9,439 research outputs found
Stirring apparatus for plural test tubes Patent
Design of mechanical device for stirring several test tubes simultaneousl
Frequency-Tunable Josephson Junction Resonator for Quantum Computing
We have fabricated and measured a high-Q Josephson junction resonator with a
tunable resonance frequency. A dc magnetic flux allows the resonance frequency
to be changed by over 10 %. Weak coupling to the environment allows a quality
factor of 7000 when on average less than one photon is stored in the
resonator. At large photon numbers, the nonlinearity of the Josephson junction
creates two stable oscillation states. This resonator can be used as a tool for
investigating the quality of Josephson junctions in qubits below the single
photon limit, and can be used as a microwave qubit readout at high photon
numbers.Comment: 3 pages, 5 figure
Deexcitation nuclear gamma-ray line emission from low-energy cosmic rays in the inner Galaxy
Recent observations of high ionization rates of molecular hydrogen in diffuse
interstellar clouds point to a distinct low-energy cosmic-ray component.
Supposing that this component is made of nuclei, two models for the origin of
such particles are explored and low-energy cosmic-ray spectra are calculated
which, added to the standard cosmic ray spectra, produce the observed
ionization rates. The clearest evidence of the presence of such low-energy
nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the
interstellar medium would be a detection of nuclear \gamma-ray line emission in
the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with
the interstellar gas and dust. Using a recent \gamma-ray cross section
compilation for nuclear collisions, \gamma-ray line emission spectra are
calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0
decay, the latter providing normalization of the absolute fluxes by comparison
with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV.
Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy
are well below the detection sensitivies of INTEGRAL, but a detection,
especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray
telescopes based on available technology. We predict also strong \gamma-ray
continuum emission in the 1-8 MeV range, which in a large part of our model
space for low-energy cosmic rays exceeds considerably estimated instrument
sensitivities of future telescopes.Comment: 22 pages, 7 figures, accepted for publication in ApJ; figures 6 and 7
replace
Propagation of cosmic-ray nucleons in the Galaxy
We describe a method for the numerical computation of the propagation of
primary and secondary nucleons, primary electrons, and secondary positrons and
electrons. Fragmentation and energy losses are computed using realistic
distributions for the interstellar gas and radiation fields, and diffusive
reacceleration is also incorporated. The models are adjusted to agree with the
observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and
convection do not account well for the observed energy dependence of B/C, while
models with reacceleration reproduce this easily. The height of the halo
propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc
for diffusion/convection models and 4-12 kpc for reacceleration models. For
convection models we set an upper limit on the velocity gradient of dV/dz < 7
km/s/kpc. The radial distribution of cosmic-ray sources required is broader
than current estimates of the SNR distribution for all halo sizes. Full details
of the numerical method used to solve the cosmic-ray propagation equation are
given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses
emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty.
To be published in ApJ 1998, v.509 (December 10 issue). More details can be
found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references
are correcte
Lifetime Constraints for Late Dark Matter Decay
We consider a class of late-decaying dark-matter models, in which a dark
matter particle decays to a heavy stable daughter of approximately the same
mass, together with one or more relativistic particles which carry away only a
small fraction of the parent rest mass. Such decays can affect galactic halo
structure and evolution, and have been invoked as a remedy to some of the small
scale structure-formation problems of cold dark matter. There are existing
stringent limits on the dark matter lifetime if the decays produce photons. By
considering examples in which the relativistic decay products instead consist
of neutrinos or electron-position pairs, we derive stringent limits on these
scenarios for a wide range of dark matter masses. We thus eliminate a sizable
portion of the parameter space for these late decay models if the dominant
decay channel involves Standard Model final states.Comment: 13 pages, 7 figures. Replaced to match published version. Discussion
expanded. References added. Accepted by Phys. Rev
The SNS Cryogenic Control System: Experiences in Collaboration
The cryogenic system for the Spallation Neutron Source (SNS) is designed by
Jefferson Laboratory (JLab) personnel and is based on the existing JLab
facility. Our task is to use the JLab control system design [2] as much as
practical while remaining consistent with SNS control system standards. Some
aspects of the systems are very similar, including equipment to be controlled,
the need for PID loops and automatic sequences, and the use of EPICS. There are
differences in device naming, system hardware, and software tools. The
cryogenic system is the first SNS system to be developed using SNS standards.
This paper reports on our experiences in integrating the new and the old.Comment: 3 page
STS-1 operational flight profile. Volume 5: Descent, cycle 3
The trajectory data presented are to be used for orbiter systems and subsystems evalation, flight and mission control center software verification, flight techniques and timeline development, crew training, and evaluation of operational mission suitability. The entry profile is very similar to cycle 2, however, elevon and body flap temperature margins have increased and the elevon schedule was changed. The terminal area energy management (TAEM) profile was completely reshaped to conform with new angle of attack constraints and left hand turn around the heading alignment cylinder. Also, the entry/TAEM interface was adjusted to minimize guidance induced angle of attack transients across the interface. The approach and landing phase was reshaped for a 20 deg glideslope and reduced velocity at touchdown. The definition of the runway threshold was standardized for all landing sites. This results in a shift at Edwards Air Force Base in aim points and touchdown relative to the threshold of 1000 feet. The rollout remains essentially unchanged with the exception of the speedbrake, which is now deployed to 50 percent at touchdown
Float zone processing in a weightless environment
Results are given for investigations into: (1) the physical limits which set the maximum practical diameters of Si crystals that can be processed by the float-zone method in a near weightless environment, and (2) the economic impact of large, space-produced Si crystals on the electronics industry. The stability of the melt is evaluated. Heat transfer and fluid flow within the melt as dependent on the crystal size and the degree and type of rotation imparted to the melt are studied. Methods of utilizing the weightless environment for the production of large, stress-free Si crystals of uniform composition are proposed. The economic effect of large size Si crystals, their potential applications, likely utilization and cost advantages in LSI, integrated circuits, and power devices are also evaluated. Foreseeable advantages of larger diameter wafers of good characteristics and the possibilities seen for greater perfection resulting from stress-free growth are discussed
Production and propagation of cosmic-ray positrons and electrons
We have made a new calculation of the cosmic-ray secondary positron spectrum
using a diffusive halo model for Galactic cosmic-ray propagation. The code
computes self-consistently the spectra of primary and secondary nucleons,
primary electrons, and secondary positrons and electrons. The models are first
adjusted to agree with the observed cosmic-ray Boron/Carbon ratio, and the
interstellar proton and Helium spectra are then computed; these spectra are
used to obtain the source function for the secondary positrons/electrons which
are finally propagated with the same model parameters. The primary electron
spectrum is evaluated, again using the same model. Fragmentation and energy
losses are computed using realistic distributions for the interstellar gas and
radiation fields, and diffusive reacceleration is also incorporated. Our study
includes a critical re-evaluation of the secondary decay calculation for
positrons.
The predicted positron fraction is in good agreement with the measurements up
to 10 GeV, beyond which the observed flux is higher than that calculated. Since
the positron fraction is now accurately measured in the 1-10 GeV range our
primary electron spectrum should be a good estimate of the true interstellar
spectrum in this range, of interest for gamma ray and solar modulation studies.
We further show that a harder interstellar nucleon spectrum, similar to that
suggested to explain EGRET diffuse Galactic gamma ray observations above 1 GeV,
can reproduce the positron observations above 10 GeV without requiring a
primary positron component.Comment: 25 pages including 8 figures and 1 table, latex, aaspp4.sty. To be
published in ApJ 1998, v.493 (February 1 issue). Details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
- …