Session FG: Mini-Symposium on Super Heavy and Heavy Elements II

New information on survival probabilities in hot fusion reactions

Recently we studied the fission-neutron emission competition in highly excited (E$^{\ast}$ = 63 MeV) $^{274}$Hs (Z=108) (where the fission barrier is due to shell effects) formed by a hot fusion reaction. At this excitation energy, these shell effects are expected to be ``washed out'' leaving a barrier height of $\leq$ 1 MeV. Matching cross bombardments ($^{26}$Mg + $^{248}$Cm and $^{25}$Mg + $^{248}$Cm) were used to identify the properties of first chance fission of $^{274}$Hs. A Harding-Farley analysis of the fission neutrons emitted in the $^{25,26}$Mg + $^{248}$Cm reaction was performed to identify the pre- and post-scission components of the neutron multiplicities in each system. ($\Gamma$$_{n}$/$\Gamma$$_{t}$) for the first chance fission of $^{274}$Hs is 0.89 $\pm$ 0.13, i.e., $\sim$ 90\% of the highly excited nuclei survive. The high value of that survival probability is due to dissipative effects (Kramers) during de-excitation. A proper description of the survival probabilities of excited superheavy nuclei formed in hot fusion reactions requires consideration of both dynamic and static (shell-related) effects. A re-analysis of several hot fusion survival probabilities under these constraints is presented.   

Isospin Dependence of Quasifission

Past successes in the formation of superheavy elements have been the result of fusion reactions. These reactions, however, are hindered by orders of magnitude by quasifission. The probability for quasifission to occur depends on many reaction conditions, including isospin. Previous attempts to understand the isospin dependence have resulted in conflicting conclusions. The prevalence of the quasifission reaction channel was systematically investigated through a series of reactions of Cr beams with W targets at energies above the fusion barrier at the Australia National University. The mass angle distributions of the fission-like fragments, known to be extremely sensitive to the quasifission process, were measured. The mass widths could not be explained by theoretical fusion-fission mass widths alone. It is shown that quasifission is a prominent reaction channel in these reactions. The likelihood of the quasifission reaction channel decreased with increasing neutron richness of the compound nucleus. This result supports the use of radioactive beams in future superheavy element production reactions.   

Microscopic analysis of fusion hindrance in heavy systems

It is well known in fusion reactions of heavy systems that the fusion hindrance occurs where the fusion probability is strongly hindered around the Coulomb barrier energy, compared with lighter systems. Quasi-fission process is considered to be mostly responsible for this hindrance. Recently, we proposed a method to extract nucleus-nucleus potential and one-body energy dissipation from the relative motion of colliding nuclei to nuclear intrinsic excitations in fusion reactions from time-dependent Hartree-Fock calculations [1,2]. In this contribution, we apply the above method to heavy systems, $^{96}$Zr+$^{124,132}$Sn, $^{136}$Xe and $^{70}$Zn+$^{208}$Pb, and analyze the property of nucleus-nucleus potential and energy dissipation. We show that the obtained potentials show a disappearance of a barrier and monotonic increase at short relative distances, which are different from lighter systems. We also analyze energy dissipation for heavy systems and show that origins of fusion hindrance come mainly from a dynamical increase in extracted potentials at short distances.\\[4pt] [1] K. Washiyama and D. Lacroix, Phys. Rev. C {\bf 78}, 024610 (2008).\\[0pt] [2] K. Washiyama, D. Lacroix, and S. Ayik, Phys. Rev. C {\bf 79}, 024609 (2009).   

Towards Identification of Super Heavy Elements by Means of Mass Spectroscopy

The present standard technique for determining the identity of Super Heavy Elements is by alpha-decay spectroscopy, wherein chains of alpha-decays to well-known species provide unique fingerprints to identify the parent nucleus. However, as advances in production capabilities bring us closer to the much-anticipated ``island of stability,'' decay spectroscopy will become less tenable. It is already seen that the heaviest elements, those above Z=113, decay chains all terminate in spontaneous fission before reaching well-known nuclei. As the island of stability is more closely approached, alpha-decay will be replaced by beta-decay and spontaneous fission while half-lives become exceedingly long. To work towards overcoming the looming limitations in identification via decay spectroscopy, we have installed a multi-reflection time-of-flight mass spectrograph coupled to the GARIS-II separator at RIKEN. The device has been proven to be highly efficient and capable of accurate high-precision mass measurements [1]. In initial studies we will aim to make precision mass measurements of trans-uranium elements up through Lr to validate the device. We will describe the progress of this project and describe the long-range strategy. \\[4pt] [1] P. Schury et al., Nucl. Instrum. Meth. B 335, 39 (2014).   

Energy Dependence of Neutron-Induced Fission Product Yields of $^{235}$U, $^{238}$U and $^{239}$Pu Between 0.5 and 14.8 MeV

Under a joint collaboration between TUNL-LANL-LLNL, a set of absolute fission product yield measurements have been performed. The energy dependence of a number of cumulative fission products between 0.5 and 14.8 MeV have been measured using quasi-monoenergetic neutron beams for three actinide targets, $^{235}$U, $^{238}$U and $^{239}$Pu, between 0.5 and 14.8 MeV. The FPYs were measured by a combination of activation utilizing specially designed dual-fission chambers and $\gamma$-ray counting. The dual-fission chambers are back-to-back ionization chambers encasing a target with thin deposits of the same target isotope in each chamber. This method allows for the direct measurement of the fission rate in the activation target with no reference to the fission cross-section, reducing uncertainties. $\gamma$-ray counting was performed on well-shield HPGe detectors over a period of 2 months per activation to properly identify fission products. Reported are absolute cumulative fission product yields for incident neutron energies of 0.5, 1.37, 2.4, 4.6 and 14.8 MeV.   

Fragment Angular Distributions in Neutron-Induced Fission of 235U and 239Pu using a Time Projection Chamber

Fission fragment angular distributions can lend insights into fission barrier shapes and level densities at the scission point, both important for fission theory development. Fragment emission anisotropies are also valuable for precision cross section ratio measurements, if the distributions are different for the two isotopes used in the ratio. Available angular data is sparse for 235U and even more so for 239Pu, especially at neutron energies above 5 MeV. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) time projection chamber, which enables precise tracking of charged particles, can be used to study angular distributions and emission anisotropies of fission fragments in neutron-induced fission. Analysis of in-beam data collected at the Los Alamos Neutron Science Center with a 239Pu/235U target will provide angular distributions as a function of incident neutron energy for these isotopes. Preliminary angular distributions for 235U and 239Pu using the NIFFTE time projection chamber will be presented.   

Fission cross section uncertainties with the NIFFTE TPC

Nuclear data such as neutron-induced fission cross sections play a fundamental role in nuclear energy and defense applications. In recent years, understanding of these systems has become increasingly dependent upon advanced simulation and modeling, where uncertainties in nuclear data propagate in the expected performances of existing and future systems. It is important therefore that uncertainties in nuclear data are minimized and fully understood. For this reason, the Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) uses a Time Projection Chamber (TPC) to measure energy-differential (n,f) cross sections with unprecedented precision. The presentation will discuss how the capabilities of the NIFFTE TPC allow to directly measures systematic uncertainties in fission cross sections, in particular for what concerns fission-fragment identification, and target and beam uniformity. Preliminary results from recent analysis of 238U/235U and 239Pu/235U data collected with the TPC will be presented. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Effects of fission fragments on the angular distribution of scission particles

The angular distribution of the scission neutron is a key to separate it from post-scission neutrons. We investigate the effects of the fission fragments on the angular distribution of scission particles, i.e., neutrons and protons. The time evolution of the wave function of the scission particle is obtained by integrating the time-dependent Schrodinger equation. The effects of the re-absorption and scattering by the fission fragments are taken into account by means of the optical potential. We investigate the fission of 236U that corresponds to the neutron induced fission of 235U as an example. The angular distribution of the scission neutron is found to be strongly modified by the presence of the fragments. The attractive nuclear potential enhances the yields around 0 and 180 degrees, while the absorptive potential diminishes them. We also investigate the angular distribution of scission protons, which are normally supposed to be focused around 90 degrees due to the Coulomb repulsion from the fragments. It has been found, contrary to the naive picture, that the strong attraction of the fragments enhances the yields around 0 and 180 degrees. Comparison of the results of the neutrons and protons will be given.   

Measuring Neutron-Induced, Angular-Momentum-Dependent Fission Probabilities Using Direct Reactions

The surrogate method has previously been used to successfully measure $(n,f)$ cross sections on a variety of difficult to produce actinde isotopes. These measurements have larger uncertainties at excitation energies below 1.5 MeV where the distribution of angular momentum states populated in the compound nucleus created by neutron absorption differs from that arising from direct reactions. A method to measure the fission probability of individual angular momentum states arising from the $^{239}$Pu$(d,pf)$ reaction is under development. This detector system utilizes an array of photovoltaic (solar) cells to measure the angular distribution of fission fragments with high resolution. This distribution uniquely identifies the angular momentum states populated. These are fit to expected distributions of angular momentum states to determine the contribution of each state. Protons are detected with 40 keV FWHM resolution at 16 angles in the forward and backward directions. The matrix obtained from these measurements determines fission probabilities of specific angular momentum states in the transition nucleus. Progress in the development of this system will be presented.   

Searching for U-235m produced by Nuclear Excitation by Electronic Transition

Nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that is predicted to occur in numerous isotopes, including U-235. When a nuclear transition matches the energy and the multipolarity of an electronic transition, there is a possibility that NEET will occur. If NEET were to occur in U-235, the nucleus would be excited to its 1/2$+$ isomeric state that subsequently decays by internal conversion with a decay energy of 77 eV and a half-life of 26 minutes. Theory predicts that NEET can occur in partially ionized uranium plasma with a charge state of 23$+$. A pulsed Nd:YAG laser operating at 1064 nm with a pulse energy of 780 mJ and a pulse width of 9 ns was used to generate the uranium plasma. The laser was focused on small samples of both depleted uranium and highly enriched uranium. The plasma conditions created by the intense laser pulse were varied by changing the spot size of the laser on the target. The resulting plasma was collected on a plate and the internal conversion electrons were focused onto a microchannel plate detector by a series of electrostatic lenses. First results will be presented.