Session L04: Superheavy Elements and Fission

Survival of excited, heavy compound nuclei

A number of recent searches for new elements with atomic numbers 119 and 120 by major laboratories worldwide have not reported any decay chains that are widely accepted in the community.  This has led to a discussion of the factors that influence the survival of excited, heavy compound nuclei.  The Cyclotron Institute at Texas A&M University has a longstanding program to study the factors that influence survival, and a number of experiments to produce nearly spherical compound nuclei near ²⁰⁸Pb have been conducted.  These experiments demonstrate the critical role of the difference in the neutron binding energy and the fission barrier in determining the final reaction cross section.  This talk will discuss these experiments and the newest results on excitation functions measured in the ⁴⁴Ca + ^{154,156,157,160}Gd and ⁴⁸Ti + ^158,160Gd reactions.  The latter reactions also allow for the investigation of the role of deformation on survival.  This talk will also discuss upgrades to the maximum magnetic rigidity and the data acquisition electronics of the AGGIE gas-filled separator, and future plans.   

Barrier distribution and excitation function measurements of the 51V + 159Tb fusion reaction for estimating the optimal reaction energy for the synthesis of element 119

We, the nSHE Research Group, are searching for new elements beyond element 118 (oganesson), i.e. element 119, at RIKEN. The probability of producing superheavy elements by fusion reactions is extremely low. Therefore, it is crucially important to determine the optimal experimental conditions to maximize the production rate, especially by predicting the optimal incident energy that maximizes the cross section. For this purpose, we have been developing a method to estimate the optimal energy based on the experimental data of quasielastic (QE) barrier distributions [1,2]. In this study, we measured the QE barrier distribution and excitation function of the evaporation-residue cross section for the ⁵¹V+¹⁵⁹Tb fusion reaction at the RIKEN superconducting heavy-ion linear accelerator facility (SRILAC [3]). This reaction system was chosen because ¹⁵⁹Tb has a large quadrupole deformation similar to that of ²⁴⁸Cm used in the search for element 119. The comparison of the QE barrier distribution and the excitation function was used to clarify the fusion reaction mechanism involving the deformed nuclei and to improve the accuracy of the method for estimating the optimal incident energy. In this presentation, we will show preliminary results of the measurement.   

Energy Dependence of Fission Product Yields

Under a joint collaboration between LANL-LLNL-TUNL, a consistent set of absolute energy dependent fission product yield measurements has been performed. For ²³⁵U, ²³⁸U and ²³⁹Pu, between 0.5 and 14.8 MeV, the energy dependence of a select number of cumulative fission product yields (FPY) have been measured using quasi-monoenergetic neutron beams at the Triangle Universities Nuclear Laboratory (TUNL). The FPYs were measured by a combination of fission counting using specially designed dual-fission chambers and γ-ray counting. Each dual-fission chamber is a back-to-back ionization chamber encasing an activation target in the center with thin deposits of the same target isotope in each chamber. This method allows for the direct measurement of the total number of fissions in the activation target with no reference to the fission cross-section, thus reducing uncertainties. Reported are absolute cumulative fission product yields for incident neutron energies of 0.5 - 14.8 MeV in nearly 1 MeV steps. New data in the second chance fission region of 5.5 – 11 MeV have recently been published.    

Studying Fission Cross Sections near 198Pb with AT-TPC at FRIB

In the region of the neutron deficient pre-actinides, around ¹⁹⁸Pb and ¹⁸⁰Hg, there is an unexpected island of asymmetric fission. To develop a method to determine fission barriers for rare isotopes and to study the transition between symmetric and asymmetric fission near ¹⁹⁸Pb, rare isotopes were produced in the A1900 separator at the NSCL and fused with ⁴He target nuclei in the Active Target Time Projection Chamber (AT-TPC). In the experiment, the incident rare isotope beam particles were isotopically identified by the HEavy ISotope Tagger (HEIST), which allowed for the identification of the fissioning nucleus on an event by event basis. A combination of analytical and machine learning methods provided the identification of fission events. The stopping power of the beam particles in the helium gas has been measured by analyzing the Z and velocity of fission fragments at large folding angles. These experimental techniques and the energy dependence of isotopically resolved fusion fission cross-sections will be presented. Preliminary fission barriers extracted from the cross sections will also be presented.   

How well can a multi-task learning model extract collective coordinates for nuclear fission?

Describing nuclear fission with a microscopic theory remains one of the most important and challenging problems in nuclear theory. A conventional approach involves a use of empirical parameters, such as quadrupole moments, for collective coordinates. However, dynamics of a nucleus does not necessarily select these parameters, and thus they do not guarantee an accurate description of fission. Current challenges persist, particularly in accurately reproducing observables such as half-lives with low-order multiple-pole moments. In this presentation, we will propose a novel method for deriving collective coordinates with multi-task learning, that is a kind of deep learning. This method involves extracting collective information from randomly generated nuclear density and energy data using Density Functional Theory (DFT), and it is independent of empirical coordinates. By implementing this method, we successfully extracted a minimal number of degrees of freedom that aptly describe the deformation dynamics with imposed axial symmetry of states near the ground state of 236U. We will show that Q20 and Q30, which have been often employed for fission study, do not contain as much information as one would have thought. This presentation will delve into this innovative approach and the nature of these new coordinates.   

Branching Ratio Measurements of the Harvested Isotope 111Ag

Our understanding on the distribution fragment masses following fission, or fission yields, impacts many different applications including the estimation of decay heat in nuclear reactors, reactor-neutrino studies, radio-isotope production for medical applications, and stockpile stewardship. Determining the isotopes produced in fission can be done in a straightforward way by measuring the characteristic γ rays emitted during the β decay of fission products. However, much of the nuclear data on fission-product β decay contain high uncertainties, and we aim to improve these data to better inform applications and basic science. Using an experimental method that takes advantage of radioactive ions beam at the CARIBU facility and a very well characterized detection system at Texas A&M, we recently performed a measurement of the ¹¹¹Ag isotope to a high degree of precision. We will present our branching ratio results and will also discuss future plans to continue these types of measurements at LLNL.    

Water Coordination Chemistry of Nobelium and Lawrencium

SHE (Super Heavy Elements, Z>103) chemistry is of great interest due to the impact of relativistic effects. These changes can lead to behavior that deviates from expected. Investigating the chemistry of these elements experimentally is challenging as they must be produced one-atom-at-a-time at particle accelerators and live for seconds or less before decaying away. This creates a difficulty in characterizing the complexes produced in chemical reactions with SHEs. At Lawrence Berkeley National Laboratory 88 - Inch Cyclotron facility, FIONA (For the Identification Of Nuclide A) measures the mass of the SHE’s complex produced in our system. These mass characterizations give us information about complex formation, coordination spheres, and oxidation states. We will present on experiments performed with  254No and 255Lr, through the reactions 208Pb(48Ca, 2n)254No and 209Bi(48Ca, 2n)255Lr respectively. These elements were reacted with water, and the coordination species observed will give insight into the bonding character of these nuclides. Calculations are also produced that will give us further information of the electronic configuration in their bonding configurations. Analysis and results of these will be presented.