A Zero-Gravity Instrument to Study Low Velocity Collisions of Fragile Particles at Low Temperatures

We discuss the design, operation, and performance of a vacuum setup constructed for use in zero (or reduced) gravity conditions to initiate collisions of fragile millimeter-sized particles at low velocity and temperature. Such particles are typically found in many astronomical settings and in regions of planet formation. The instrument has participated in four parabolic flight campaigns to date, operating for a total of 2.4 hours in reduced gravity conditions and successfully recording over 300 separate collisions of loosely packed dust aggregates and ice samples. The imparted particle velocities achieved range from 0.03-0.28 m s^-1 and a high-speed, high-resolution camera captures the events at 107 frames per second from two viewing angles separated by either 48.8 or 60.0 degrees. The particles can be stored inside the experiment vacuum chamber at temperatures of 80-300 K for several uninterrupted hours using a built-in thermal accumulation system. The copper structure allows cooling down to cryogenic temperatures before commencement of the experiments. Throughout the parabolic flight campaigns, add-ons and modifications have been made, illustrating the instrument flexibility in the study of small particle collisions.Comment: D. M. Salter, D. Hei{\ss}elmann, G. Chaparro, G. van der Wolk, P. Rei{\ss}aus, A. G. Borst, R. W. Dawson, E. de Kuyper, G. Drinkwater, K. Gebauer, M. Hutcheon, H. Linnartz, F. J. Molster, B. Stoll, P. C. van der Tuijn, H. J. Fraser, and J. Blu

Full-Folding Optical Potentials for Elastic Nucleon-Nucleus Scattering based on Realistic Densities

Optical model potentials for elastic nucleon nucleus scattering are calculated for a number of target nuclides from a full-folding integral of two different realistic target density matrices together with full off-shell nucleon-nucleon t-matrices derived from two different Bonn meson exchange models. Elastic proton and neutron scattering observables calculated from these full-folding optical potentials are compared to those obtained from `optimum factorized' approximations in the energy regime between 65 and 400 MeV projectile energy. The optimum factorized form is found to provide a good approximation to elastic scattering observables obtained from the full-folding optical potentials, although the potentials differ somewhat in the structure of their nonlocality.Comment: 21 pages, LaTeX, 17 postscript figure

Starbursts and High-Redshift Galaxies are Radioactive: High Abundances of 26^{26}Al and Other Short Lived Radionuclides

Short lived radionuclides (SLRs) like $^{26}$Al are synthesized by massive stars and are a byproduct of star formation. The abundances of SLRs in the gas of a star-forming galaxy are inversely proportional to the gas consumption time. The rapid evolution of specific star formation rate (SSFR) of normal galaxies implies they had mean SLR abundances ~3--10 times higher at z = 2. During the epoch of Solar system formation, the background SLR abundances of the Galaxy were up to twice as high as at present, if SLR yields from massive stars do not depend on metallicity. If SLRs are homogenized in the gas of galaxies, the high SSFRs of normal galaxies can partly explain the elevated abundance of SLRs like $^{60}$Fe and $^{26}$Al in the early Solar system. Starburst galaxies have much higher SSFRs still, and have enormous mean abundances of $^{26}$Al ($^{26}$Al/$^{27}$Al ~ $10^{-3}$ for Solar metallicity gas). The main uncertainty is whether the SLRs are mixed with the star-forming molecular gas: they could be trapped in hot gas and decay before entering the colder phases, or be blown out by starburst winds. I consider how variability in star-formation rate affects the SLR abundances, and I discuss how SLR transport may differ in these galaxies. The enhanced $^{26}$Al of starbursts might maintain moderate ionization rates ($10^{-18}$ -- $10^{-17}$ s$^{-1}$), possibly dominating ionization in dense clouds not penetrated by cosmic rays. Similar ionization rates would be maintained in protoplanetary discs of starbursts, if the SLRs are well-mixed, and the radiogenic heating of planetesimals would likewise be much higher. In this way, galaxy evolution can affect the geological history of planetary systems.Comment: Published in MNRAS, 12 pages, 3 figure

Role of uteroplacental and fetal Doppler in identifying fetal growth restriction at term.

Identification of the fetus at risk of adverse outcome at term is a challenge to both clinicians and researchers alike. Despite the fact that fetal growth restriction (FGR) is a known risk factor for stillbirth, at least two thirds of the stillbirth cases at term are not small for gestational age (SGA) - a commonly used proxy for FGR. However, the majority of SGA fetuses are constitutionally small babies and do not suffer from adverse perinatal outcome. The cerebroplacental ratio (CPR) is emerging as a marker of failure to reach growth potential at term. CPR is an independent predictor of intrapartum fetal distress, admission to the neonatal unit at term, stillbirth, perinatal death and neonatal morbidity. Raised uterine artery Doppler resistance in the third trimester is independently associated with significantly lower birthweight and CPR. The combination of the estimated fetal weight, CPR and uterine Doppler in the third trimester can identify the majority of fetuses at risk of stillbirth

Assessement of tensile strength of graphites by the iosipescu coupon test

Polycrystalline graphites are widely used in the metallurgical, nuclear and aerospace industries. Graphites are particulated composites manufactured with a mixture of coke with pitch, and changes in relative proportions of these materials cause modifications in their mechanical properties. Uniaxial tension tests must be avoided for mechanical characterization in this kind of brittle material, due to difficulties in making the relatively long specimens and premature damages caused during testing set-up. On other types of tests, e.g. bending tests, the specimens are submitted to combined stress states (normal and transverse shear stresses). The Iosipescu shear test, is performed in a beam with two 90° opposite notches machined at the mid-length of the specimens, by applying two forces couples, so that a pure and uniform shear stress state is generated at the cross section between the two notches. When a material is isotropic and brittle, a failure at 45° in relation to the beam long axis can take place, i.e., the tensile normal stress acts parallel to the lateral surface of the notches, controls the failure and the result of the shear test is numerically equivalent to the tensile strength. This work has evaluated a graphite of the type used in rocket nozzles by the Iosipescu test and the resulted stress, ~11 MPa, was found to be equal to the tensile strength. Thus, the tensile strength can be evaluated just by a single and simple experiment, thus avoiding complicated machining of specimen and testing set-up

Close-to-threshold Meson Production in Hadronic Interactions

Studies of meson production at threshold in the hadron--hadron interaction began in the fifties when sufficient energies of accelerated protons were available. A strong interdependence between developments in accelerator physics, detector performance and theoretical understanding led to a unique vivid field of physics. Early experiments performed with bubble chambers revealed already typical ingredients of threshold studies, which were superseded by more complete meson production investigations at the nucleon beam facilities TRIUMF, LAMPF, PSI, LEAR and SATURNE. Currently, with the advent of the new cooler rings as IUCF, CELSIUS and COSY the field is entering a new domain of precision and the next step of further progress. The analysis of this new data in the short range limit permits a more fundamental consideration and a quantitative comparison of the production processes for different mesons in the few--body final states. The interpretation of the data take advantage of the fact that production reactions close-to-threshold are characterized by only a few degrees of freedom between a well defined combination of initial and exit channels. Deviations from predictions of phase-space controlled one-meson-exchange models are indications of new and exciting physics. Precision data on differential cross sections, isospin and spin observables -- partly but by no means adequately available -- are presently turning up on the horizon. There is work for the next years and excitement of the physics expected. Here we try to give a brief and at the same time comprehensive overview of this field of hadronic threshold production studies.Comment: 100 pages, Review article to be published in Prog. Part. Nucl. Phys. Vol. 49, issue 1 (2002

Social Class

Discussion of class structure in fifth-century Athens, historical constitution of theater audiences, and the changes in the comic representation of class antagonism from Aristophanes to Menander

Spike-Timing Precision and Neuronal Synchrony Are Enhanced by an Interaction between Synaptic Inhibition and Membrane Oscillations in the Amygdala

The basolateral complex of the amygdala (BLA) is a critical component of the neural circuit regulating fear learning. During fear learning and recall, the amygdala and other brain regions, including the hippocampus and prefrontal cortex, exhibit phase-locked oscillations in the high delta/low theta frequency band (∼2–6 Hz) that have been shown to contribute to the learning process. Network oscillations are commonly generated by inhibitory synaptic input that coordinates action potentials in groups of neurons. In the rat BLA, principal neurons spontaneously receive synchronized, inhibitory input in the form of compound, rhythmic, inhibitory postsynaptic potentials (IPSPs), likely originating from burst-firing parvalbumin interneurons. Here we investigated the role of compound IPSPs in the rat and rhesus macaque BLA in regulating action potential synchrony and spike-timing precision. Furthermore, because principal neurons exhibit intrinsic oscillatory properties and resonance between 4 and 5 Hz, in the same frequency band observed during fear, we investigated whether compound IPSPs and intrinsic oscillations interact to promote rhythmic activity in the BLA at this frequency. Using whole-cell patch clamp in brain slices, we demonstrate that compound IPSPs, which occur spontaneously and are synchronized across principal neurons in both the rat and primate BLA, significantly improve spike-timing precision in BLA principal neurons for a window of ∼300 ms following each IPSP. We also show that compound IPSPs coordinate the firing of pairs of BLA principal neurons, and significantly improve spike synchrony for a window of ∼130 ms. Compound IPSPs enhance a 5 Hz calcium-dependent membrane potential oscillation (MPO) in these neurons, likely contributing to the improvement in spike-timing precision and synchronization of spiking. Activation of the cAMP-PKA signaling cascade enhanced the MPO, and inhibition of this cascade blocked the MPO. We discuss these results in the context of spike-timing dependent plasticity and modulation by neurotransmitters important for fear learning, such as dopamine