possible sources of positrons at high intensities and instrumentation, and possible applications (defect profiles at surfaces/interfaces, composite materials, superconductors). Separate abstracts and indexing were prepared for the 23 papers.
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The interest of the Positron Working Group, attended by about 20 participants during this workshop, was focused on two main topics: positron production and the motivation for implementing a positron beam. We tried to answer the qu...
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The interest of the Positron Working Group, attended by about 20 participants during this workshop, was focused on two main topics: positron production and the motivation for implementing a positron beam. We tried to answer the questions: how, how many, and why. We had no time left to work on the cost of the beam, and the project itself is still rather indefinite. Nevertheless, we think that the order of magnitude, given in the previous report, namely, a few percent of the total CEBAF cost, remains correct. Positron production is discussed in section 1. Reactions using directly the positrons themselves are presented in section 2. The characteristics of secondary photon beams which can be produced by positron beams are detailed in section 3, together with some applications. (ERA citation 11:055201)
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The Maximum Likelihood Estimator (MLE) algorithm for tomographic image reconstruction is being investigated in substantial detail by a number of research groups, as it appears to promise images with very low noise and increased sh...
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The Maximum Likelihood Estimator (MLE) algorithm for tomographic image reconstruction is being investigated in substantial detail by a number of research groups, as it appears to promise images with very low noise and increased sharpness when compared with filtered backprojection techniques. Recently, however, it has been found that the reconstruction of data from uniform activity distributions exhibits strong peaks and valleys when the number of iterations increases toward a maximum in the likelihood function. This problem has now been investigated with our Positron Emitter Beam Analyzer (PEBA) camera, which, because of its small size and favorable geometry, has allowed an analysis with enough detail to find the origin of that apparent instability. The findings can be summarized as follows: The very low noise of the MLE reconstructions comes about by the ability of the Poisson-based MLE algorithm to generate an image which favors the matching of experimental data (detector pairs) containing few counts; the image instability at a high number of iterations is a direct consequence of the above characteristic; and the matrix of probability elements needed for the MLE reconstruction provides the link between the two above observed phenomena. It appears that, by proper system design, it is possible to obtain the favorable low noise characteristic without the instability. The applicability of the above findings to true tomography (PEBA does not carry out a true tomographic reconstruction) seems direct, but confirmation should be obtained by further research on the question. (ERA citation 11:038641)
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Positronium fraction measurements using positron beams have been utilized to extract information about the diffusion properties of positrons as well as defect concentrations in the near surface region of materials under a variety ...
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Positronium fraction measurements using positron beams have been utilized to extract information about the diffusion properties of positrons as well as defect concentrations in the near surface region of materials under a variety of experimental conditions. Owing to this recent interest we have undertaken to study some of the systematics and uncertainties associated with measurements of the positronium fraction, f. We restrict our discussion to determinations of f based on the peak:total ratio of counting rates for a single detector, only briefly considering alternate ways of obtaining f. We conclude with several recommendations that should be of particular interest to practitioners in the field. (ERA citation 10:014899)
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Positronium fraction measurements using positron beams have been utilized to extract information about the diffusion properties of positrons as well as defect concentrations in the near surface region of materials under a variety ...
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Positronium fraction measurements using positron beams have been utilized to extract information about the diffusion properties of positrons as well as defect concentrations in the near surface region of materials under a variety of experimental conditions. Owing to this recent interest we have undertaken to study some of the systematics and uncertainties associated with measurements of the positronium fraction, f. We restrict our discussion to determinations of f based on the peak:total ratio of counting rates for a single detector, only briefly considering alternate ways of obtaining f. We conclude with several recommendations that should be of particular interest to practitioners in the field. (ERA citation 10:014899)
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Progress on the positron microscope during the past year has been steady, and we currently project that initial microscope images can be collected during mid to late summer of 1992. Work during the year has mainly been divided amo...
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Progress on the positron microscope during the past year has been steady, and we currently project that initial microscope images can be collected during mid to late summer of 1992. Work during the year has mainly been divided among four areas of effort: ...
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The authors have been working to improve the quality of barium fluoride scintillators for the fast component and subsequently improve the coincidence timing. The authors are now able to obtain approximately 400 psec timing and les...
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The authors have been working to improve the quality of barium fluoride scintillators for the fast component and subsequently improve the coincidence timing. The authors are now able to obtain approximately 400 psec timing and less than 20% energy resolution for barium fluoride using quartz faced photomultiplier tubes. One major problem with the use of barium fluoride and quartz windows on the PMT's, the coupling of the scintillator to the photomultiplier tube. The best available coupling compound is viscasil from GE which is a silicon grease. It is highly efficient for transmitting the 220 nm uv light from the scintillator. (ERA citation 12:031868)
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