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91.
J. L. Shinn J. W. Wilson W. Schimmerling M. R. Shavers J. Miller E. V. Benton A. L. Frank F. F. Badavi 《Radiation and environmental biophysics》1995,34(3):155-159
Abtract The use of Green's function has played a fundamental role in transport calculations for high-charge high-energy (HZE) ions. Two recent developments have greatly advanced the practical aspects of implementation of these methods. The first was the formulation of a closedform solution as a multiple fragmentation perturbation series. The second was the effective summation of the closedform solution through nonperturbative techniques. The nonperturbative methods have been recently extended to an inhomogeneous, two-layer transport media to simulate the lead scattering foil present in the Lawrence Berkeley Laboratories (LBL) biomedical beam line used for cancer therapy. Such inhomogeneous codes are necessary for astronaut shielding in space. The transport codes utilize the Langley Research Center atomic and nuclear database. Transport code and database evaluation are performed by comparison with experiments performed at the LBL Bevalac facility using 670A MeV20Ne and 600A MeV56Fe ion beams. The comparison with a time-of-flight and E detector measurement for the20Ne beam and the plastic nuclear track detectors for56Fe show agreement up to 35%–40% in water and aluminium targets, respectively.Submitted paper presented at the International Symposium on Heavy Ion Research: Space, Radiation Protection and Therapy, Sophia-Antipolis, France 21–24 March 1994 相似文献
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94.
Benton MJ 《Trends in ecology & evolution》1997,12(12):490-495
The diversification of life through geological time a rise from presumably one species to many millions today. The diversification of marine families in the past 600 million years (Myr) appears to have followed two or three logistic curves, with equilibrium levels that lasted for up to 200 Myr. In contrast, continental organisms clearly show an exponential pattern of diversification, and although it is not clear whether the empirical diversification patterns are real or are artefacts of a poor fossil record, the latter explanation seems unlikely. Perhaps marine and continental organisms diversified in different ways, or perhaps the appearance of equilibrium patterns for marine organisms is an artefact of taxonomic structures. 相似文献
95.
Mark N. Puttick Gavin H. Thomas Michael J. Benton 《Evolution; international journal of organic evolution》2016,70(4):873-886
Dating the origin of Placentalia has been a contentious issue for biologists and paleontologists. Although it is likely that crown‐group placentals originated in the Late Cretaceous, nearly all molecular clock estimates point to a deeper Cretaceous origin. An approach with the potential to reconcile this discrepancy could be the application of a morphological clock. This would permit the direct incorporation of fossil data in node dating, and would break long internal branches of the tree, so leading to improved estimates of node ages. Here, we use a large morphological dataset and the tip‐calibration approach of MrBayes. We find that the estimated date for the origin of crown mammals is much older, ~130–145 million years ago (Ma), than fossil and molecular clock data (~80–90 Ma). Our results suggest that tip calibration may result in estimated dates that are more ancient than those obtained from other sources of data. This can be partially overcome by constraining the ages of internal nodes on the tree; however, when this was applied to our dataset, the estimated dates were still substantially more ancient than expected. We recommend that results obtained using tip calibration, and possibly morphological dating more generally, should be treated with caution. 相似文献
96.
Virus in Water. II. Evaluation of Membrane Cartridge Filters for Recovering Low Multiplicities of Poliovirus from Water 总被引:9,自引:9,他引:0
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William F. Hill Jr. Elmer W. Akin William H. Benton Theodore G. Metcalf 《Applied microbiology》1972,23(5):880-888
The efficiency of a Millitube MF cartridge filter, a membrane filter, for recovery of poliovirus from 100-gal volumes of both fresh (tap) and estuarine water was determined. In the high multiplicity of virus input-output experiments, recovery of 97% or greater of input virus was achieved in both types of water when the final concentration of divalent cation as Mg(2+) was 1,200 mug/ml and the pH was 4.5. Virus was effectively eluted from the membrane cartridge with 5x nutrient broth in 0.05 M carbonate-bicarbonate buffer at pH 9.0. Four elutions of 250 ml each were used. In the low multiplicity of virus input-output experiments under the same cationic and pH conditions, up to 67% of the input virus was recovered when the virus was further concentrated from the eluates by the aqueous polymer two-phase separation technique. The volume reduction was 126,000-190,000 to 1. The use of the combined techniques, i.e., membrane adsorption followed by aqueous polymer two-phase separation, provided a highly sensitive, simple, and remarkably reliable sequential methodology for the quantitative recovery of poliovirus occurring at multiplicities as low as 1 to 2 plaque-forming units per 5 gal of water. 相似文献
97.
Biochemical Evidence of Selective Nerve Cell Changes in the Normal Ageing Human and Rat Brain 总被引:6,自引:6,他引:0
S. J. Allen J. S. Benton M. J. Goodhardt E. A. Haan N. R. Sims C. C. T. Smith J. A. Spillane D. M. Bowen A. N. Davison 《Journal of neurochemistry》1983,41(1):256-265
Abstract: Atrophy with ageing of human whole brain, entire temporal lobe, and caudate nucleus was assessed in autopsy specimens, by biochemical techniques. Only the caudate nucleus showed changes. Markers for several neurotransmitter systems were also examined for changes with age. In neocortex and temporal lobe of human brain, small decreases were detected in markers of cholinergic nerve terminals, whereas a large decrease (79%) occurred in the caudate nucleus. Findings were similar in striatum from 3–33-month-old rats. No change occurred in binding of [3H]quinuclidinyl benzilate by human samples. Markers of serotonergic terminals were also unchanged in human and rat brain. By contrast, binding of [3H]lysergic acid diethylamide and [3H]serotonin was decreased (32–81%) in human neocortex and temporal lobe, but not in caudate nucleus. A 43% loss of a marker of γ-aminobutyrate terminals occurred in human neocortex, while [3H]muscimol binding increased (179%). No changes were detected in markers of catecholamine synapses in temporal lobe or rat striatum. Hence, with human ageing there appears to be a loss of markers of γ-aminobutyrate neurones intrinsic to neocortex and acetylcholine cells intrinsic to the caudate nucleus, as well as a change in postsynaptic serotonin receptors in neocortex. These losses are accompanied by relative preservation of markers of ascending projections from basal forebrain and brain stem. 相似文献
98.
Benton M. Stidd 《Review of Palaeobotany and Palynology》1981,32(1):63-101
The compression and permineralized fossil record of presumed medullosan plants is reviewed and evaluated. Recent studies of stelar architecture suggest that stems are monostelic rather than polystelic as commonly believed. Imparipinnate fronds are mostly of the Neuralthopteris/Alethopteris type with Myeloxylon anatomy; the affinity between medullosan stems and neuropterid fronds is uncertain. It is suggested that the reproductive organs of the family should be restricted to those pollen organs believed to have produced Monoletes pollen and to ovules, for the most part, of the Trigonocarpus/Pachytesta type. It is doubtful that reproductive organs replaced lateral pinnules; it is more likely that they terminated pinnae or were borne abaxially along the pinna rachis. In some cases they may have been borne directly on penultimate axes or were, as in Codonotheca and Goldenbergia, borne on fertile fronds or fertile portions of vegetative fronds. Recent research involving pollen organs is emphasized and a phylogeny of Monoletespollen organs is proposed. It is suggested that the fossil record of medullosans is best interpreted from a punctuational perspective. 相似文献
99.
100.
Differentiation of Trichoplusia ni MEV and Autographa californica MEV by macrophage migration inhibition tests 总被引:2,自引:0,他引:2
The macrophage migration inhibition test (MMI), an in vitro correlate of delayed hypersensitivity, was found to be an effective means of differentiating Trichoplusia ni and Autographa californica multiple embedded nuclear polyhedrosis viruses (NPV). Peritoneal exudate cells from guinea pigs sensitized to virions of T. ni NPV demonstrated significantly different MMI when challenged with T. ni vs A. californica virions. Similarly, when virions of A. californica NPV were employed as the sensitizing antigen, different percentages of MMI were observed in the homologuus versus heterologous challenges.The susceptibility of the fall armyworm, Spodoptera frugiperda, to these two pathogens was very similar as revealed by a comparison of LD50's, slopes, and fiducial limits of dose-mortality regression lines. In contrast, the cabbage looper, T. ni, was much more susceptible to A. californica NPV.The polyhedral sizes, shapes, and virion occlusion patterns of the two species of NPV were virtually indistinguishable. 相似文献