Gonadesiklus van die kroonkiewiet Vanellus coronatus op enkele binnelandse lughawens

Ondersoek is ingestel na die gonadesiklus van 474 kroonkiewiete Vanellus coronatus wat gedurende 1986/87 op twee binnelandse lughawens versamel is. Geoordeel aan die ovariummassa en deursnit van die grootste sigbare follikel vind ’n opvallende aktiwiteitspiek in die ovariumsiklus van wyfies gedurende die voorsomer plaas. Geoordeel aan die massa, volume, histologiese ontwikkelingstadia en saadbuisdeursnit van die testes kom ’n ooreenstemmende maar langer aktiwiteitspiek ook by mannetjies voor. Hoewel die testes van onvolwasse mannetjies oor die algemeen kleiner is as dié van volwassenes, is sommige individue binne hul eerste lewensjaar potensieel tot voortplanting in staat. Klaarblyklik word die broeisiklus van die kroonkiewiet in ’n groter mate deur die relatiewe beskikbaarheid van voedsel as omgewingsfaktore soos daglengte, temperatuur of reënval beïnvloed.English Title: Gonadal cycle of the crowned plover Vanellus coronatus at two inland airportsEnglish AbstractGonadal cycle of the crowned plover Vanellus coronatus at two inland airports. The gonadal cycle of 474 crowned plovers Vanellus coronatus collected from two inland airports during 1986/87 was investigated. Judged by the ovarian mass and the diameter of the largest visible follicle, a prominent activity peak occurs in the ovarian cycle of females during early summer. Judged by the mass, volume, histological development stages and diameter jof the seminiferous tubules of the testes, a corresponding but longer activity peak also occurs among males. Although the testes of immature males are generally smaller than those of adults, some individuals are potentially capable of reproduction within their first year of life. Evidently the breeding cycle of the crowned plover is influenced to a greater extent by the relative availability of food than by environmental factors such as day length, temperature or rainfall

Titanium additions to MgB2 conductors

A series of doping experiments are reported for MgB2 conductors that have been synthesized using doped boron fibers prepared by chemical vapor deposition(CVD) methods. Undoped MgB2 samples prepared from CVD prepared fibers consistently give critical current densities, Jc, in the range of 500,000 A/cm^2 in low field at 5K. These values fall by a factor of about 100 as the magnetic field increases to 3T. For heavily Ti-doped boron fibers where the B/Ti ratio is comparable to 1, there is a substantial suppression of both Tc, superconducting volume fraction, and Jc values. If, however, a sample with a few percent Ti in B is deposited on a carbon coated SiC substrate and reacted at 1100 degrees C for 15 min, then Tc is suppressed only a couple of degrees Kelvin and critical current densities are found to be approximately 2-5 x 10^6 A/cm^2 for superconducting layers ranging from 4-10 micrometers thick. These materials show Jc values over 10,000 A/cm^2 at 25K and 1.3 T.Comment: 10 pages, 6 figure

Dosimetrie van Heelliggaamsbestraling met Elektrone vir die Behandeling van Mycosis Fungoides

A six-field method for whole-body radiation with 3,4 MeV electrons obtained from a Mevatron 8 linear accelerator, is described. The physical properties of the electrons are given, and a method is described whereby the wholebody isodose distributions can be predicted during wholebody irradiation

Standardization, calibration and innovation: a special issue on lithic microwear method

YesThis paper introduces a special issue of the Journal of Archaeological Science that considers the current state and future directions in lithic microwear analysis. There is considerable potential for lithic microwear analysis to reconstruct past human behaviour as it can provide direct insight into past activities. Consequently, it is a technique worthy of significant additional investment and continued development. To further the cause of methodological maturation within microwear analysis and to promote standardization, calibration, and innovation, the following collection of papers present various approaches and perspectives on how greater methodological refinement and increased reliability of results can and should be achieved. Many of these papers were part of a session held at the 2011 Society for American Archaeology Meeting (SAA) in Sacramento, California, while others were selected from the 2012 International Conference on Use-Wear Analysis in Faro, Portugal. The purpose of the SAA session and this special themed issue is essentially two-fold. The first is to promote awareness of the need for methodological standardization, calibration, and continuing innovation. The second is to open a serious dialogue about how these aims could be pursued and achieved.AAE was supported by the AHRC (AH/J007935/1). HJL was supported by L’Équipe Archéometrie at the Laboratoires d’ Archéologie, a part of the Centre Interuniversitaire d’Études sur les Lettres, les Arts et les Traditions (CELAT) at Université Laval. DAM was supported by Fondation Fyssen and the University of Toronto. WJS was supported by Faculty Development Grant from Keene State College

Toward a Broader View of Security Protocols

Computer and network security researchers usually focus on the security of computers and networks. Although it might seem as if there is more than enough insecurity here to keep all of us fully occupied for the foreseeable future, this narrow view of our domain may actually be contributing to the very problems that we are trying to solve. We miss important insights from, and opportunities to make contributions to, a larger world that has been grappling with security since long before the computer was invented

Canonical theory of spherically symmetric spacetimes with cross-streaming null dusts

The Hamiltonian dynamics of two-component spherically symmetric null dust is studied with regard to the quantum theory of gravitational collapse. The components--the ingoing and outgoing dusts--are assumed to interact only through gravitation. Different kinds of singularities, naked or "clothed", that can form during collapse processes are described. The general canonical formulation of the one-component null-dust dynamics by Bicak and Kuchar is restricted to the spherically symmetric case and used to construct an action for the two components. The transformation from a metric variable to the quasilocal mass is shown to simplify the mathematics. The action is reduced by a choice of gauge and the corresponding true Hamiltonian is written down. Asymptotic coordinates and energy densities of dust shells are shown to form a complete set of Dirac observables. The action of the asymptotic time translation on the observables is defined but it has been calculated explicitly only in the case of one-component dust (Vaidya metric).Comment: 15 pages, 3 figures, submitted to Phys. Rev.

Order of Two-Dimensional Isotropic Dipolar Antiferromagnets

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite N\'eel-temperature is calculated for a square lattice by means of linear spin-wave theory.Comment: 10 pages, REVTEX, 1 figure available upon request, TUM-CP-93-0

Pseudogap and Conduction Dimensionalities in High-T_c Superconductors

The nature of normal state charge-carriers' dynamics and the transition in conduction and gap dimensionalities between 2D and 3D for YBa_2 Cu_3 O_{7-delta} and Bi_2 Sr_2 Ca_{1-x} Y_x Cu_2 O_8 high-T_c superconductors were described by computing and fitting the resistivity curves, rho(T,delta,x). These were carried out by utilizing the 2D and 3D Fermi liquid (FL) and ionization energy (E_I) based resistivity models coupled with charge-spin (CS) separation based t-J model [Phys. Rev. B 64, 104516 (2001)]. rho(T,delta,x) curves of Y123 and Bi2212 samples indicate the beginning of the transition of conduction and gap from 2D to 3D with reduction in oxygen content (7-delta) and Ca^{2+} (1-x) as such, c-axis pseudogap could be a different phenomenon from superconductor and spin gaps. These models also indicate that the recent MgB_2 superconductor is at least not Y123 or Bi2212 type.Comment: To be published in Physica

Superconductivity in MgB_2 doped with Ti and C

Measurements of the superconducting upper critical field, H_{c2}, and critical current density, J_c, have been carried out for MgB_2 doped with Ti and/or C in order to explore the problems encountered if these dopants are used to enhance the superconducting performance. Carbon replaces boron in the MgB_2 lattice and apparently shortens the electronic mean free path thereby raising H_c2. Titanium forms precipitates of either TiB or TiB_2 that enhance the flux pinning and raise J_c. Most of these precipitates are intra-granular in the MgB_2 phase. If approximately 0.5% Ti and approximately 2% C are co-deposited with B to form doped boron fibers and these fibers are in turn reacted in Mg vapor to form MgB_2, the resulting superconductor has H_{c2}(T=0) ~ 25 T and J_c ~ 10,000 A/cm**2 at 5 K and 2.2 T.Comment: 11 pages, 10 figure