A simple model of electron beam initiated dielectric breakdown

A steady state model that describes the internal charge distribution of a planar dielectric sample exposed to a uniform electron beam was developed. The model includes the effects of charge deposition and ionization of the beam, separate trap-modulated mobilities for electrons and holes, electron-hole recombination, and pair production by drifting thermal electrons. If the incident beam current is greater than a certain critical value (which depends on sample thickness as well as other sample properties), the steady state solution is non-physical

A study of the relationship between student achievement and teacher-student interaction in secondary classrooms

The purpose of this study was to investigate the relationship between the amount of verbal interaction between a student and a teacher and that student\u27s achievement in the class taught by that teacher at the secondary school level. The student was used as the unit of study.;The sample was selected from a secondary school (9-12) in southeast Virginia with an enrollment of approximately 1800 students. One hundred and twenty-eight students from three intact Algebra II classes and three intact English 11 classes were included in the study.;All data were collected by three trained observers who coded the frequency of student-initiated and teacher-initiated interactions. Only instructional interactions between the teacher and the student were coded.;It was hypothesized that a positive correlation existed between the amount of teacher-student interactions and student achievement. It was assumed that a positive correlation between ability and achievement existed.;It was concluded that a positive correlation did exist between the amount of teacher-student interaction and student achievement in the English classes but not in the mathematics classes. The correlation between ability and achievement was not significant.;It was also discovered that a few students in each classroom were involved in the majority of the teacher-student interactions while the rest of the class sat quietly.;Further study is needed to determine the effect of balancing the amount of teacher-student interaction on achievement in classes where there is an obvious disparity in the involvement of the students. In addition, the relationship between the quality of interaction and achievement should be studied in classrooms where the quantity of interactions has been balanced. and lastly, the relationship between ability and achievement should be examined in secondary classrooms

Selected Major Issues in the Philosophy, Design, and Operation of a Simulated Model Office

In many cases, past business graduates from secondary schools were unable to secure job-entry office positions due to their lack of business experience. Concern over this problem caused many business educators to seek improved methods of training for practical office work experience. Today, much attention is being focused on the Simulated Model Office Method for work experience. However, simulated model office programs have been hampered by a lack of organizational principles concerning the philosophy, design, and operation of simulated model offices. The problem in this study was threefold: (1) to determine the philosophy, design, and operation of model office simulation, as indicated in relevant business education literature; (2) to identify from this literature pertinent issues regarding model office simulation; and (3) to investigate the attitudes of Canadian business educators towards selected issues concerning simulated model office instruction. Eighty Canadian business educators participated in the study. The researcher found that simulated model offices are used in many high schools, particularly in the provinces of Ontario, Saskatchewan, and Alberta. The study revealed almost unanimous agreement (80 to 100 per cent) with each of the following: A simulated model office should reproduce in the classroom those tasks and procedures that are representative of local offices in the community. Published practice sets for simulated model office training are not preferred by a significant majority of the respondents. Model office simulation should be concerned with creating a variety of job situations encountered by an office worker in a specific POSITION. The following facilities, systems, or items should be placed within a simulated model office: telephones, paper cutter, stencil duplicator, spirit duplicator, photo copier, electric typewriters, legal-size file cabinet, alphabetical file system, and a transcribing machine. The following items received less than a 50 per cent majority, and could be considered highly controversial: How many periods per week, and what length of period should be devoted to simulated model office instruction? What proportion of the school year should be devoted to model office simulation? The following facilities, systems, or items should be placed within a simulated model office: fixed partitions, movable partitions, carpeting, imitation flowers and planter, intercom system, switchboard, time-clock, letter-size file cabinet, geographical file .system, bookkeeping machine, full-keyboard adding-listing machine, rotary calculator, electronic calculator, postage meter, addressograph, offset duplicator, collator, electric stapler, coffee percolator, cheque protector, and copyholder. Based on the findings of this study, the significant conclusions are: A simulated model office complements, rather than replaces the Cooperative Office Work Experience Program. The Concurrent Operations Plan lends itself to realistic model office simulation experiences. Canadian business educators prefer one general model office simulation that can be applied to all students— bookkeeping, clerical, and secretarial

The stellar content of the Hamburg/ESO survey. IV. Selection of candidate metal-poor stars

We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, B-V colors (both measured directly from the digital HES spectra), as well as J-K colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 Angstrom, and a calibration of the HES B-V colors, using CCD photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color range 0.3 < B-V < 1.4. These accuracies make it possible to reliably reject stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 square degrees of the southern high Galactic latitude sky. The candidate sample consists of 20,271 stars in the magnitude range 10 < B < 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e.g., the three most heavy-element deficient stars currently known.Comment: 11 pages, 10 figures, accepted for publication in A&

Continued development of a detailed model of arc discharge dynamics

Using a previously developed set of codes (SEMC, CASCAD, ACORN), a parametric study was performed to quantify the parameters which describe the development of a single electron indicated avalanche into a negative tip streamer. The electron distribution function in Teflon is presented for values of the electric field in the range of four-hundred million volts/meter to four billon volts/meter. A formulation of the scattering parameters is developed which shows that the transport can be represented by three independent variables. The distribution of ionization sites is used to indicate an avalanche. The self consistent evolution of the avalanche is computed over the parameter range of scattering set

Finding the First Stars: The Hamburg/ESO Objective Prism Survey

We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by automatic spectral classification. Extensive simulations show that the selection efficiency for turnoff stars of [Fe/H]25% at B<16.5. Since the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992), the HES offers the possibility to efficiently increase the total number of metal-poor stars by at least a factor of 4.Comment: To appear in: Proceedings of ESO/MPA conference "The First Stars". 2 pages, 1 figur

Chemical abundances of the metal-poor horizontal-branch stars CS 22186-005 and CS 30344-033

We report on a chemical-abundance analysis of two very metal-poor horizontal-branch stars in the Milky Way halo: CS 22186-005 ([Fe/H]=-2.70) and CS 30344-033 ([Fe/H]=-2.90). The analysis is based on high-resolution spectra obtained at ESO, with the spectrographs HARPS at the 3.6 m telescope, and UVES at the VLT. We adopted one-dimensional, plane-parallel model atmospheres assuming local thermodynamic equilibrium. We derived elemental abundances for 13 elements for CS 22186-005 and 14 elements for CS 30344-033. This study is the first abundance analysis of CS 30344-033. CS 22186-005 has been analyzed previously, but we report here the first measurement of nickel (Ni; Z = 28) for this star, based on twenty-two NiI lines ([Ni/Fe]=-0.21$\pm$0.02); the measurement is significantly below the mean found for most metal-poor stars. Differences of up to 0.5 dex in [Ni/Fe] ratios were determined by different authors for the same type of stars in the literature, which means that it is not yet possible to conclude that there is a real intrinsic scatter in the [Ni/Fe] ratios. For the other elements for which we obtained estimates, the abundance patterns in these two stars match the Galactic trends defined by giant and turnoff stars well. This confirms the value of horizontal-branch stars as tracers of the chemical properties of stellar populations in the Galaxy. Our radial velocities measurements for CS 22186-005 differ from previously published measurements by more than the expected statistical errors. More measurements of the radial velocity of this star are encouraged to confirm or refute its radial velocity variability

The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars

The detailed chemical composition of most metal-poor halo stars has been found to be highly uniform, but a minority of stars exhibit dramatic enhancements in their abundances of heavy neutron-capture elements and/or of carbon. The key question for Galactic chemical evolution models is whether these peculiarities reflect the composition of the natal clouds, or if they are due to later mass transfer of processed material from a binary companion. If the former case applies, the observed excess of certain elements was implanted within selected clouds in the early ISM from a production site at interstellar distances. Our aim is to determine the frequency and orbital properties of binaries among these chemically peculiar stars. This information provides the basis for deciding whether mass transfer from a binary companion is necessary and sufficient to explain their unusual compositions. This paper discusses our study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element enhanced VMP and EMP stars. High-resolution, low signal-to-noise spectra of the stars were obtained at roughly monthly intervals over 8 years with the FIES spectrograph at the Nordic Optical Telescope. From these spectra, radial velocities with an accuracy of ~100 m/s were determined by cross-correlation against an optimized template. 14 of the programme stars exhibit no significant RV variation over this period, while 3 are binaries with orbits of typical eccentricity for their periods, resulting in a normal binary frequency of ~18+-6% for the sample. Our results confirm our preliminary conclusion from 2011, based on partial data, that the chemical peculiarity of the r-I and r-II stars is not caused by any putative binary companions. Instead, it was imprinted on the natal molecular clouds of these stars by an external, distant source. Models of the ISM in early galaxies should account for such mechanisms.Comment: 14 pages, 3 figures, accepted for publication in Astronomy and Astrophysic