Μελέτη επιπέδων ραδιενέργειας και συγκεντρώσεων βαρέων μετάλλων σε παράκτιες περιοχές του Λαυρίου

Εθνικό Μετσόβιο Πολυτεχνείο. Μεταπτυχιακή εργασία. Διεπιστημονικό - Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.

Measurement of the 235U(n,f) cross section relative to the 10B(n,α) reaction with Micromegas detectors at the CERN n_TOF facility: first results

Funding from the European Union's Horizon 2020 research and innovation programme SANDA (Supplying Accurate Nuclear Data for energy and non-energy Applications) under grant agreement no 847552, is gratefully acknowledged.Neutron cross section measurements are often made relative to a neutron cross section standard. Thus, the accuracy of the neutron standards determines the best possible accuracy of the neutron measurements. The U-235(n,f) cross section is widely used as reference, while it is considered a standard at thermal point and between 0.15 to 200 MeV. For this reason, additional cross section data for the U-235(n,f) reaction are useful in order to improve the accuracy and to extend the energy range of the standard. In this work, preliminary results of the measurement of the 235U(n,f) cross-section relative to the standard (10) B(n,a) reaction are presented. The high accuracy measurement was performed at the experimental area EAR-1 of the n_TOF facility at CERN, aiming at covering the energy range from the thermal region up to approximately 100 keV. The samples were produced at JRC-Geel in Belgium, while the experimental setup was based on Micromegas detectors.European Union's Horizon 2020 research and innovation programme SANDA 84755

NEAR at n_TOF/CERN: Preparing the first multi-foil activation measurement

The n_TOF facility at CERN is a neutron Time-Of-Flight facility based on a spallation neutron source. During the Second Long Shutdown (LS2) , a new experimental zone was designed and delivered. This new experimental area -the NEAR station - is located very close to the lead spallation target, at a distance of just ~3m. In this way, the high luminosity of the n_TOF neutron spallation source can be fully exploited. Towards the characterization of the new experimental area as well for the benchmarking of the performed simulations, the multi-foil activation measurement will be implemented. Eleven threshold and seven capture reference reactions will be utilized for the unfolding of the NEAR neutron beam energy spectrum that stretches from the meV to the GeV region

Measurement of the neutron-induced fission cross section of Th 230 at the CERN n_TOF facility

The neutron-induced fission cross section of 230 Th has been measured at the neutron time-of-flight facility n_TOF located at CERN. The experiment was performed at the experimental area EAR-1 with a neutron flight path of 185 m, using Micromegas detectors for the detection of the fission fragments. The 230 Th(n, f ) cross section was determined relative to the 235 U(n, f ) one, covering the energy range from the fission threshold up to 400 MeV. The results from the present work are compared with existing cross-section datasets and the observed discrepancies are discussed and analyzed. Finally, using the code EMPIRE 3.2.3 a theoretical study, based on the statistical model, was performed leading to a satisfactory reproduction of the experimental results with the proper tuning of the respective parameters, while for incident neutron energy beyond 200 MeV the fission of 230 Th was described by Monte Carlo simulations.This project received funding from the Euratom “Support safe operation of nuclear systems” program 2014–2018 under Grant Agreement No. 847552 (SANDA) and by the funding agencies of the participating institutes. This research is imple- mented through the IKY scholarships program and cofinanced by the European Union (European Social Fund ’ESF) and Greek national funds through the action entitled “Reinforce- ment of Postdoctoral Researchers - 2nd call (MIS 5033021)”, in the framework of the Operational Programme “Human Resources Development Program, Education and Lifelong Learning” of the National Strategic Reference Framework.Article signat per 137 autors/es: V. Michalopoulou, A. Stamatopoulos, M. Diakaki, A. Tsinganis, R. Vlastou, M. Kokkoris, N. Patronis, Z. Eleme, D. Macina, L. Tassan-Got, N. Colonna, E. Chiaveri, A. Ventura, P. Schillebeeckx, J. Heyse, G. Sibbens, G. Alaerts, A. Borella, A. Moens, D. Vanleeuw, O. Aberle, V. Alcayne, S. Amaducci, J. Andrzejewski, L. Audouin, V. Babiano-Suarez, M. Bacak, M. Barbagallo, S. Bennett, E. Berthoumieux, J. Billowes, D. Bosnar, A. Brown, M. Busso, M. Caamaño, L. Caballero, F. Calviño, M. Calviani, D. Cano-Ott, A. Casanovas, F. Cerutti, G. Cortés, M. A. Cortés-Giraldo, L. Cosentino, S. Cristallo, L. A. Damone, P. J. Davies, M. Dietz, C. Domingo-Pardo, R. Dressler, Q. Ducasse, E. Dupont, I. Durán, B. Fernández-Domínguez, A. Ferrari, P. Finocchiaro, V. Furman, K. Göbel, R. Garg, A. Gawlik-Ramiega, S. Gilardoni, I. F. Gonçalves, E. González-Romero, C. Guerrero, F. Gunsing, H. Harada, S. Heinitz, D. G. Jenkins, A. Junghans, F. Käppeler, Y. Kadi, A. Kimura, I. Knapová, Y. Kopatch, M. Krticka, D. Kurtulgil, I. Ladarescu, C. Lederer-Woods, H. Leeb, J. Lerendegui-Marco, S. J. Lonsdale, A. Manna, T. Martínez, A. Masi, C. Massimi, P. Mastinu, M. Mastromarco, E. A. Maugeri, A. Mazzone, E. Mendoza, A. Mengoni, P. M. Milazzo, F. Mingrone, J. Moreno-Soto, A. Musumarra, A. Negret, R. Nolte, F. Ogállar, A. Oprea, A. Pavlik, J. Perkowski, C. Petrone, L. Piersanti, E. Pirovano, I. Porras, J. Praena, J. M. Quesada, D. Ramos-Doval, T. Rauscher, R. Reifarth, D. Rochman, Y. Romanets, C. Rubbia, M. Sabaté-Gilarte, A. Saxena, D. Schumann, A. Sekhar, A. G. Smith, N. V. Sosnin, P. Sprung, G. Tagliente, J. L. Tain, A. Tarifeño-Saldivia, Th. Thomas, P. Torres-Sánchez, J. Ulrich, S. Urlass, S. Valenta, G. Vannini, V. Variale, P. Vaz, D. Vescovi, V. Vlachoudis, A. Wallner, P. J. Woods, T. Wright, and P. Žugec.Postprint (published version

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

Study of the fission cross-section of 230Th at the CERN n_TOF facility

Accurate data on neutron induced reactions on actinides is of considerable importance for the design of advanced nuclear systems and alternative fuel cycles. Specifically, 230Th is present in the thorium cycle produced from the decay of 234U. Thus, knowledge of the 230Th(n,f) reaction cross-section is strongly required. However, few experimental datasets exist in literature with large deviations among them, covering the energy range between 0.2 to 25 MeV. In addition, the study of fission cross-sections is of great interest in the research of the fission process related to the structure of the fission barriers. In the case of the thorium isotopes, narrow resonances and fine structures appear in the threshold region of the fission cross-section, known as the thorium anomaly. Specifically, previous measurements on the fission cross-section of 230Th revealed a main resonance at En =715 keV and additional fine structures, but with high discrepancies among the measured cross-section values. The scope of this work is to provide cross-section data for the 230Th(n,f) reaction, covering a wide energy range from the fission threshold up to 400 MeV. For this reason the experiment was performed in both experimental areas EAR-1 and EAR-2 of the CERN n_TOF facility. Seven high purity targets of the natural, unstable but very rare isotope 230Th, were produced at JRC-Geel in Belgium. The experimental setup was based on Micromegas detectors, while the 235U(n,f) cross-section was used as reference. A detailed description of the experimental setup, the analysis procedure, starting from the raw data to the final cross-section results, including the Monte-Carlo simulations performed and the theoretical study of the 230Th(n,f) reaction with the EMPIRE code are presented

Μελέτη της ενεργού διατομής σχάσης του 230Τh στα πλαίσια του πειράματος n_TOF στο CERN

Accurate data on neutron induced reactions on actinides is of considerable importance for the design of advanced nuclear systems and alternative fuel cycles. Specifically, 230Th is present in the thorium cycle produced from the decay of 234U. Thus, knowledge of the 230Th(n,f) reaction cross-section is strongly required. However, few experimental datasets exist in literature with large deviations among them, covering the energy range between 0.2 to 25 MeV. In addition, the study of fission cross-sections is of great interest in the research of the fission process related to the structure of the fission barriers. In the case of the thorium isotopes, narrow resonances and fine structures appear in the threshold region of the fission cross-section, known as the thorium anomaly. Specifically, previous measurements on the fission cross-section of 230Th revealed a main resonance at En =715 keV and additional fine structures, but with high discrepancies among the measured cross-section values. The scope of this work is to provide cross-section data for the 230Th(n,f) reaction, covering a wide energy range from the fission threshold up to 400 MeV. For this reason the experiment was performed in both experimental areas EAR-1 and EAR-2 of the CERN n_TOF facility. Seven high purity targets of the natural, unstable but very rare isotope 230Th, were produced at JRC-Geel in Belgium. The experimental setup was based on Micromegas detectors, while the 235U(n,f) cross-section was used as reference. A detailed description of the experimental setup, the analysis procedure, starting from the raw data to the final cross-section results, including the Monte-Carlo simulations performed and the theoretical study of the 230Th(n,f) reaction with the EMPIRE code are presented.Τα δεδομένα ενεργών διατομών για τις ακτινίδες παίζουν σημαντικό ρόλο στη μελέτη και το σχεδιασμό προηγμένων πυρηνικών συστημάτων παραγωγής ενέργειας και εναλλακτικών πυρηνικών κύκλων καυσίμων. Συγκεκριμένα, το 230Th συναντάται στον κύκλο του θορίου, όπου παράγεται από την αποδιέγερση του 234U. Για το λόγο αυτό απαιτείται η γνώση της ενεργού διατομή της αντίδρασης σχάσης 230Th(n,f). Μέχρι σήμερα, λίγα πειραματικά δεδομένα για την αντίδραση αυτή συναντώνται στη βιβλιογραφία, τα οποία καλύπτουν το ενεργειακό εύρος από 0.2 έως 25 MeV και παρουσιάζουν μεγάλες αποκλίσεις μεταξύ τους. Επίσης, η μελέτη των ενεργών διατομών αντιδράσεων σχάσης αποτελεί ένα μέσο για τη μελέτη της διαδικασίας της σχάσης και της δομής του δυναμικού της σχάσης. Όσο αναφορά τα ισότοπα του θορίου, παρουσιάζονται λεπτοί συντονισμοί στην περιοχή του κατωφλίου της σχάσης. Συγκεκριμένα, προηγούμενες μετρήσεις στην ενεργό διατομή σχάσης του 230Th έχουν αναδείξει ένα βασικό συντονισμό με ενέργεια Εn =715 keV με επιπλέον δομές, αλλά μεγάλες αποκλίσεις μεταξύ των υπάρχοντων πειραματικών ενεργών διατομών. Στόχος της παρούσας διδακτορικής διατριβής είναι η παροχή πειραματικών δεδομένων για την αντίδραση σχάσης του 230Th, καλύπτοντας το ενεργειακό εύρος από το κατώφλι της σχάσης μέχρι τα 400 MeV. Για το σκοπό αυτό πραγματοποιήθηκαν μετρήσεις στις πειραματικές περιοχές EAR-1 και EAR-2 της εγκατάστασης n_TOF στο CERN, με τη χρήση επτά στόχων υψηλής καθαρότητας του φυσικού και πολύ σπάνιου ισοτόπου 230Th, οι οποίοι κατασκευάστηκαν στο JRC-Geel στο Βέλγιο. Η πειραματική διάταξη βασίστηκε στους ανιχνευτές αερίου Micromegas, ενώ η αντίδραση 235U(n,f) χρησιμοποιήθηκε σαν αντίδραση αναφοράς. Η αναλυτική περιγραφή της πειραματικής διάταξης, της διαδικασίαςτης ανάλυσης των δεδομένων και των προσομοιώσεων Monte-Carlo που πραγματοποιήθηκαν ώστε να υπολογιστεί η ενεργός διατομή της αντίδρασης 230Th(n,f), καθώς και η θεωρητική μελέτη της αντίδρασης που υλοποιήθηκε με τον κώδικα EMPIRE, παρουσιάζονται στην παρούσα διδακτορική διατριβή