89 research outputs found
Sub-barrier fusion for 24,26Mg + 12C. New techniques of gamma-particle and heavy ion-particle coincidences for 30Si + 12C
No full textIl lavoro di tesi consiste nello studio della dinamica di fusione di sistemi con Q-valore positivo ad energie molto al disotto della barriera Coulombiana. Lo studio di sistemi con Q-valore positivo è di particolare interesse per l'astrofisica, ma stabilire la presenza dell'effetto di hindrance per questi sistemi richiede misure impegnative. Per questo motivo, lo studio di sistemi leggermente più pesanti è molto importante. Il sistema 24Mg+12C è di particolare interesse poichè presenta due caratteristiche particolari. La prima è il valore molto alto della sezione d'urto alla soglia dell'hindrance rispetto ad altri sistemi simili, la seconda sta nel fatto che ad energie molto basse i dati sperimentali sono riprodotti da un semplice modello unidimensionale basato sulla penetrazione della barriera. La funzione di eccitazione del 24Mg+12C è stata riprodotta usando due modelli diversi, il primo è una formula empirica nello spirito del modello adiabatico. La seconda è una parametrizzazione dell'hindrance usato per riprodurre la funzione di eccitazione di molti sistemi con Q-valore positivo. Per capire meglio la ragione dietro l'alto valore della sezione d'urto alla soglia dell'hindrance, è stato misurato il sistema 26Mg+12C. In questo esperimento la sezione d'urto di fusione è stata ottenuta rivelando direttamente i residui di evaporazione (ER) con il set-up PISOLO che consiste in un deflettore elettrostatico usato per separare gli ER dal fascio trasmesso, due MCP che misura il tempo di volo, una camera di ionizzazione che misura l'energia persa e un rivelatore al silicio che misura l'energia residua. Combinando le informazioni fornite da PISOLO è possibile estrarre la sezione d'urto di fusione fino a 6 μb insieme alla derivata logaritmica e al fattore astrofisico S. Confrontando i due sistemi si può notare che nel caso 26Mg+12C il massimo del fattore S è meno evidente e il valore della sezione d'urto alla soglia dell'hindrance è più piccola rispetto al caso del 24Mg+12C. La ragione potrebbe essere la struttura α-like del 24Mg non presente per il 26Mg. Si può anche notare che la derivata logaritmica di entrambi i sistemi presenta delle oscillazioni che potrebbero essere collegate alla bassa densità dei livelli del nucleo composto. Per discriminare fra i due modelli usati per riprodurre la sezione d'urto sperimentale del 24Mg+12C è necessario eseguire misure precise ad energie molto al disotto della barriera. Però, PISOLO non può essere usato per misurare sezioni d'urto inferiore a pochi μb, per questo, sono stati testati due metodi alternativi per misurare funzioni di eccitazione nell'ordine dei nanobarn. Il primo è un miglioramento del set-up PISOLO: due rivelatori al silicio sono stati installati nella camera di reazione per rivelare le particelle cariche evaporate dal nucleo composto in coincidenza con gli ER rivelati da PISOLO. Questa tecnica è stata testata con il sistema 30Si+12C misurando le energie 47, 40 e 37 MeV. I risultati ottenuti mostrano una forte riduzione del fondo che può permettere di misurare sezioni d'urto di fusione inferiori a 1μb con PISOLO, al costo di una efficienza minore del set-up. Il secondo metodo si basa sulle coincidenze fra le particelle evaporate dal nucleo composto e i raggi-γ emessi dagli ER. Per questo esperimento sono stati usati lo spettrometro-γ AGATA e l'array di rivelatori al silicio a due stadi EUCLIDES per studiare l'esperimento 30Si+12C misurando le energie di 47 e 40 MeV. L'analisi è stata fatta selezionando gli eventi riconosciuti come protoni e particelle α negli spettri E-∆E per poi studiare gli spettri-γ in coincidenza. Questo permette di identificare le transizioni che vanno direttamente al fondamentale in modo da estrarre la sezione d'urto di fusione. Il testo ha prodotto buoni risultati e un esperimento basato sulle coincidenze γ-particelle cariche è stato fatto per misurare la sezione d'urto del sistema 28Si+12C usando AGATA e due DSSD.The thesis work focuses on the study of the fusion dynamics of systems with positive Q-value at energies far below the Coulomb barrier. The study of systems with positive Q-value is of particular interest for astrophysics, but establishing the presence of the hindrance effect for these systems requires challenging measurements. For this reason, the study of slightly heavier systems is very important. The system 24Mg + 12C is of particular interest since it presents two peculiar features. The first one is the very high value of the cross section at the hindrance threshold compared with other similar systems, the second one is that the experimental data at very low energies are well reproduced by a simple one-dimensional barrier penetration calculation. The excitation function of 24Mg + 12C has been reproduced with two different models, the first one is an empirical formula in the spirit of the adiabatic model. The second one is a hindrance parametrization used to reproduce the excitation function of several systems with positive Q-values. To better understand the reason behind the high value of the cross section at the hindrance threshold, the system 26Mg + 12C has been measured. The fusion cross section in this experiment has been obtained detecting the evaporation residue (ER) by the set-up PISOLO consisting of an electrostatic deflector to separate the ER from the transmitted beam, two MCP to measure the time of flight, an ionization chamber to measure the energy loss and a silicon detector giving the residual energy. Combining the information provided by PISOLO it has been possible to extract the fusion cross section down to about 6 μb together with the logarithmic slope and the astrophysical S factor. Comparing the two systems one can notice that for the 26Mg + 12C case the maximum for the astrophysical S factor is narrower and the value of the cross section at the hindrance threshold is smaller compared with what has been obtained for the 24Mg + 12C case. The reason may be the α-like structure of the 24Mg not owned by the 26 Mg. One can also notice that the logarithmic derivative presents some oscillations for both systems that could be related to the low level density of the compound nucleus. To discriminate between the two models used reproducing the experimental cross section of 24Mg + 12C at deep sub-barrier energies it is necessary to perform precise measurements at lower energies. However, PISOLO can not be used to measure cross section below a few μb, therefore, two alternative methods to measure the excitation function in the nanobarns range have been tested. The first one is an upgrade of the PISOLO set-up: two two-stage silicon detectors have been installed inside the reaction chamber to detect the light charged particles evaporated by the compound nucleus in coincidence with the ER detected by PISOLO. This technique has been tested for the system 30Si + 12C measuring three energies 47, 40 and 37 MeV. The results obtained show a large reduction of the background that could allow the measurement of fusion cross section below 1μb with PISOLO, at the cost of a lower efficiency of the set-up. The second method is based on the coincidences between the charged particles evaporated by the compound nucleus and the γ-rays emitted by the ER. To perform this measurement the γ-spectrometer AGATA and the array of two-stage silicon detectors EUCLIDES have been used to study the system 30Si + 12C at the energies of 47 and 40 MeV. The analysis has been performed by selecting the protons and α particles events in the E-∆E spectra and studying the γ-spectra in coincidence. This allows to identify the transitions of the ER that go directly to the ground state so to extract the fusion cross section. The test has been successful and an experiment based on the γ-charged particles coincidence technique has been performed to measure the fusion cross section of the system 28Si + 12C using AGATA and two DSSD detectors
Fusion of
No full textRecent experiments on 12C + 24Mg and preliminary data on 12C + 26Mg show that we observe the hindrance phenomenon in these two systems that are close to the lighter ones relevant for astrophysics. The cross section at the hindrance threshold has a remarkably high value for 12C + 24Mg, while it is closer to an empirical systematics for 12C + 26Mg. The lowest-energy fusion cross sections of 12C + 24Mg are consistent with simple one-dimensional barrier penetration calculations, i.e. the coupling strengths seem to be strongly damped far below the barrier. Measurements at slightly lower energies would be essential to discriminate between different models and to allow reliably extrapolating to the lighter systems producing energy and elemental synthesis in stellar environments
Particle-gamma coincidences with Agata+Euclides for study of light-ion fusion at astrophysical energies
No full textopenThe study of light ion fusion at sub-barrier energies has a crucial role in astrophysics because they determine the dynamics of stellar evolution. In particular, the discovery of fusion hindrance in these systems increased the interest in their study because this phenomen appears as a smaller cross-section than expected by theoretical calculations and this affects the abundances of many isotopes.
In this thesis, it is described the data analysis of a text experiment performed on the reaction 30Si+12C where fusion hindrance was already observed in the past. The purpose of the test was to demostrate the feasibility of cross-section measurements in the nano-barn range using the particle-gamma coincidences technique exploiting the Agata+Euclides setup. With this technique the cross-section is obtained by counting the coincidences between the charged particles evaporated from the compund nucleus and the gamma-rays emitted by the evaporation residues.The study of light ion fusion at sub-barrier energies has a crucial role in astrophysics because they determine the dynamics of stellar evolution. In particular, the discovery of fusion hindrance in these systems increased the interest in their study because this phenomen appears as a smaller cross-section than expected by theoretical calculations and this affects the abundances of many isotopes.
In this thesis, it is described the data analysis of a text experiment performed on the reaction 30Si+12C where fusion hindrance was already observed in the past. The purpose of the test was to demostrate the feasibility of cross-section measurements in the nano-barn range using the particle-gamma coincidences technique exploiting the Agata+Euclides setup. With this technique the cross-section is obtained by counting the coincidences between the charged particles evaporated from the compund nucleus and the gamma-rays emitted by the evaporation residues
Fusion Hindrance and Pauli Blocking in 58Ni +64Ni
No full textThe argument of this thesis is the measurement of deep sub-barrier fusion cross sections for 58Ni +64Ni. In this system the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced in a famous experiment by Beckerman et al.. Subsequent experiments for the two symmetric systems 58Ni +58Ni and 64Ni +64Ni showed that fusion hindrance is clearly present in both cases. The lowest measured cross section for 58Ni +64Ni, however, was relatively large (0.1 mb), so that no hindrance was observed. The present measurements have been recently performed at the XTU Tandem accelerator of LNL and the excitation function has been extended by two orders of magnitude downward.
The case of 58Ni +64Ni is very similar to 40Ca+96Zr because of the flat shape of the two sub-barrier fusion excitation functions, originating from the couplings to several Q>0 neutron pick-up channels. 40Ca+96Zr was studied to very small cross sections (2b) and fusion hindrance does not show up, suggesting that this unusual behavior is due to the Q>0 transfer couplings, since the valence nucleons can flow freely from one
nucleus to the other without being hindered by Pauli blocking.
The present experiment indicates that the flat trend of the sub-barrier cross sections for 58Ni +64Ni continues down to the level of b and fusion hindrance is not observed. This trend at far sub-barrier energies reinforces the suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the effect of Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.ope
Fusion Hindrance and Pauli Blocking in 58Ni +64Ni
No full textThe argument of this thesis is the measurement of deep sub-barrier fusion cross sections for 58Ni +64Ni. In this system the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced in a famous experiment by Beckerman et al.. Subsequent experiments for the two symmetric systems 58Ni +58Ni and 64Ni +64Ni showed that fusion hindrance is clearly present in both cases. The lowest measured cross section for 58Ni +64Ni, however, was relatively large (0.1 mb), so that no hindrance was observed. The present measurements have been recently performed at the XTU Tandem accelerator of LNL and the excitation function has been extended by two orders of magnitude downward.
The case of 58Ni +64Ni is very similar to 40Ca+96Zr because of the flat shape of the two sub-barrier fusion excitation functions, originating from the couplings to several Q>0 neutron pick-up channels. 40Ca+96Zr was studied to very small cross sections (2b) and fusion hindrance does not show up, suggesting that this unusual behavior is due to the Q>0 transfer couplings, since the valence nucleons can flow freely from one
nucleus to the other without being hindered by Pauli blocking.
The present experiment indicates that the flat trend of the sub-barrier cross sections for 58Ni +64Ni continues down to the level of b and fusion hindrance is not observed. This trend at far sub-barrier energies reinforces the suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the effect of Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier
Speciation Investigation of Gadolinium Based Contrasting Agents in Wastewater and Seawater of the Northern Adriatic Sea
No full textThe contaminants of emerging concern is a class in constant evolution since new compounds which may present toxicity towards the biota are continuously detected in the environment, especially in water bodies. Among these substances, gadolinium based contrasting agents (GBCAs) are regarded as possible contaminants of relevance due to their widespread use in medicine1. Substances containing gadolinium are used as contrasting agents in magnetic resonance imaging (MRI) analyses due to the paramagnetic properties of Gd; since gadolinium ion in its free from, Gd3+, is highly toxic, it is administered before MRI examination as a complex with an organic ligand which ensure a high stability of the compound avoiding the release of free Gd ions into the body fluids1. The high stability implicates that GBCAs are persistent also in the environment, and indeed their concentration in the aquatic ecosystems is expected to increase, possibly leading to adverse effects.
In this work, we investigated the presence of two GBCAs, namely gadoterate meglumine (Gd-DOTA) and gadoteridol (Gd-HP-DO3A), in two different aqueous matrices, wastewater both at the entrance and at the exit of a wastewater treatment plant in Trieste, and seawater collected in the gulf of Trieste near the WWTP’s pipeline.The wastewater samples were pre-concentrated by evaporation, while the seawater samples, presenting a high salinity, needed a specific treatment consisting in a solid phase extraction to remove the saline components in order to prevent interferences during analysis2. The determination of GBCAs was carried out by HPLC-ICP-MS using an anion exchange column and a mixture of 125 mM ammonium nitrate buffer at pH 9 and MilliQ water in a 25:75 ratio as a mobile phase. The isotopes 157Gd and 158Gd were monitored.
The high recoveries obtained for Gd-DOTA and Gd-HP-DO3A proved the validity of the two sample preparation methods. The results showed the presence of both substances in the samples analysed, in the range between 0.1 and 50 ng/L; indeed, higher concentrations were found in the wastewater matrices, while the concentration was lower in the seawater samples, providing information on the environmental pathway of these compounds.
References:
[1] R. Brünjes, T. Hofmann, Water Research 2020, 182, 115966.
[2] M. Horstmann, R. Gonzalez de Vega, D.P. Bishop, U. Karst, P.A. Doble, D. Clases, Journal of Analytical Atomic Spectrometry 2021, 36, 767 – 775
24 Mg + 12 C fusion reaching the no coupling limit far below the barrier
No full textIn the present work, the fusion cross section of the 12C+24Mg system has been
measured down to energies far below the coulomb barrier around 4μb. This system is slightly heavier than those of astrophysical interest, like 12C+12C and 16O+16O. The data points highlight the presence of hindrance in 12C+24Mg because the excitation function is overestimated by standard Coupled-Channels calculations, and a clear maximum of the S factor has been observed. The cross section at the hindrance threshold is found to be remarkably large (σ ≈0.75mb). The S-factor maximum is nicely fitted using both an empirical interpolation in the spirit of the adiabatic model, and the hindrance parametrisation. The data far below the barrier may suggest that the coupling strengths gradually decrease and vanish so that the excitation function seems to be well reproduced by a simple one-dimensional tunnelling through the potential barrier in that energy range. On the other hand, the equally good fit obtained with the hindrance model, indicates that discriminating between the two approaches would require further precise measurements at slightly lower energies
A review of South African perspectives on serial murder
No full textDissertation (MA (Clinical Psychology))--University of Pretoria, 2007.Pistorius (1996) was the first individual to introduce research on serial murder in the South African context with her psychodynamic approach to the phenomenon. Since then, others have followed, namely Labuschagne (1998, 2001) with his search for interactional and communication connections; Du Plessis (1998) from a grounded theory approach; and Hodgskiss (2001) with his assessment of South African offender characteristics and behaviours. Four more studies have been conducted, namely Hook’s post-structural deconstruction of narratives of a person who committed serial murder (2003); De Wet’s psychosocial perspective (2005); Barkhuizen’s intrapsychic object relations approach (2006); and Del Fabbro’s exploration of the phenomenon from a family systems paradigm (2006). To date, no attempt has been made to review these research endeavors. In this article, these studies are reviewed and their strengths and weaknesses with regard to theory, methodology and research findings debated. The author concludes with several recommendations for future research on the phenomenon of serial murder: greater communication between various disciplines (e.g. psychology, criminology, law) investigating serial murder; expansion of Hodgskiss’ research (2001) on offence characteristics for profiling purposes; and the relationship between psychopathology and serial murder.PsychologyMAunrestricte
ANATOMIC ASSESSMENT OF THE ANTERIOR MANDIBLE AND RELATIVE HEMORRHAGE RISK IN IMPLANT DENTISTRY. A CADAVERIC AND CT SCAN STUDY.
No full textAbstract
Objectives: To evaluate prevalence, size, location and content of foramina and bony canals located on the lingual aspect of the mandibular midline.
Material and methods: The prevalence and the size of midline lingual foramina and canals visible above and/or below genial spines and their distances from the alveolar crest and the mandibular base were measured either in 60 dry mandibles from adult human cadavers or in 100 CT scans from patients scheduled for dental implant surgery in mandible. In addition, macro-anatomic dissections were performed on another 20 mandibles injected with red latex to investigate the vascular canal contents associated with these midline lingual foramina and canals.
Results: All mandibles investigated exhibited at least one lingual foramen at the midline above the genial spines (100% incidence). On the whole, a total of 118 and 188 foramina have been detected at the mandibular midline respectively in 60 dry mandibles and in 100 CT scans. The mean distance of the superior foramina and canals was 10.8 ± 2.7 (SD) mm from the alveolar crest by cadaveric analysis and 11.1 ± 2.1 (SD) mm in the CT scan study.
Such distances were found to be far reduced in Class V to VI mandibles displaying a severe osseous atrophy, confirming the deep correlation between the “Class” of resorption of the anterior mandible (according to Cawood & Howell’s classification of 1988) and the distance of the lingual foramina to the alveolar crest. The mean diameter of the superior foramina, measured at the entrance of the bony canal, averaged 0.9 ± 0.5 mm (SD). Macro-anatomic dissections showed clear sublingual vascular branches entering the midline mandible in 19 out of 20 mandibles studied (95%).
Conclusions: Blood vessels in the floor of the mouth may be in close proximity to the lingual cortical plate of the mandibular midline in most cases. This implies that bleeding can occur when the mandibular cortical plate is perforated even minimally and especially in case of severe osseous atrophies. Consequently, the Author suggests a careful planning of dental implant positioning at mandibular midline, possibly opting for the use of an even number of implants, in order to reduce the risk of violation of sublingual vessels afferent to midline lingual foramina
Architecture as total structure
No full textTekst odnosi się do twórczości dwóch architektów, którzy odegrali znaczącą rolę w rozwoju unikalnej architektonicznej i urbanistycznej wyobraźni na światowej mapie architektury współczesnej. Autor opisuje projekty argentyńskiego architekta Amancio Williamsa (1913-1989): Casa del Puente w Mar del Plata i cienkościenne konstrukcje „Umbrellas”, a także dwa najbardziej reprezentatywne dzieła włosko-argentyńskiego architekta Clorindo Testy (1923-2013): Bank of London and South America i Bibliotekę Narodową w Buenos Aires. Prace analizowane są według tematów odnoszących się do architektury betonowej i jej możliwych językowych i konstruktywnych odmian. Zwłaszcza w odniesieniu do znaczenia formy w funkcji strukturalno-materialnego składnika i do praw regulujących związek między materiałem a rozwiązaniami architektonicznymi.The article reflects on the two architects who played a significant role in building a unique architectural and urban imagination on the world-wide panorama of contemporary architecture. The author refers to the Argentinian architect Amancio Williams (1913-1989) and his design of Casa del Puente in Mar del Plata and the “Umbrellas” – special thin shell structures – and to the Italian-Argentinian architect Clorindo Testa (1923-2013) and his two most representative works, both built in Buenos Aires, the Bank of London and South America and the National Library. The works are analysed according to themes that refer to concrete architecture and its possible linguistic and constructive declinations. Particularly to the meaning of form in the function of the structural-material component, and to the laws governing the relationship between static solids and architectural solutions
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