Nuclear-fission studies with relativistic secondary beams: Analysis of fission channels

Abstract

Nuclear fission of several neutron-deficient actinides and pre-actinides from excitation energies around 11 MeV was studied at GSI Darmstadt by use of relativistic secondary beams. The characteristics of multimodal fission of nuclei around 226 Th are systematically investigated and interpreted as the superposition of three fission channels. Properties of these fission channels have been determined for 15 systems. A global view on the properties of fission channels including previous results is presented. The positions of the asymmetric fission channels are found to be constant in element number over the whole range of systems investigated.2 data of 15 of the systems, which show features of multi-modal fission. The systematic survey of fissioning systems in the transition from single-humped to double-humped element distributions around 226 Th extends the systematic view on how the intensities and other relevant parameters of the fission channels vary as a function of the nuclear composition of the fissioning nucleus. EXPERIMENTAt GSI Darmstadt, a new technique to investigate low-energy fission has been developed [4,5]. Relativistic secondary projectiles are produced via fragmentation of a 1 A GeV primary beam of 238 U and identified in nuclear mass and charge number by the fragment separator FRS [6]. In a dedicated experimental set-up, the giant resonances, mostly the giant dipole resonance, are excited by electromagnetic interactions in a secondary lead target, and fission from excitation energies around 11 MeV is induced. The fission fragments are identified in nuclear charge, and their velocity vectors are determined. From these data, the element yields and the total kinetic energies are deduced. Details of the experimental technique are given elsewhere [1].

Evolution of isotopic fission-fragment yields with excitation energy

“…At higher bombarding energy, pre-equilibrium particles emission may occur before the formation of the compound nucleus and consequently, the different compound nuclei are produced over a range of A and E * . Fission at a low excitation energy induces the hump around Z ≈ 54 from shell effects which stabilise the mass and atomic-number distributions of heavy fission fragments [10]. This trend is confirmed by data from spallation-fission reactions performed at GSI [11], for 238 U at 1 A GeV impinging on a deuterium target (squares).…”

Neutron-induced astrophysical reaction rates for translead nuclei

“…The deformation energies of the fragments were assumed to be specific to the individual fission channels. They were deduced from experimental data (Wahl 1988;Böckstiegel et al 2008, and references therein) on total kinetic energies and neutron yields. Kinetic energies were then calculated applying the energy conservation law.…”

Modelling the widths of fission observables in GEF

“…The mass distributions of the asymmetric fission channels, Standard 1 and Standard 2, which are distributed to shell effects, are much narrower: σ A is around 3, respectively 5 mass units [15]. The widths increase slightly with growing initial excitation energy of the fissioning system.…”

“…A value of σ Z = 4 units was found, which is nearly constant within the experimental uncertainties in the range from 212 Ac to 226 Th for excitation energies of a few MeV above the fission barrier. From this value one can deduce a value of σ A = 10 mass units for the mass width of the symmetric channel [14,15]. There are no experimental data available on the evolution of the width with excitation energy for the symmetric fission channel in low-energy fission.…”