A CF4 based positron trap

All buffer-gas positron traps in use today rely on N2 as the primary trapping gas due to its conveniently placed ${{\rm{a}}}^{1}{\rm{\Pi }}$ electronic excitation cross-section. The energy loss per excitation in this process is 8.5 eV, which is sufficient to capture positrons from low-energy moderated beams into a Penning-trap configuration of electric and magnetic fields. However, the energy range over which this cross-section is accessible overlaps with that for positronium (Ps) formation, resulting in inevitable losses and setting an intrinsic upper limit on the overall trapping efficiency of ~25%. In this paper we present a numerical simulation of a device that uses CF4 as the primary trapping gas, exploiting vibrational excitation as the main inelastic capture process. The threshold for such excitations is far below that for Ps formation and hence, in principle, a CF4 trap can be highly efficient; our simulations indicate that it may be possible to achieve trapping efficiencies as high as 90%. We also report the results of an attempt to re-purpose an existing two-stage N2-based buffer-gas positron trap. Operating the device using CF4 proved unsuccessful, which we attribute to back scattering and expansion of the positron beam following interactions with the CF4 gas, and an unfavourably broad longitudinal beam energy spread arising from the magnetic field differential between the source and trap regions. The observed performance was broadly consistent with subsequent simulations that included parameters specific to the test system, and we outline the modifications that would be required to realise efficient positron trapping with CF4. However, additional losses appear to be present which require further investigation through both simulation and experiment

Positron transport in water vapour

Transport properties of positron swarms in water vapour under the influence of electric and magnetic fields are investigated using a Monte Carlo simulation technique and a multi-term theory for solving the Boltzmann equation. Special attention is paid to the correct treatment of the non-conservative nature of positronium (Ps) formation and its explicit and implicit influences on various positron transport properties. Many interesting and atypical phenomena induced by these influences are identified and discussed. Calculated transport properties for positrons are compared with those for electrons, and the most important differences are highlighted. The significant impact of a magnetic field on non-conservative positron transport in a crossed field configuration is also investigated. In general, the mean energy and diffusion coefficients are lowered, while for the measurable drift velocity an unexpected phenomenon arises: for certain values of the reduced electric field, the magnetic field enhances the drift. The variation of transport coefficients with the reduced electric and magnetic fields is addressed using physical arguments with the goal of understanding the synergistic effects of Ps formation and magnetic field on the drift and diffusion of positrons in neutral gases

Positron kinetics in an idealized PET environment

The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the 'gas-phase' assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations

A Case Study on the Danube Limes in Serbia: Valorisation and Cartographic Analyses of Selected Tourism Products

Cultural assets in the area of the Danube Limes in Serbia are an integral part of the world heritage “Roman Empire Borders”. The research presented in this paper includes the tourist and cartographic visualization of 19 Roman sites in the Danube Limes region of Golubac–Radujevac, to determine the real possibilities of tourism development in this area. The historical and cultural heritage of this area is among the most attractive tourist destinations in Serbia, Djerdap National Park and Djerdap Geopark. Despite its diverse cultural and historical values and the specific and unique natural environment, this area is not sufficiently used for tourism. The research included the evaluation of localities, which may serve as the basis to establish which activities should be undertaken in order to plan, use, preserve, and protect such important cultural assets, under the principles of sustainable tourism development. Information based on spatially referenced data in the research process requires cartographic support, in order to understand the geospatial relations of the site significance. Cartographic visualization enabled efficiently systematized data organization, spatial identification, presentation, and the use of complex information from the mapped area in the data analysis in this paper

World virtual globes

The article deals with some possibilities of presenting and visualising geospatial data. The Internet is especially useful for analyzing and detection of geospatial reality and its features. In the field of geodesy and cartography the use of the Internet is very impressive. Virtual atlases have become very attractive

Application of GIS from the multifunctionality aspect

Application of computer technology in the Earth science has led to the creation and development of new scientific disciplines, particularly digital cartography and GIS. This paper describes the application of GIS from several points of view such as: multi-scale, multi-thematic, multi-viewing, accommodation to standard software platforms and multi-user data accesses. Introduction The premise of a successful usage of topographic data nowadays implies their disposal in a digital form and organisation in a modern way, suitable for further computer processing. It is a unanimous opinion that it can be successfully completed only through creating data basis of space developing GIS. The theme of the written work is the development and application of GIS, and management of topographic data in accordance with international standards and users' needs. Moving to digital technology of creating and using topographic data should not be literal translation of the analogue map into a digital picture or just the automation of the map making using digital technology. The new method implies the formation of the central topographic data base which would generate displays of arbitrary scales, desired volumes of content, changeable sheet dimensions, various thematic displays, altogether in accordance with users' demands and needs. Digital cartography and GIS The development and application of computers in the area of earth sciences caused the creation of new definitions and disciplines among which are noticeable computer supported cartography and GIS. The computer-supported cartography was in its later phase called digital cartography and it can be used in two ways: like modern technology of date processing about space and like a new discipline. The main principle on which digital cartography as a discipline is based represents the processing and visualization of data about space but with computer supported technology. Application of GIS from various aspects Topographic maps have existed for many years in more or less similar or the same traditional form. By creating the basic data base about space through the application of GIS technology, a wide range of possibilities for dynamic selection of content appears. This content will further be shown on the screen or as a drawing on a piece of paper. The aspects of multidisciplinarity promote topographic data base as a significant part of general spatial data infrastructure. Application from the multiscale aspect There are different information levels which differ among themselves in quantity and content of data necessary for various levels of observation management and decision making. One of the principal tasks which emerges is a choice of the basic scale and resolution of the display for which the data are gathered. In cartography, that is a scale of 1:25,000, based on which the topographic map (TM) is created. Mutual complement of GIS and WWW will completely change the classic delivery of spatial data. Various topographic and thematic maps will be created and obtained from the unique data base. Application from the multithematic aspect The main objective of the formation of spatial data infrastructure is the creation of the standard GIS platform but also the possibility of multi purpose application of GIS for environment (farming, forestry, waterpower engineering) land development (commercial and business), itinerary analysis, tracing of retail and wholesale routes distributional nets (shops and warehouse locations) media and marketing planning (locations, sales territory, sales prospects, market analysis according to users), finances - insurance (from the users aspect, investors and areas of low/high risk), healthcare (general healthcare, appearance of epidemic and its spreading) and non-productive activities (tourism, archaeology, environment protection). Application from the aspect of multidisplay Once formed, the spatial data base, or GIS, should offer to the user a lot of graphic, descriptive and other important information which is, in fact, a digital or traditional display in a form of a determined output. One of the ways of using GIS implies cartographic publishing (getting the reproductive originals and printing geographic map sheets) in a traditional form. Application from the aspect of adjustment to standard software platforms In the groundbreaking period of civilization with radical changes in a scientific - technological sphere, it was necessary to select a new, most suitable software platform with attaching hardware components. After thorough analyses (analyses of foreign and national literature and the usage of demo versions of various GIS software), the American company ESRI software platform was selected. Application from the aspect of multiusers data access Internet and Intranet are deeply rooted technologies and have been increasingly used as technologies to accomplish the growing number of distributed informational systems. Therefore, the above-mentioned technologies are needed for distributed work with spatial data. Maintenance of GIS data The maintenance of the topographic data denotes a process of updating old-fashioned content and entering new geospatial data as well as the elimination and the correction of details which are not in accordance with the real state on the field. In connection with the volume and the method of work, difference should be made between the spatial data maintenance aspects for the basic scale, which refer to the entire content and specific elements of the content. Regarding periods in which the present space data maintenance is done, there are continuous and periodical ones. Conclusion When establishing the unique GIS, we ought to apply a principle according to which all the data once gathered in one State institution can be used variedly as well as in many other institutions (military, administrative, economic, scientific, cultural, etc.). This principle does not apply only to the rationalization but also to uniqueness and quality of the research of common geospace. In the present state of the development, it is hard to predict the trends of cartographic and generally speaking GIS processes, but the most important is that topographic data have digital form the meaning of which is universal, regardless of the current development of technique and technology

Evaluation of terrain geomorphometric characteristics for ground clearance charts production

Geomorphometric exploration applied in the military terrain analysis is based on the GIS methodology of spatial analyses and is related primarily to military terrain analyses. It includes relief assessment aiming at producing ground clearance charts for the analysis of terrain maneuverability and its deployment, cover and concealment possibilities. An evaluation analysis of geomorphological parameters was performed for the Avala test area using a few terrain parameters (visibility, terrain aspect and slope) as well as some terrain qualitative categories (e.g. vegetation density). Terrain slope Slope and aspect are morphometric terrain parameters that can be derived directly from the DTM using some GIS operations. Slope is the first derivative of a surface and has both magnitude and direction. Slope is perhaps the most important aspect of the surface form, since surfaces are formed completely of slopes, and slope angles control the gravitational force available for geomorphic work. Mathematically, the tangent of the slope angle is the first derivative of altitude, and it is a tangent or percent slope as this surface parameter is generally referred to. Slope is defined at each point as the slope of a plane tangent to the surface at that point. In practice, however, slope is generally measured over a finite distance, especially when data are obtained from a contour map. Terrain aspect Aspect is also the first derivative of a surface and has both magnitude and direction. The term aspect is defined as the direction of the biggest slope vector on the tangent plane projected onto the horizontal plane. Aspect is the bearing (or azimuth) of the slope direction, and its angle ranges from 0 to 360°. Analyses of terrain slope and ground clearance for military forces The analysis of land assessment of the Avala test area included the definition of relief categories in relation to cover and concealment purposes with the aim to include the geomorphological basis into the standard military procedure OCOKA (Observation and fields of fires; Cover and concealment; Obstacles and movement; Key terrain; Avenues of approach). A few parameters of relief significantly influencing the possibilities for cover and concealment (visibility, slope and aspect) were included into the definition of the model of terrain spatial analysis The morphometric data included in partial assessment categories were determined on the basis of the digital model relief analysis and by using GIS tools and given morphometric relief exploration methods. Analysis of vegetation effects on ground clearance for military forces Vegetation, in addition to terrain slope, presents one of the main factors in cross-country analyses and ground clearance assessments. In classification and extraction of vegetation from satellite images, numerous algorithms of two basic classification types, supervised and unsupervised classification, are applied. Supervised classification requires the identification of cover types of interest by user. Samples of pixels are then selected, based on available ground real information to represent each cover type. These samples are called training areas. The selection of appropriate training areas is based on the analyst's familiarity with the geographical area and his knowledge of the actual surface cover types presented in the image. Thus, the analyst 'supervises' the categorization of a set of specific classes. Unsupervised classification basically reverses the supervised classification process. Spectral classes are grouped first, based solely on the numerical information in the data, and then they are matched by the analyst to information classes (if possible). Programs, called clustering algorithms, are used to determine the natural (statistical) groupings or structures in the data. The analyst usually specifies how many groups or clusters are to be looked for in the data. In addition to specifying the desired number of classes, the analyst may also specify parameters related to the separation distance among the clusters and the variation within each cluster. The result of this iterative clustering process may result in some clusters that the analyst will want to combine subsequently, or clusters that should be broken down further on - each of these requiring a further application of the clustering algorithm. Thus, unsupervised classification is not completely without human intervention. Study area and THE used data The Avala hill test area, located in the central part of the Republic of Serbia, has been mapped extensively over the years and several GIS layers are available at various scales. The study area corresponds with one sheet topographic map at a scale of 1:25 000 (TM25 430-3-1) produced in the Military Geographical Institute (MGI). The main geomorphometric features of the test area include hill summits and shoulders, eroded slopes of small valleys, valley bottoms, a large abandoned river channel, and the river terrace. The elevation of the area ranges from 70 m to 500 m with an average of 195.05 m and a standard deviation of 56.7 m. The basic sources of information were the SPOT 5 image and the digital elevation model generated from the contour lines

Monte Carlo simulation and Boltzmann equation analysis of non-conservative positron transport in H2

This work reports on a new series of calculations of positron transport properties in molecular hydrogen under the influence of spatially homogeneous electric field. Calculations are performed using a Monte Carlo simulation technique and multi term theory for solving the Boltzmann equation. Values and general trends of the mean energy, drift velocity and diffusion coefficients as a function of the reduced electric field E/n0 are reported here. Emphasis is placed on the explicit and implicit effects of positronium (Ps) formation on the drift velocity and diffusion coefficients. Two important phenomena arise; first, for certain regions of E/n0 the bulk and flux components of the drift velocity and longitudinal diffusion coefficient are markedly different, both qualitatively and quantitatively. Second, and contrary to previous experience in electron swarm physics, there is negative differential conductivity (NDC) effect in the bulk drift velocity component with no indication of any NDC for the flux component. In order to understand this atypical manifestation of the drift and diffusion of positrons in H2 under the influence of electric field, the spatially dependent positron transport properties such as number of positrons, average energy and velocity and spatially resolved rate for Ps formation are calculated using a Monte Carlo simulation technique. The spatial variation of the positron average energy and extreme skewing of the spatial profile of positron swarm are shown to play a central role in understanding the phenomena

Positrons in gas filled traps and their transport in molecular gases

In this paper we give a review of two recent developments in positron transport, calculation of transport coefficients for a relatively complete set of collision cross sections for water vapour and for application of they Monte Carlo technique to model gas filled subexcitation positron traps such as Penning Malmberg Surko (Surko) trap. Calculated transport coefficients, very much like those for argon and other molecular gases show several new kinetic phenomena. The most important is the negative differential conductivity (NDC) for the bulk drift velocity when the flux drift velocity shows no sign of NDC. These results in water vapour are similar to the results in argon or hydrogen. The same technique that has been used for positron (and previously electron) transport may be applied to model development of particles in a Surko trap. We have provided calculation of the ensemble of positrons in the trap from an initial beam like distribution to the fully thermalised distribution. This model, however, does not include plasma effects (interaction between charged particles) and may be applied for lower positron densities