1,417 research outputs found
Axions from wall decay
We discuss the decay of axion walls bounded by strings and present numerical
simulations of the decay process. In these simulations, the decay happens
immediately, in a time scale of order the light travel time, and the average
energy of the radiated axions is for . is found to increase approximately linearly with
. Extrapolation of this behaviour yields in axion models of interest.Comment: 6 pages, 7 figures, to be published in the Proc. of the 5th IFT Axion
workshop Gainesville FL, Mar 13-15 199
A broadband THz receiver for low background space applications
We have developed a sensitive bolometric receiver for low background space applications. In a 10 percent bandwidth at 1 THz, this receiver is approximately 100 times more sensitive than a quantum limited heterodyne receiver with a 1 GHz IF bandwidth. This receiver is designed to be used for the long wavelength band (200-700 microns) in the MIPS instrument on NASA's SIRTF satellite. The bolometers are cooled to 100 mK by an adiabatic demagnetization refrigerator. Roughly 60 g of cesium chrome alum salt is partially demagnetized to 100 mK, followed by a slow regulated downramp to compensate for the heat leak. The hold time of the ADR system is about 18 hours with a temperature stability of delta T(sub rms) approx. equals 10 micro-K. The composite bolometers have electrical responsivities of 10(exp 9)V/W and electrical NEP's of about 3x10(exp -17) W/square root of Hz. The bolometer signals are read out by JFET preamplifiers located on the helium plate and operated at 120 K. We have addressed a number of space qualification issues, such as the development of an analog magnet controller, construction of a cryogenic shake-table for bolometers and selection of the paramagnetic salt CCA which can survive a bakeout at 50 C. The receiver is scheduled to be flown in the spring of 1992 on a balloon telescope. This flight has a dual purpose. One is to provide realistic test of the capabilities of the new receiver. The other is to search for anisotropies in the cosmic microwave background on scales of a few degrees
The Rydberg-Atom-Cavity Axion Search
We report on the present progress in development of the dark matter axion
search experiment with Rydberg-atom-cavity detectors in Kyoto, CARRACK I and
CARRACK II. The axion search has been performed with CARRACK I in the 8 % mass
range around , and CARRACK II is now ready for the search in
the wide range . We have also developed
quantum theoretical calculations on the axion-photon-atom system in the
resonant cavity in order to estimate precisely the detection sensitivity for
the axion signal. Some essential features on the axion-photon-atom interaction
are clarified, which provide the optimum experimental setup for the axion
search.Comment: 8 pages, 2 figures, Invited talk presented at the Dark2000,
Heidelberg, Germany,10-15 July, 200
Functional mapping of the human visual cortex with intravoxel incoherent motion MRI.
Functional imaging with intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) is demonstrated. Images were acquired at 3 Tesla using a standard Stejskal-Tanner diffusion-weighted echo-planar imaging sequence with multiple b-values. Cerebro-spinal fluid signal, which is highly incoherent, was suppressed with an inversion recovery preparation pulse. IVIM microvascular perfusion parameters were calculated according to a two-compartment (vascular and non-vascular) diffusion model. The results obtained in 8 healthy human volunteers during visual stimulation are presented. The IVIM blood flow related parameter fD* increased 170% during stimulation in the visual cortex, and 70% in the underlying white matter
How Much Confidence Do We Have in a MRI Tractography Experiment?
When performing a tractography experiment it is essential to know whether a reconstructed tract results from the diffusion signal itself or from some random effect or noise. In this study, we introduce a way to associate to every connection a confidence level. The reason why the latter greatly varies with the length of the tract is analyzed. We use this method to filter out the connections likely to be the result of noise and show the effect on the connectivity of the human visual system
- …