Fission and Properties of Neutron-Rich Nuclei

The following sections are included:

• Program Committee and Advisory Committee
• Preface
• Contents

After nearly 80 years of research on nuclear fission, we still do not fully understand the fission process. A combination of new requirements for accurate fission nuclear data, the ability to calculate sophisticated models of fission, and the development of new experimental techniques has re-energized the field. This paper concentrates on recent developments in the experimental realm and what the new results can contribute to the database for applications and to improving the underlying physical models. This overview is intended to be an introduction to fission presentations at this meeting.

We examine the published experimental data on the ²³⁵U(n,f) reaction with a goal to quantify cross section fluctuations above the fission barrier. For neutron energies in the range 10 keV - 25 keV we find that there are significant fluctuations of the order of 5% measured at 0.25 keV energy resolution. Here we analyze the fluctuations by the auto-correlation function with the following conclusions: 1) The correlation width associated with peak at zero energy offset is smaller than the experimental energy resolution; 2) there are peaks in the autocorrelation function on an energy scale of several keV. Such a multi-peaked structure is incompatible with Ericson’s statistical treatment of compound nucleus fluctuations but may arise in more detailed models of the fission dynamics.

We present results of the Hartree-Fock-Bogolyubov calculations performed using nuclear energy density functionals based on regularized functional generators at next-to-leading and next-to-next-to-leading order. We discuss properties of binding energies and pairing gaps determined in semi-magic spherical nuclei. The results are compared with benchmark calculations performed for the functional generator SLyMR0 and functional UNEDF0.

The enhancement observed below 2 MeV in the radiative strength function of nuclei near closed shells is explained by shell model calculations as M1 transitions between excited states. In the open-shell a change to a bimodal structure composed of the zero energy spike and a scissors resonance is found. The features are caused by realignment of high-j orbitals.

We present applications of the In-Medium Similarity Renormalization Group (IMSRG) framework in ab initio nuclear structure theory. We present two complementary strategies for the computation of nuclear ground- and excited- state properties, namely direct evaluations and the derivation of valence-space interactions for the nuclear configuration interaction (Shell model) approach. We highlight recent results for ground- and excited state observables of sd- and pf-shell nuclei, and look ahead at the next steps towards a predictive ab initio theory of medium-mass nuclei.

Laser spectroscopy is a prominent tool in the study of nuclear ground and isomeric state properties. Being able to measure nuclear spins, magnetic dipole and electric quadrupole moments, and changes in mean-square charge radii, make it an effective probe of both single particle and collective phenomena. Topics of recent investigation have included: evolution of single particle levels and changing magicity in the vicinity of shell closures, emerging collectivity, establishing new isomeric states, and the first optical measurements of a transfermium element.

Experiments for producing heaviest elements by a cold-fusion type reaction were performed using a gas-filled recoil ion separator, GARIS, at RIKEN. After confirmation experiments of the reported nuclides, ²⁷¹Ds, ²⁷²Rg, and ²⁷⁷Cn, a new superheavy nuclide ²⁷⁸113 has been searched by the reaction of ⁷⁰Zn on ²⁰⁹Bi. Totally 3 decay chains due to ²⁷⁸113 were observed during the net irradiation time of 576 days. The 1st and 2nd decay chains consist of four alpha decays from ²⁷⁸113 to ²⁶⁶Bh, and terminate by spontaneous fission of ²⁶²Db. The 3rd chain consists of 6 consecutive alpha decays down to ²⁵⁴Md. Observed decay properties from 3 decay chains were consistent with each other. We also examined the decay properties of ²⁶⁶Bh produced by the reaction of ²³Na on ²⁴⁸Cm to establish the cross-reaction of ²⁷⁸113 decay chain. It was found that 266Bh can be regarded as an anchor nuclide of 278113 decay chain, and thus the production of ²⁷⁸113 was confirmed.

Review of discovery of isotopes of elements 113–118 produced in the ⁴⁸Ca-induced reactions with actinide target nuclei is presented. These superheavy nuclei were synthesized at DGFRS; the decay properties of the majority of these nuclei were subsequently reproduced in experiments carried out at SHIP, BGS, TASCA, and GARIS. The increased role of shell effects in the stability of superheavy nuclei with rise of their neutron number is demonstrated by comparison of the experimental data and results of theoretical calculations.

After a successful production of isotopes of element 116 in the reaction ⁴⁸Ca + ²⁴⁸Cm, the reaction ⁵⁴Cr + ²⁴⁸Cm → ³⁰²120* was investigated at the velocity filter SHIP at GSI, Darmstadt, aiming to search for isotopes of element 120. One chain of events was observed, which is not created by chance with high probability. Parts of the chain agree with model predictions and measured data from a decay chain which could start at the isotope ²⁹⁹120. In a complementary study, model dependent shell-correction energies and related heights of fission barriers were deduced from measured Q_α values. The results are compared with predictions of macroscopic-microscopic models. The consequences for calculations of crosssections are discussed.

All elements up to Z = 118 are discovered, and several search experiments for elements beyond have been carried out. None of them, however, have reported the discovery of a heavier element. Relevant aspects for a successful search for elements with Z ≥ 119 are discussed.

The properties of the mass and energy distributions of fissionlike fragments formed in the reactions ⁴⁸Ca,⁵⁸Fe + ²⁰⁸Pb, ³⁶S,⁴⁸Ca,⁴⁸Ti,⁶⁴Ni + ²³⁸U, ⁴⁸Ca + ²³²Th,²⁴⁴Pu,²⁴⁸Cm at energies around the Coulomb barrier have been analyzed to define the systematic trend of compound nucleus fission and quasifission in cold and hot fusion reactions. The measurements have been carried out at the U400 cyclotron of the FLNR, JINR using the double-arm time-of-flight spectrometer CORSET. The fusion probabilities have been deduced from the analysis of mass and energy distributions. It was found that for the studied reactions fusion probability depends exponentially on mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

We show that the microscopic TDHF approach provides an important tool to shed some light on the nuclear dynamics leading to the formation of superheavy elements. In particular, we discuss studying quasifission dynamics and calculating ingredients for compound nucleus formation probability calculations.

Recent experiments using actinide + ⁴⁸Ca reactions have led to the discovery of five new elements and more than 50 new isotopes on the “hot fusion” island. These new nuclei, all having a large neutron excess over protons, provide strong evidence for the existence of the proposed island of stability for superheavy nuclei. As atomic number Z increases beyond Z = 115, the reaction cross sections are decreasing, requiring new approaches for continued progress. Ion beams heavier than ⁴⁸Ca will be required to reach elements above Z = 118, and new actinide target technologies and separation capabilities will be needed to accommodate higher beam intensities and optimize the use of heavier ²⁵¹Ca targets in experiments. In this paper, we review the availability of actinide materials, the use of new target/projectile combinations, and related experimental opportunities, including prospects for new element discoveries and for approaching the presumed closed shell at N = 184. Research opportunities include ongoing experiments to produce the heaviest nuclei to date using mixed Cf targets, and measurements of excitation functions for ⁴⁸Ca and ⁵⁰Ti on Pu and Cm targets to estimate cross sections for planned experiments using ⁵⁰Ti beams on ²⁴⁹Bk and ²⁵¹Cf for synthesis of elements 119 and 120. New developments for the production of ²⁴⁹Bk and for increasing the availability of ²⁵¹Cf and possibly ²⁵⁴Es are described.

During the last 18 years, six new elements have been discovered and confirmed and over 50 new isotopes have been synthesized in nuclear reactions using ⁴⁸Ca beams and actinide targets (²³⁷Np, ^{239,240,242,244}Pu, ²⁴³Am, ^245,248Cm, ²⁴⁹Bk, ^249,251Cf). The production properties of Fl isotopes will be discussed briefly with regards to fission. The extent of the region of enhanced stability near Z = 114 and N = 184 is not completely known. To attempt to produce the heaviest isotopes of element 118, new experiments were performed using ⁴⁸Ca projectiles and a ²⁵¹Cf (mixed isotope) target at the Dubna Gas Filled Recoil Separator (DGFRS) located in the Flerov Laboratory of Nuclear Reactions in Dubna. Progress on the production of ²⁹⁴Og and heavier isotopes of element 118 will be discussed. The probability that observed decay chains are due to random events occurring in the detectors or electronics rather than correlated decay chains, using the LLNL-developed method of Monte Carlo Random Probability analysis will be discussed.

Microscopic theories of alpha decay and cluster radioactivity explain these decay modes as a quantum tunneling of a preformed cluster at the nuclear surface. In the present work we show for the first time that in a spontaneous cold fission process the shell plus pairing corrections calculated with Strutinsky’s procedure may give a strong argument for preformation of a light fission fragment near the nuclear surface. It is obtained when the radius of the light fragment, R₂, is increased linearly with the separation distance, R, of the two fragments, while for R₂ = constant one gets the well known two hump potential barrier. Nuclear physics community also contributed to nanocluster physics by applying the macroscopic-microscopic method to explain the shell effects experimentally observed since 1984.

Gamma-ray spectroscopy continues to be an important tool for the study of nuclei. Excitation energies can be measured directly and in model-independent ways, and thus are among the key observables that can guide our understanding of atomic nuclei. The Gamma-Ray Energy Tracking Array (GRETA) is a full 4π γ-ray tracking detector, capable of reconstructing the energy and three-dimensional position of γ-ray interactions within a compact sphere of high-purity germanium crystals. GRETA, with its combination of high efficiency, good background rejection, and excellent energy and position resolution, will be a key instrument at FRIB, and will add new capabilities to existing facilities.

ISOLDE is the CERN facility dedicated to the production of radioactive ion beams for many different experiments in the fields of nuclear and atomic physics, materials science and life sciences. By a clever combination of target and ion source units pure beams of over 1000 different nuclei of 74 elements have been produced and delivered to experiments. Since more than ten years ISOLDE offers the largest variety of post-accelerated radioactive beams in the world today. The HIE-ISOLDE intensity and energy upgrade has finished its phase 1 dedicated to upgrade the energy up to 5.5 MeV/u, producing the first radioactive beams at this energy in September 9th 2016. The results from this campaign of post-accelerated beams will be described together with very recent ISOLDE highlights of neutron-rich nuclei.

SPES is a new infrastructure of the Legnaro National Laboratories (Italy) dedicated to nuclear physics and nuclear physics based applications. The facility will provide high intensity and high-quality beams of unstable neutron-rich nuclei with the aim to perform forefront research in nuclear structure, reaction dynamics and interdisciplinary fields like medical, biological and material sciences. SPES represents a second generation ISOL radioactive ion beam facility with technical solutions which define it as an intermediate step toward the future European ISOL facility EURISOL. Due to its specific technical solutions, it is part of the network of ISOL-facilities presently under construction in Europe constituting the so-called EURISOL distributed facility (EURISOL-DF). To produce radioactive beams it uses the ISOL method with a uranium carbide (UCx) Direct Target able to sustain a maximum power of 8 kW. The primary proton beam impinging on the target is delivered by a Cyclotron accelerator with an energy up to 70 MeV and a beam current up to 750 μA. Neutron-rich radioactive ions are produced by fission at an expected maximum rate in the target of the order of 10¹³ fissions per second. The exotic isotopes, after extraction and separation, are re-accelerated by the ALPI superconducting linac of the Legnaro National Laboratories at energies of 10 MeV/A and higher, for masses up to A=200 amu, with an expected rate on the secondary target of 10⁷-10⁹ pps for the most prolific species.

The octupole strength present in the neutron-rich, radiocative nucleus ¹⁴⁶Ba has been experimentally determined for the first time using Coulomb excitation. To achieve this, A=146 fission fragments from CARIBU were post-accelerated by the Argonne Tandem Linac Accelerator System (ATLAS) and impinged on a thin ²⁰⁸Pb target. Using the GRETINA γ-ray spectrometer and the CHICO2 heavy-ion counter, the reduced transition probability B(E3; 3⁻→0⁺) was determined as $48\left(\begin{array}{l}+21\\ -29\end{array}\right)$ W.u. The new result provides further experimental evidence for the presence of a region of octupole deformation surrounding the neutron-rich barium isotopes.

Gamma rays emitted by fission fragments of ²⁵²Cf and measured using the Gammasphere array continue to give insight into neutron rich nuclei. Using our high statistics data, we have reexamined high-spin states and linking transitions associated with octupole correlations in ¹⁴⁴Ba and ¹⁴⁸Ce. In an even-A system, it is possible that rotational band structures are explainable by simplex quantum numbers, s = +1 and s = -1. In ¹⁴⁴Ba and 148Ce we have deduced spin values from angular correlations and assigned parities from mixing ratios for s = -1 characterized states. These are the first examples of both simplex bands in even-even nuclei. Extensions to higher spins of bands in ¹⁴⁴Ba and in ¹⁴⁸Ce are also reported. In addition to collective model analysis of energy displacement and rotational frequency, the intrinsic dipole moment via B(E2)/B(E1) ratios are also analyzed.

High-spin level structures of neutron-rich ¹⁴⁰Xe and ¹⁴¹Xe nuclei have been reinvestigated by analyzing the triple and four-fold γ coincidence data obtained from the spontaneous fission of ²⁵²Cf. Several new levels and transitions are identified in both nuclei. The doublet octupole bands are observed in ¹⁴⁰Xe and expanded in ¹⁴¹Xe. The characteristics of octupole deformation and octupole correlations are discussed in both nuclei.

Mass spectrometry is a well-established technique for nuclear structure studies in exotic nuclei far off stability. The Penning-trap mass spectrometer SHIP-TRAP at GSI has focused on mass measurements of the heaviest elements to map shell effects around N = 152. Several upgrades are presently ongoing to extend these measurements to heavier elements. A major step was the implementation of the phase-imaging technique (PI-ICR) that has been used for mass measurements of long-lived and stable nuclides in the context of neutrino physics. In this contribution recent achievements are briefly reviewed.

Production of neutron-rich nuclei around N = 126 by multinucleon transfer reactions has recently gained a renewal of interest both theoretically and experimentally. We are now advancing the KISS project at RIKEN to produce those nuclei by multinucleon transfer reactions and measure their lifetimes to investigate the astrophysical environments of the r-process. To investigate the relevance of the production mechanism, we performed measurements for the reaction system ¹³⁶Xe + ¹⁹⁸Pt, using the spectrometer VAMOS++ and the gamma-ray array EXOGAM at GANIL. Cross sections deduced from the measurements for neutron-rich nuclei around N = 126 show a modest enhancement when compared with the GRAZING calculations, indicating the advantage of the multinucleon transfer reactions as compared with fragmentation for production of N = 126 neutron-rich isotones. The possibility for production of neutron-rich nuclei around uranium beyond N = 126 by multinucleon transfer reactions is also discussed.

The use of α-transfer reactions to populate states in nuclear species that cannot be produced in the present radioactive beam facilities is presented. Some examples of the use of these type of reactions to measure nuclear g factors and lifetimes will be discussed. The advantages and challenges for the use of α-transfer reactions with stable and radioactive beams will be discussed.

We address the theoretical interpretation of rotational bands and low-lying states in odd-odd prolate fission-products Nb and La nuclei, which are characteristic of shape transition and coexistence. Our Projected Shell Model (PSM), Total Routhian Surface (TRS) and Potential Energy Surface (PES) calculations were applied to the odd-odd ^104,106Nb. Results for ¹⁴²La show theoretically a high-spin state as band-head in the case of both odd neutron and proton in K = 1/2 states.

A new Table of Recommended Nuclear Electric Quadrupole Moments was published recently [1]. The main feature of the table is the adoption, wherever possible, of the best available calculation of the electric field gradient (efg), required to extract the moment from the measured quantity in most measurement methods. This paper presents a short discussion of the accuracy to which quadrupole moments of nuclear states of all elements are known, highlighting that fact that few, if any, are known to better than a few percent. For some 19 elements no reliable efg has been adopted and up-to-date calculation for these elements remains an outstanding need.

High spin states of the neutron-rich ¹⁰⁴Mo nucleus have been reinvestigated by analyzing the γ-rays in the spontaneous fission of ²⁵²Cf with Gammasphere. Both γ-γ-γ and γ-γ-γ-γ coincidence data were analyzed. A new ΔI = 1 band has been discovered with a tentative 5^- band-head, and is proposed to form a class of chiral doublets with another 4^- band previously found. Angular correlation measurements have been performed to determine the spins and parities. The origin of the chiral doublet bands in ¹⁰⁴Mo is interpreted as neutron h_11/2 particle and mixed d_5/2, g_7/2 hole coupled to the short and long axis, respectively.

In a previous study of ¹⁵⁴Gd spectrum, using the dynamic pairing plus quadrupole model, data for eight K-bands were analyzed. Here we present a revised analysis of the new data and new concepts. A discussion is given on the view of the shape coexistence of β-band with ground band and on the nature of the K^π = 0⁺ excited bands. Comparison is made with the X(5) symmetry. The validity of the multi-phonon view of the K^π = 4⁺ and 2₂⁺ bands is also illustrated.

The GODDESS coupling of charged-particle and photon spectrometers has been commissioned using a beam of ¹³⁴Xe at 10 MeV/A. This measurement demonstrates the capability of the device for measuring inelastic scattering and (d,pγ) reactions in inverse kinematics under radioactive beam conditions, and provides the first data on the single-particle structure of states in ¹³⁵Xe, including previously unobserved states based on the orbitals above the N = 82 shell closure.

New transitions in the deformed neutron-rich ¹⁰⁰Y uniquely identified using the VAMOS++ spectrometer (A, Z) in conjunction with EXOGAM, produced in fission reactions of ²³⁸U on a ⁹Be target at energies around the barrier are reported. High spin states were obtained from an analysis of the large fold γ coincidence data from the spontaneous fission of ²⁵²Cf using the Gammasphere detector array. The new K^π = 4⁺ band decaying to an isomeric state (1s) is assigned to be the high-K Gallagher-Moszkowski (GM) partner of the known K^π = 1⁺ band, and is also proposed to be the pseudo spin (PS) partner of another new K^π = 5⁺ band. Constrained triaxial covariant density functional and quantal particle rotor model calculations for the band structure and electromagnetic transition probabilities are in good agreement with the measurements. The work shows for the first time a coexistance of GM and PS bands in the same nucleus and could point towards a common source of these bands despite their very different origins.

This paper reviews the systematic investigations of the shape transitions and coexistence with regard to triaxial deformations in neutron-rich nuclei with A ˜ 100-126, Z from Zr (Z = 40) to Cd (Z = 48). Shape changes with Z, N and spins are addressed. γ vibrations, chiral symmetry breaking and wobbling motions identified in the region are discussed.

Superheavy elements are primarily formed through heavy ion fusion reactions. Formation of a fully equilibrated compound nucleus is a critical step in this reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. The interplay between the fusion-fission and quasifission reaction channels was explored by measuring fission mass distributions in eight different combinations of Cr+W reactions, with varying neutron-richness, at the Australian National University. The reactions were measured in two energy regimes: one at 13% above the Bass fusion barrier and one at 52.0 MeV of excitation energy in the compound nucleus, ${\text{E}}_{\text{CN}}^{*}$. For the systems measured at E_c.m./ V_Bass = 1.13 the dependence on the neutron-richness is clear. However, for the reactions at ${\text{E}}_{\text{CN}}^{*}$ = 52.0 MeV, the dependence is less clear and additional factors are shown to play a vital role, especially the influence of deformation on the effective fusion barrier. The present work demonstrates that quasifission is an important process in competition with heavy-ion fusion in reactions with intermediate mass projectiles, particularly with more neutron-rich systems.

Beta-delayed neutrons emitted from the decay of fission fragments play an important role in the r-process and nuclear reactor physics. Neutron energy spectra provide information necessary to accurately reconstruct a level scheme that reflects nuclear structure. Decay cascades where neutron emission populate excited states in the (N-1) daughter complicate level scheme construction. These excited states in the (N-1) daughter often emit gamma-rays. These situations necessitate measurement of three-fold coincidence between the beta, neutron and gamma-ray. However, there is a scarcity of measurements providing these data.

To address this need, we designed the Versatile Array of Neutron Detectors at Low Energy (VANDLE), which measures neutron energies via the time-of-flight technique. VANDLE is instrumented with a digital data acquisition system. The digital system provides a neutron detection threshold as low as 70 keV and time resolutions better than 1 ns. A high-efficiency gamma-ray detection system enables complete calorimetry of the decay by detecting gamma-rays that follow neutron emission. High purity Ge clovers present isotope identification, but suffer from poor geometric efficiency. A high efficiency array of NaI and LaBr₃ detectors boost neutron-gamma coincidences.

Total Absorption Spectrometers are characterized by high efficiency detection of γ-ray radiation, which is due to their large volume and nearly 4π solid angle coverage. This high efficiency, used in the study of β-decay of unstable nuclei, allows for the total detection of the deexcitation path of daughter nuclei. This makes total absorption spectroscopy an ideal technique to establish true β-decay feeding patterns, which helps determine β-strength distributions as well as to understanding decay heat and the anti-neutrino spectrum emitted from nuclear reactors. The Modular Total Absorption Spectrometer (MTAS) constructed at the Holifield Radioactive Ion Beam Facility has been used to study the β decay of over 70 fission products. The measurements were focused on nuclei abundantly produced in the reactor core. In this contribution we present aspects of the analysis of the β-decay of ⁸⁹Rb measured with the MTAS detector.

This talk contains results from several applications of the mean-field model for nuclei. A brief discussion is give of a comparison of nuclear shapes in heavy-ion collisions from time-dependent Hartree-Fock (TDHF) and the asymmetric two-center oscillator shell model (TCSM). The remainder of the talk is devoted to the structure of light nuclei: analysis in terms of α-clusters, the exotic 4-α chain state in ¹⁶O, and a rotating torus configuration for ⁴⁰Ca.

We review state-of-the-art no-core shell model (NCSM) work that exploits symmetries which bare the emergence of simple patterns from Quantum Chromodynamics (QCD)-inspired effective interactions. Our theory of choice for this is the Symplectic Model (SpM) and a recent implementation that includes mixing of configurations of the no-core shell-model type, called NCSpM. The SpM is a fully microscopic theory that respects exact and exploits partial symmetries and hence, can uncover simple patterns hidden within the complexities of nuclei. Making use of the algebraic features of the SpM, the NCSpM complements the NCSM by “vertically extending” a physically relevant subspace into a model space that is needed to fully capture the collectivity of nuclei.

In order to advance our understanding of the fission process we need to measure changes in fission product yields as a function of excitation energy of the fissioning system, as well as study the correlations between kinetic energy, mass and charge of the products. In addition, fission product yields are used for diagnostics in nuclear technology and there is therefore interest in reducing the uncertainties in their yield.

The SPIDER instrument is based on the 2v-2E technique for measuring the mass of fission products, and has been used to study spontaneous fission of ²⁵²Cf, as well as thermal neutron-induced fission of ²³³U, ²³⁵U, and ²³⁹Pu. The current instrument has two spectrometer “arms”, which provides sufficient detection efficiency for studying fission at thermal neutron energies. In order to study the change in fission yields at fast energies higher detection efficiency is needed, and a new version of SPIDER is therefore under construction. The new instrument will have a total of 16 individual arms, bringing the total solid angular coverage to about 1%.

We present a consistent framework for treating the energy and angular-momentum dependence of the nuclear shape evolution in the fission process. It combines microscopically calculated level densities with the Metropolis walk method and it contains no new parameters. The treatment can elucidate the influence on the shape dynamics of warm nuclei of pairing and shell effects as a function of the excitation energy.

We examine the role of non-collective degrees of freedom in fission. we focus in particular on the use of the quasiparticle random-phase approximation (QRPA) to study the population mechanisms of the fissioning nucleus, and the Schrödinger collective intrinsic model (SCIM) to allow for quasiparticle excitations during fission. The QRPA calculations are performed with the D1S finite-range effective interaction for ²⁴⁰Pu while the SCIM is applied to a level-density calculation for the schematic multi-O (4) model.

We have measured the total kinetic energy release (TKE), its variance and associated fission product distributions for the neutron induced fission of ²³²Th and ²³⁵U for E_n = 2-90 MeV. The neutron energies were determined on an event by event basis by time of flight measurements with the white spectrum neutron beam from LANSCE. The TKE decreases non-linearly with increasing neutron energy for both systems, while the TKE variances are sensitive indicators of n^th chance fission. The associated fission product distributions show the decrease in TKE with increasing beam energy that is due to the increasing probability of symmetric fission — which has a lower associated TKE — and the decreasing TKE associated with asymmetric fission, presumably due to the decreasing importance of the A ∼ 132 shell structures.

We investigate the angular distribution of scission particles taking account of the effects of fission fragments. The time evolution of the wave function of the scission particle is obtained by integrating the time-dependent Schrödinger equation. The effects of the fission fragments are taken into account by means of the optical potentials. The angular distribution is strongly modified by the presence of the fragments. In the case of asymmetric fission, it is found that the heavy fragment has stronger effects. The angular distribution of the energy of scission neutrons is also discussed.

A consistent set of high-precision measurements have been performed to study the energy dependence of the fission product yields of ²³⁵U, ²³⁸U, and ²³⁹Pu using monoenergetic neutrons between 0.5 and 14.8 MeV. The results confirm the progression towards symmetric fission at higher incident neutron energy, i.e., 14.8 MeV. However, at lower energies (E_n < ∼ 3 MeV) the experimental data reveal a peculiar energy dependence of some of the fission-product yield from neutron-induced fission of ²³⁹Pu: a positive slope up to about 4-5 MeV which then turns negative as the incident neutron energy increases. This latter finding at low-energy is in conflict with present theoretical predictions.

The Chi-Nu experiment aims to accurately measure the prompt fission neutron spectrum (PFNS) for the major actinides. At the Los Alamos Neutron Science Center (LANSCE), fission can be induced using the white neutron source. Using a two arm time of flight (T.O.F) technique; Chi-Nu presents a preliminary result of the low energy component of the ²³⁵U PFNS measured using an array of 22-Lithium glass scintillators.

Despite the discovery of fission nearly 80 years ago and its importance to nuclear energy, national security, and astrophysics, there are very few measurements of the correlations between multiple fission products. An experiment is underway at Oak Ridge National Laboratory to measure the energy and angle correlations between prompt fission neutrons, γ rays, and fragments. These measurements could reveal the existence of scission neutrons, the energy balance between these products, and other details needed for benchmarking advanced fission models. The setup includes an array of VANDLE neutron detectors and a γ-ray detector positioned opposite a suite of detectors to determine the mass of one fragment. Preliminary results with a spontaneous ²⁵²Cf source show expected correlations between prompt fragments and neutrons along the fission axis. Further refinements will improve the resolution of the fragment mass and increase efficiency for triple-coincident events. Future experiments could be done with a neutron beam to study induced fission on thin targets.

In light of a new experiment which claims an identification of tetraneutron, we discuss the results of experimental search of trineutron and tetraneutron in different nuclear reactions. A summary of theoretical studies for trineutron and tetraneutron within variety of approaches are presented.

Over the last two decades transfer reactions have seen a resurgence following developments in methods to use them with exotic beams. An important step in this evolution was the ability to perform the (d,p) reaction on fission fragment beams using the inverse kinematics technique, built on the experience with light beams. There has been renewed interest in using (⁹Be, ⁸Be) and (¹³C, ¹²C) reactions to selectively populate single-particle like states that can be studied via their subsequent γ decay. These reactions have been successfully utilized in the ¹³²Sn region. Additionally, our collaboration has recently performed experiments with GODDESS, a combination of the full ORRUBA detector and Gammasphere arrays. Another new direction is measuring neutrons from (d,n) reactions, performed in inverse kinematics, with the VANDLE array of plastic scintillators. Presented below is an overview of these new techniques and some of the early data from recent experiments.

The multinucleon transfer reactions are investigated by using the improved quantum molecular dynamics (ImQMD) model, the dinuclear system (DNS) model and the DNS+GRAZING model. In the framework of ImQMD, the total-kinetic-energy-mass distributions of primary binary fragments are studied. The isotope production cross sections of ¹³⁶Xe+²⁰⁸Pb, ²³⁸U+²⁴⁸Cm and ⁶⁴Ni+²³⁸U systems are analyzed via three models. The calculations show that although each model has its successes in description of multinucleon transfer reactions, it also has its limitations for the results.

Starting from the chiral N³LO potential and using many-body perturbation theory (MBPT), we have calculated the structures of finite nuclei. In order to speed up the convergence of calculations, the similarity renormalization group (SRG) has been employed. The MBPT has been developed in the Hartree-Fock (HF) basis within the angular momentum coupling representation. The developed MBPT calculation has been successfully applied to closed-shell nuclei. It has been demonstrated that in the HF basis the corrections up to the third order in energy and up to the second order in radius can give converged results, while in the harmonic oscillator (HO) basis one would have to go to 30^th-order corrections to obtain converged solutions. The calculated binding energies and radii of the ⁴He and ¹⁶O nuclei agree well with experimental data. The present MBPT calculations have been compared with other ab-initio calculations with the same potential.

Single-particle transfer reactions are typically measured at energies where only a peripheral reaction can occur, without probing the interior of the nuclear wave function. At low energies (≈5 MeV/u), spectroscopic factors cannot be reliably extracted without a detailed description of the bound-state potential. Mukhamedzhanov and Nunes have proposed a method to constrain the shape of the bound state potential by combining transfer reaction measurements at two different energies. The external contribution of the wave function is extracted using a peripheral reaction, and is combined with a higher energy measurement which probes the nuclear interior more deeply. These two measurements should constrain the single-particle asymptotic normalization coefficient, ANC, and enable spectroscopic factors to be deduced with uncertainties dominated by the cross-section measurements rather than the bound-state potential. Published measurements of ⁸⁶Kr(d,p) at 5.5 MeV/u were used to determine the external contribution of this reaction. At less-peripheral energies, ⁸⁶Kr(d,p) at 35 MeV/u has been measured in inverse kinematics at the NSCL using the OR- RUBA and SIDAR arrays of silicon strip detectors. Preliminary analysis shows that the single-particle ANC can be constrained. The details of the analysis and prospects for measurements with neutron-rich beams will be presented.

Nuclear clusters are intricately linked to astrophysical processes involving light nuclei. Here, several examples of the role played in the evolution of the universe by these exotic structures in light nuclei are given, concluding with an outlook for future direct measurements.

Impressive progress has been made in the course the last decades in understanding astrophysical objects. Increasing precision of nuclear physics data has contributed significantly to this success, but now a better understanding of several important findings is frequently limited by uncertainties related to the available nuclear physics data. Consequently it is desirable to improve significantly the quality of these data. An important step towards higher precision is an excellent signal to background ratio of the data. Placing an accelerator facility inside an underground laboratory to reduce the cosmic ray induced background by six orders of magnitude is a powerful method to reach this goal. Nevertheless careful reduction of environmental and beam induced background must still be considered. Experience in the field of underground nuclear astrophysics has been gained since 20 years due to the pioneering work of the LUNA Collaboration (Laboratory for Underground Nuclear Astrophysics) operating inside the underground laboratories of the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. Based on the success of this work presently also several other projects for underground laboratories dedicated to nuclear astrophysics are being pursued worldwide. This contribution gives a survey of the past experience in underground nuclear astrophysics as well as an outlook on future developments

We discuss the possibility to build a neutron target for nuclear reaction studies in inverse kinematics utilizing a storage ring and radioactive ion beams. The proposed neutron target is a specially designed spallation target surrounded by a large moderator of heavy water (D₂O).

Nuclear isomers usually suggest unexpected nuclear structure properties and may play an important role in nuclear astrophysics. One of the most interesting cases is the 0⁺(T_1/2=6.3 s) isomer in ²⁶Al, located just 228-keV above the ground state (5⁺, T_1/2=740 ky). Proton captures on both, the ground state and the isomeric state, have a direct impact on the abundance of ²⁶Al in the Galaxy. We have developed a high-quality isomeric ²⁶Al^m beam via In-flight technique. By tunning the production energy we can favor the population of the isomeric state or the ground state in ²⁶Al. In this work, our efforts to develop and characterize an isomeric ²⁶Al^m beam are discussed as well as plans for experimental studies using this isomeric beam to constrain the destruction rate of Galactic ²⁶Al in relevant astrophysical environments.

Prompt neutrons and γ rays emitted shortly after scission provide valuable information to constrain fission physics models and their parameters. Numerical results obtained with event generators that simulate the de-excitation of the primary fission fragments can indeed be directly compared to various post-scission observables. The CGMF code implements the Hauser-Feshbach equations in a Monte Carlo approach to simulate the decay of the primary fission fragments that are produced in various configurations in energy, spin and parity. The neutron and decay channel widths are computed at each stage of the decay chain, thereby following successive emissions of particles on an event-by-event basis. Experimental as well as theoretical studies of correlations among the emitted particles shed some light on the mechanisms of excitation energy sorting at scission, the production of angular momentum in the fission fragments, and on the statistical nature of the fission fragment decay. Nuclear spectroscopy of the fission fragments is crucial for the accurate prediction of isomeric ratios and late-time promptγ-ray emissions. The CGMF code was recently integrated into the MCNP6.2 transport code, providing new and powerful tools for the interpretation of complex experimental data.

The investigation of the dynamics of the nuclear fission process has been a standing research topic at the JRC-Geel during the past decades. Recently the focus was put on the de-excitation of fission fragments through the emission of prompt neutrons and gamma-rays.

To this end new detector systems were developed at JRC-Geel, e.g. a position sensitive ionization chamber used in conjunction with the neutron scintillator array SCINTIA. The array has been tested using the spontaneous fission of ²⁵²Cf. The goal is to study correlations of fission fragments with prompt neutron emission in the resolved resonance region. No strong fluctuations of the average prompt neutron multiplicity for the strongest resonances in ²³⁵U were observed. From the present data the mass-dependent neutron multiplicity, ν(A), was generated. The ν(A) distribution shows a more pronounced dip around the doubly magic mass A = 132 and at very low masses around A ∼ 80 compared to the literature. In addition, a steeper slope for ν(TKE) is observed. Cross checking with fragment data clearly shows a narrower mass and total kinetic energy (TKE) distribution.

The 2E-2v spectrometer VERDI (VElocity foR Direct mass Identification) became operational. For ²⁵²Cf(sf) superior mass resolution is observed compared to a twin Frisch-grid ionization chamber. For post-neutron mass distributions still some issues need to be solved and ν(A), being the difference of pre- and post-neutron mass distributions, is still deviating from literature data. Eventually, VERDI will provide a complementary measurement technique to assess ν(A) and ν(TKE). In addition, an experimental campaign to measure ν(A) as a function of incident neutron energy for different actinides has been started. First tests show promising results.

For many years, the state of the art for handling fission in radiation transport codes has involved sampling from average distributions. Such “average” fission models have limited interaction-by-interaction capabilities. Energy is not explicitly conserved and no correlations are available because all particles are emitted isotropically and independently. However, in a true fission event, the energies, momenta and multiplicities of emitted particles are correlated. The FREYA (Fission Reaction Event Yield Algorithm) code generates complete fission events. Event-by-event techniques such as those of FREYA are particularly useful because it is possible to obtain complete kinematic information on the prompt neutrons and photons emitted during the fission process. It is therefore possible to extract any desired correlation observables. We describe FREYA and compare our results with neutron-neutron and neutron-light fragment correlation data. We also present preliminary results studying the sensitivities of various neutron observables to the yield distribution Y (A, Z, TKE) used as input to the code. All results are shown for the spontaneous fission of ²⁵²Cf.

The 4π multi-detector γ-ray calorimeter DANCE at the Los Alamos Neutron Science Center and its recent upgrade with an array of neutron detectors NEUANCE are described. DANCE has proven its ability to measure prompt fission γ rays with a large number of γ-ray multiplicities. The prompt fission γ rays may exceed a multiplicity of 25 and total energy of 20 MeV. NEUANCE adds the option of neutron measurement in addition to γ-ray measurement with DANCE and thus to study the fission γ-n correlations. In addition, a pair of silicon detectors measuring the kinetic energy of the fission fragments enables research on the γ-n-TKE correlations. Preliminary results of correlated data from ²⁵²Cf(sf) and ²³⁵U(n, f) measurements are presented.

A major part of the scientific program of the CERN n_TOF facility involves the study of neutron-induced fission reactions, which is essential for nuclear technology, as well as for the development of the theoretical models of fission. An overview of the fission studies performed and the corresponding detection systems is given, with a short description of the latest highlights.

Information of isotopic fission-fragment yields is important for both fundamental and applied nuclear physics. In the first applications of the Brownian shape motion (BSM) method fission fragment yields were obtained versus nucleon number A. Charge yields were obtained by obvious transformations assuming constant proton Z to neutron N ratios in the emerging fragments, identical to that of the compound nucleus. In these applications the method was implemented as a Metropolis random walk on five-dimensional (5D) potential-energy surfaces calculated on a discrete grid. The surfaces were functions of five nuclear shape degrees of freedom, namely elongation, spheroidal deformation of each of the nascent fragments, neck diameter, and left-right shape asymmetry. We envisioned that if we could devise a method to calculate these surfaces versus Z and N in the emerging fragments, rather than overall shape asymmetry, then a straightforward implementation of the standard BSM method as a random walk, now on 6D surfaces, would yield fragment distributions Y (Z, N) versus both neutron and proton numbers. Once the fragment identities can be taken into account, pairing effects arising from odd-odd or even-even nucleon divisions could also be introduced. We review the steps recently taken to obtain such surfaces and discuss some results.

Angular distributions from sub-barrier and near-barrier photofission can yield insights into excited states and level densities at the fission barrier. Photofission experiments were performed on targets of ²³²Th, ^233,235,238U, ²³⁷Np, and ^239,240Pu using nearly 100% linearly polarized, high intensity, and nearly-monoenergetic γ-ray beams having energies between 5.3 and 7.6 MeV at the High Intensity γ-ray Source (HIγS) located at Duke University and Triangle Universities Nuclear Laboratory. An array of 12–18 liquid scintillators was used to measure prompt fission neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, the differences between the in-plane and out-of-plane yields divided by their sums, were measured. Asymmetries close to zero were found for ^233,235U, ²³⁷Np, and ²³⁹Pu while significant asymmetries (∼0.2–0.5) were found for ²³²Th, ²³⁸U, and ²⁴⁰Pu. Predictions of the polarization asymmetries based on previously measured photofission fragment angular distributions combined with a model of prompt neutron emission agree well with the experimental results.

In addition to providing insights into the structure of the fission barrier, these polarization asymmetries could be used to measure the fissile versus non-fissile content of special nuclear material. These potential applications provide an incentive to utilize polarization effects in active interrogation of special nuclear material.

Accurate measurement of the prompt fission neutron spectrum is extremely important to several aspects of nuclear engineering. The spectrum for the spontaneous fission of ²⁵²Cf is particularly of use since other spectra are often measured as a ratio to Californium and the efficiency of neutron detectors can be measured using it. A measurement was performed at Rensselaer Polytechnic Institute in order to provide a new high accuracy measurement of the spectrum particularly in the regions of little data below 0.5 MeV and above 10 MeV. A measurement was performed using the multiple gamma tagging method and provided a new dataset in the region from 50 keV to 7 MeV which agreed well with previous measurements and theory. Preliminary measurements have also been performed to test the feasibility of using this method to measure the high portion of the spectrum from 5 MeV up to 15 MeV.

We study the fission dynamics of ²⁴⁰Pu within an implementation of the Density Functional Theory (DFT) extended to superfluid systems and real-time dynamics. We demonstrate the critical role played by the pairing correlations. The evolution is found to be much slower than previously expected in this fully non-adiabatic treatment of nuclear dynamics, where there are no symmetry restrictions and all collective degrees of freedom (CDOF) are allowed to participate in the dynamics.

Many shape degrees of freedom play crucial roles in determining fission barriers. In multidimensionally constrained covariant density functional theories (MDC-CDFTs) which we have developed in recent years, both the axial and the spatial reflection symmetries are broken and all deformations described by β_λμ with even μ, including β₂₀, β₂₂, β₃₀, β₃₂, β₄₀, etc., are considered self-consistently. The MDC-CDFTs have been applied to the study of fission barriers and potential energy surfaces of actinide nuclei, third minima in potential energy surfaces of light actinides, shapes and potential energy surfaces of superheavy nuclei, the Y₃₂ correlations in N = 150 isotones and Zr isotopes, and the shape of hypernuclei. In this contribution we will present an introduction of the MDC-CDFTs and a short review of its applications on fission barriers.

In this work we study a stabilization of internal cluster structure of the nuclear system under the fluctuations of the asymmetry value used as a constraint in the shell correction calculations. Persistent cluster magic structures play a main role in origination of various fission modes. We get quantitative results for clustering in theoretical description of actinide multimodal fission by means of analyzing the high dimensional potential energy surface’s topological complexity. It is shown that traditional adiabatic description is insufficient in the regions of the deformation characterized by the strong internal shell structures rearrangement and therefore it is necessary to take nonequilibrium effects of reclusterization into account.

It is shown that the multi-nucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multi-nucleon transfer channels of the reactions of ¹⁸O+²³²Th, ¹⁸O+²³⁸U and ¹⁸O+²⁴⁸Cm are used to study fission for various nuclei from many excited states. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel. The experimental data are compared to a calculation based on the fluctuation-dissipation model. Role of multi-chance fission in fission fragment mass distributions is discussed.

The nuclear fission process involves a drastic rearrangement of the nuclear matter of the fissioning system as it undergoes shape changes from a single, compact nucleus to two separated fission fragments. Understanding the dynamics of this process has been the focus of much research for decades, both theoretical and experimental, using a number of different approaches. Early on, Kramers [1] proposed that the fission rate is not well described by the statistical counting of transition states as proposed by Bohr and Wheeler [2] but that the friction or viscosity of the nuclear matter can play a substantial role in slowing down the rate by large factors. Aside from the theoretical estimates of fission time scales, a number of different approaches have been used to obtain experimental information on this quantity. Although the divergence of results obtained by the different experimental approaches have been known for years, these discrepancies have not yet been resolved and they have recently received renewed attention [3]. In this paper I will recount some of the experimental approaches to estimate the fission time scales and discuss their respective strengths and weaknesses without coming close to provide a resolution of the discrepancies.

Multipole mixing ratios have been measured, using ²⁵²Cf spontaneous-fission, γ-ray coincidence data, for transitions from states in the γ-vibrational-bands to states in the ground state bands of deformed, neutron-rich isotopes, ^{102,104,106,108}Mo, ^108,110,112Ru, ^112,114,116Pd. These mixing ratios have been found to be pure, or nearly pure, E2, in agreement with theory.

A natural connection exists between the fission product yields and the prompt neutron emission. The average neutron emission for a fission of a particular nucleus $\overline{v}$ is typically known to a very high level of certainty. In addition, the average neutron emission as a function of the fragment mass $\overline{v}$(A) also provides a connection between not only average quantities, such as $\overline{v}$, but also the prompt neutron emission distribution. In this way, one can utilize established prompt neutron data as a constraint on fission product yields.

Using quadruple coincidence events of prompt fission gamma rays from Gammasphere data on spontaneous fission of ²⁵²Cf, we made a careful analysis of the yield matrix of coincident pairs of barium (Z = 56) and molybdenum (Z = 42) fission fragments. The accuracy of previously determined yield matrices is improved upon with the use of high accuracy quadruple coincidences, the increased statistics of the most recent Gammasphere data, and improved level schemes for barium and molybdenum isotopes. The previously proposed extra hot fission mode (up to ten neutron evaporated) has been confirmed in our reanalysis. Our results are well in agreement with the results from the 1995 Gammasphere data analysis of the Ba-Mo yields.

High spin states of neutron rich ^146,147La have been reinvestigated by γ-γ-γ and γ-γ-γ-γ coincidence data from a ²⁵²Cf spontaneous fission experiment by using Gammasphere. Two new bands in ¹⁴⁶La have been established. One of them is proposed to be the octupole parity partner of the previously known band. The ground state band of ¹⁴⁷La has been established with a proposed 5/2⁺ bandhead. Angular correlations of cascades have been used to study the spins and parities of the states. The B(E1)/B(E2) ratios and dipole moments between the proposed octupole bands in ^146,147La have been measured showing a decreasing trend from ¹⁴⁴La to ¹⁴⁶La, and from ¹⁴⁵La to ¹⁴⁷La.

The Zr-Ce fission fragment yield matrix has been determined from the spontaneous fission of ²⁵²Cf using Gamasphere data. About 1.9*10¹¹ quadruple-coincidence events of fission gamma rays were used to reconstruct the yield matrix of coincident pair of zirconium [Z=40] and cerium [Z=58] fission fragments from 0 to 8 neutron multiplicities. The data used in the current analysis have better statistics than that used in the previous study on the spontaneous fission of ²⁵²Cf. Also, the previous work only had triple gamma coincidence events. Thus, the present study was expected to yield more accurate results. However, the results from this study were similar to those from the previous study, with smaller error limits.

The following sections are included:

• Poem by Walter Greiner
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