Key words
experimental nuclear physics (nucl-ex), reaction mechanisms, fission, transfer reactions, surrogate method, fusion, actinides, shell structure, super heavy elements
Reaction mechanisms in the superheavy element region
The quest for new elements is a powerful engine to stretch the limits of our knowledge in nuclear physics. So far, the heaviest confirmed element is Livermorium (Z = 116) and the heaviest claimed one is Z = 118. Methods used to produce them involve nuclear physics processes. The TASCA collaboration have been playing a leading role in the field of super-heavy elements (Z > 104). We have recently confirmed the existence of new elements Z = 115 and Z = 117. Furthermore, tremendous efforts using state-of-the-art apparatus and techniques (gas-filled separator, digital electronics) were deployed to produce, for the first time, the elements Z = 119 and Z = 120. After months of experiments with TBs of data accumulated, only a few nuclei may have been produced.
During the experiments, many nuclear reactions occurred, mostly transfers ones, leading to the production of already known nuclei. So far, these data were considered as background and of lower interest. The “Super-Heavy Element Chemistry” group at GSI, core of the TASCA collaboration, is now investigating these data in order to understand the nuclear reaction mechanisms involved. This may lead to a better understanding of the reactions leading to new super-heavy elements. It may also allow better theoretical predictions to be made.
Fission studies in the neutron-deficient Pb region
Nuclear fission is a process which allows, under certain conditions, a given nucleus to split into two fragments. The way this nucleus splits (symmetrically or asymmetrically) depends on its excitation energy and its inner structure. Nuclei generally fission symmetrically, except in the actinide region (Z = 89-103). Beta-delayed fission is a very rare process (concerning only a couple of known nuclei) which allows a nucleus to beta-decay (where the nucleus transforms one of its protons into a neutron, or vice-versa, to reach a higher stability) and then fission spontaneously. Recent experiments using beta-delayed fission at CERN/ISOLDE have demonstrated the unexpected asymmetric fission of 180Hg (A. Andreyev et al., Phys. Rev. Lett. 105, 252502 (2010)).
In order to understand this observation and investigate the inner structure of the nuclei is this mass region, we performed an experiment with Dr Katsuhisa Nisho (ASRC/JAEA) and Pr Andrei Andreyev (University of York) at the ASRC/JAEA Tandem (Tokai-Mura, Japan) to study the fission of 180,184,190,194Hg and 178,182Pt induced by complete fusion.
Limits of the surrogate method with transfer reactions
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In order to improve our understanding of the fission process, and thus obtaining the data required to manage nuclear waste and design the next generation of reactors, new experimental methods have to be developed. The surrogate method (J. D.Cramer and H. C. Britt, Phys. Rev. C 2, 2350-2358 (1970)) appeared in the 1970’s to study the nuclei which are not accessible to neutron-induced fission (due to their lifetime and radioactivity). One of the key points is to investigate the limits of this method, by extending the range of nuclear reactions used and by taking advantage of the new detectors and apparatus available nowadays. Several experiments were performed at GANIL (CEA/DSM-CNRS/IN2P3, Caen, France), using multi-nucleon transfer reactions in inverse kinematics between an 238U beam and a 12C target (X. Derkx, Ph.D. thesis, Université de Caen, (2010)).