2,695 research outputs found
A trap-based pulsed positron beam optimised for positronium laser spectroscopy
We describe a pulsed positron beam that is optimised for positronium (Ps) laser-spectroscopy experiments. The system is based on a two-stage Surko-type buffer gas trap that produces 4 ns wide pulses containing up to 5 Ă 105 positrons at a rate of 0.5-10 Hz. By implanting positrons from the trap into a suitable target material, a dilute positronium gas with an initial density of the order of 107 cmâ3 is created in vacuum. This is then probed with pulsed (ns) laser systems, where various Ps-laser interactions have been observed via changes in Ps annihilation rates using a fast gamma ray detector. We demonstrate the capabilities of the apparatus and detection methodology via the observation of Rydberg positronium atoms with principal quantum numbers ranging from 11 to 22 and the Stark broadening of the n = 2 â 11 transition in electric fields
Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms
Predicted twenty years ago, positron binding to neutral atoms has not yet
been observed experimentally. A new scheme is proposed to detect positron-atom
bound states by colliding Rydberg positronium (Ps) with neutral atoms.
Estimates of the charge-transfer reaction cross section are obtained using the
first Born approximation for a selection of neutral atom targets and a wide
range of incident Ps energies and principal quantum numbers. We also estimate
the corresponding Ps ionization cross section. The accuracy of the calculations
is tested by comparison with earlier predictions for Ps charge transfer in
collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps
beam suitable for producing positron-atom bound states and estimate signal
rates based on the calculated cross sections and realistic experimental
parameters. We conclude that the proposed methodology is capable of producing
such states and of testing theoretical predictions of their binding energies.Comment: 19 pages, 11 figures, submitted to Phys. Rev.
Canadaâs Earthquakes: âThe Good, the Bad, and the Uglyâ
Much of Canada is âearthquake countryâ. Tiny earthquakes (that can only be recorded by seismographs) happen every day. On average, earthquakes large enough to be felt occur every week in Canada, damaging earthquakes are years to decades apart, and some of the worldâs largest earthquakes are typically separated by intervals of centuries. In this article, we provide details on the most significant earthquakes that have been recorded in, or near, Canada, including where and when they occurred, how they were felt, and the effects of those earthquakes. We also provide a brief review of how earthquakes are monitored across Canada and some recent earthquake hazard research. It is the results of this monitoring and research, which provide knowledge on earthquake hazard, that are incorporated into the National Building Code of Canada. This, in turn, will contribute to reduced property losses from future earthquakes across Canada.
SOMMAIRE
Un bonne partie du Canada est un âpays de sĂ©ismesâ. De petits sĂ©ismes (que seuls les sĂ©ismographes peuvent enregistrer) sây produisent quotidiennement. En moyenne, un sĂ©isme assez fort pour quâon le ressente sây produit Ă intervalle dâune semaine; assez fort pour causer des dommages sây produit Ă intervalle de quelques annĂ©es Ă quelques dĂ©cennies; alors que lâintervalle de rĂ©currence des plus grands sĂ©ismes est de lâordre des siĂšcles. Dans le prĂ©sent article on trouvera des dĂ©tails sur les plus importants sĂ©ismes sâĂ©tant produits sur ou Ă proximitĂ© du territoire canadien, incluant le lieu et le moment, leurs manifestations et leurs rĂ©percussions. On y dĂ©crit sommairement les moyens de dĂ©tection dĂ©ployĂ©s sur le territoire canadien ainsi que quelques-unes des recherches rĂ©centes sur les risques sismiques. Ce sont les rĂ©sultats des efforts de surveillance et des recherches sur les tremblements de terre qui ont Ă©tĂ© intĂ©grĂ©s dans le Code national du bĂątiment du Canada. Et cela aidera Ă amoindrir les rĂ©percussions des sĂ©ismes Ă venir sur la propriĂ©tĂ©
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
Nonlinear Quantum Mechanics at the Planck Scale
I argue that the linearity of quantum mechanics is an emergent feature at the
Planck scale, along with the manifold structure of space-time. In this regime
the usual causality violation objections to nonlinearity do not apply, and
nonlinear effects can be of comparable magnitude to the linear ones and still
be highly suppressed at low energies. This can offer alternative approaches to
quantum gravity and to the evolution of the early universe.Comment: Talk given at the International Quantum Structures 2004 meeting, 16
pages LaTe
Energetic charged particle fluxes relevant to Ganymede's polar region
The JEDI instrument made measurements of energetic charged particles near Ganymede during a close encounter with that moon. Here we find ion flux levels are similar close to Ganymede itself but outside its magnetosphere and on near wake and open field lines. But energetic electron flux levels are more than a factor of 2 lower on polar and near-wake field lines than on nearby Jovian field lines at all energies reported here. Flux levels are relevant to the weathering of the surface, particularly processes that affect the distribution of ice, since surface brightness has been linked to the open-closed field line boundary. For this reason, we estimate the sputtering rates expected in the polar regions due to energetic heavy ions. Other rates, such as those related to radiolysis by plasma and particles that can reach the surface, need to be added to complete the picture of charged particle weathering
Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics
We consider two approaches to evading paradoxes in quantum mechanics with
closed timelike curves (CTCs). In a model similar to Politzer's, assuming pure
states and using path integrals, we show that the problems of paradoxes and of
unitarity violation are related; preserving unitarity avoids paradoxes by
modifying the time evolution so that improbable events bewcome certain. Deutsch
has argued, using the density matrix, that paradoxes do not occur in the "many
worlds interpretation". We find that in this approach account must be taken of
the resolution time of the device that detects objects emerging from a wormhole
or other time machine. When this is done one finds that this approach is viable
only if macroscopic objects traversing a wormhole interact with it so strongly
that they are broken into microscopic fragments.Comment: no figure
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"Am iz kwiin" (I'm his queen): Combining interpretative phenomenological analysis with a feminist approach to work with gems in a resource-constrained setting
This article focuses on working with gems using a feminist approach to interpretative phenomenological analysis (IPA) in a resource-constrained setting. The research explores the experiences of maternal disclosure of HIV to children of HIV positive mothers in Kingston, Jamaica. A feminist approach helps recognise power imbalances within research relationships and the womenâs lived experiences. We present three âgemsâ which illuminate womenâs lived experiences and explore how popularised representations of womenâs sexuality and mothering influence disclosure discourses. We use emotion work as a conceptual resource to structure the womenâs narratives and challenge existing policy discourses, which arguably represent disclosure within a binary, rationalist, decision-making framework. This article adds to global literature on maternal HIV disclosure and problematises policy discourses by bringing into relief the emotion work women engage in when deciding if and how to communicate their HIV status to their children. It adds to the body of research using IPA, particularly in resource-constrained settings where IPA has thus far had little application
Resolved Mid-IR Emission in the Narrow Line Region of NGC 4151
We present subarcsecond resolution mid infrared images of NGC 4151 at 10.8
micron and 18.2 micron. These images were taken with the University of Florida
mid-IR camera/spectrometer OSCIR at the Gemini North 8-m telescope. We resolve
emission at both 10.8 micron and 18.2 micron extending ~ 3.5" across at a P.A.
of ~ 60 degrees. This coincides with the the narrow line region of NGC 4151 as
observed in [OIII] by the Hubble Space Telescope. The most likely explanation
for this extended mid-IR emission is dust in the narrow line region heated by a
central engine. We find no extended emission associated with the proposed torus
and place an upper limit on its mid-IR size of less than or equal to ~ 35 pc.Comment: accepted for publication in the Astrophysical Journal, 19 pages
including 5 figure
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