Determination of (n, 2n) Reaction Cross-Sections for 71Ga, 106Cd & 138Ba Using 14 MeV Neutrons

The increasing trend towards the use of activation analysis as an analytic means of identifying elements and determining their relative abundance demands more accurate values of reaction cross-sections. Accurate values of cross-sections are also important in the investigation of possible shell effects in reaction mechanisms. Many activation cross-sections for reactions produced by 14 MeV neutrons have been measured because of the relative ease with which this energy of neutrons can be produced. Low voltage accelerators are capable of producing 14. MeV neutrons by the 3H( d,n)4He reaction. Although many people(1-5) have measured the (n,2n) reaction cross-sections for a multitude of stable isotopes, their values differ considerably for a particular range or they report large uncertainties in their measured values. By using improved counting and flux monitoring techniques, an attempt will be made to more accurately determine the values of the (n,2n ) reaction cross-sections for 71Ga, 106Cd, and 138Ba. The values for the (n,2n) reaction cross-sections for these three isotopes are determined by the simultaneous activation of the isotope in question and two copper disks. Each sample is sandwiched between the two foils during activation. Since the 63Cu(n,2n) 62Cu reaction cross-section for 14 MeV neutrons has been accurately measured, the activity produced in the copper disks is used in the determination of the (n,2n) reaction cross-sections. The purpose of this thesis is to more accurately determine *he values of the (n,2n) reaction cross-sections for 71Ga, 106Cd, and 138Ba by improved techniques

The Case for International Emission Trade in the Absence of Cooperative Climate Policy

We evaluate the efficacy of international trade in carbon emission permits when countries are guided strictly by their national self-interest. To do so, we construct a calibrated general equilibrium model that jointly describes the world economy and the strategic incentives that guide the design of national abatement policies. Countries' decisions about their participation in a trading system and about their initial permit endowment are made noncooperatively; so a priori it is not clear that permit trade will induce participation in international abatement agreements or that participation will result in significant environmental gains. Despite this, we find that emission trade agreements can be effective; that smaller groupings pairing developing and developed-world partners often perform better than agreements with larger rosters; and that general equilibrium responses play an important role in shaping these outcomes.Global warming, coalitions, general equilibrium, tradable permits

Annular Lichen Planus with Central Clearing

Annular lichen planus is an uncommon clinical variant of lichen planus. Characteristics include an annular configuration associated with a narrow rim of erythema and inflammation and a depressed center. There is a tendency toward central clearing. We present an illustrative case of annular lichen planus in an 82-year-old woma

The Case for International Emission Trade in the Absence of Cooperative Climate Policy

We evaluate the efficacy of international trade in carbon emission permits when countries are guided strictly by their national self-interest. To do so, we construct a calibrated general equilibrium model that jointly describes the world economy and the strategic incentives that guide the design of national abatement policies. Countries' decisions about their participation in a trading system and about their initial permit endowment are made noncooperatively; so a priori it is not clear that permit trade will induce participation in international abatement agreements or that participation will result in significant environmental gains. Despite this, we find that emission trade agreements can be effective; that smaller groupings pairing developing and developed-world partners often perform better than agreements with larger rosters; and that general equilibrium responses play an important role in shaping these outcomes

3D-CFD Design Study And Optimization Of A Centrifugal Turbo Compressor For The Operation In A Hybrid Sorption/ Compression Heat Pump Cycle

In various applications the use of sorption chillers and heat pumps is limited by the available temperature level of the driving heat source or the heat sink for export of reject heat. These constraints can be overcome by integrating an efficient high-speed transonic turbo-compressor into the internal cycle of a thermally driven water/lithium bromide absorption heat pump. The operation in a hybrid heat pump with the refrigerant water implies specific challenges for the design of the compressor: Saturation pressures in the sub-atmospheric range, low vapor density, high volume flows and a targeted pressure ratio of 3 result in high impeller tip speed up to 660 m/s and transonic flow phenomena in the flow channel of impeller and diffusor. Here the authors present a theoretical design study based on a 3D-simulation of a centrifugal compressor, targeted at the given operating conditions for a hybrid heat pump. Key figures are investigated to figure out the relationship between impeller tip speed, compressor pressure ratio and operating range of the compressor meeting the requirements, wide stable operating range between surge and choke line and appropriate pressure ratio. The optimization of the impeller geometry comprises both fluid dynamic behavior and structural stability

Examining a Relationship Between Chronic Dietary Folic Acid Deficiency and Activation of p53 Gene in Down Syndrome Ts65Dn Mice

Seaver Undergraduate Researc

Exploring learners' mathematical understanding through an analysis of their solution strategies

The purpose of this study is to investigate various solution strategies employed by Grade 7 learners and their teachers when solving a given set of mathematical tasks. This study is oriented in an interpretive paradigm and is characterised by qualitative methods. The research, set in nine schools in the Eastern Cape, was carried out with nine learners and their mathematics teachers and was designed around two phases. The research tools consisted of a set of 12 tasks that were modelled after the Third International Mathematics and Science Study (TIMSS), and a process of clinical interviews that interrogated the solution strategies that were used in solving the 12 tasks. Aspects of grounded theory were used in the analysis of the data. The study reveals that in most tasks, learners relied heavily on procedural understanding at the expense of conceptual understanding. It also emphasises that the solution strategies adopted by learners, particularly whole number operations, were consistent with those strategies used by their teachers. Both learners and teachers favoured using the traditional, standard algorithm strategies and appeared to have learned these algorithms in isolation from concepts, failing to relate them to understanding. Another important finding was that there was evidence to suggest that some learners and teachers did employ their own constructed solution strategies. They were able to make sense of the problems and to 'mathematize' effectively and reason mathematically. An interesting outcome of the study shows that participants were more proficient in solving word problems than mathematical computations. This is in contrast to existing research on word problems, where it is shown that teachers find them difficult to teach and learners find them difficult to understand. The findings of this study also highlight issues for mathematics teachers to consider when dealing with computations and word problems involving number sense and other problem solving type problems

A prediction method for flow in axial compressors

Thesis (M. Ing.) -- University of Stellenbosch, 1995.ENGLISH ABSTRACT: A procedure is presented for predicting the flow through axial compressors. The matrix throughflow equation is transformed to yield an expression of radius dependent on stream function and axial position. The solution of the resultant equation combines the advantages of following streamlines through the calculation domain (as in the streamline curvature method, SCM) with the stability of the matrix throughflow method (MTFM), and is correspondingly called the streamline through flow method (STFM). The predictions of the method were compared to analytical results for a number of inviscid test cases and gave good results. As with the SCM and MTFM, using STFM to predict turbomachinery flows requires empirical models for cascade loss and deflection as well as endwall loss. The off-design loss and deflection model of Howell was used as the basis for a new off-design correlation, H2, valid for axial velocity density ratios (AVDR) of unity. The H2 correlation was developed from the NACA 65-Series database as carpet-plotted by Felix. A simple subcritical Reynolds number correlation for loss and deflection was adapted from a method of Roberts, using inlet blade chord Reynolds number, camber angle, pitch-chord ratio, maximum blade thickness-chord ratio and turbulance factor as parameters. The H2 correlation together with the adapted Roberts correlation gave good predictions of loss and deflection for low-Reynolds number cascade flows at AVDR values of unity. Measurements were taken at the compressor inlet and behind each blade row of a low speed, three stage axial flow compressor at three flowrates: near-design, near-surge and near-choke. The predictions of STFM using Howell's endwall loss models, the modified low Reynolds number correlation and respectively Howell's original off design method and H2 were compared with the experimental results. Howell's method predicted pressure rise to within 3% at design and 10% at off-design, compared to 4% at design and 9% at off-design for the H2 method. The prediction of flow angles for H2 were considerably worse than that of Howell. This was deemed to be caused by AVDR effects. An interim AVDR correlation, dependent on stagger angle, was used together with H2. Choosing values of AVDR for the interim correlation which together with H2 would predict flow angles to match the experimental values, predictions of total pressure rise within 3% at design and 8% at off-design were achieved. As a measure of confidence can be placed in H2 and the modified low Reynolds number correlation, the endwall loss correlation of Howell was determined to be the cause of lack of further gains in accuracy.AFRIKAANSE OPSOMMING: 'n Prosedure vir die voorspel van vloei deur aksiaalkompressors word daargestel. Die matriksdeurvloeivergelyking word getransformeer om 'n uitdrukking van radius, wat afhanklik is van stroomfunksie en aksiale posisie, te gee. Die oplossing van die gevolglike vergelyking span die voordele verbonde aan die volg van stroomlyne deur die berekeningsgebied (soos in the stroomlynkrommingsmetode, SKM) met die stabiliteit van die matriksdeurvloeimetode (MDVM), saam. Dienoorkomstig word die prosedure die stroomlyndeurvloeimetode (SDVM) genoem. Die voorspellings van die metode is met analitiese resultate van 'n aantal nie-viskeuse proefgevalle vergelyk en goeie resultate is behaal. Soos met die SKM en MDVM, genoodsaak die gebruik van SDVM om vloei deur turbomasjiene te voorspel empiriese kaskade verlies- en defleksiemodelle, asook wandverliesmodelle. Die af-ontwerp verlies- en defleksiemodel van Howell is as 'n basis gebruik vir 'n nuwe af-ontwerp korrelasie, H2, wat geldig is vir aksiaalsnelheid-digtheidsverhoudings (ASDV) van een. Die H2 korrelasie is vanuit die NACA 65-Reeks databasis soos deur Felix geplot, ontwikkel. 'n Subkritiese Reynoldsgetal korrelasie vir verlies en defleksie is vanuit 'n metode van Roberts aangepas. Hierdie metode maak gebruik van inlaat lemkoord Reynoldsgetal, krommingshoek, lemspasie-koordverhouding, rnaksimum lemdikte-koordverhouding en turbulensiefaktor as parameters. Die H2 korrelasie, saam met die aangepaste Roberts korrelasie, het goeie voorspellings van verlies en defleksie vir lae Reynoldsgetal kaskadevloeie met ASDV waardes van een gegee. Metings is geneem by die kompressorinlaat en agter elke lemry van 'n lae-spoed, driestadium aksiaalvloeikompressor teen drie massavloeie: naby die ontwerppunt, naby die stollyn en naby die wurglyn. Die voorspellings van SDVM met gebruik van Howell se wandverliesmodelle, die gemodifiseerde lae Reynoldsgetal korrelasie en onderskeidelik Howell se oorspronklike af-ontwerp metode en H2, is met die eksperimentele resultate vergelyk. Howell se metode het drukstyging tot binne 3% by ontwerp en 10% by af-ontwerp voorspel, in vergeleke met 4% by ontwerp en 9% by af-ontwerp vir die H2 metode. Die voorspelling van vloeihoeke deur H2 is heelwat swakker as die van Howell. Dit is as gevolg van ASDV-effekte geag. 'n Interim ASDV korrelasie, afhanklik van lemstelhoek, is saam met H2 gebruik. Deur van ASDV-waardes vir die interim korrelasie te kies wat saam met H2 die eksperimentele vloeihoeke sal voorspel, is totale druk voorspellings binne 3% by ontwerp en 8% by af-ontwerp behaal. Aangesien 'n mate van vertroue in H2 en die aangepasde lae Reynoldsgetal korrelasie geplaas kan word, is die wandverlies korrelasie van Howell uitgesonder as die oorsaak van die gebrek aan verdere verbeterings in akkuraatheid.Maste