Perspectives on the measurement of competitive double gamma decay with the AGATA tracking array

In this thesis, the capability of AGATA to measure the rare competitive double-gamma decay has been studied with a detailed simulation. This consists in a rare process, in which an excited state of a nucleus decays emitting two gamma rays despite the emission of a single photon is allowed. From a theoretical standpoint, the decay is correctly described as a second order process in perturbative QED. The measurement, would not only return more detailed physical information, namely on the energy and angular distributions of the photons, but also test the capabilities of the advanced gamma-ray spectrometer AGATA. The main challenge that the measurement faces is the suppression of Compton events without relying on timing properties due to the resolution limitations of germanium detectors. The thesis is focused on assessing whether the good energy resolution and spatial reconstruction properties of the detector, together with a strict event selection and optimization, can overcome the difficulties of such measurement. The emission of single and double-gamma photons has been simulated and optimal values of the algorithms were found for this specific measurement. The performance of the tracking was characterized and a further event selection was performed. While the measurement of the competitive double-gamma decay has proved itself challenging from an experimental standpoint, the current work only represents a starting point and a statement of the current capabilities. In fact, while the tracking algorithm showed a good performance with its optimal parameters, the need for a specific algorithm is argued in this thesis. In that sense, the community is working towards new tracking algorithms based on a different statistical approach. Since the array is in continuous development and improvement by the AGATA community, many progresses are expected as the research proceeds and many of them will likely be of crucial importance for the measurement of the competitive double-gamma decay. At last, a preliminary analysis of data taking with AGATA and a 137Cs source was performed. The background activity as well as the neutron damage correction and the stability of the calibration over time during the time span of the experiment have been discussed. While the analysis is only preliminary, a final comparison between the experimental data and the simulation has been performed.ope

Fusion Hindrance and Pauli Blocking in 58Ni + 64Ni

58Ni +64Ni is the first case where the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced, in a very well known experiment by Beckerman et al., by comparing with the two systems 58Ni + 58Ni and 64Ni+64Ni. Subsequent measurements on 64Ni + 64Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58Ni + 64Ni down to the measured level of 0.1 mb. In the present experiment the excitation function has been extended by two orders of magnitude downward. The cross sections for 58Ni + 64Ni continue decreasing very smoothly below the barrier, down to '1 µb. The logarithmic slope of the excitation function increases slowly, showing a tendency to saturate at the lowest energies. No maximum of the astrophysical S -factor is observed. Coupled-channels (CC) calculations using a Woods-Saxon potential and includinginelastic excitations only, underestimate the sub-barrier cross sections by a large amount. Good agreement is found by adding two-neutron transfer couplings to a schematical level. This behaviour is quite different from what already observed for 64Ni+ 64Ni (no positive Q-value transfer channels available), where a clear low-energy maximum of the S -factorappears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in 58Ni+ 64Ni in the measured energy range. This trend at deep sub-barrier energies reinforces the recent suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier

Multinucleon transfer reactions and proton transfer channels

Transfer reactions have always been of great importance for nuclear structure and reaction mechanism studies. So far, in multinucleon transfer studies, proton pickup channels have been completely identified in atomic and mass numbers at energies close to the Coulomb barrier only in few cases. We measured the multinucleon transfer reactions in the 40Ar+208Pb system near the Coulomb barrier, by employing the PRISMA magnetic spectrometer. By using the most neutron-rich stable 40Ar beam we could populate, besidesneutron pickup and proton stripping channels, also neutron stripping and proton pickup channels. Comparison ofcross sections between different systems with the 208Pb target and with projectiles going from neutron-poor to neutron-rich nuclei, as well as between the data and GRAZING calculations, was carried out.Finally, recent results concerning the measurement of the excitation function from the Coulomb barrier to far below for the 92Mo+54Fe system, where both proton stripping and pickup channels were populated with similar strength, will be discussed

Investigating the 46Ar proton wave function with the 46Ar(3He,d)47K direct reaction

L'evoluzione della chiusura di shell N = 28 ha suscitato molto interesse a causa di alcune discrepanze osservate tra il ben consolidato modello a shell con l'interazione SDPF-U ed alcuni esperimenti condotti per l'isotopo semi-magico 46Ar. In particolare, nonostante la notevole corrispondenza tra i valori teorici e sperimentali di energia di separazione dei neutroni, le probabilità di transizione misurate tramite eccitazione Coulombiana alle medie energie divergono di più di un fattore due dai valori previsti. Il motivo di questa mancata corrispondenza è stato individuato in una sbagliata descrizione degli elementi di matrice protonici, suggerendo una descrizione errata dello spazio protonico sd al di sotto di Z = 20. L'esperimento analizzato e discusso nella presente tesi ha affrontato il problema sondando direttamente la componente protonica della funzione d'onda tramite una reazione diretta di pick-up in cinematica inversa: 46Ar(3He, d)47K ad un'energia di 9 MeV/u. Lo scopo della misura, effettuata a GANIL presso Spiral 1 con un fascio radioattivo post-accelerato di 46Ar ed un target criogenico 3He, è quello di quantificare la sezione d'urto per il trasferimento al livello 3/2+ rispetto all'1/2+ (ground state) del 47K, basandosi su un apparato sperimentale all'avanguardia per una precisa ricostruzione della cinematica della reazione. Il frammento di reazione pesante è stato identificato dallo spettrometro magnetico ad alta accettanza, VAMOS, mentre il rivelatore DSSD di silicio ad alta granularità, MUGAST, ha consentito la misurazione della distribuzione angolare del frammento di reazione leggero, eseguendo anche l'identificazione delle particelle. L'array di germanio con tracciamento di raggi gamma AGATA ha misurato i fotoni prodotti dal decadimento degli stati eccitati del 47K. L'evidenza sperimentale indica un trasferimento in onda L=2 sostanzialmente soppresso al primo stato eccitato del 47K, in contrasto con i calcoli del modello shell che predicono gli orbitali s1/2 e d3/2 come quasi degeneri e non completamente occupati. I risultati sono discussi nel corso della tesi nell'ambito dei calcoli ab initio e di campo medio. In questi risultati teorici, la bassa occupazione dell'orbitale s1/2, in accordo con l'alto fattore spettroscopico relativo misurato, implica una riduzione della densità centrale nella funzione d'onda del protone: una cosiddetta struttura a bolla.The evolution of the nuclear shell closure at N=28 has gathered much interest due to the observed discrepancies between the well-established shell model with SDPF-U interaction and measurements of the semi-magic 46Ar isotope. In particular, while a remarkable agreement was observed between theoretical and experimental neutron separation energies, transition probabilities measured with intermediate Coulomb excitation diverge by more than a factor of two from their predicted values. The reason behind this mismatch has been pinned down to the proton transition matrix elements and hints at an incorrect description of the sd proton space below Z=20. The experiment analyzed and discussed in the present thesis addressed this problem by directly probing the proton component of the wave function via a proton-pickup direct reaction in inverse kinematics: 46Ar(3He, d)47K at an energy of 9 MeV/u. The measurement, performed at the Spiral 1 facility in GANIL with a post-accelerated radioactive 46Ar beam impinging on a high-density cryogenic 3He target, aimed at quantifying the transfer cross section to the 3/2+ level relative to the 1/2+ ground state in 47K, relying on a state-of-the-art experimental setup for a precise reconstruction of the kinematics of the reaction. The heavy reaction fragment was identified by the high acceptance magnetic spectrometer, VAMOS, while the high-granularity Silicon DSSD detector, MUGAST, allowed the measurement of the angular distribution of the light ejectile while also performing particle identification. The AGATA gamma-ray tracking Germanium array measured the photons produced by the decay of the 47K excited states. The experimental evidence indicates a substantially suppressed L=2 transfer to the first excited state of 47K, at odds with shell-model calculations that predict the s1/2 and d3/2 orbitals as almost degenerate and not entirely occupied. The results will also be discussed in the framework of ab initio and mean-field calculations. In these theoretical results, the low occupancy of the s1/2 orbital, in agreement with the high relative spectroscopic factor measured, implies a central depletion of the proton wavefunction: a so-called bubble structure

Perspectives on the measurement of competitive double gamma decay with the AGATA tracking array

In this thesis, the capability of AGATA to measure the rare competitive double-gamma decay has been studied with a detailed simulation. This consists in a rare process, in which an excited state of a nucleus decays emitting two gamma rays despite the emission of a single photon is allowed. From a theoretical standpoint, the decay is correctly described as a second order process in perturbative QED. The measurement, would not only return more detailed physical information, namely on the energy and angular distributions of the photons, but also test the capabilities of the advanced gamma-ray spectrometer AGATA. The main challenge that the measurement faces is the suppression of Compton events without relying on timing properties due to the resolution limitations of germanium detectors. The thesis is focused on assessing whether the good energy resolution and spatial reconstruction properties of the detector, together with a strict event selection and optimization, can overcome the difficulties of such measurement. The emission of single and double-gamma photons has been simulated and optimal values of the algorithms were found for this specific measurement. The performance of the tracking was characterized and a further event selection was performed. While the measurement of the competitive double-gamma decay has proved itself challenging from an experimental standpoint, the current work only represents a starting point and a statement of the current capabilities. In fact, while the tracking algorithm showed a good performance with its optimal parameters, the need for a specific algorithm is argued in this thesis. In that sense, the community is working towards new tracking algorithms based on a different statistical approach. Since the array is in continuous development and improvement by the AGATA community, many progresses are expected as the research proceeds and many of them will likely be of crucial importance for the measurement of the competitive double-gamma decay. At last, a preliminary analysis of data taking with AGATA and a 137Cs source was performed. The background activity as well as the neutron damage correction and the stability of the calibration over time during the time span of the experiment have been discussed. While the analysis is only preliminary, a final comparison between the experimental data and the simulation has been performed

Dynamic Language Model Focusing for Automatic Transcription of Talk-Show TV Programs

In this paper, an approach for unsupervised dynamic adaptation of the language model used in an automatic transcription task is proposed. The approach aims to build language models ”focused” on linguistic content and speaking style of audio documents to transcribe by adapting a general purpose language model on a running window of text derived from automatic recognition hypotheses. The text in each window is used to automatically select documents from the same corpus utilized for training the general purpose language model. In particular, a fast selection approach has been developed and compared with a more traditional one used in the information retrieval area. The new proposed approach allows for a real time selection of documents and, hence, for a frequent language model adaptation on a short (less than 100 words) window of text. Experiments have been carried out on six episodes of two Italian TV talk-shows programs, by varying the size and advancement step of the running window and the corresponding number of words selected for focusing language models. A relative reduction in word error rate of about 5.0% has been obtained using the running window for focusing the language models, to be compared with a corresponding relative reduction of 3.5% achieved using the whole automatic transcription of each talk-show episode for focusing the language models

A APLICABILIDADE DO DIREITO AO ESQUECIMENTO NA ESFERA CIVIL: UMA RELAÇÃO COM A EXPOSIÇÃO DA IMAGEM DE CRIANÇAS E ADOLESCENTES NA INTERNET

A pesquisa tem como tema o direito ao esquecimento e como problema a possibilidade de invocação do direito ao esquecimento quando há exposição de crianças e adolescentes na internet por meio de perfil em rede social gerenciado por genitores influenciadores. Os objetivos consistem em verificar a aplicação do direito ao esquecimento na hipótese apresentada, analisar de maneira breve o direito ao esquecimento na esfera civil e no âmbito da internet e apresentar conflitos entre os direitos de personalidade e o poder familiar em relação às imagens de crianças e adolescentes expostos pelos pais influenciadores. O método de pesquisa utilizado foi o dedutivo, com auxílio do método comparativo. A conclusão final indica que a aplicação do direito ao esquecimento é um meio de proteção que deve ser analisado em casos concretos, buscando sempre a proteção da criança e do adolescente em detrimento ao poder familiar