15,683 research outputs found
Design and application of gas-gap heat switches
Gas-gap heat switches can serve as an effective means of thermally disconnecting a standby cryocooler when the primary (operating) cooler is connected and vice versa. The final phase of the development and test of a cryogenic heat switch designed for loads ranging from 2 watts at 8 K, to 100 watts at 80 K are described. Achieved heat-switch on/off conductance ratio ranged from 11,000 at 8 K to 2200 at 80 K. A particularly challenging element of heat-switch design is achieving satisfactory operation when large temperatures differentials exist across the switch. A special series of tests and analyses was conducted and used in this Phase-2 activity to evaluate the developed switches for temperature differentials ranging up to 200 K. Problems encountered at the maximum levels are described and analyzed, and means of avoiding the problems in the future are presented. A comprehensive summary of the overall heat-switch design methodology is also presented with special emphasis on lessons learned over the course of the 4-year development effort
Chapter 4: Normal and Anomalous Coronary Arteries in Humans. Part 2: Angiographic Atlas of Coronary Anomalies
Cost-effectiveness analysis of the management of distal ureteral stones in children
Objective
To determine the most cost-effective approach to the management of distal ureteral stones in children given the potential for recurrent renal colic during a trial of passage versus potential stent discomfort and complications of ureteroscopy.
Methods
We developed a decision tree to project costs and clinical outcomes associated with observation, medical explusive therapy (MET), and ureteroscopy for the management of an index patient with a 4mm distal ureteral stone. We determined which strategy would be least costly and offer the most pain-free days within 30 days of diagnosis. We performed a one-way sensitivity analysis on the probability of successful stone passage with MET. We obtained probabilities from the literature and costs from the 2016 Pediatric Health Information System Database.
Results
Ureteroscopywas the costliest strategy but maximized the number of pain-free days within 30 days of diagnosis (615/21.8 pain-free days). Observation cost more than MET and was also less effective ($2,139/15.5 pain-free days). The one-way sensitivity analysis on the probability of successful stone passage with MET demonstrated that ureteroscopyalways has the highest net monetary benefits value and is therefore the recommended strategy given a fixed WTP.
Discussion
Using a rigorous decision-science approach, we found that ureteroscopy is the recommended strategy in children with small distal ureteral stones. Although it cost more than MET, it resulted in more pain-free days in the first 30 days following diagnosis given the faster resolution of the stone episode
Cosmological Supergravity from a Massive Superparticle and Super Cosmological Black Holes
We describe in superspace a classical theory of two dimensional
dilaton supergravity with a cosmological constant, both with and without
coupling to a massive superparticle. We give general exact non-trivial
superspace solutions for the compensator superfield that describes the
supergravity in both cases. We then use these compensator solutions to
construct models of two-dimensional supersymmetric cosmological black holes.Comment: 20 pages, Late
Onset voltage shift due to non-zero Landau ground state level in coherent magnetotransport
Coherent electron transport in double-barrier heterostructures with parallel
electric and magnetic fields is analyzed theoretically and with the aid of a
quantum simulator accounting for 3-dimensional transport effects. The
onset-voltage shift induced by the magnetic field in resonant tunneling diodes,
which was previously attributed to the cyclotron frequency inside the
well is found to arise from an upward shift of the non-zero ground (lowest)
Landau state energy in the entire quantum region where coherent transport takes
place. The spatial dependence of the cyclotron frequency is accounted for and
verified to have a negligible impact on resonant tunneling for the device and
magnetic field strength considered. A correction term for the onset-voltage
shift arising from the magnetic field dependence of the chemical potential is
also derived. The Landau ground state with its nonvanishing finite harmonic
oscillator energy is verified however to be the principal
contributor to the onset voltage shift at low temperatures.Comment: 13 pages, and 3 figures. Accepted for publication in Phys. Rev.
High scale impact in alignment and decoupling in two-Higgs doublet models
The two-Higgs doublet model (2HDM) provides an excellent benchmark to study
physics beyond the Standard Model (SM). In this work we discuss how the
behaviour of the model at high energy scales causes it to have a scalar with
properties very similar to those of the SM -- which means the 2HDM can be seen
to naturally favor a decoupling or alignment limit. For a type II 2HDM, we show
that requiring the model to be theoretically valid up to a scale of 1 TeV, by
studying the renormalization group equations (RGE) of the parameters of the
model, causes a significant reduction in the allowed magnitude of the quartic
couplings. This, combined with -physics bounds, forces the model to be
naturally decoupled. As a consequence, any non-decoupling limits in type II,
like the wrong-sign scenario, are excluded. On the contrary, even with the very
constraining limits for the Higgs couplings from the LHC, the type I model can
deviate substantially from alignment. An RGE analysis similar to that made for
type II shows, however, that requiring a single scalar to be heavier than about
500 GeV would be sufficient for the model to be decoupled. Finally, we show
that not only a 2HDM where the lightest of the CP-even scalars is the 125 GeV
one does not require new physics to be stable up to the Planck scale but this
is also true when the heavy CP-even Higgs is the 125 GeV and the theory has no
decoupling limit for the type I model.Comment: 28 pages, 19 figure
Evidence for a diffusion-controlled mechanism for fluorescence blinking of colloidal quantum dots
Fluorescence blinking in nanocrystal quantum dots is known to exhibit power-law dynamics, and several different mechanisms have been proposed to explain this behavior. We have extended the measurement of quantum-dot blinking by characterizing fluctuations in the fluorescence of single dots over time scales from microseconds to seconds. The power spectral density of these fluctuations indicates a change in the power-law statistics that occurs at a time scale of several milliseconds, providing an important constraint on possible mechanisms for the blinking. In particular, the observations are consistent with the predictions of models wherein blinking is controlled by diffusion of the energies of electron or hole trap states
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