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Search Results: 1 - 10 of 157534 matches for " F. William Summers "
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The Librarian as an Agent of Change
F. William Summers
Journal of Educational Media & Library Sciences , 1989,
Abstract: 無 無
Sham Peer Review: A Psychiatristís Experience and Analysis
Summers, William K.
Journal of American Physicians and Surgeons , 2005,
Abstract:
Current Cosmological Simulations, Advice on their Interpretation, and an IMAX Film Preview
F J Summers
Physics , 1995,
Abstract: This general talk is geared toward non-specialists and addresses three questions: (1) What types of cosmological simulations are done? (2) What are the relevant problems to address and what results can one expect? (3) How can one tell if the results are reasonable?
Cosmological Simulations of Galaxy Formation Including Hydrodynamics (hyper-abridged)
F J Summers
Physics , 1994,
Abstract: The formation of galaxies in hierarchical cosmogonies is studied using high resolution N-body plus SPH hydrodynamics simulations. The collapse of structure is followed self-consistently from Mpc scale filamentary structures to kpc scale galactic objects. The characteristics and formation processes of the galaxy like objects are studied in detail, along with the aggregation into a poor cluster. Related studies consider the effects of modelling star formation, the reliability of tracing galaxies in simulations, and tests of SPH methods. This submission serves first to notify that the full text and figures of my thesis are available in compressed PostScript form via anonymous ftp from astro.princeton.edu in the directory /summers/thesis (122 files, 19 MB compressed, 65 MB uncompressed). See the README file first. Second, this submission contains the title page, abstract, table of contents, introductory chapter, summary chapter, and references for my thesis. Those who are curious about the work may scan these pages to identify which chapters may be interesting to get via ftp.
Galaxy Formation in Hierarchical Models
F J Summers
Physics , 1996,
Abstract: High resolution gravity plus smoothed particle hydrodynamics simulations are used to study the formation of galaxies within the context of hierarchical structure formation. The simulations have sufficient dynamic range to resolve from ten kpc scale galactic disks up to many Mpc scale filaments. Over this range of scales, we find that hierarchical structure development proceeds through a series of increasingly larger filamentary collapses. The well resolved simulated galaxies contain hundreds to thousands of particles and have varied morphologies covering the entire expected range from disks to tidally distorted objects. The epoch of galaxy formation occurs early, about redshift 2.5 for 10^12 M_sun galaxies. Hierarchical formation naturally produces correlations among the mass, age, morphology, and local density of galaxies which match the trends in the observed morphology--density relation. We also describe a method of spiral galaxy formation in which galactic disks form through the discrete accretion of gas clouds which transport most of the angular momentum to the inner regions. Such a process is characteristic of the somewhat chaotic nature of hierarchical structure formation where simple analytical ideas of spherical collapse appear incongruous.
Sensitivity of 8B breakup cross section to projectile structure in CDCC calculations
N. C. Summers,F. M. Nunes
Physics , 2004, DOI: 10.1088/0954-3899/31/12/005
Abstract: Given the Astrophysical interest of $^7$Be$(p,\gamma)^8$B, there have been several experiments applying the Coulomb dissociation method for extracting the capture rate. Measurements at Michigan State are dominated by $E1$ contributions but have a small $E2$ component. On the other hand, a lower energy measurement at Notre Dame has a much stronger $E2$ contribution. The expectation was that the two measurements would tie down the $E2$ and thus allow for an accurate extraction of the $E1$ relevant for the capture process. The aim of this brief report is to show that the $E2$ factor in breakup reactions does not translate into a scaling of the $E2$ contribution in the corresponding capture reaction. We show that changes to the $^8$B single particle parameters, which are directly related to the $E2$ component in the capture reaction, do not effect the corresponding breakup reactions, using the present reaction theory.
Extracting (n,g) direct capture cross sections from Coulomb dissociation: application to $^{14}$C(n,$γ$)$^{15}$C
N. C. Summers,F. M. Nunes
Physics , 2008, DOI: 10.1103/PhysRevC.78.069908
Abstract: A methodology for extracting neutron direct capture rates from Coulomb dissociation data is developed and applied to the Coulomb dissociation of 15C on 208Pb at 68 MeV/nucleon. Full Continuum Discretized Coupled Channel calculations are performed and an asymptotic normalization coefficient is determined from a fit to the breakup data. Direct neutron capture calculations using the extracted asymptotic normalization coefficient provide $(n,\gamma)$ cross sections consistent with direct measurements. Our results show that the Coulomb Dissociation data can be reliably used for extracting the cross section for 14C(n,g)15C if the appropriate reaction theory is used. The resulting error bars are of comparable magnitude to those from the direct measurement. This procedure can be used more generally to extract capture cross sections from breakup reactions whenever the desired capture process is fully peripheral.
Testing linear-theory predictions of galaxy formation
Ben Sugerman,F. J. Summers,Marc Kamionkowski
Physics , 1999, DOI: 10.1046/j.1365-8711.2000.03107.x
Abstract: The angular momentum of galaxies is routinely ascribed to a process of tidal torques acting during the early stages of gravitational collapse, and is predicted from the initial mass distribution using second-order perturbation theory and the Zel'dovich approximation. We have tested this theory for a flat hierarchical cosmogony using a large N-body simulation with sufficient dynamic range to include tidal fields, allow resolution of individual galaxies, and thereby expand on previous studies. We find relatively good correlation between the predictions of linear theory and actual galaxy evolution. While structure formation from early times is a complex history of hierarchical merging, salient features are well described by the simple spherical-collapse model. Most notably, we test several methods for determining the turnaround epoch, and find that turnaround is succesfully described by the spherical collapse model. The angular momentum of collapsing structures grows linearly until turnaround, as predicted, and continues quasi-linearly until shell crossing. The predicted angular momentum for well-resolved galaxies at turnaround overestimates the true turnaround and final values by a factor of ~3 with a scatter of ~70 percent, and only marginally yields the correct direction of the angular momentum vector. We recover the prediction that final angular momentum scales as mass to the 5/3 power. We find that mass and angular momentum also vary proportionally with peak height.
Core excitation in the elastic scattering and breakup of $^{11}$Be on protons
N. C. Summers,F. M. Nunes
Physics , 2007, DOI: 10.1103/PhysRev.C77.049901
Abstract: The elastic scattering and breakup of $^{11}$Be from a proton target at intermediate energies is studied. We explore the role of core excitation in the reaction mechanism. Comparison with the data suggests that there is still missing physics in the description.
7Be breakup on heavy and light targets
N. C. Summers,F. M. Nunes
Physics , 2004, DOI: 10.1103/PhysRevC.70.011602
Abstract: In this paper we present all-order quantum mechanical calculations of 7Be breakup on heavy (208Pb) and light (12C) targets. We examine the issues concerning the extraction of the astrophysical S-factor from the breakup data. We discuss the interplay between Coulomb and nuclear breakup, and the importance of higher-order couplings on the cross section. We show that nuclear and Coulomb contributions are not separable using the standard angular selection criterion as nuclear breakup remains large for small scattering angles, even for the heavy target. However, by selecting an upper limit on the relative energy between the final fragments, the contribution from the nuclear breakup can be significantly reduced such that Coulomb breakup is the main reaction mechanism. We show that the extraction of the asymptotic normalization coefficient may require more careful consideration of the nuclear interior than previously used.
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