Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions

Abstract

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of 48 Ca, 50 Ti, and 54 Cr with 208 Pb. Moving from 48 Ca to 54 Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured massangle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P CN (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for 50 Ti and 15 for 54 Cr in comparison to 48 Ca. The energy dependence of P CN indicates that cold fusion reactions (involving 208 Pb) are not driven by a diffusion process.

Deformed shell effects in Ca48+Bk249 quasifission fragments

“…= 0 (forward angles) to 180 degrees (backward angles). This wide angular distribution motivates the development of larger angular acceptance detectors [9,10]. Note that each angular momentum range leads itself to a broad distribution of angles.…”

“…Quasifission occurs when the collision of two heavy nuclei produces two fragments with similar characteristics to fusionfission fragments, but without the intermediate formation of a fully equilibrated compound nucleus [1][2][3][4]. It is the main mechanism that hinders fusion of heavy nuclei and consequently the formation of superheavy elements [5][6][7][8][9][10]. It is thus crucial to achieve a deeper insight of quasifission in order to minimize its impact and maximize the formation of compound nuclei for heavy and superheavy nuclei searches.…”

Search for elements 119 and 120

“…From a nuclear reaction point of view (σ cap and P CN ), one can assume that the present 50 Ti + 249 Bk and 50 Ti + 249 Cf reactions are similar to the 48 Ca-induced reactions because of the target deformation. There should be some deviations due to the influence of the nuclear structure of the reactants, which largely impacts fusion-evaporation cross sections in Pb-target based reactions [60,[63][64][65] but is not yet fully understood in reactions with deformed target nuclei [66,67]. Nevertheless, broad ER excitation functions of the present reactions, similar to the ones for 48 Ca-induced reactions, can be assumed.…”

Fusion Dynamics of Low-Energy Heavy-Ion Collisions for Production of Superheavy Nuclei

“…Experimentally, the measurement of fusion probability is required to distinguish quasifission between fusion-fission and fast fission [71][72][73][74][75][76]. The experimental characteristics of the quasifission process are different from the fusion-fission process [77]. Therefore, it is important to distinguish the fusion and quasifission fragments for a better understanding of the fusion mechanism.…”