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  • American Association for the Advancement of Science (AAAS)  (4)
  • American Institute of Physics (AIP)
  • 2005-2009
  • 1990-1994  (4)
  • 1980-1984  (3)
  • 1965-1969
  • 1991  (4)
  • 1983  (3)
  • 1
    Electronic Resource
    Electronic Resource
    High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 2926-2938 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349317
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  • 2
    Electronic Resource
    Electronic Resource
    Twistor representation of Yang–Mills fields on Minkowski space (1991)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 32 (1991), S. 259-265 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: A correspondence between (not necessarily self-dual nor anti-self-dual) Yang–Mills fields on Minkowski space, and pairs of cohomology classes γ, φ, on null twistor space PN(large-closed-square) is established; γ∈H1CR (PN(large-closed-square),@sg) defines a deformed Cauchy–Riemman (C–R)(γ) structure on a principal bundle over PN, and φ∈H1CR(γ) (PN(large-closed-square),O(−4)⊗@sg). The correspondence depends on the choice of a spacelike hyperplane in Minkowski space. Here, γ and φ provide initial values in the spin bundle over this hyperplane for a system of evolution equations, along the null geodesic spray congruence in the spin bundle over Minkowski space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.529129
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  • 3
    Electronic Resource
    Electronic Resource
    An experimental study of the molecular mixing process in an axisymmetric laminar vortex ring (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1385-1392 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results are presented from an experimental study of the molecular mixing of a dynamically passive conserved scalar quantity in an axisymmetric laminar vortex ring. The experiments are based on highly resolved laser-induced fluorescence imaging measurements of the scalar field ζ(x,t) in the diametral plane of the ring, from which the evolution of the molecular mixing rate field ∇ζ⋅∇ζ(x,t) can be directly examined. In particular, the structure and dynamics of the mixing process are addressed during the three characteristic stages in the ring evolution, namely, (i) the ring generation stage, (ii) the ring pinch-off stage, and (iii) the asymptotic stage of the ring. Results show a layering of the mixing process in which the diffusional cancellation term ∇(∇ζ):∇(∇ζ) plays a major role in setting the overall mixing rate achieved. The scalar field measurements are also used to extract detailed information about the underlying velocity field in the ring.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858069
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  • 4
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    Evidence for biased gene conversion in concerted evolution of ribosomal DNA (1991)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1991-01-18
    Description: Concerted evolution is the production and maintenance of homogeneity within repeated families of DNA. Two mechanisms--unequal crossing over and biased gene conversion--have been the principal explanations of concerted evolution. Concerted evolution of ribosomal DNA (rDNA) arrays is thought to be largely the result of unequal crossing over. However, concerted evolution of rDNA in parthenogenetic lizards of hybrid origin is strongly biased toward one of two parental sequences, which is consistent with biased gene conversion as the operative mechanism. The apparent gene conversions are independent of initial genome dosage and result in homogenization of rDNA arrays across all nucleolar organizer regions.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Hillis, D M -- Moritz, C -- Porter, C A -- Baker, R J -- New York, N.Y. -- Science. 1991 Jan 18;251(4991):308-10.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Zoology, University of Texas, Austin 78712.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/1987647" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; *Biological Evolution ; Blotting, Southern ; DNA, Ribosomal/*genetics ; Gene Conversion ; Karyotyping ; Lizards ; Nucleic Acid Hybridization ; Parthenogenesis ; Restriction Mapping
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 5
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    Plasticity of motor behavior in monkeys with crossed forelimb nerves (1983)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1983-04-22
    Description: Monkeys in which nerves innervating the flexor muscles of the forearm and hand (the ulnar or the median nerve) had been surgically cross-united with the nerve innervating the extensor muscles (the radial nerve), and vice versa, showed excellent (ulnar-radial crosses) to moderate (median-radial crosses) control of movement performance after regeneration. Antagonistic movement responses were seen occasionally, but these were corrected almost immediately. Stimulation of the crossed nerves showed that they had innervated the antagonistic muscle groups. The results reveal the capacity of the primate central nervous system to adapt to gross disturbances imposed on the execution of movements by changes in peripheral innervation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Brinkman, C -- Porter, R -- Norman, J -- New York, N.Y. -- Science. 1983 Apr 22;220(4595):438-40.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/6836289" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; Fingers/physiology ; Forearm/*innervation/physiology ; Hand/innervation/physiology ; Humans ; Macaca fascicularis ; Macaca nemestrina ; Median Nerve/physiology ; *Movement ; *Neuronal Plasticity ; Radial Nerve/physiology ; Thumb/physiology ; Ulnar Nerve/physiology
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 6
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    Corollary discharge provides accurate eye position information to the oculomotor system (1983)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1983-09-16
    Description: The saccadic system accurately compensates for perturbations of eye position produced by microstimulation of the superior colliculus. This requires that information about the stimulation-induced change in eye position be provided by an extraretinal source--either proprioceptive endings in extraocular muscles or a centrally generated corollary discharge. It is shown that compensation remains intact after elimination of extraocular muscle proprioception, demonstrating that corollary discharge provides accurate eye position information.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Guthrie, B L -- Porter, J D -- Sparks, D L -- F32 EY05651/EY/NEI NIH HHS/ -- P30 EY03039/EY/NEI NIH HHS/ -- R01 EY01189/EY/NEI NIH HHS/ -- New York, N.Y. -- Science. 1983 Sep 16;221(4616):1193-5.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/6612334" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; *Eye Movements ; Macaca mulatta ; Oculomotor Muscles/*physiology ; Photic Stimulation ; Proprioception ; *Saccades ; Superior Colliculi/*physiology
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 7
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    Spermidine requirement for cell proliferation in eukaryotic cells: structural specificity and quantitation (1983)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1983-03-04
    Description: Six structural homologs of spermidine and five of its precursor, putrescine, were studied for their ability to prevent cytostasis of cultured L1210 leukemia cells induced by alpha-difluoromethylornithine (DFMO), a specific inhibitor of putrescine biosynthesis. High-performance liquid chromatography and competition studies with spermidine indicated that the homologs, which vary in the length of the carbon chain separating the amines, penetrated the cells. The structural specificity of the spermidine carrier was defined. Three of the six spermidine homologs supported cell growth during a 48-hour incubation in the presence of DFMO, indicating that a two-carbon extension of spermidine structure was tolerated for biological function. Two of the five putrescine homologs supported growth after being converted by the cells to their respective spermidine homologs. The central nitrogen of spermidine appears to be essential for function since diamines of chain length comparable to that of spermidine did not prevent DFMO cytostasis. No more than 15 percent of the spermidine normally present in L1210 cells was required for cell proliferation in the presence of DFMO.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Porter, C W -- Bergeron, R J -- CA-22153/CA/NCI NIH HHS/ -- CA-24538/CA/NCI NIH HHS/ -- New York, N.Y. -- Science. 1983 Mar 4;219(4588):1083-5.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/6823570" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; *Cell Division ; *Cell Physiological Phenomena ; Eukaryotic Cells/*physiology ; Leukemia L1210/pathology ; Mice ; Ornithine Decarboxylase Inhibitors ; Putrescine/physiology ; Spermidine/analogs & derivatives/*physiology ; Structure-Activity Relationship
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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