577 research outputs found
Correlation effects in sequential energy branching: an exact model of the Fano statistics
Correlation effects in in the fluctuation of the number of particles in the
process of energy branching by sequential impact ionizations are studied using
an exactly soluble model of random parking on a line. The Fano factor F
calculated in an uncorrelated final-state "shot-glass" model does not give an
accurate answer even with the exact gap-distribution statistics. Allowing for
the nearest-neighbor correlation effects gives a correction to F that brings F
very close to its exact value. We discuss the implications of our results for
energy resolution of semiconductor gamma detectors, where the value of F is of
the essence. We argue that F is controlled by correlations in the cascade
energy branching process and hence the widely used final-state model estimates
are not reliable -- especially in the practically relevant cases when the
energy branching is terminated by competition between impact ionization and
phonon emission.Comment: 11 pages, 4 figures. Submitted to Physical Review
Fragmentation efficiency of explosive volcanic eruptions: A study of experimentally generated pyroclasts
Products of magma fragmentation can pose a severe threat to health, infrastructure, environment, and aviation. Systematic
evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be
observed directly and their deposits undergo transport-related sorting. However, enhanced knowledge is required for hazard
assessment and risk mitigation. Laboratory experiments on natural samples allow the precise characterization of the generated
pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes. They hold the promise
of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and
the physical conditions at fragmentation.
We performed a series of rapid decompression experiments on three sets of natural samples from Unzen volcano, Japan. The
analysis comprised grain-size analysis and surface area measurements. The grain-size analysis is performed by dry sieving for
particles larger than 250 Am and wet laser refraction for smaller particles. For all three sets of samples, the grain-size of the most
abundant fraction decreases and the weight fraction of newly generated ash particles (up to 40 wt.%) increases with experimental
pressure/potential energy for fragmentation. This energy can be estimated from the volume of the gas fraction and the applied
pressure. The surface area was determined through Argon adsorption. The fragmentation efficiency is described by the degree of fineparticle
generation. Results show that the fragmentation efficiency and the generated surface correlate positively with the applied
energy
Field-based density measurements as tool to identify preeruption dome structure: set-up and first results from Unzen volcano, Japan
For an improvement in the quality of conduit flow and dome-related explosive eruption models, knowledge of the
preeruption or precollapse density of the rocks involved is necessary. As close investigation is impossible during eruption, the
best substitute comes from quantitative investigation of the eruption deposits. The porosity of volcanic rocks is of primary
importance for the eruptive behaviour and, accordingly, a key-parameter for realistic models of dome stability and conduit flow.
Fortunately, this physical property may be accurately determined via density measurements.
We developed a robust, battery-powered device for rapid and reliable density measurements of dry rock samples in the
field. The density of the samples (sealed in plastic bags at 250 mbar) is determined using the Archimedean principle. We
have tested the device on the deposits of the 1990–1995 eruption of Unzen volcano, Japan. Short setup and operation
times allow up to 60 measurements per day under fieldwork conditions. The rapid accumulation of correspondingly large
data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block-and-ash
flow deposits.
More than 1100 samples with a total weight of 2.2 tons were measured. The data set demonstrates that the deposits of the last
eruptive episode at Unzen display a bimodal density distribution, with peaks at 2.0F0.1 and 2.3F0.1 g/cm3, corresponding to
open porosity values of 20 and 8 vol.%, respectively. We use this data set to link the results of laboratory-based fragmentation
experiments to field studies at recently active lava domes
Fall-experiments on Merapi basaltic andesite and constraints on the generation of pyroclastic surges
International audienceWe have performed fall-experiments with basaltic andesite rock samples from Merapi volcano, using an apparatus designed to analyze samples heated up to 850°C. Relative pressure changes during impact and fragmentation of the samples were measured by a pressure transducer. From 200°C, dynamic pressure waves were formed on impact and fragmentation. Peak and duration of the pressure signal, and degree of fragmentation were found to strongly increase with increasing temperature of rock samples. The pressure waves are most likely generated by sudden heating of air forcing it to expand. We propose that the observed pressure changes are analogues to pyroclastic surges that may be generated on impact and fragmentation of large blocks during passage of a pyroclastic flow over a steep cliff. We infer that rock temperatures of ca. 400°C are sufficient for this process to occur, a temperature common in pyroclastic flows even in distal reaches
The fragmentation threshold of pyroclastic rocks
In response to rapid decompression, porous magma may fragment explosively. This occurs when the melt can no
longer withstand forces exerted upon it due to the overpressure in included bubbles. This occurs at a critical pressure
difference between the bubbles and the surrounding magma. In this study we have investigated this pressure threshold
necessary for the fragmentation of magma. Here we present the first comprehensive, high temperature experimental
quantification of the fragmentation threshold of volcanic rocks varying widely in porosity, permeability, crystallinity, and
chemical composition. We exposed samples to increasing pressure differentials in a high temperature shock tube apparatus
until fragmentation was initiated. Experimentally, we define the fragmentation threshold as the minimum pressure
differential that leads to complete fragmentation of the pressurized porous rock sample. Our results show that the
fragmentation threshold is strongly dependent on porosity; high porosity samples fragment at lower pressure differentials
than low porosity samples. The fragmentation threshold is inversely proportional to the porosity. Of the other factors,
permeability likely affects the fragmentation threshold at high porosity values, whereas chemical composition, crystallinity
and bubble size distribution appear to have minor effects. The relationship for fragmentation threshold presented here can
be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation
of porous magma
Evaluation of game templates to support programming activities in schools
Game creation challenges in schools potentially provide engaging, goal-oriented, and interactive experiences in classes; thereby supporting the transfer of knowledge for learning in a fun and pedagogic manner. A key element of the ongoing European project No One Left Behind (NOLB) is to integrate a game-making teaching framework (GMTF) into the educational app Pocket Code. Pocket Code allows learners to create programs in a visual Lego®-style way to facilitate learning how to code at secondary high schools. The concept of the NOLB GMTF is based on principles of the Universal Design for Learning (UDL) model. Its focus lies on three pillars of learning: the what, how, and why. Thereby, the NOLB GMTF is a common set of concepts, practices, pedagogy, and methods. This framework provides a coherent approach to learning and teaching by integrating leisure oriented gaming methods into multi-discipline curricula. One output of this framework is the integration of game-based methods via game templates that refer to didactical scenarios that include a refined set of genres, assets, rules, challenges, and strategies. These templates allows: 1) teachers to start with a well-structured program, and 2) pupils to add content and adjust the code to integrate their own ideas. During the project game genres such as adventure, action, and quiz, as well as rewards or victory point mechanisms, have been embedded into different subjects, e.g., science, mathematics, and arts. The insights gained during the class hours were used to generate 13 game templates, which are integrated in Create@School (a new version of the Pocket Code app which targets schools). To test the efficiency of these templates, user experience (UX) tests were conducted during classes to compare games created by pupils who used templates and those who started to create a game from scratch. Preliminary results showed that these templates allow learners to focus on subject-relevant problem solving activities rather than on understanding the functionality of the app. This directly leads to more time to express their creativity in different levels and more time for extra tasks
Intranasal oxytocin reduces provoked symptoms in female patients with posttraumatic stress disorder despite exerting sympathomimetic and positive chronotropic effects in a randomized controlled trial
Background: Posttraumatic stress disorder (PTSD) is a severe psychiatric disease accompanied by neuroendocrine changes such as adrenergic overdrive and hence an elevated cardiovascular morbidity. Current pharmacotherapeutic options for PTSD are less than suboptimal, necessitating the development of PTSD-specific drugs. Although the neuropeptide oxytocin has been repeatedly suggested to be effective in PTSD treatment, there are, to our knowledge, only three studies that have assessed its efficacy on the intensity of PTSD symptoms in PTSD patients - among them one symptom provocation study in male veterans. Methods: To evaluate for the first time how oxytocin influences the intensity of provoked PTSD symptoms and, furthermore, cardiac control in female PTSD patients, we assessed their psychic and cardiac response to trauma-script exposure with and without oxytocin pretreatment in a double-blind randomized placebo-controlled study. We used a within-subject design to study 35 female PTSD patients who received oxytocin and placebo in a 2-week interval. Furthermore, we performed a small pilot study to get an idea of the relation of the stress-modulated endogenous oxytocin levels and heart rate - we correlated oxytocin serum levels with the heart rate of 10 healthy individuals before and after exposure to the Trier Social Stress Test (TSST). Results: Intranasal oxytocin treatment was followed by a reduction of provoked total PTSD symptoms, in particular of avoidance, and by an elevation in baseline and maximum heart rate together with a drop in the pre-ejection period, a marker for sympathetic cardiac control. Furthermore, we found a positive correlation between endogenous oxytocin levels and heart rate both before and after TSST challenge in healthy control subjects. Conclusions: This study provides the first evidence that oxytocin treatment reduces the intensity of provoked PTSD symptoms in female PTSD patients. The small size of both samples and the heterogeneity of the patient sample restrict the generalizability of our findings. Future studies have to explore the gender dependency and the tolerability of the oxytocin- mediated increase in heart rate
The EBEX Experiment
EBEX is a balloon-borne polarimeter designed to measure the intensity and
polarization of the cosmic microwave background radiation. The measurements
would probe the inflationary epoch that took place shortly after the big bang
and would significantly improve constraints on the values of several
cosmological parameters. EBEX is unique in its broad frequency coverage and in
its ability to provide critical information about the level of polarized
Galactic foregrounds which will be necessary for all future CMB polarization
experiments. EBEX consists of a 1.5 m Dragone-type telescope that provides a
resolution of less than 8 arcminutes over four focal planes each of 4 degree
diffraction limited field of view at frequencies up to 450 GHz. The experiment
is designed to accommodate 330 transition edge bolometric detectors per focal
plane, for a total of up to 1320 detectors. EBEX will operate with frequency
bands centered at 150, 250, 350, and 450 GHz. Polarimetry is achieved with a
rotating achromatic half-wave plate. EBEX is currently in the design and
construction phase, and first light is scheduled for 2008.Comment: 13 pages, 10 figures. Figure 1 is changed from the one which appeared
in the Proceedings of the SPI
Constructionist Attempts at Supporting the Learning of Computer Programming: A Survey
Although programming is often seen as a key element of constructionist approaches, the research on learning to program through a constructionist strategy is somewhat limited, mostly focusing on how to bring the abstract and formal nature of programming languages into \u201cconcrete\u201d, possibly tangible objects, graspable even by children with limited abstraction power. We survey the literature in programming education and analyse some programming languages designed to help novices from a constructionist perspective
- …