Construction, test and performance analysis of the Tracker flight towers for the Large Area Telescope of GLAST.

Construction, test and performance analysis of the Tracker flight towers for the Large Area Telescope of GLAST. Riassunto della tesi candidato: Melissa Pesce-Rollins relatore della tesi: Prof. Ronaldo Bellazzini L'interesse verso l'astronoma gamma, sia nello spazio che a terra, e` andato notevolmente crescendo negli ultimi decenni. Lo studio dei fotoni di maggiore energia dello spettro elettromagnetico ha mostrato alcuni dei fenomeni celesti piu` energetici e dinamici conosciuti ad oggi. Inoltre la scarsa probabilita` di interazione con la materia che caratterizza i raggi gamma e l'insensibilita` ai campi magnetici galattici ed extragalattici permette di osservare, con relativa facilita`, sorgenti a distanze notevoli da noi. L'esperimento GLAST (Gamma-Ray Large Area Telescope) e` una missione spaziale internazionale ideata con lo scopo di studiare il cielo gamma nell'intervallo di energia cha va da 10 KeV a 300 GeV, che costituisce ad oggi una finestra osservativa ancora largamente inesplorata. GLAST rappresenta la naturale evoluzione delle precedenti misssioni spaziali, in particolare il Compton Gamma-Ray Observatory (CGRO) , oltre che un fondamentale punto di incontro tra esse e gli esperimenti a terra che sono in grado di rivelare fotoni di energie superiori al TeV. Lo strumento principale a bordo della missione e` il LAT (Large Area Telescope), un telescopio a produzione di coppie realizzato utilizando le piu` moderne tecniche sviluppate nel campo della fisica delle particelle elementari. Esso include essenzialmente un tracciatore-convertitore a microstrip di silicio ed un calorimentro di CsI(Tl), il tutto circondato da uno schermo di anticoincidenza (ACD, Anti Coincidence Detector) per la reiezione del fondo dovuto ai raggi cosmici. La collaborazione italiana (in particolare l'INFN) e` responsabile della costruzione e caratterizzazione del tracciatore al silicio, che con i suoi 80 m^2 di superficie attiva e` di gran lunga il piu` grande mai costruito per una missione spaziale. La struttura del LAT e` completamente modulare e strutturata in una matrice 4 per 4 di torri identiche. La costruzione dei 16 moduli di volo del tracciatore si e` conclusa con estremo sucesso alla fine del 2005; oltre a questi sono stati realizzati due moduli spare che permetteranno la calibrazione dello strumento su fascio. Questo lavoro di tesi descrive in dettaglio le attivita`, legate alla costruzione ed ai successivi test per la verifica delle prestazioni, cui ho preso parte direttamente. Ho partecipato ai test elettrici che sono stati eseguiti sui singoli wafer di silicio con lo scopo di verificarne la qualita`. Ho collaborato alla scrittura del software online per la gestione dell'acquisizione dati e all'implementazione delle procedure di test per la verifica delle funzionalita` dell'hardware a tutti i livelli della costruzione, oltre ovviamente all'esecuzione materiale dei test stessi. La tesi contiene una descrizione esaustiva dei risultati ottenuti ed in particolare una caratterizzazione completa dei moduli del tracciatore in termini di rumore, efficienza di rivelazione ed allineamento. L'ultima parte dell'elaborato e` dedicata ai test ambientali (vibrazioni e termo vuoto) che sono stati eseguiti su ogni singolo modulo prima dell'integrazione sulla griglia di volo

Gas pixel detectors

Abstract With the Gas Pixel Detector (GPD), the class of micro-pattern gas detectors has reached a complete integration between the gas amplification structure and the read-out electronics. To obtain this goal, three generations of application-specific integrated circuit of increased complexity and improved functionality has been designed and fabricated in deep sub-micron CMOS technology. This implementation has allowed manufacturing a monolithic device, which realizes, at the same time, the pixelized charge-collecting electrode and the amplifying, shaping and charge measuring front-end electronics of a GPD. A big step forward in terms of size and performances has been obtained in the last version of the 0.18 μm CMOS analog chip, where over a large active area of 15×15 mm 2 a very high channel density (470 pixels/mm 2 ) has been reached. On the top metal layer of the chip, 105,600 hexagonal pixels at 50 μm pitch have been patterned. The chip has customable self-trigger capability and includes a signal pre-processing function for the automatic localization of the event coordinates. In this way, by limiting the output signal to only those pixels belonging to the region of interest, it is possible to reduce significantly the read-out time and data volume. In-depth tests performed on a GPD built up by coupling this device to a fine pitch (50 μm) gas electron multiplier are reported. Matching of the gas amplification and read-out pitch has let to obtain optimal results. A possible application of this detector for X-ray polarimetry of astronomical sources is discussed

Multiple Sources of Solar High-energy Protons

During the 24th solar cycle, the Fermi Large Area Telescope (LAT) has observed a total of 27 solar flares possessing delayed gamma-ray emission, including the exceptionally well-observed flare and coronal mass ejection (CME) on 2017 September 10. Based on the Fermi/LAT data, we plot, for the first time, maps of possible sources of the delayed >100 MeV gamma-ray emission of the 2017 September 10 event. The long-lasting gamma-ray emission is localized under the CME core. The gamma-ray spectrum exhibits intermittent changes in time, implying that more than one source of high-energy protons was formed during the flare-CME eruption. We find a good statistical correlation between the gamma-ray fluences of the Fermi/LAT-observed delayed events and the products of corresponding CME speed and the square root of the soft X-ray flare magnitude. Data support the idea that both flares and CMEs jointly contribute to the production of subrelativistic and relativistic protons near the Sun.</p

Interplanetary Protons versus Interacting Protons in the 2017 September 10 Solar Eruptive Event

We analyze the relativistic proton emission from the Sun during the eruptive event on 2017 September 10, which caused a ground-level enhancement (GLE 72) registered by the worldwide network of neutron monitors. Using the neutron monitor data and interplanetary transport modeling both along and across interplanetary magnetic field (IMF) lines, we deduce parameters of the proton injection into the interplanetary medium. The inferred injection profile of the interplanetary protons is compared with the profile of the >100 MeV γ-ray emission observed by the Fermi Large Area Telescope, attributed to pion production from the interaction of >300 MeV protons at the Sun. GLE 72 started with a prompt component that arrived along the IMF lines. This was followed by a more prolonged enhancement caused by protons arriving at the Earth across the IMF lines from the southwest. The interplanetary proton event is modeled using two sources—one source at the root of the Earth-connected IMF line and another source situated near the solar western limb. The maximum phase of the second injection of interplanetary protons coincides with the maximum phase of the prolonged >100 MeV γ-ray emission that originated from a small area at the solar western limb, below the current sheet trailing the associated coronal mass ejection (CME). A possible common source of interacting protons and interplanetary protons is discussed in terms of proton acceleration at the CME bow shock versus coronal (re-)acceleration in the wake of the CME

A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices $(0.7 < \Gamma <1.4)$ and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J. Kn\"odlseder, N. Webb, B. Pancraz

Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe

The Extragalactic Background Light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts (GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of gamma-ray flux attenuation by the EBL. We place upper limits on the gamma-ray opacity of the Universe at various energies and redshifts, and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. (2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A. Reimer, L.C. Reye