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[en] Positron annihilation rates have been studied in polymers and graphite-polymer composites as a function of their moisture content. The annihilation rates have been found to increase linearly with increasing moisture content in epoxies and polyamides, whereas no definite trends have been observed in polyimides. These experimental results have been used as the basis for the calculation of moisture content of several polymeric test specimens. For example, the directly measured moisture content of a Kevlar specimen was 45.5 + or - 5.0% of saturation value, whereas the moisture content on the basis of the decrease in positron lifetime was calculated to be 46.5 + or - 3.5%. Similarly, the directly measured moisture content of a graphite-epoxy composite (55 v/o fiber) was 19.2 + or - 0.6% of saturation value as opposed to a calculated value of 16.0 + or - 5.0%
[en] Large extra dimensions (LED) have been proposed to account for the apparent weakness of gravitation. These theories also indicate that the postulated massive Kaluza-Klein (KK) gravitons may be produced by nucleon-nucleon bremsstrahlung in the course of core collapse of supernovae. Hannestad and Raffelt have predicted energy spectra of gamma ray emission from the decay of KK gravitons trapped by the gravity of the remnant neutron stars (NS). These and other authors have used EGRET data on NS to obtain stringent limits on LED. Fermi-LAT is observing radio pulsar positions obtained from radio and x-ray catalogs. NS with certain characteristics are unlikely emitter of gamma rays, and emit in radio and perhaps x-rays. This talk will focus on the blind analysis we plan to perform, which has been developed using the 1st 2 months of all sky data and Monte Carlo simulations, to obtain limits on LED based on about 1 year of Fermi-LAT data. Preliminary limits from this analysis using these first 2 months of data will be also be discussed.
[en] In 1934 Fano received a Sc. D. degree in mathematics at University of Turin, Italy (the city of his birth in 1912). He was then led to physics by his cousin Guilio Racah, and received postdoctoral training from Fermi at Rome and from Heisenberg at Leipzig. He worked at institutions near Washington, D. C. during the war, and joined the staff of the National Bureau of Standards in 1946. He became a professor of physics at The University of Chicago in 1966. His contributions to radiation physics, atomic and molecular physics, and statistical physics are extensive and outstanding. Recognition includes many honors such as the Fermi Award by the DOE, and terms such as the Beutler-Fano profile of certain spectral lines, the Fano factor characterizing the fluctuations of the radiation-induced ionization, the Fano-Lichten mechanism for inelastic atomic collisions, and the Fano effect leading to spin-polarized photoelectrons. His work follows a style inherited from Fermi and is characterized by incisive insight into the physics behind experimental data, penetrating mathematical analysis, and close communications with many colleagues. Because he took a leading role in developing new areas of research and in nurturing young scientists, his influence now permeates many topics of physics. They include far uv and soft x-ray spectroscopy with synchrotron radiation and fundamental radiological physics, both stemming from his time at NBS, as well as multi-channel quantum-defect theory and hyperspherical-coordinate approach, both pioneered at Chicago. Fuller accounts of his life and science are seen in Inokuti , in Rau , and in a forthcoming special issue of Physics Essays in his honor. The present work is supported by U. S. DOE, Office of Science, Nuclear Physics Division, under Contract No. W-31-109-Eng-38. References 1. M. Inokuti, in Fundamental Processes of Atomic Dynamics, J. S. Briggs et al. (eds.), (Plenum, New York, 1988), p. 1. 2. A. R. P. Rau, Comments At. Mol. Phys. 33, 181 (1997)
[en] In this paper we discuss representations of charge particle densities in particle-in-cell simulations, analyze the sources and profiles of the intrinsic numerical noise, and present efficient methods for their removal. We devise two alternative estimation methods for charged particle distribution which represent significant improvement over the Monte Carlo cosine expansion used in the 2D code of Bassi et al. (G. Bassi, J.A. Ellison, K. Heinemann and R. Warnock Phys. Rev. ST Accel. Beams 12 080704 (2009)G. Bassi and B. Terzic, in Proceedings of the 23rd Particle Accelerator Conference, Vancouver, Canada, 2009 (IEEE, Piscataway, NJ, 2009), TH5PFP043), designed to simulate coherent synchrotron radiation (CSR) in charged particle beams. The improvement is achieved by employing an alternative beam density estimation to the Monte Carlo cosine expansion. The representation is first binned onto a finite grid, after which two grid-based methods are employed to approximate particle distributions: (i) truncated fast cosine transform; and (ii) thresholded wavelet transform (TWT). We demonstrate that these alternative methods represent a staggering upgrade over the original Monte Carlo cosine expansion in terms of efficiency, while the TWT approximation also provides an appreciable improvement in accuracy. The improvement in accuracy comes from a judicious removal of the numerical noise enabled by the wavelet formulation. The TWT method is then integrated into the CSR code (G. Bassi, J.A. Ellison, K. Heinemann and R. Warnock Phys. Rev. ST Accel. Beams 12 080704 (2009)), and benchmarked against the original version. We show that the new density estimation method provides a superior performance in terms of efficiency and spatial resolution, thus enabling high-fidelity simulations of CSR effects, including microbunching instability.
[en] The authors calculate the energy spectrum for N interacting bosons confined by a harmonic trap. Due to the rapidly growing number of states available, the authors are limited practically to relatively low-lying states. They consider parameters for the atoms and the trap similar to those from the recent Bose-Einstein condensation experiment of Anderson et al. and discuss the relevance of their results to the experiment
[en] The theory of neutron beta decay provides for the possibility of a violation of time-reversal (T) invariance due to the presence of a manifestly T-odd term. This term is a triple correlation involving the neutron spin and the momenta of the electron and neutrino decay products. The degree of correlation (usually denoted by the coefficient D) may be measured using a beam of cold, polarized neutrons and detecting beta-decay electrons in coincidence with recoil protons. The emiT experiment will improve on previous neutron-based measurements of D by employing PIN diode proton detectors in an octagonal detector array at the Cold Neutron Research Facility, NIST. The expected sensitivity to D is 3 x 10-4, which may permit restrictions to be placed on several extensions to the Standard Model that allow values of D near 10-3. The detector system is currently nearing completion and preliminary runs will be made this summer at NIST
[en] The method of complex-coordinate rotation is used to investigate electric-field effects on the doubly-excited 2s2p 1Po and 2p21De states of He, the two neighboring states with a field-free seperation of 0.017 Ryd. Strong electric-field strengths up to F = 0.02 Ryd are used in the present study. Products of Slater orbitals are used to represent the two-electron wave functions, with lmax = 8 is employed for individual electron. Block matrices with up to Lmax = 7 (K-states) are used to investigate the convergence behaviors for the resonance parameters (resonance energy and width). When the external electric field is turned on, open-quotes classicclose quotes Stark effect is observed for these two neighboring states. Results for electric-field effects on the lowest-lying 2s21Se state are also given. Comparisons are made with other calculations when available
[en] The authors have extended previous studies of n=10 Rydberg states of H2 to include the other two stable isotopes, HD and D2. The Rydberg fine structure is found to be very similar in all three systems. In HD, perturbing effects of the core dipole moment are observable, but relatively small. The main difference between the isotopes results from the influence of nuclear statistics on the relative populations of different Rydberg levels. This factor favors even R levels in D2 by 2:1, odd R levels in H2 by 3:1, and is absent in HD