11,661 research outputs found
NTF: Soldering Technology Development for Cryogenics
The advent of the National Transonic Facility (NTF) brought about a new application for an old joining method, soldering. Soldering for use at cryogenic temperatures requires that solders remain ductile and free from tin-pest (grey tin), have toughness to withstand aerodynamic loads associated with flight research, and maintain their surface finishes. Solders are used to attach 347 Stainless-Steel tubing in surface grooves of models. The solder must fill up the gap and metallurgically bound to the tubing and model. Cryogenic temperatures require that only specific materials for models can be used, including: Vasco Max 200 CVM, lescalloy A-286 Vac Arc, pH 13-8 Mo. Solders identified for testing at this time are: 50% Sn - 49.5% Pb - 0.5% Sb, 95% Sn - 5% Sb, 50% In 50% Pb, and 37.5% Sn - 37.5% Pb - 25% In. With these materials and solders, it is necessary to determine their solderability. After solderability is determined, tube/groove specimens are fabricated and stressed under cryogenic temperatures. Compatible solders are then used for acutual models
Secondary gamma-ray production in a coded aperture mask
The application of the coded aperture mask to high energy gamma-ray astronomy will provide the capability of locating a cosmic gamma-ray point source with a precision of a few arc-minutes above 20 MeV. Recent tests using a mask in conjunction with drift chamber detectors have shown that the expected point spread function is achieved over an acceptance cone of 25 deg. A telescope employing this technique differs from a conventional telescope only in that the presence of the mask modifies the radiation field in the vicinity of the detection plane. In addition to reducing the primary photon flux incident on the detector by absorption in the mask elements, the mask will also be a secondary radiator of gamma-rays. The various background components in a CAMTRAC (Coded Aperture Mask Track Chamber) telescope are considered. Monte-Carlo calculations are compared with recent measurements obtained using a prototype instrument in a tagged photon beam line
Vortices in a Bose-Einstein Condensate
We have created vortices in two-component Bose-Einstein condensates. The
vortex state was created through a coherent process involving the spatial and
temporal control of interconversion between the two components. Using an
interference technique, we map the phase of the vortex state to confirm that it
possesses angular momentum. We can create vortices in either of the two
components and have observed differences in the dynamics and stability.Comment: 4 pages with 3 figure
Taming the Runaway Problem of Inflationary Landscapes
A wide variety of vacua, and their cosmological realization, may provide an
explanation for the apparently anthropic choices of some parameters of particle
physics and cosmology. If the probability on various parameters is weighted by
volume, a flat potential for slow-roll inflation is also naturally understood,
since the flatter the potential the larger the volume of the sub-universe.
However, such inflationary landscapes have a serious problem, predicting an
environment that makes it exponentially hard for observers to exist and giving
an exponentially small probability for a moderate universe like ours. A general
solution to this problem is proposed, and is illustrated in the context of
inflaton decay and leptogenesis, leading to an upper bound on the reheating
temperature in our sub-universe. In a particular scenario of chaotic inflation
and non-thermal leptogenesis, predictions can be made for the size of CP
violating phases, the rate of neutrinoless double beta decay and, in the case
of theories with gauge-mediated weak scale supersymmetry, for the fundamental
scale of supersymmetry breaking.Comment: 31 pages, including 3 figure
Operating characteristics of a prototype high energy gamma-ray telescope
The field of gamma-ray astronomy in the energy range from ten to several hundred MeV is severely limited by the angular resolution that can be achieved by present instruments. The identification of some of the point sources found by the COS-B mission and the resolution of detailed structure existing in those sources may depend on the development of a new class of instrument. The coded aperture mask telescope, used successfully at X-ray energies hold the promise of being such an instrument. A prototype coded aperture telescope was operated in a tagged photon beam ranging in energy from 23 to 123 MeV. The purpose of the experiment was to demonstrate the feasibility of operating a coded aperture mask telescope in this energy region. Some preliminary results and conclusions drawn from some of the data resulting from this experiment are presented
Probabilistic study of the speed of approach to equilibrium for an inelastic Kac model
This paper deals with a one--dimensional model for granular materials, which
boils down to an inelastic version of the Kac kinetic equation, with
inelasticity parameter . In particular, the paper provides bounds for
certain distances -- such as specific weighted --distances and the
Kolmogorov distance -- between the solution of that equation and the limit. It
is assumed that the even part of the initial datum (which determines the
asymptotic properties of the solution) belongs to the domain of normal
attraction of a symmetric stable distribution with characteristic exponent
\a=2/(1+p). With such initial data, it turns out that the limit exists and is
just the aforementioned stable distribution. A necessary condition for the
relaxation to equilibrium is also proved. Some bounds are obtained without
introducing any extra--condition. Sharper bounds, of an exponential type, are
exhibited in the presence of additional assumptions concerning either the
behaviour, near to the origin, of the initial characteristic function, or the
behaviour, at infinity, of the initial probability distribution function
Communications Biophysics
Contains research objectives and reports on three research projects.U.S. Air Force under Contract AF19(604)-411
Sensing electric fields using single diamond spins
The ability to sensitively detect charges under ambient conditions would be a
fascinating new tool benefitting a wide range of researchers across
disciplines. However, most current techniques are limited to low-temperature
methods like single-electron transistors (SET), single-electron electrostatic
force microscopy and scanning tunnelling microscopy. Here we open up a new
quantum metrology technique demonstrating precision electric field measurement
using a single nitrogen-vacancy defect centre(NV) spin in diamond. An AC
electric field sensitivity reaching ~ 140V/cm/\surd Hz has been achieved. This
corresponds to the electric field produced by a single elementary charge
located at a distance of ~ 150 nm from our spin sensor with averaging for one
second. By careful analysis of the electronic structure of the defect centre,
we show how an applied magnetic field influences the electric field sensing
properties. By this we demonstrate that diamond defect centre spins can be
switched between electric and magnetic field sensing modes and identify
suitable parameter ranges for both detector schemes. By combining magnetic and
electric field sensitivity, nanoscale detection and ambient operation our study
opens up new frontiers in imaging and sensing applications ranging from
material science to bioimaging
Vortex precession in Bose-Einstein condensates: observations with filled and empty cores
We have observed and characterized the dynamics of singly quantized vortices
in dilute-gas Bose-Einstein condensates. Our condensates are produced in a
superposition of two internal states of 87Rb, with one state supporting a
vortex and the other filling the vortex core. Subsequently, the state filling
the core can be partially or completely removed, reducing the radius of the
core by as much as a factor of 13, all the way down to its bare value. The
corresponding superfluid rotation rates, evaluated at the core radius, vary by
a factor of 150, but the precession frequency of the vortex core about the
condensate axis changes by only a factor of two.Comment: 4 pages, 3 figure
Supersymmetric Monojets at the Large Hadron Collider
Supersymmetric monojets may be produced at the Large Hadron Collider by the
process qg -> squark neutralino_1 -> q neutralino_1 neutralino_1, leading to a
jet recoiling against missing transverse momentum. We discuss the feasibility
and utility of the supersymmetric monojet signal. In particular, we examine the
possible precision with which one can ascertain the neutralino_1-squark-quark
coupling via the rate for monojet events. Such a coupling contains information
on the composition of the neutralino_1 and helps bound dark matter direct
detection cross-sections and the dark matter relic density of the neutralino_1.
It also provides a check of the supersymmetric relation between gauge couplings
and gaugino-quark-squark couplings.Comment: 46 pages, 10 figures. The appendix has been rewritten to correct an
error that appears in all previous versions of the appendix. This error has
no effect on the results in the main body of the pape
- …