1,150 research outputs found
Energy Calibration of the JLab Bremsstrahlung Tagging System
In this report, we present the energy calibration of the Hall B
bremsstrahlung tagging system at the Thomas Jefferson National Accelerator
Facility. The calibration was performed using a magnetic pair spectrometer. The
tagged photon energy spectrum was measured in coincidence with pairs
as a function of the pair spectrometer magnetic field. Taking advantage of the
internal linearity of the pair spectrometer, the energy of the tagging system
was calibrated at the level of . The absolute energy scale
was determined using the rate measurements close to the end-point of
the photon spectrum. The energy variations across the full tagging range were
found to be MeV.Comment: 15 pages, 12 figure
Emotions and actions associated with altruistic helping and punishment
Evolutionary altruism (defined in terms of fitness effects) exists in the context of punishment in addition to helping. We examine the proximate psychological mechanisms that motivate altruistic helping and punishment, including the effects of genetic relatedness, potential for future interactions, and individual differences in propensity to help and punish. A cheater who is a genetic relative provokes a stronger emotional reaction than a cheater who is a stranger, but the behavioral response is modulated to avoid making the transgression public in the case of cheating relatives. Numerous behavioral differences are not accompanied by emotional differences, suggesting that other psychological mechanisms dictate the specific response to emotion-provoking events. Paradoxically, there is a positive correlation between temptation to cheat and propensity to punish others for cheating, leading to a concept of ?selfish punishment? that has been substantiated by a computer simulation model. This study demonstrates that fictional scenarios can provide an important methodological tool for studying the psychological basis of helping and punishment
Understanding depletion forces beyond entropy
The effective interaction energy of a colloidal sphere in a suspension
containing small amounts of non-ionic polymers and a flat glass surface has
been measured and calculated using total internal reflection microscopy (TIRM)
and a novel approach within density functional theory (DFT), respectively.
Quantitative agreement between experiment and theory demonstrates that the
resulting repulsive part of the depletion forces cannot be interpreted entirely
in terms of entropic arguments but that particularly at small distances
( 100 nm) attractive dispersion forces have to be taken into account
Untangling the Conceptual Isssues Raised in Reydon and Scholz’s Critique of Organizational Ecology and Darwinian Populations
Reydon and Scholz raise doubts about the Darwinian status of organizational ecology by arguing that Darwinian principles are not applicable to organizational populations. Although their critique of organizational ecology’s typological essentialism is correct, they go on to reject the Darwinian status of organizational populations. This paper claims that the distinction between replicators and interactors, raised in modern philosophy of biology but not discussed by Reydon and Scholz, points the way forward for organizational ecologists. It is possible to conceptualise evolving Darwinian populations providing the inheritance mechanism is appropriately specified. By this approach, adaptation and selection are no longer dichotomised, and the evolutionary significance of knowledge transmission is highlightedPeer reviewe
Depletion forces near curved surfaces
Based on density functional theory the influence of curvature on the
depletion potential of a single big hard sphere immersed in a fluid of small
hard spheres with packing fraction \eta_s either inside or outside of a hard
spherical cavity of radius R_c is calculated. The relevant features of this
potential are analyzed as function of \eta_s and R_c. There is a very slow
convergence towards the flat wall limit R_c \to \infty. Our results allow us to
discuss the strength of depletion forces acting near membranes both in normal
and lateral directions and to make contact with recent experimental results
Photofission of heavy nuclei at energies up to 4 GeV
Total photofission cross sections for 238U, 235U, 233U, 237Np, 232Th, and
natPb have been measured simultaneously, using tagged photons in the energy
range Egamma=0.17-3.84 GeV. This was the first experiment performed using the
Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the
photofission cross section for 238U relative to that for 237Np is about 80%,
implying the presence of important processes that compete with fission. We also
observe that the relative photofission cross sections do not depend strongly on
the incident photon energy over this entire energy range. If we assume that for
237Np the photofission probability is equal to unity, we observe a significant
shadowing effect starting below 1.5 GeV.Comment: 4 pages of RevTex, 6 postscript figures, Submitted to Phys. Rev. Let
Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution
Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication
Depletion potential in hard-sphere mixtures: theory and applications
We present a versatile density functional approach (DFT) for calculating the
depletion potential in general fluid mixtures. In contrast to brute force DFT,
our approach requires only the equilibrium density profile of the small
particles {\em before} the big (test) particle is inserted. For a big particle
near a planar wall or a cylinder or another fixed big particle the relevant
density profiles are functions of a single variable, which avoids the numerical
complications inherent in brute force DFT. We implement our approach for
additive hard-sphere mixtures. By investigating the depletion potential for
high size asymmetries we assess the regime of validity of the well-known
Derjaguin approximation for hard-sphere mixtures and argue that this fails. We
provide an accurate parametrization of the depletion potential in hard-sphere
fluids which should be useful for effective Hamiltonian studies of phase
behavior and colloid structure
Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature
We explore the interplay of membrane curvature and nonspecific binding due to
excluded-volume effects among colloidal particles inside lipid bilayer
vesicles. We trapped submicron spheres of two different sizes inside a
pear-shaped, multilamellar vesicle and found the larger spheres to be pinned to
the vesicle's surface and pushed in the direction of increasing curvature. A
simple model predicts that hard spheres can induce shape changes in flexible
vesicles. The results demonstrate an important relationship between the shape
of a vesicle or pore and the arrangement of particles within it.Comment: LaTeX with epsfig; ps available at
http://dept.physics.upenn.edu/~nelson/index.shtml Phys Rev Lett in press
(1997
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