Isotopic fission fragment distributions as a deep probe to fusion-fission dynamics

Abstract

Abstract. During the fission process, the atomic nucleus deforms and elongates up to the two fragments inception and their final separation at the scission deformation. The evolution of the nucleus energy with deformation defines a potential energy landscape in the multidimensional deformation space. It is determined by the macroscopic properties of the nucleus, and is also strongly influenced by the single-particle structure of the nucleus, which modifies the macroscopic energy minima. The fission fragment distribution is a direct consequence of the deformation path the nucleus has encountered, and therefore is the most genuine experimental observation of the potential energy landscape of the deforming nucleus. Very asymmetric fusionfission reactions at energy close to the Coulomb barrier, produce well-defined conditions of the compound nucleus formation, where processes such as quasi-fission, pre-equilibrium emission and incomplete fusion are negligible. In the same time, the excitation energy is sufficient to reduce significantly structural effects, and mostly the macroscopic part of the potential is responsible for the formation of the fission fragments. We use inverse kinematics combined with a spectrometer to select and identify the fission fragments produced in 238 U+ 12 C at a bombarding energy close to and well-above the Coulomb barrier. For the first time, the isotopic yields are measured over the complete atomic-number distribution, between Z=30 and Z=63. In the experimental set-up, it is also possible to identify transfer-induced reactions, which lead to low-energy fission where the nuclear shell structure shows a strong influence on the fission-fragment distributions. The resulting set of data gives the possibility to observe the fission fragment properties over a wide range of excitation energy, and they reveal the vanishing of the shell effects in the potential energy of the fissioning nucleus, as well as the influence of fission dynamics. IntroductionFragment mass distributions produced in low excitation-energy fission of trans-uranium actinides are known for a long time to present in general a double-humped structure, the fragments being divided in two distinct groups, a heavy-fragment group and a light-fragment one [1]. With increasing mass of the fissioning nucleus, the heavy-fragment group shows a constant average mass, close to the value A=140, whereas the average mass of the light-fragment group increases. This double-humped structure has been understood as a signature for shell-structure influence in the potential energy describing the deforming nucleus from the ground state to the scission point deformation. The constant position of the heavy fragment distribution lead to the comprehension that the shell influence is in fact the single-particle structure inside the nascent fragments, and not in the fissioning nucleus. The most spread model for the description of the fission fragment distribution is based on the macro-microscopic description of the fissioning nucleus potential energy [2, 3...

Complete characterization of the fission fragments produced in reactions induced byPb208projectiles on proton at500AMeV

“…Recently, new experiments at GANIL also took advantage of the inverse kinematics technique together with transfer-and fusion-induced fission reactions [41][42][43]. In these experiments transfer reactions were identified and reconstructed with an annular segmented silicon detector [43].…”

Proton-induced fission cross sections onPb208at high kinetic energies

“…Following its discovery 76 years ago, a complete microscopic description of the fission process is not yet available. Significant progress was achieved in the last years by new microscopic descriptions of the nuclear potential governing the fission process [1,2], detailed measurements of fission cross sections [3][4][5][6], and the characteristics of the final fission fragments [7][8][9][10].…”