15,296 research outputs found
Underground study of 15N(p,gamma)16O at stellar energies
No full textSince the ancient times, mankind has been wondering about the role of stars, both in the sky at night and in his life. Actually we know that stars are the forges of the elements which constitute our universe and of course ourselves. As a matter of fact, we are made of stardust. Since the Eddington hypothesis that the energy which powers the sun and other stars comes from thermonuclear reactions, the scientists have done a lot of work to under- stand the mechanisms of nuclear reactions inside stars. All the elements of the periodic table (except hydrogen, helium and lithium which are synthesized during the Big Bang Nu- cleosynthesis) are produced inside the stars. Many theories were formulated and supported by experimental measurements, however there are still many questions to be answered.
Many models have been developed in order to describe stellar systems. In recent years, due to the resources provided by the new high performance computers, the models can describe the stellar behavior also in three dimensions extending our knowledge of the stellar interior. One critical input of these models is the cross section of the nuclear reactions. The knowledge of the cross section at the stellar energies lies at the heart of nuclear astrophysics. At these energies nuclear reactions are strongly suppressed by the Coulomb repulsion of the colliding nuclei and the cross section is extremely small. As a matter of fact, the reaction rates are too low to be measured due to the overwhelming experimental background. In overground laboratories the nuclear reactions can be measured at higher energies than those inside the stars, and then extrapolated to stellar energies. To reduce the uncertainties on the extrapolation procedure considerable efforts have been spent in recent years to push the cross section measurements to lower and lower energies.
LUNA (Laboratory of Underground Nuclear Astrophysics), thanks to its unique position in the Laboratori Nazionali del Gran Sasso (LNGS), can study nuclear reactions at energies much lower than ever reached before. Thanks to the natural shielding of the 1.4 km of rock offered by Gran Sasso, it has been possible, in particular, to directly determine the rate of 3He(3He,2p)4He, one of the most important reaction of the p-p chain, at the energies of the center of the sun. 14N(p,γ)15O, the key reaction of the CNO cycle, has also been studied in great details obtaining a cross section smaller by a factor two as compared to previous extrapolations. The other important reaction of the cycle is 15N(p,γ)16O which links the first to second CNO cycle. In this thesis I will discuss the study of 15N(p,γ)16O performed with a gas target and a solid target setup in two different experiments. The first chapter is a brief description of the general characteristics of thermonuclear reactions in the stars. In the second chapter the LUNA2 accelerator will be reviewed and the two beam channels, dedicated to the gas target and solid target experiments, will be described. Finally, the third and the fourth chapters are devoted to the 15N(p,γ)16O measurements. Specifically in the third chapter the analysis of the gas target data will be discussed and the results presented. The fourth chapter is dedicated to the analysis of the solid target data, in particular to the different measurements performed to characterize the targets. We measured the cross section of 15N(p,γ)16O for the first time down to 70 keV obtaining values smaller by about a factor two as compared to the extrapolation in the NACRE (Nuclear Astrophysics Compilation of REaction Rates) database. Finally, the influence of our results on isotope production in nova explosions will be discussed.Sin dall'antichità, l’uomo si è chiesto quale fosse il ruolo delle stelle nell’universo e nella sua vita. Adesso sappiamo che le stelle sono le fucine dove vengono prodotti gli elementi che costituiscono l’universo e, di conseguenza, anche il nostro corpo. Per questa ragione ci possiamo considerare polvere di stelle.
Da quando Eddington formulò la sua ipotesi, secondo la quale l’energia che alimenta il sole e le altre stelle viene fornita dalle reazioni termonucleari che trasformano gli elementi leggeri in quelli via via più pesanti, gli scienziati si sono prodigati a lungo per comprendere i dettagli delle reazioni nucleari all’interno dei corpi celesti. Tutti gli elementi della tavola periodica (con l’eccezione di idrogeno, elio e litio che sono prodotti durante la Nucleosintesi del Big Bang) sono generati all’interno delle stelle. Fino ad ora sono state formulate molte teorie supportate dai risultati sperimentali, ma rimangono ancora diversi quesiti irrisolti.
Vari modelli sono stati sviluppati per descrivere i sistemi stellari. In particolare, negli ultimi anni, grazie alla potenza di calcolo dei computer di nuova generazione, i modelli riescono a descrivere la struttura e lo sviluppo delle stelle anche in tre dimensioni. In questo modo si riesce a studiare in maniera sempre più approfondita la struttura interna dei corpi celesti. Un input fondamentale per questi modelli è la sezione d’urto delle reazioni nucleari. La conoscenza della sezione d’urto all’energia propria delle stelle rappresenta il cuore dell’astrofisica nucleare. A queste energie le reazioni nucleari sono fortemente soppresse dalla repulsione Coulombiana dei nuclei interagenti e la sezione d’urto è molto piccola. E' possibile misurare queste reazioni nucleari in laboratorio, ma solo ad energie più elevate di quelle di interesse astrofisico. Infatti la sezione d’urto estremamente piccola si traduce in un rateo di eventi molto basso e non separabile dal fondo ambiente. Le sezioni d’urto così ottenute devono essere estrapolate alle energie proprie delle stelle introducendo una notevole incertezza.
Negli ultimi anni sono stati fatti considerevoli sforzi in modo da misurare la sezione d’urto ad energie sempre più vicine a quelle di interesse astrofisico. In particolare, LUNA (Laboratory for Underground Nuclear Astrophysics), grazie alla sua posizione privilegiata all’interno dei Laboratori Nazionali del Gran Sasso (LNGS), sfrutta la soppressione dei raggi cosmici e può quindi studiare le reazioni nucleari alle basse energie non esplorabili negli esperimenti di superficie. La schermatura naturale di 1.4 km di roccia fornita dal Gran Sasso ha reso possibile, ad esempio, lo studio della reazione 3He(3He,2p)4He, una delle più importanti reazioni della catena protone-protone, alle energie alle quali avviene nel sole. 14N(p,γ)15O, la reazione chiave del ciclo carbonio azoto ossigeno (CNO), è stata anch’essa studiata da LUNA ottenendo una sezione d’urto minore di un fattore due rispetto alle precedenti estrapolazioni. L’altra importante reazione del ciclo e` 15N(p,γ)16O che collega il primo con il secondo ciclo CNO.
In questo lavoro di tesi presenterò lo studio di questa reazione, svolto utilizzando due differenti tipologie di setup: con bersaglio gassoso e con bersaglio solido, rispettivamente. Il primo capitolo è una breve descrizione delle principali caratteristiche delle reazioni termonucleari nelle stelle. Nel secondo capitolo verrà presentato l’acceleratore LUNA2 insieme ai due canali di misura utilizzati e verranno descritte le caratteristiche del rivelatore BGO. Infine, il terzo e il quarto capitolo vertono sulla misura della reazione 15N(p,γ)16O. Nello specifico il capitolo tre è dedicato all’analisi delle misure eseguite con il bersaglio gassoso ed ai risultati sperimentali ottenuti con questo setup. Nel capitolo quattro, invece, vengono presentati i risultati delle misure eseguite con bersaglio solido soffermandosi in particolare sulle metodologie di analisi dei bersagli utilizzati.
In sintesi, la sezione d’urto della reazione 15N(p,γ)16O è stata misurata per la prima volta sino all’energia di 70 keV nel centro di massa. Il risultato ottenuto riduce di un fattore due la precedente estrapolazione delle misure dirette riportata nel database NACRE (Nuclear Astrophysics Compilation of REaction Rates). Alla fine, verranno discusse le implicazioni di questi nuovi risultati per la sintesi degli elementi nelle esplosioni di novae
Study of the
No full textThe NeNa-MgAl cycles are involved in the synthesis of Ne, Na, Mg, and Al isotopes. The 20Ne(p,γ)21Na (Q = 2431.68 keV) reaction is the first and slowest reaction of the NeNa cycle and it controls the speed at which the entire cycle proceeds. At the state of the art, the uncertainty on the 20Ne(p,γ)21Na reaction rate affects the production of the elements in the NeNa cycle. In particular, in the temperature range from 0.1 GK to 1 GK, the rate is dominated by the 366 keV resonance corresponding to the excited state of EX = 2797.5 keV and by the direct capture component. The present study focus on the study of the 366 keV resonance and the direct capture below 400 keV. At LUNA (Laboratory for Underground Nuclear Astrophysics) the 20Ne(p,γ)21Na reaction has been measured using the intense proton beam delivered by the LUNA 400 kV accelerator and a windowless differential-pumping gas target. The products of the reaction are detected with two high-purity germanium detectors. The experimental details and preliminary results on the 366 keV resonance and on the direct capture component at very low energies will be shown, together with their possible impact on the 20Ne(p,γ)21Na reaction rate
Declaration of Intention for Antonio Franchihtti
No full textDeclaration of Intention to become a citizen of the United States, as filled out and signed by Antonio Franchitti. Applicant lists himself as a 47 year old farmer residing in Hammonton, New Jersey, born in Campombosso, Italy on 18 April 1870, who sailed on the US bound vessel Britania. Declaration submitted and sworn April 21, 1917
The role of educative thought in the life and work of Antonio Gramsci
No full textMany philosophers have propounded a vision of an improved society, what distinguishes Antonio Gramsci is his continuous effort to make it happen by understanding the process in order to put into practice. Gramsci's conviction about the importance of educative development came from both theory and experience. While there has been considerable examination of Gramsci's work in relation to the Prison Notebooks, this study will seek to address a lacuna in Gramsci scholarship. Using Gramsci's philological method, I analyse Gramsci's pre-prison activity; his pre-prison articles and letters, which, together with his letters from prison, formed part of his educative mission. This educative process was necessary, in order to construct a new party which would develop a collective will, collaboratively, with the masses.In this study therefore, I explore the contexts and formative experiences of the first part of his life together with the intellectual sources from which Gramsci developed his later theories, making central hitherto underemphasised connections between them which informed his writing and ideas. I intend to illustrate that Gramsci's underlying purpose in his writing, and political activity, was not only practical, on how to create a new socialist ruling class, but also educative in forming the mindset and values of his comrades. So that in addition to outlining his vision of a new order, he implicitly guided or explicitly explained the processes by which the necessary changes in social relations and moral climate could be made in order to achieve it. Each person had to engage with the values of the new order so that each could contribute to the construction of a new robust state. It was essential to build a hegemony at the most profound level, one which was dependent on collective understandings and a collective will
Edna Ferber in St. Anthony Hotel, San Antonio, Tex., 1948
No full text''Ferber's novel, Giant, set on a fictitious south Texas ranch, was published in 1952.''''Edna Ferber was in San Antonio on Saturday for a search of the surrounding ranch country for material and characters for a new book. The author of Show boat was registered at the St. Anthony Hotel.'
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