286 research outputs found
Fluorescence and phosphorescence of photomultiplier window materials under electron irradiation
The fluorescence and phosphorescence of photomultiplier window materials under electron irradiation were investigated using a Sr-90/Y-90 beta emitter as the electron source. Spectral emission curves of UV grade, optical grade, and electron-irradiated samples of MGF2 and LiF, CaF2, BaF2, sapphire, fused silica, and UV transmitting glasses were obtained over the spectral range of 200 nm to 650 nm. Fluorescence yields, expressed as the number of counts in a solid angle of 2 pi steradian per 1MeV of incident electron energy deposited, were determined on these materials utilizing photomultiplier tubes with cesium telluride, bialkali, and trialkali (S-20) photocathodes, respectively
Assessment of Subjective Experiences of Boys Entering the Community From a Correctional School
The purpose of this study was to explore, classify, and evaluate the personal experiences of juvenile offenders following release from a state training school.
During a two month period beginning in the fall of 1967, a group of twenty-three releasees from the MacLaren School for Boys in Woodburn, Oregon were interviewed on a weekly basis in the Portland metropolitan area. The boys, ranging in age from thirteen to seventeen were contacted in their homes, schools, and other settings to learn of positive, negative, and critical experiences encountered in the community.
Open ended questions based upon a twenty- seven item schedule were administered to the boys, focusing upon personal experiences in six major areas: school, family, peer group, community, work and authority. Responses were recorded verbatim and classified according to the boys\u27 judgments of positive and negative connotations. Critical experiences were also recorded to determine specific incidents which brought the boys into contact with legal authorities or otherwise seriously affected their parole status.
Positive, negative, and critical experiences from this study were analyzed both descriptively and statistically. Null hypotheses were tested to determine the quality, quantity, and intensity of experiences with respect to such major variables as age of the boys, race, length of stay in the community, records of school attendance, and personality inventory characteristics. Experiences were similarly related with respect to other variables found to be present as a result of the interviewing experience.
Findings yielded evidence to indicate a predominance of negative experiences in the boysâ contacts with the community following release. The highest proportion of these occurred within the boysâ families, followed by contacts with authority and school, respectively. Peer group relationships for the boys indicated the highest proportion of positive experiences when compared with other major categories
Neutron Stars in a Varying Speed of Light Theory
We study neutron stars in a varying speed of light (VSL) theory of gravity in
which the local speed of light depends upon the value of a scalar field .
We find that the masses and radii of the stars are strongly dependent on the
strength of the coupling between and the matter field and that for
certain choices of coupling parameters, the maximum neutron star mass can be
arbitrarily small. We also discuss the phenomenon of cosmological evolution of
VSL stars (analogous to the gravitational evolution in scalar-tensor theories)
and we derive a relation showing how the fractional change in the energy of a
star is related to the change in the cosmological value of the scalar field.Comment: 15 pages, 2 figures. Added solutions with a more realistic equation
of state. To be published in PR
The influence of Galactic aberration on precession parameters determined from VLBI observations
The influence of proper motions of sources due to Galactic aberration on
precession models based on VLBI data is determined. Comparisons of the linear
trends in the coordinates of the celestial pole obtained with and without
taking into account Galactic aberration indicate that this effect can reach 20
as per century, which is important for modern precession models. It is
also shown that correcting for Galactic aberration influences the derived
parameters of low-frequency nutation terms. It is therefore necessary to
correct for Galactic aberration in the reduction of modern astrometric
observations
The Laser Astrometric Test of Relativity Mission
This paper discusses new fundamental physics experiment to test relativistic
gravity at the accuracy better than the effects of the 2nd order in the
gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR)
mission uses laser interferometry between two micro-spacecraft whose lines of
sight pass close by the Sun to accurately measure deflection of light in the
solar gravity. The key element of the experimental design is a redundant
geometry optical truss provided by a long-baseline (100 m) multi-channel
stellar optical interferometer placed on the International Space Station. The
geometric redundancy enables LATOR to measure the departure from Euclidean
geometry caused by the solar gravity field to a very high accuracy. LATOR will
not only improve the value of the parameterized post-Newtonian (PPN) parameter
gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach
ability to measure effects of the next post-Newtonian order (1/c^4) of light
deflection resulting from gravity's intrinsic non-linearity. The solar
quadrupole moment parameter, J2, will be measured with high precision, as well
as a variety of other relativistic. LATOR will lead to very robust advances in
the tests of fundamental physics: this mission could discover a violation or
extension of general relativity, or reveal the presence of an additional long
range interaction in the physical law. There are no analogs to the LATOR
experiment; it is unique and is a natural culmination of solar system gravity
experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International
Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12
December 2003, Washington, D
Long-term evolution of orbits about a precessing oblate planet: 1. The case of uniform precession
It was believed until very recently that a near-equatorial satellite would
always keep up with the planet's equator (with oscillations in inclination, but
without a secular drift). As explained in Efroimsky and Goldreich (2004), this
opinion originated from a wrong interpretation of a (mathematically correct)
result obtained in terms of non-osculating orbital elements. A similar analysis
carried out in the language of osculating elements will endow the planetary
equations with some extra terms caused by the planet's obliquity change. Some
of these terms will be nontrivial, in that they will not be amendments to the
disturbing function. Due to the extra terms, the variations of a planet's
obliquity may cause a secular drift of its satellite orbit inclination. In this
article we set out the analytical formalism for our study of this drift. We
demonstrate that, in the case of uniform precession, the drift will be
extremely slow, because the first-order terms responsible for the drift will be
short-period and, thus, will have vanishing orbital averages (as anticipated 40
years ago by Peter Goldreich), while the secular terms will be of the second
order only. However, it turns out that variations of the planetary precession
make the first-order terms secular. For example, the planetary nutations will
resonate with the satellite's orbital frequency and, thereby, may instigate a
secular drift. A detailed study of this process will be offered in the
subsequent publication, while here we work out the required mathematical
formalism and point out the key aspects of the dynamics
Photoacoustic Experimental System To Confirm Infrared Absorption Due to Greenhouse Gases
An experimental system for detecting infrared absorption using the photoacoustic (PA) effect is described. It is aimed for use at high-school level to illustrate the difference in infrared (IR) absorption among the gases contained in the atmosphere in connection with the greenhouse effect. The experimental system can be built with readily available components and is suitable for small-group experiments. The PA signal from a greenhouse gas (GHG), such as CO2, H2O, and CH4, can be detected down to a concentration of 0.1%. Since the basic theory of the PA effect in gases due to IR absorption is straightforward, the experiments with this PA system are accessible to students. It can be shown that there is a significant difference in IR absorption between GHGs and the major components of the atmosphere, N2, O2, and Ar, which helps students understand that the minor components, that is, the GHGs, determine the IR absorptivity of the atmosphere
Astrometric and Timing Effects of Gravitational Waves from Localized Sources
A consistent approach for an exhaustive solution of the problem of
propagation of light rays in the field of gravitational waves emitted by a
localized source of gravitational radiation is developed in the first
post-Minkowskian and quadrupole approximation of General Relativity. We
demonstrate that the equations of light propagation in the retarded
gravitational field of an arbitrary localized source emitting quadrupolar
gravitational waves can be integrated exactly. The influence of the
gravitational field on the light propagation is examined not only in the wave
zone but also in cases when light passes through the intermediate and near
zones of the source. Explicit analytic expressions for light deflection and
integrated time delay (Shapiro effect) are obtained accounting for all possible
retardation effects and arbitrary relative locations of the source of
gravitational waves, that of light rays, and the observer. It is shown that the
ADM and harmonic gauge conditions can both be satisfied simultaneously outside
the source of gravitational waves. Their use drastically simplifies the
integration of light propagation equations and those for the motion of light
source and observer in the field of the source of gravitational waves, leading
to the unique interpretation of observable effects. The two limiting cases of
small and large values of impact parameter are elaborated in more detail.
Explicit expressions for Shapiro effect and deflection angle are obtained in
terms of the transverse-traceless part of the space-space components of the
metric tensor. We also discuss the relevance of the developed formalism for
interpretation of radio interferometric and timing observations, as well as for
data processing algorithms for future gravitational wave detectors.Comment: 43 pages, 4 Postscript figures, uses revtex.sty, accepted to Phys.
Rev. D, minor corrections in formulae regarding algebraic sign
Nonhuman humanitarianism: when âAI for goodâ can be harmful
Artificial intelligence (AI) applications have been introduced in humanitarian operations in order to help with the significant challenges the sector is facing. This article focuses on chatbots which have been proposed as an efficient method to improve communication with, and accountability to affected communities. Chatbots, together with other humanitarian AI applications such as biometrics, satellite imaging, predictive modelling and data visualisations, are often understood as part of the wider phenomenon of âAI for social goodâ. The article develops a decolonial critique of humanitarianism and critical algorithm studies which focuses on the power asymmetries underpinning both humanitarianism and AI. The article asks whether chatbots, as exemplars of âAI for goodâ, reproduce inequalities in the global context. Drawing on a mixed methods study that includes interviews with seven groups of stakeholders, the analysis observes that humanitarian chatbots do not fulfil claims such as âintelligenceâ. Yet AI applications still have powerful consequences. Apart from the risks associated with misinformation and data safeguarding, chatbots reduce communication to its barest instrumental forms which creates disconnects between affected communities and aid agencies. This disconnect is compounded by the extraction of value from data and experimentation with untested technologies. By reflecting the values of their designers and by asserting Eurocentric values in their programmed interactions, chatbots reproduce the coloniality of power. The article concludes that âAI for goodâ is an âenchantment of technologyâ that reworks the colonial legacies of humanitarianism whilst also occluding the power dynamics at play
Lit up and left dark: Failures of imagination in urban broadband networks
The design and deployment of urban broadband infrastructures inscribe particular imaginations of Internet access onto city streets. The different manifestations and locations of these networks, their uses, and access points often expose material excesses of urban broadband networks, as well as failures of Internet service providers, urban planners, and public officials to imagine the diverse ways that people incorporate Internet connection into their everyday lives. We approach the study of urban broadband networks through the juxtaposition of invisible networks that are buried under the streets and have always been âturned offâ (dark fiber) versus hypervisible that are âturned onâ and prominently displayed on city streets (LinkNYC). In our analysis of these two case studies, we critique themes of visibility and invisibility as indexes of power and access. Our findings are meant to provide a critical analysis of urban technology policy as well as theories of infrastructure, visibility, and access
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