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Measuring the Fission Barrier of 254No Gregoire Henning

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Measuring the Fission Barrier of 254No Gregoire Henning Stability of Transfermium Elements at High Spin : Measuring the Fission Barrier of 254No Gregoire Henning To cite this version: Gregoire Henning. Stability of Transfermium Elements at High Spin : Measuring the Fission Bar- rier of 254No. Other [cond-mat.other]. Université Paris Sud - Paris XI, 2012. English. NNT : 2012PA112143. tel-00745915 HAL Id: tel-00745915 https://tel.archives-ouvertes.fr/tel-00745915 Submitted on 26 Oct 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés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bstract Super heavy nuclei provide opportunities to study nuclear struc- ture near three simultaneous limits: in charge Z, spin I and excita- tion energy E∗. These nuclei exist only because of a fission barrier, created by shell effects. It is therefore important to determine the fission barrier and its spin dependence Bf (I), which gives informa- tion on the shell energy Eshell(I). Theoretical calculations predict different fission barrier heights from Bf (I =0)=6.8 MeV for a macro-microscopic model [1, 2] to 8.7 MeV for Density Functional Theory calculations using the Gogny or Skyrme interactions [3–5]. Hence, a measurement of Bf provides a test for theories. To investigate the fission barrier, an established method is to measure the rise of fission with excitation energy, characterised by the ratio of decay widths Γfission/Γtotal, using transfer reactions [6,7]. However, for heavy elements such as 254No, there is no suitable tar- get for a transfer reaction. We therefore rely on the complementary decay widths ratio Γγ/Γfission and its spin dependence, deduced from the entry distribution (I,E∗). Measurements of the gamma-ray multiplicity and total energy for 254No have been performed with beam energies of 219 and 223 MeV in the reaction 208Pb 48Ca, 2n at ATLAS (Argonne Tandem Linac Accelerator System). The 254No gamma rays were detected using the Gammasphere array as a calorimeter – as well as the usual high- resolution γ-ray detector. Coincidences with evaporation residues at the Fragment Mass Analyzer focal plane separated 254No gamma rays from those from fission fragments, which are > 106 more in- tense. From this measurement, the entry distribution – i.e. the initial distribution of I and E∗ – is constructed. Each point (I,E∗) of the entry distribution is a point where gamma decay wins over 3 fission and, therefore, gives information on the fission barrier. The measured entry distributions show an increase in the max- imum spin and excitation energy from 219 to 223 MeV of beam energy. The distributions show a saturation of E∗ for high spins. The saturation is attributed to the fact that, as E∗ increases above the saddle, Γfission rapidly dominates. The resulting truncation of the entry distribution at high E∗ allows a determination of the fis- sion barrier height. The experimental entry distributions are also compared with en- try distributions calculated with decay cascade codes which take into account the full nucleus formation process, including the cap- ture process and the subsequent survival probability as a function of E∗ and I. We used the KEWPIE2 [8] and NRV [9] codes to sim- ulate the entry distribution. 4 Stabilité des Éléments Trans-fermium à Haut Spin : Mesure de la Barrière de Fission de 254No Les noyaux super lourds offrent la possibilité d’étudier la structure nucléaire à trois limites simultanément: en charge Z, spin I et énergie d’excitation E∗. Ces noyaux existent seulement grâce à une barrière de fission créée par les effets de couche. Il est donc important de déterminer cette barrière de fission et sa dépendance en spin Bf (I), qui nous renseigne sur l’énergie de couche Eshell(I). Les théories prédisent des valeurs différentes pour la hauteur de la barrière de fission, allant de Bf (I =0)=6.8 MeV dans un modèle macro- microscopique [1, 2] à 8.7 MeV pour des calculs de la théorie de la fonctionnelle de la densité utilisant l’interaction Gogny ou Skyrme [3–5]. Une mesure de Bf fournit donc un test des théories. Pour étudier la barrière de fission, la méthode établie est de mesurer, par réaction de transfert, l’augmentation de la fission avec l’énergie d’excitation, caractérisée par le rapport des largeurs de décroissance Γfission/Γtotal, [6,7]. Cependant, pour les éléments lourds comme 254No, il n’existe pas de cible appropriée pour une réaction de transfert. Il faut s’en remettre à un rapport de largeur de décrois- sance complémentaire: Γγ/Γfission et sa dépendance en spin, déduite de la distribution d’entrée (I,E∗). Des mesures de la multiplicité et l’énergie totale des rayons γ de 254No ont été faites aux énergies de faisceau 219 et 223 MeV pour la réaction 208Pb 48Ca, 2n à ATLAS (Argonne Tandem Linac Ac- celerator System). Les rayons γ du 254No ont été détectés par le multi-détecteur Gammasphere utilisé comme calorimètre – et aussi 5 comme détecteur de rayons γ de haute résolution. Les coïncidences avec les résidus d’évaporation au plan focal du Fragment Mass An- alyzer ont permis de séparer les rayons γ du 254No de ceux issus de la fission, qui sont > 106 fois plus intenses. De ces mesures, la distribution d’entrée – c’est-à-dire la distribution initiale en I et E∗ – est reconstruite. Chaque point (I,E∗) de la distribution d’entrée est un point où la décroissance γ l’a emporté sur la fission, et ainsi, contient une information sur la barrière de fission. La distribution d’entrée mesurée montre une augmentation du spin maximal et de l’énergie d’excitation entre les énergies de fais- ceau 219 et 223 MeV. La distribution présente une saturation de E∗ à hauts spins. Cette saturation est attribuée au fait que, lorsque E∗ augmente au-dessus de la barrière, Γfission domine rapidement. Il en résulte une troncation de la distribution d’entrée à haute énergie qui permet la détermination de la hauteur de la barrière de fission. La mesure expérimentale de la distribution d’entrée est également comparée avec des distributions d’entrée calculées par des simula- tions de cascades de décroissance qui prennent en compte le pro- cessus de formation du noyau, incluant la capture et la survie, en fonction de E∗ et I. Dans ce travail, nous avons utilisé les codes KEWPIE2 [8] et NRV [9] pour simuler les distributions d’entrée. 6 7 8 On a qu’à parler avec une robe et un bonnet, tout galimatias devient savant, et toute sottise devient raison. Molière Le Malade imaginaire, Acte III, Scène 14. 10 Foreword Reaching the end of those three years of work, I can say that I met a lot of great people. I enjoyed working with them and in the process, gained colleagues and, dare I say, friends. Ifirstwanttogivethebiggestthankstomysupervisors:WaelyandTengLek.Icould not have dreamed of anybody better to help me during those three years. They let me all the autonomy I needed to work while guiding me and being available for all my questions. Thank you to Silvia Leoni and Peter Reiter for accepting to review my (long) manuscript during the summer. Thanks to Philippe Quentin for being part of the defense jury and to Elias Khan for presiding it. I am grateful to all the jury members who asked challenging questions and provided insightful comments that will help move the analysis to even more significant results. Thanks to David Potterveld for helping with the experiment data that would have been lost without him. Thank you to Darek Seweryniak, Mike Carpente, Torben Lauritsen and Shaofei Zhu for their support and involvement in the experiment and the analysis. Thank you to Neil Rowley and David Boiley for their interest in my work and fruitful discussions. Thank you to Peter Reiter for providing many information on the previous experiment. Thanks to Andreas Heinz, with whom I learnt a lot about nuclear physics, and who provided comments and encouragement. I thank all my colleagues at Argonne National Lab.: Andrew, Ben, Birger, Calem, Chris, Chitra, Darek, David, Filip, Gulan, Kim, Libby, Martin, Mike, Peter, Robert and Shaofei, and at CSNSM: Carole, Catherine, Dave, Georges, Georgi, Joa, Karl, Pierre and Stephanie and specially my office-mates and fellow thésards: Asenath, Asli, Cédric, Christophe, Meng. Thank you to the administrative staff: Annie, Christine, Patricia, Réjane and Sonia in Orsay and Coleen, Barbara and Janet at Argonne, for their essential support.
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