Secondary emission by positrons

In Chapter I the existing literature on secondary emission is reviewed from the point of view of possible effects which might be observed with positrons. Secondary emission by electrons of energy ~ 2 KeV has been extensively studied, and work on positive ions has been reported. No work at all on positrons has been published, and in the energy region covered by the present research ( ~ 5 - 500 KeV) there has been no theoretical work and practically no experimental work on electrons. Some simple theoretical ideas concerning secondary emission by positrons are put forward in Chapter II. Secondary emission by ''Potential Ejection", which has been observed for positive ions but is impossible for electrons, is considered as a possible process for positrons; if such a process can occur for positrons it would only be predominent for particles with an energy ~ 1eV. Some known differences in the behaviour of positrons and electrons are then discussed, from the point of view of any effect these might have on the secondary emission of the particles. It is concluded that no large differences are to be expected in the energy range which can be investigated by experiments which are feasible at present. The second part of Chapter II outlines the basic principle of the experiment, which was to compare the secondary emission by positrons and electrons of the same energy under identical conditions of geometry and target surface, A ?-spectrometer and a copper 64 source, which emitted positrons and electrons, provided focused beams of particles. The secondary electrons were detected with an Allen type electron multiplier, and the number of primary particles was counted with a thin windowed Geiger counter. Chapter III describes the electron multiplier and the associated electronics, and discusses briefly some measurements on its performance. In Chapter IV a preliminary experiment on secondary emission without using the spectrometer is described, which confirmed that there were no large differences in secondary emission by electrons and positrons at high energies. Chapter V describes the ?-spectrometer and the rotating coil method used to measure the magnetic field. Chapter VI describes the main experiments to determine, the relative secondary emission of electrons to that of positrons. Some absolute measurements were also made. It was found that above ~ 50 KeV was about 1.04; as the energy was reduced began to rise rapidly, exceeding 2 below 10 KeV. As such large values of were not expected on any existing theory, a very thorough investigation was carried out to establish that the results were not due to any instrumental errors. The final results for platinum, after all the corrections, none of which was very large, had been applied, were as follows;- Energy KeV 6.5 10 20 50-500 m 3.25 + 1.15 1.7 + 0.3 1.2 + 0.1 1.04. + 0.025 A copper-beryllium target gave similar results. In the final Chapter some tentative explanations for the results are put forward, For energies greater than 100 KeV a semi- quantitative theory is given. It was assumed that the secondary yield was proportional to the energy loss of the particles, and that a primary could produce a secondary as it entered the target, or as it left the target, if it did so as a result of scattering within the target. Using recent data on the energy loss of positrons and electrons, and the results of Seliger, who found that electrons were backscattered by ~30% more than positrons, values of m of the right order of magnitude are predicted. Below ~100 KeV the simple theory breaks down, but other factors which become important at lower energies enable this theory to be extended, so that it can possibly account for the results down to 20 KeV. This extended theory does not seem adequate to explain the large values of m observed below 20 KeV. Some very tentative ideas are put forward concerning processes by which positrons and electrons might liberate secondary electrons, which suggest qualitatively that electrons may be favoured. It is concluded that more experimental and theoretical work is required before the results at low energies can be understood

Cracking in asphalt materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.Peer ReviewedPostprint (author's final draft

Statistical features of edge turbulence in RFX-mod from Gas Puffing Imaging

Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the Gas Puffing Imaging Diagnostics. Statistical features of the signal are quantified in terms of the Probability Distribution Function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al [J.P. Graves, et al, Plasma Phys. Control. Fusion 47, L1 (2005)]. On the other hand, pulses with external methods for plasma control feature modified PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDFs is common to other devices as well, and has been suggested to be due to the simultaneous presence of different mechanisms driving respectively coherent bursts and gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.Comment: 27 pages. Published in PPC

Revision of the genus Tapholeon Wells, 1967 (Copepoda, Harpacticoida, Laophontidae)

To date, only two species are known in the laophontid genus Tapholeon Wells, 1967 (Copepoda, Harpacticoida). In the present contribution, a redescription of the type species T. ornatus Wells, 1967, based on the type material, is provided. Furthermore, two new species are described from the coast of Kenya, T. inconspicuus sp. nov. and T. tenuis sp. nov. Two species, formerly attributed to Asellopsis Brady and Robertson, 1873 (namely A. arenicola Chappuis, 1954 and A. chappuisius Krishnaswamy, 1957), are allocated to Tapholeon based on the absence of sexual dimorphism in the swimming legs P2-P4. The former of the two species is redescribed based on additional material from the Comoros. An updated generic diagnosis and a key to the six species of Tapholeon are included

Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first Super-Earth with measured radius

We report the discovery of very shallow (DF/F = 3.4 10-4), periodic dips in the light curve of an active V = 11.7 G9V star observed by the CoRoT satellite, which we interpret as due to the presence of a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion. Methods. We use CoRoT color information, good angular resolution ground-based photometric observations in- and out- of transit, adaptive optics imaging, near-infrared spectroscopy and preliminary results from Radial Velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star are derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of the companion. We examine carefully all conceivable cases of false positives, and all tests performed support the planetary hypothesis. Blends with separation larger than 0.40 arcsec or triple systems are almost excluded with a 8 10-4 risk left. We conclude that, as far as we have been exhaustive, we have discovered a planetary companion, named CoRoT-7b, for which we derive a period of 0.853 59 +/- 3 10-5 day and a radius of Rp = 1.68 +/- 0.09 REarth. Analysis of preliminary radial velocity data yields an upper limit of 21 MEarth for the companion mass, supporting the finding. CoRoT-7b is very likely the first Super-Earth with a measured radius.Comment: Accepted in Astronomy and Astrophysics; typos and language corrections; version sent to the printer w few upgrade

Taxing the Informal Economy: The Current State of Knowledge and Agendas for Future Research

This paper reviews the literature on taxation of the informal economy, taking stock of key debates and drawing attention to recent innovations. Conventionally, the debate on whether to tax has frequently focused on the limited revenue potential, high cost of collection, and potentially adverse impact on small firms. Recent arguments have increasingly emphasised the more indirect benefits of informal taxation in relation to economic growth, broader tax compliance, and governance. More research is needed, we argue, into the relevant costs and benefits for all, including quasi-voluntary compliance, political and administrative incentives for reform, and citizen-state bargaining over taxation

Equilibrium reconstruction for Single Helical Axis reversed field pinch plasmas

Single Helical Axis (SHAx) configurations are emerging as the natural state for high current reversed field pinch (RFP) plasmas. These states feature the presence of transport barriers in the core plasma. Here we present a method for computing the equilibrium magnetic surfaces for these states in the force-free approximation, which has been implemented in the SHEq code. The method is based on the superposition of a zeroth order axisymmetric equilibrium and of a first order helical perturbation computed according to Newcomb's equation supplemented with edge magnetic field measurements. The mapping of the measured electron temperature profiles, soft X-ray emission and interferometric density measurements on the computed magnetic surfaces demonstrates the quality of the equilibrium reconstruction. The procedure for computing flux surface averages is illustrated, and applied to the evaluation of the thermal conductivity profile. The consistency of the evaluated equilibria with Ohm's law is also discussed.Comment: Submitted to Plasma Physics and Controlled Fusio

Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV

A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7 TeV is presented. The data were collected at the LHC, with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. No significant excess is observed above the background expectation, and upper limits are set on the Higgs boson production cross section. The presence of the standard model Higgs boson with a mass in the 270-440 GeV range is excluded at 95% confidence level.Comment: Submitted to JHE