Study of the electronic recoil background of the XENON1T experiment

Uno dei principali protagonisti della caccia alla Materia Oscura è il Progetto XENON presso i LNGS, con l'obiettivo di rivelare le WIMP. Forti dell'esperienza proveniente dalle precedenti fasi del Progetto, l'attuale esperimento XENON1T è il primo che contiene circa 3.2 t di xenon liquido, di cui circa 2 t costituiscono la massa attiva della TPC a doppia fase. E questa è la prima TPC con massa attiva superiore a 1 t e con il più basso livello di fondo tra tutti gli esperimenti di questo tipo. Nel 2017, con un tempo di esposizione di soli 34.2 giorni, XENON1T ha ottenuto uno dei miglior limiti di esclusione per la sezione d'urto di interazione WIMP-nucleo non dipendente dallo spin. Nella prima parte del presente lavoro di tesi, verifico la possibilità che il neutron generator (NG), una sorgente di neutroni per la calibrazione della risposta del rivelatore ai rinculi nucleari (NR), possa essere una sorgente di fondo per il rivelatore essendo posizionato vicino alla TPC. Dalla stima del rate di eventi in presenza o meno del NG, nessuna differenza è stata osservata per gli eventi da rinculo elettronico (ER) a bassa energia. Successivamente alla valutazione dell'attività di U238 e Th232 nei materiali del NG, è possibile stimare il fondo indotto da neutroni radiogenici atteso dal NG: poiché risulta essere due ordini di grandezza inferiore a quanto atteso dai materiali di costruzione del rivelatore, può essere considerato un contributo trascurabile. Nella parte finale della tesi si presentano tutte le possibili sorgenti di fondo per eventi ER nel rivelatore di XENON1T e la simulazione, con il programma GEANT4, di tale fondo. In particolare, è esaminata e discussa la nuova implementazione della simulazione per il doppio decadimento beta dell'isotopo Xe136. Lo stato dell'arte del confronto delle simulazioni Monte Carlo con i dati reali è mostrato alla fine del lavoro di tesi: i risultati preliminari evidenziano una buona conoscenza del fondo dell'ER nel rivelatore XENON1T

Asymmetric invasion in anisotropic porous media

We report and discuss, by means of pore-scale numerical simulations, the possibility of achieving a directional-dependent two-phase flow behavior during the process of invasion of a viscous fluid into anisotropic porous media with controlled design. By customising the pore-scale morphology and heterogeneities with the adoption of anisotropic triangular pillars distributed with quenched disorder, we observe a substantially different invasion dynamics according to the direction of fluid injection relative to the medium orientation, that is depending if the triangular pillars have their apex oriented (flow aligned) or opposed (flow opposing) to the main flow direction. Three flow regimes can be observed: (i) for low values of the ratio between the macroscopic pressure drop and the characteristic pore-scale capillary threshold, i.e., for Δp0/pc≤1, the fluid invasion dynamics is strongly impeded and the viscous fluid is unable to reach the outlet of the medium, irrespective of the direction of injection; (ii) for intermediate values, 1<Δp0/pc≤2, the viscous fluid reaches the outlet only when the triangular pillars are flow-opposing oriented; (iii) for larger values, i.e., for Δp0/pc>2, the outlet is again reached irrespective of the direction of injection. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering. We thus argue that the invasion process occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness, which in the case of flow-opposing pillars is halved. We present a simple approximated model, based on Darcy\u27s assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening, to predict the asymmetric invasion dynamics. This peculiar behavior opens up the possibility of fabrication of porous capillary valves to control the flow along certain specific directions

Preparazione di materiali nanocompositi a base carbonio per applicazioni tecnologiche

La sintesi e le applicazioni dei materiali nanocompositi sono di importanza strategica nel campo della scienza dei materiali. In questo lavoro di tesi sono stati sviluppati diverse tipologie di materiali nanocampositi: Resina epossidica/nanotubi di carbonio (CNT), Poli(3,4-etilendiossitiofene) polistirensulfonato (PEDOT:PSS)/CNT, metallo/CNT. Questi materiali sono stati caratterizzati con le seguenti tecniche: Microscopia elettronica a scansione (SEM), Microscopia a forza atomica (AFM), microscopia acustica a forza atomica (AFAM), spettroscopia Raman e tecniche elettroanalitiche. La caratterizzazione funzionale è stata condotta testando i materiali in differenti applicazioni: Sensori resistivi, sensori di tipo nano-bilancia al quarzo e gestione del calore. I risultati sperimentali sono riportati nel lavoro di tesi.The synthesis and applications of nanocomposites are an important and strategical field of nanomaterial science. In this work has been developed several kind of nanocomposit materials, in particular: Epoxy resins and carbon nanotubes (CNT), Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and CNT, metal and CNT. This material has been characterized by means of: scanning electron microscopy (SEM), Atomic force miscroscpy (AFM), acoustic atomic force microscopy (AFAM), Raman spectroscopy and Electroanalitical techniques. The developed nanocomposite material has been tested by means different kind of application: Resistive sensors, quartz nano-balance and thermal management. The results are reported in the thesis work

Time-resolved optical studies, heat dissipation and melting of Ag and Au nanoparticle systems and arrays

Transient absorption spectroscopy has been extensively used in recent years to examine the temporal response of isolated nanoparticles (NPs) to the absorption of light [1]. These studies are largely based on the use of the surface plasmon resonance (SPR) to monitor characteristics of the NP such as electronic and lattice temperature, shape and morphology as a function of time. In the case of extended Au/Ag NP structures the plasmon resonance is strongly distorted due to the inter-particle coupling effects. For example, we have observed this effect in Rhodamine dye functionalized Au nanoparticles which undergo self-assembly to form nanostructures due to the interactions between the dye molecules attached to the surfaces of the nanoparticles. Indeed the SPR splits into two with one resonance remaining in the vicinity of that of the isolated AuNPs and is generally called the transverse SPR while a second resonance due to an extended excitation spanning across multiple particles appears to the lower energies. The precise spectral energy and shape of the extended plasmon resonance depends on the inter-particle distance, the particle disposition and the number of particles involved. When the plasmon band or interband spectral region of the NP is excited by an intense pulse the photon energy absorbed by the electrons is transferred to the lattice of the NP as heat through electron-phonon coupling. Depending on the intensity of the light pulse and thus the initial electron temperature a number of outcomes are possible. The first aim of this work is to use low intensity pump pulses to study the wavelength dependence of the sub 10 ps dynamics which reflects the electron-photon scattering within the nanoparticle structure. On the other hand, the interaction of more intense light with the NPs can modify the morphology of NP systems, for example by reshaping gold nanorods into nanospheres or, in general, mediate the synthesis of metallic nanostructures. At medium intensities the initial temperature is sufficient to induce melting of the NPs which can lead to morphological changes of the NP structure. Higher intensities can cause other effects such as photofragmentation of the NPs, release of stabiliser molecules from the surface of the NPs or even Coulomb explosion due to multiple ionisation events. The second aim of this work is to concentrate on the effects of medium intensity laser excitation of a self-assembled Au/Ag NP systems. The NP system is excited by a femtosecond laser pulse of different wavelengths allowing selective deposition of energy and the subsequent heat dissipation through phonon-phonon coupling and morphological changes are monitored in time by recording transient absorption spectra in the visible range. This wavelength range makes it possible to follow the phonon-phonon coupling effects on the recovery of the bleaching of both the transverse and extended plasmon resonances of the NP system. As the intensity of the pump pulse is increased it can be seen that the NPs are no longer able to dissipate all of the heat before arrival of subsequent laser pulses thus leading to melting of the NP structure and strong changes in the plasmon response of the system. The overall aim of this study is to fully understand the delocalized electron-phonon coupling in the extended plasmon region of the NP structures and to use this knowledge to control the melting in nanostructures. The methods developed can be useful for plasmon mediated nano-engineerin

Magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-SQUID readout for high-resolution X-ray emission spectroscopy

We present two variants of a magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-SQUID readout for high-resolution X-ray emission spectroscopy. Each variant employs two overhanging gold absorbers with a sensitive area of 150$\mu$m x 150$\mu$m and a thickness of 3$\mu$m, thus providing a quantum efficiency of 98% for photons up to 5keV and 50% for photons up to 10keV. The first variant turned out to be fully operational, but, at the same time, to suffer from Joule power dissipation of the Josephson junction shunt resistors, athermal phonon loss, and slew rate limitations of the overall setup. Overall, it only achieved an energy resolution $\Delta E_\mathrm{FWHM} = 8.9eV$. In the second variant, we introduced an innovative `tetrapod absorber geometry' as well as a membrane-technique for protecting the temperature sensors against the power dissipation of the shunt resistors. By this, the second variant achieves an outstanding energy resolution of $\Delta E_\mathrm{FWHM} =1.25(18)eV$ and hence provides, to our knowledge, the present best energy resolving power $E/\Delta E_\mathrm{FWHM}$ among all existing energy-dispersive detectors for soft and tender X-rays.Comment: submitted to Applied Physics Letter

Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows

Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereas aggregate breakup occurs when the local hydrodynamic stress $\sigma\sim \varepsilon^{1/2}$, with $\varepsilon$ being the energy dissipation at the position of the aggregate, overcomes a given threshold $\sigma_\mathrm{cr}$, which is characteristic for a given type of aggregates. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a universal scaling among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, theresults are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.Comment: 15 pages, 12 figures, Refinded discussion in Section 2.1, results unchange

Self-standing 3D-printed PEGDA–PANIs electroconductive hydrogel composites for pH monitoring

Additive manufacturing (AM), or 3D printing processes, is introducing new possibilities in electronic, biomedical, sensor-designing, and wearable technologies. In this context, the present work focuses on the development of flexible 3D-printed polyethylene glycol diacrylate (PEGDA)- sulfonated polyaniline (PANIs) electrically conductive hydrogels (ECHs) for pH-monitoring applications. PEGDA platforms are 3D printed by a stereolithography (SLA) approach. Here, we report the successful realization of PEGDA–PANIs electroconductive hydrogel (ECH) composites produced by an in situ chemical oxidative co-polymerization of aniline (ANI) and aniline 2-sulfonic acid (ANIs) monomers at a 1:1 equimolar ratio in acidic medium. The morphological and functional properties of PEGDA–PANIs are compared to those of PEGDA–PANI composites by coupling SEM, swelling degree, I–V, and electro–chemo–mechanical analyses. The differences are discussed as a function of morphological, structural, and charge transfer/transport properties of the respective PANIs and PANI filler. Our investigation showed that the electrochemical activity of PANIs allows for the exploitation of the PEGDA–PANIs composite as an electrode material for pH monitoring in a linear range compatible with that of most biofluids. This feature, combined with the superior electromechanical behavior, swelling capacity, and water retention properties, makes PEGDA–PANIs hydrogel a promising active material for developing advanced biomedical, soft tissue, and biocompatible electronic applications

Optimum filter-based analysis for the characterization of a high-resolution magnetic microcalorimeter

Ultrasensitive cryogenic calorimeters have become a favored technology with widespread application where eV-scale energy resolutions are needed. In this article, we characterize the performance of an x-ray magnetic microcalorimeter (MMC) using a $^{55}Fe$ source. Employing an optimum filter-based amplitude estimation and energy reconstruction, we demonstrate that a full-width half-maximum (FWHM) resolution of$ΔE_{FWHM}=(1.25±0.17(stat)^{+0.05}_{−0.07}(syst))  eV$ can be achieved, leading to an unprecedented energy resolving power $E/ΔE_{FWHM}∼4700$ among existing energy-dispersive detectors for soft and tender x-rays. We also derive the best possible resolution and discuss limiting factors affecting the measurement. The analysis pipeline for the MMC data developed in this paper is furthermore an important step for the realization of the proposed superfluid helium-based experiment DELight, which will search for direct interaction of dark matter particles with masses below $100  MeV/c^2$

Optimum filter-based analysis for the characterization of a high-resolution magnetic microcalorimeter towards the DELight experiment

Ultra-sensitive cryogenic calorimeters have become a favored technology with widespread application where eV-scale energy resolutions are needed. In this article, we characterize the performance of an X-ray magnetic microcalorimeter (MMC) using a Fe-55 source. Employing an optimum filter-based amplitude estimation and energy reconstruction, we demonstrate that an unprecedented FWHM resolution of $ΔE_{FWHM}=(1.25±0.17(stat)^{+0.05}_{−0.07}(syst))eV$ can be achieved. We also derive the best possible resolution and discuss limiting factors affecting the measurement. The analysis pipeline for the MMC data developed in this paper is furthermore an important step for the realization of the proposed superfluid helium-based experiment DELight, which will search for direct interaction of dark matter with masses below 100 MeV/c$^2$

OBLIQUE PHOTOGRAMMETRY SUPPORTING 3D URBAN RECONSTRUCTION OF COMPLEX SCENARIOS

Accurate 3D city models represent an important source of geospatial information to support various “smart city” applications, such as space management, energy assessment, 3D cartography, noise and pollution mapping as well as disaster management. Even though remarkable progress has been made in recent years, there are still many open issues, especially when it comes to the 3D modelling of complex urban scenarios like historical and densely-built city centres featuring narrow streets and non-conventional building shapes. Most approaches introduce strong building priors/constraints on symmetry and roof typology that penalize urban environments having high variations of roof shapes. Furthermore, although oblique photogrammetry is rapidly maturing, the use of slanted views for façade reconstruction is not completely included in the reconstruction pipeline of state-of-the-art software. This paper aims to investigate state-of-the-art methods for 3D building modelling in complex urban scenarios with the support of oblique airborne images. A reconstruction approach based on roof primitives fitting is tested. Oblique imagery is then exploited to support the manual editing of the generated building models. At the same time, mobile mapping data are collected at cm resolution and then integrated with the aerial ones. All approaches are tested on the historical city centre of Bergamo (Italy)