Resonant tunneling and the bimodal symmetric fission ofFm258

Abstract

The concept of resonant tunneling is invoked to explain the sharp drop in the measured spontaneousfission half-life when going from 256 Fm to 258 Fm. Various consequences of such a suggestion on the other observed characteristics of the bimodal symmetric fission of 258 Fm are briefly discussed.PACS numbers: 25.85.Ca, 27.90.+b Bimodal symmetric fission of the heaviest known elements has been of considerable interest 1 " 9 over the past few years. It exhibits two distinctly different components in the total-kinetic-energy (TKE) distributions of the fission fragments occurring concurrently in the symmetric mass division in the spontaneous fission of 258 Fm and of a few other elements in this region of the actinide nuclei. Such results are qualitatively understood in terms of two distinguishable modes of spontaneous fission, each of which is derived from the effects of the shell structure. The first mode corresponds to the effects of the shell structure in the parent fissioning nucleus and leads to fission along the so-called "old" valley resulting in liquid-drop-type elongated scission shapes similar to the scission configuration for the lighter actinides, with relatively low TKE values centered around 200 MeV and a broadly symmetric fission-fragment mass distribution. On the other hand, the second mode produces a sharply symmetric fission-fragment mass distribution with rather large values of the TKE of the fragments ( -235 MeV) approaching the Q value for the fission reaction. This mode is believed to occur because of the strong shell effects in the emerging fission fragments approaching the doubly magic l32 Sn nuclei with closed proton and neutron shells (Z=50, TV =82). Such fragment-shelldirected symmetric fission is found to occur along the so-called "new" valley leading to very compact scission configuration of two touching spheres. The two such valleys have been found in recent potential-energy-surface calculations 2,4 " 6 to be separated by a rather high ridge which serves to prevent reequilibration during the descent to scission. Recent results from the neutron multiplicity measurements 10 in the spontaneous fission of 260 Md provide rather conclusive evidence in support of explaining the bimodal symmetric fission in terms of two such valleys corresponding to distinctly different scission configurations.Spontaneous-fission properties of the actinide nuclei are known to vary fairly smoothly as one goes from uranium (Z =92) to fermium (Z = 100). An increase in the proton number of the fissioning nucleus leads to a gradual decrease in fission-fragment mass asymmetry, an increase in fission-fragment kinetic energies, and a decrease in fission half-lives. Except for the fission halflives these quantities vary fairly slowly with the neutron number of the fissioning nucleus. However, at 258 Fm there are sudden dramatic changes in all of these quantities. Fission becomes symmetric with a very narrow mass distribution, the average TKE values are about 35 MeV higher than in the asymmetric fission of 256 Fm, and th...

How an interacting many-body system tunnels through a potential barrier to open space

“…When the potential is unbound in one direction, the quantum nature of the systems allows them to overcome potential barriers for which they classically would not have sufficient energy and, as a result, a fraction of the many-particle system is emitted to open space. For example, in fusion, fission, photoassociation, and photodissociation processes, the energetics or life times are of primary interest (5)(6)(7)(8)(9). The physical analysis was made under the assumption that the correlation between decay products (i.e., between the remaining and emitted fractions of particles) can be neglected.…”

“…By finding the answers to these questions we will gain a deeper theoretical understanding of many-body tunneling, which is of high relevance for future technologies and applied sciences. In particular, this knowledge will allow us to determine whether the studied ultracold atomic clouds qualify as candidates for atomic lasers (11,(25)(26)(27) or as a toolbox for the study of ionization or decay processes (5)(6)(7)(8).…”

Tunneling Dynamics in Open Ultracold Bosonic Systems

“…With this very general condition for its occurrence, quantum tunneling is characteristic for many processes and is omnipresent in nature and its physical systems. It occurs in the field of nuclear physics of fusion [40] and fission [41], in the field of chemistry in photodissociation [42] as well as photoassociation [43], to name just a few. It is important to stress here, that almost all systems in which the tunneling phenomenon occurs are many-body systems which are, in principle, open.…”

“…The tunneling effect lies at the very heart of quantum mechanics (QM), as there is no classical analogon to it and there is plenty of realizations of it in nature: α-decay, fusion and fission in nuclear physics, photoassociation and photodissociation in biology and chemistry [37,[40][41][42][43] and many others more. The effect has been a matter of discussion since the advent of QM, see Refs [38,100,101] and a theoretical overview in Ref.…”

“…And how does the correlation (coherence) between the emitted particles and the source evolve? Finding the answers to these questions will allow for a deeper theoretical understanding of many-body tunneling and explains whether the studied ultracold atomic clouds qualify as candidates for atomic lasers [24,[102][103][104][105][106] or as a toolbox for the study of ionization or decay processes [37,[40][41][42]. Am …”