School-Sponsored and Self-Sponsored at the Same Time

Numerous compositionists have argued that it is desirable to allow students to select their own writing topics, especially in extensive projects like research papers. But few researchers have compared students' work on assigned and self-designed projects where the students have kept extensive process journals. In this essay, I will demonstrate that when the context supports sustained writing, both assigned and self-designed tasks can foster learning and commitment to writing. Substantial confusion exists over different types of instructor-designed and self-designed tasks. For example, few studies distinguish between such teacher-designed tasks as (1) placement essays; (2) single-episode, in-class writing; (3) single-episode, out-of-class writing; and (4) writing designed as part of a larger sequence. Even for self-designed tasks, it is essential to distinguish between (5) single-episode, in-class writing; (6) single-episode, out-of-class writing; and (7) writing sustained over a period of time. George Hillocks warns of a simplistic view of school-sponsored writing, and he claims that research studies may not represent what is actually happening in pedagogical situations: [Researchers] assume, for example, that assignments are given without preparation, that no specific criteria for judging writing are presented, and that no specific provisions are made for feedback and revision ... Demonstrating that school-sponsored writing results in a lack of commitment to writing requires that evidence be collected over a range of programs with different characteristics. The studies at hand have not done that

Study of Counting Characteristics of Porous Radiation Detectors

This paper presents the development of a new technology of registration of ionizing radiation and a new type of detectors - single-cathode multiwire porous detector with neither a gaseous nor semiconductor, but a porous dielectric substance, e.g., CsI, being used as working medium. It is shown that the performance of the multiwire porous detector is stable, ensuring highly efficient detection of both heavily ionizing particles and soft X-rays with a spatial resolution better than $\pm 60\mu m$. The continuous stable performance opens up new perspectives for using porous detectors in research as well as medicine. The obtained data are basic for the development of the theory of the phenomenon of electrons' drift and multiplication in porous dielectrics under the action of a strong external electric field.Comment: 43

Beam Test of a Segmented Foil SEM Grid

A prototype Secondary-electron Emission Monitor (SEM) was installed in the 8 GeV proton transport line for the MiniBooNE experiment at Fermilab. The SEM is a segmented grid made with 5 um Ti foils, intended for use in the 120 GeV NuMI beam at Fermilab. Similar to previous workers, we found that the full collection of the secondary electron signal requires a bias voltage to draw the ejected electrons cleanly off the foils, and this effect is more pronounced at larger beam intensity. The beam centroid and width resolutions of the SEM were measured at beam widths of 3, 7, and 8 mm, and compared to calculations. Extrapolating the data from this beam test, we expect a centroid and width resolutions of 20um and 25 um, respectively, in the NuMI beam which has 1 mm spot size.Comment: submitted to Nucl. Instr. Meth.

The influence of the secondary electrons induced by energetic electrons impacting the Cassini Langmuir probe at Saturn

The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current-voltage (I-V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I-V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I-V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I-V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov-Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere

Detection of a strongly negative surface potential at Saturn's moon Hyperion

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (~ −200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator

Triggering Threshold Spacecraft Charging with Changes in Electron Emission from Materials

Modest changes in spacecraft charging conditions can lead to abrupt changes in the spacecraft equilibrium, from small positive potentials to large negative potentials relative to the space plasma; this phenomenon is referred to as threshold charging. It is well known that temporal changes of the space plasma environment (electron plasma temperature or density) can cause threshold charging. Threshold charging can also result from by temporal changes in the juxtaposition of the spacecraft to the environment, including spacecraft orbit, orientation, and geometry. This study focuses on the effects of possible changes in electron emission properties of representative spacecraft materials. It is found that for electron-induced emission, the possible threshold scenarios are very rich, since this type of electron emission can cause either positive or negative charging. Alternately, modification of photon- or ion-induced electron emission is found to induce threshold charging only in certain favorable cases. Changes of emission properties discussed include modifications due to: contamination, degradation and roughening of surfaces and layered materials; biasing and charge accumulation; bandstructure occupation and density of states caused by heat, optical or particle radiation; optical reflectivity and absorptivity; and inaccuracies and errors in measurements and parameterization of materials properties. An established method is used here to quantitatively gauge the relative extent to which these various changes in electron emission alter a spacecraft’s charging behavior and possibly lead to threshold charging. The absolute charging behavior of a hypothetical flat, two-dimensional satellite panel of a single material (either polycrystalline conductor Au or the polymeric polyimide Kapton™ H) is modeled as it undergoes modification and concomitant changes in spacecraft charging in three representative geosynchronous orbit environments, from full sunlight to full shade (eclipse) are considered