Homesickness and the Great Wall

Postcard from Aimee Bertolli, during the Linfield College Semester Abroad Program at Peking University in Beijing, Chin

Energy functional for the three-level Lipkin model

We compute the energy functional of a three-level Lipkin model via a Legrendre transform and compare exact numerical results with analytical solutions obtained from the random phase approximation (RPA). Except for the region of the phase transition, the RPA solutions perform very well. We also study the case of three non-degenerate levels and again find that the RPA solution agrees well with the exact numerical result. For this case, the analytical results give us insight into the form of the energy functional in the presence of symmetry-breaking one-body potentials.Comment: 6 pages, 5 figure

Supporti alla Programmazione Grid-Aware: Implementazione del Sistema di Controllo dell'Adattività su Grid di Applicazioni ASSIST

Le piattaforme Grid mettono a disposizione grandi quantità di risorse computazionali fornendo un’astrazione per la loro gestione e il loro accesso in modo unificante a fronte della loro eterogeneità, dinamicità, autonomia e grado di utilizzazione. Le problematiche di ricerca che si presentano su tali piattaforme riguardano la natura dinamica della disponibilità delle risorse: non viene garantito né un livello minimo di performance dei nodi di elaborazione né la loro effettiva disponibilità a determinati istanti nel tempo. Un'applicazione che utilizzi le risorse di una piattaforma Grid deve essere capace di affrontare queste problematiche per garantire un livello minimo di performance, detto contratto di performance. Questo vale ancora di più se si tratta di applicazioni high-performance, cioè applicazioni che devono fornire un alto livello di performance. In questa tesi si estende l’ambiente di programmazione alla base del progetto Grid.it con un Sistema per il Controllo dell’Adattività di applicazioni ASSIST. Tale sistema implementa i meccanismi necessari a garantire la soddisfazione dei contratti di performance. La tesi comprende la descrizione di un’architettura software del sistema e una sua possibile implementazione. Infine vengono presentati i risultati del testing di tale sistema su alcune applicazioni ASSIST. Grid platforms are composed of a large number of computing resources, providing applications with abstractions for using and accessing them in a unified fashion. This is done in the face of resource heterogeneity, dynamicity, autonomy and degree of utilization. One of the main issue for these platforms is represented by their dynamic nature: it is not guaranteed neither a minimum level of performance of computing nodes, nor of their actual availability. An application for Grid platforms must be able to adapt itself to guarantee a minimum degree of performance, also called performance contract. This is even more strict in the case of High-Performance applications. In this thesis we extend the ASSIST programming environment, research base of the Grid.it project, with a system for the adaptivity control. This system implements the needed mechanisms for applications to guarantee performance contracts. The thesis includes also a description of a software architecture of the system and a prototype implementation. Finally, also experimental results for notable ASSIST applications are presented

Shear strength contribution provided by inorganic-matrix composites fully wrapped around reinforced concrete beams

Fiber-reinforced cementitious matrix (FRCM) composites have been increasingly employed as externally bonded (EB) reinforcement for existing structures. FRCMs are effective in increasing the shear strength of existing concrete members. When the fully-wrapped configuration is employed, the composite tensile failure could be attained. In this paper, an analytical approach previously proposed to describe FRCM U-wrapped RC beams is extended to fully-wrapped members. This approach provides an accurate description of the stress-transfer mechanism between the FRCM bridging the shear crack and the substrate, thus allowing for an in-depth study of the composite contribution to the member shear strength

H ingestion into He-burning convection zones in super-AGB stellar models as a potential site for intermediate neutron-density nucleosynthesis

We investigate the evolution of super-AGB (SAGB) thermal pulse (TP) stars for a range of metallicities (Z) and explore the effect of convective boundary mixing (CBM). With decreasing metallicity and evolution along the TP phase, the He-shell flash and the third dredge-up (TDU) occur closer together in time. After some time (depending upon the CBM parametrization), efficient TDU begins while the pulse-driven convection zone (PDCZ) is still present, causing a convective exchange of material between the PDCZ and the convective envelope. This results in the ingestion of protons into the convective He-burning pulse. Even small amounts of CBM encourage the interaction of the convection zones leading to transport of protons from the convective envelope into the He layer. H-burning luminosities exceed 10⁹ (in some cases 10¹⁰) L⊙. We also calculate models of dredge-out in the most massive SAGB stars and show that the dredge-out phenomenon is another likely site of convective-reactive H-¹²C combustion. We discuss the substantial uncertainties of stellar evolution models under these conditions. Nevertheless, the simulations suggest that in the convective-reactive H-combustion regime of H ingestion the star may encounter conditions for the intermediate neutron capture process (i-process). We speculate that some CEMP-s/r stars could originate in i-process conditions in the H ingestion phases of low-Z SAGB stars. This scenario would however suggest a very low electron-capture supernova rate from SAGB stars. We also simulate potential outbursts triggered by such H ingestion events, present their light curves and briefly discuss their transient properties

Data-driven methods for respiratory signal detection in positron emission tomography

Positron Emission Tomography (PET) is a nuclear medicine imaging technique which allows quantitative assessment of functional processes, by determining the distribution of radioactive tracers inside the patient body. It is mainly used in oncology. Respiration during PET data acquisition of the chest leads to blurring and other artefacts in the images, lowering their quantitative accuracy. If a respiratory signal is available, these issues can be overcome by splitting the data into different motion states. In current clinical practice this signal is obtained using external devices. However, these are expensive, require prior setup and can cause patient discomfort. This thesis develops and evaluates Data-Driven (DD) techniques based on Principal Component Analysis (PCA) to generate the signal directly from the PET data. Firstly, the arbitrary relation between the sign of the PCA signal and the respiratory motion is addressed: a maximum in the signal could refer either to end-inspiration or end-expiration, possibly causing inaccurate motion correction. A new correction method is proposed and compared with two already existing methods. Subsequently, the methods are extended to Time-of-Flight (TOF) PET data, proposing a data processing step prior to using PCA, in order to benefit from the increased spatial information provided by TOF. The proposed methods are then extensively tested on lower lung patient data (non-TOF and TOF). The obtained respiratory signal is compared with that of an external device and with internal motion observed with Magnetic Resonance Imaging (MRI). Lastly, to investigate the performance of PCA where respiratory motion is minimal, the methods are applied to patient and simulation data of the upper lung, showing that they could potentially be utilised for detecting respiratory-induced density variations in the upper lung. This study shows that the presented methods could replace external devices for obtaining a respiratory signal, providing a simple and cost-effective tool for motion management in PET