Nuclear fission: a review of experimental advances and phenomenology

Abstract

In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies. This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams. The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and γ-ray emission will also be reviewed. A particular attention will be given to the low-energy fission in the so far scarcely explored nuclei in the very neutron-deficient lead region. They recently became the focus for several complementary experimental studies, such as β-delayed fission with radioactive beams at ISOLDE(CERN), Coulex-induced fission of relativistic secondary beams at FRS(GSI), and several prompt fusion-fission studies. The synergy of these approaches allows a unique insight in the new region of asymmetric fission around [Formula: see text]Hg, recently discovered at ISOLDE. Recent extensive theoretical efforts in this region will also be outlined. The unprecedented high-quality data for fission fragments, completely identified in Z and A, by means of reactions in inverse kinematics at FRS(GSI) and VAMOS(GANIL) will be also reviewed. These experiments explored an extended range of mercury-to-californium elements, spanning from the neutron-deficient to neutron-rich nuclides, and covering both asymmetric, symmetric and transitional fission regions. Some aspects of heavy-ion induced fusion-fission and quasifission reactions will be also discussed, which reveal their dynamical features, such as the fission time scale. The crucial role of the multi-chance fission, probed by means of multinucleon-transfer induced fission reactions, will be highlighted. The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising 'next-generation' fission approaches, which might become available within the next decade.

Semiclassical origin of asymmetric nuclear fission: nascent-fragment shell effect in periodic-orbit theory

“…In Fig. 8, the Fourier peak of the quantum level density at the lengths of the prefragment diameter (2,1) and triangle (3,1) orbits are compared with the semiclassical amplitudes derived in [31], according to Eq. (21).…”

“…The solid (red) curves represent the quantum results, and the long-dashed (blue) curves represent the results of trace formula(10) and(15). The short-dashed (green) curves show the contribution of the triangle orbit family(3,1) in the prefragments (discussion will be conducted in Sec. 4.2).…”

First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments

“…One of the many open questions regarding the fission process is related to the origin of the angular momentum of the fission fragments [1]. It is thus desirable to obtain information on the angular momentum of the fragments as this can provide insight into the properties of the dynamical evolution of the fissioning nucleus from the saddle point until its descent to scission [2].…”

“…As pointed out in a recent review by Andreyev et al [1], although it is well established that the fission fragments carry a considerable amount of angular momentum [7], it is still not fully comprehended how this is generated. Competing theories on this issue exist, which among others hypothesize thermal excitation [8][9][10][11] and/or quantum-mechanical uncertainty of angular-momentum-bearing modes [12] and Coulomb excitation after scission [13].…”

Study of fission using multi-nucleon transfer reactions

“…Recently, β/EC delayed fission was investigated for the very proton-rich nucleus using radioactive beams, and 180 Hg was found to show an asymmetric fission as a new region of mass-asymmetric fission [3]. Advancement of new technique for fission studies is reviewed in [4].…”