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  • 1
    Electronic Resource
    Electronic Resource
    A high counting rate parallel-plate gas proportional counter for fluorescence extended x-ray-absorption fine structure studies : Proceedings of the 4th international conference on synchrotron radiation instrumentation (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 837-841 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Gas proportional counters (GPCs) are well suited for fluorescence extended x-ray-absorption fine structure (EXAFS) experiments in the soft x-ray (〈4 keV) region. They have a near unity detection efficiency and offer good energy resolution. Of the different types of GPC, the parallel-plate device is an attractive option for use with synchrotron radiation beam lines where its intrinsically high count rate permits optimum use of the high photon flux available. The development of a parallel-plate gas proportional counter for use on soft x-ray beam lines at the Daresbury synchrotron source is described. This device is demonstrated to have a counting rate in excess of 1 MHz, and data are presented on its performance including its energy resolution, long-term gain stability, and sample EXAFS spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142622
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  • 2
    Electronic Resource
    Electronic Resource
    A high rate preamplifier for use with a gas proportional counter : Proceedings of the 4th international conference on synchrotron radiation instrumentation (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 842-845 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In the development of detectors for use with synchrotron radiation, a serious limit to the maximum counting rate is frequently determined by the signal processing speed of the general purpose commercial electronic units that are often used. Therefore, in developing a high speed gas proportional counter for soft x-ray spectroscopy measurements, a major requirement has been for a high rate, low noise preamplifier. A crucial factor in meeting the required processing speed is the ability to separate out the fast electron pulse from the much slower, induced ion afterpulse and to reject the latter. In addition, all gas proportional counters suffer from occasional high voltage breakdown and so the circuit design must provide the sensitive front end of the preamplifier with sufficient transient protection. We present a design for a preamplifier circuit which offers a 30 MHz bandwidth and better than 2500 electrons rms noise. It features pulse shaping which permits the complete rejection of the ion afterpulse and an input protection circuit which has enabled the preamplifier to survive detector breakdown. The preamplifier has a measured gain of 0.6 μV per electron. The fast rise time of the preamplifier has allowed us to investigate the speed of the detector, a primary limitation of which is due to the electron diffusion rate within the counter. We describe an experiment conducted using this preamplifier to reduce the electron drift speed to 〈100 ns.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142623
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  • 3
    Electronic Resource
    Electronic Resource
    Photofluxonic detection: A new mechanism for infrared detection in superconducting thin films (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 2847-2849 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new model is proposed in which a photon of energy E=hf is absorbed by a superconducting film to create a pair of equal and opposite fluxons (or vortices), each with quantized flux Φ0=h/2e. An applied current sweeps these fluxons to opposite edges of the film, causing a voltage pulse with time-integrated magnitude Φ0, and leading to a time-averaged voltage responsivity Rv = Φ0/E = 1/(2ef). This is directly analogous to photoconductive detection in a semiconductor via creation of electron-hole pairs. Data on an ultrathin granular NbN film are presented which indicate a responsivity of 6000 V/W in red light, in agreement with the model. This is promising for the development of a sensitive, high-speed infrared detector using thin films of either low or high Tc superconductors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.103760
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  • 4
    Electronic Resource
    Electronic Resource
    Optimization of growth conditions for strain compensated Ga0.32In0.68As/Ga0.61In0.39As multiple quantum wells (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2311-2313 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The results of an investigation into the growth of Ga0.32In0.68As/Ga0.61In0.39As strain compensated multiple quantum well stacks are presented. We show that these structures, which contain alloys that are not prone to compositional clustering, do suffer from thickness modulated growth. A growth strategy has been developed to inhibit this phenomenon and a 50 well, strain compensated stack has been grown with planar and abrupt interfaces. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.112727
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  • 5
    Electronic Resource
    Electronic Resource
    Vacancy controlled interdiffusion of the group V sublattice in strained InGaAs/InGaAsP quantum wells (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 797-799 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this letter we present the results of a photoluminescence study of the interdiffusion of arsenic and phosphorous in the In0.66Ga0.33As/In0.66Ga0.33As0.7P0.3 system over the temperature range 950–600 °C. We have shown that the diffusion is Fickian with no dependence of the diffusion coefficient on the substrate doping type or etch pit density. For both tin- and sulfur-doped substrates the diffusion can be described by a diffusion coefficient D, which is given by D=D0 exp(−EA/kT), where D0=23 cm2/s and EA=3.7 eV for temperatures greater than 675 °C. This activation energy is the same as that determined for the group III interdiffusion in In0.2Ga0.8As/GaAs. Below this temperature a lower activation energy process takes over with D0=5×10−10 cm2/s and EA=1.7 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.109911
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  • 6
    Electronic Resource
    Electronic Resource
    Performance of the Daresbury synchrotron radiation source soft x-ray double-crystal monochromator : Proceedings of the 4th international conference on synchrotron radiation instrumentation (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 1322-1325 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The double-crystal monochromator on beam line 3.4 at the Daresbury synchrotron radiation source covers the energy region 0.8–4 keV, routinely using crystal pairs of beryl, quartz, InSb, Ge, and Si. The beam line has a unique system to cut off high-energy radiation. The whole beam line has been built on a swinging arm, which allows the glancing angle on a chromium coated premirror to be varied between 0.5° and 3°. This not only reduces the heat load on fragile crystals, but also reduces the higher order contributions. We have made a thorough study of the alignment problems arising in the double-crystal geometry. Crystal misalignment, Bragg angular offset, as well as heat-load effects, can cause severe beam movements. A beam-position monitor, which has recently been commissioned, is an invaluable diagnostic tool for these beam movements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143059
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  • 7
    Electronic Resource
    Electronic Resource
    Sites of [3H]taurine Uptake in the Rat Substantia Nigra in Relation to the Release of Taurine from the Striatonigral Pathway (1990)
    Oxford, UK : Blackwell Publishing Ltd
    European journal of neuroscience 2 (1990), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The autoradiographic localization of radiolabelled taurine taken up in the rat substantia nigra in vivo together with conditions of release of the [3H]taurine taken up into brain slices were studied to determine whether they are consistent with the hypothesis that taurine may act as a neurotransmitter in the striatonigral pathway.At the light microscopic level the main cellular elements that became radiolabelled following the injection of [3H]taurine into the substantia nigra could be identified as glial cells. Electron microscope autoradiography confirmed that a subpopulation of glial cells including astrocytes, pericytes, and oligodendrocytes were radiolabelled and that neuronal perikarya were not radiolabelled. In addition, axonal elements including both terminal and preterminal boutons were found to have silver grains overlying them and were thus considered to be radiolabelled. This was supported by a quantitative analysis of the distribution of the silver grains; whereas glial elements had a significantly higher number of grains associated with them than with any other structure, axonal elements had a significantly greater number of grains than dendritic structures.Release of the preloaded [3H]taurine from superfused slices of substantia nigra occurred in response to veratridine, was calcium-dependent and was sensitive to inhibition by high magnesium concentrations or tetrodotoxin. Following the destruction of neurons in the striatum by ibotenic acid injections, although the weight of the ipsilateral substantia nigra was reduced, the uptake of [3H]taurine was not altered. In contrast to this, the veratridine-stimulated release was markedly attenuated, implying that the destruction of striatal neurons causes the loss of sites in the substantia nigra from which exogenous taurine is released.These results add further support to previous suggestions that taurine might act as a neurotransmitter or neuromodulator in the striatonigral pathway.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-9568.1990.tb00380.x
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  • 8
    Electronic Resource
    Electronic Resource
    Descending Projections from the Substantia Nigra and Retrorubral Field to the Medullary and Pontomedullary Reticular Formation (1991)
    Oxford, UK : Blackwell Publishing Ltd
    European journal of neuroscience 3 (1991), S. 0 
    Details
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: We have investigated the projection from the substantia nigra to the pontomedullary reticular formation in the rat using both retrograde and anterograde neuroanatomical tracers. Injections of a conjugate of wheatgerm agglutinin with horseradish peroxidase into the medullary or pontomedullary reticular formation resulted in the retrograde labelling of a continuous band of cells extending from the caudal half of the dorsolateral substantia nigra into the retrorubral field. Injections of the anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L) into either the dorsolateral substantia nigra or the caudally adjacent retrorubral field revealed a descending projection to the lateral medullary and pontomedullary brainstem, which terminated mainly within the lateral (parvicellular) reticular formation. The anterograde PHA-L fibre labelling ran throughout the rostrocaudal extent of the parvicellular reticular formation and extended into the caudally continuous region, the medullary dorsal and medullary ventral reticular formation, where it tapered off. Also labelled, although more lightly, were the rostral and ventrolateral regions of the nucleus of the solitary tract and the magnocellular reticular formation. Electron microscopy established that the PHA-L-labelled fibres formed synaptic contacts with nerve cell bodies and dendrites in the parvicellular reticular formation. It is suggested that one role of this nigroreticular pathway might be to connect the basal ganglia with brainstem premotor neurons that influence orofacial musculature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-9568.1991.tb00088.x
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  • 9
    Electronic Resource
    Electronic Resource
    Net production of herbage from a continuously stocked Lolium perenne-dominated sward in late autumn (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Grass and forage science 46 (1991), S. 0 
    Details
    ISSN: 1365-2494
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Preliminary studies were carried out on the effect of stocking rate during late autumn on a continuously stocked Lolium perenne-dominated sward at an upland site in central Scotland. Measurements were made of L. perenne tiller population density on 29 September and 2 November and of L. perenne net production, mean sward height and total herbage mass in early and late October and early and late November. Stocking rates were 12 ewes per ha during October and 8 and 16 ewes per ha during November. Sward height and herbage mass declined with time and more rapidly at the higher stocking rate. L. perenne growth per tiller and per unit area was influenced by time but not by stocking rate and was closely related to the 5·5°C soil temperature at 10 cm depth. Tiller senescence was greatly reduced at the higher stocking rate and/or the consequentially lower sward height and herbage mass. Tiller net production was therefore sustained at a positive level on the higher stocked sward throughout November while on the lower stocked sward it fell below zero early in November.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2494.1991.tb02404.x
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  • 10
    Electronic Resource
    Electronic Resource
    Hatfield, Gary: The Natural and the Normative: Theories of Spatial Perception from Kant to Helmholtz : Cambridge, Mass, and London, MIT Press, 1991 (1993)
    Cambridge : Cambridge University Press
    The @British journal for the history of science 26 (1993), S. 93-95 
    Details
    ISSN: 0007-0874
    Source: Cambridge Journals Digital Archives
    Topics: History , Natural Sciences in General
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1017/S0007087400030314
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