In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies. This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams. The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and γ-ray emission will also be reviewed. A particular attention will be given to the low-energy fission in the so far scarcely explored nuclei in the very neutron-deficient lead region. They recently became the focus for several complementary experimental studies, such as β-delayed fission with radioactive beams at ISOLDE(CERN), Coulex-induced fission of relativistic secondary beams at FRS(GSI), and several prompt fusion-fission studies. The synergy of these approaches allows a unique insight in the new region of asymmetric fission around 180Hg, recently discovered at ISOLDE. Recent extensive theoretical efforts in this region will also be outlined. The unprecedented high-quality data for fission fragments, completely identified in Z and A, by means of reactions in inverse kinematics at FRS(GSI) and VAMOS(GANIL) will be also reviewed. These experiments explored an extended range of mercury-to-californium elements, spanning from the neutron-deficient to neutron-rich nuclides, and covering both asymmetric, symmetric and transitional fission regions. Some aspects of heavy-ion induced fusion-fission and quasifission reactions will be also discussed, which reveal their dynamical features, such as the fission time scale. The crucial role of the multi-chance fission, probed by means of multinucleon-transfer induced fission reactions, will be highlighted. The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising 'next-generation' fission approaches, which might become available within the next decade.

/pdf/nuclear-fission-a-review-of-experimental-advances-and-1el0tcg7me.pdf

Nuclear fission: a review of experimental advances and phenomenology.

An overview is given on some of the main advances in the experimental methods, experimental results, theoretical models and ideas of the last few years in the field of nuclear fission. New approaches have considerably extended the availability of fissioning systems for the experimental study of nuclear fission, and have provided a full identification of all fission products in A and Z for the first time. In particular, the transition from symmetric to asymmetric fission around 226Th, some unexpected structures in the mass distributions in the fission of systems around Z  =  80-84, and an extended systematics of the odd-even effect in the fission fragment Z distributions have all been measured (Andreyev et al 2018 Rep. Prog. Phys. 81 016301). Three classes of model descriptions of fission presently appear to be the most promising or the most successful. Self-consistent quantum-mechanical models fully consider the quantum-mechanical features of the fission process. Intense efforts are presently being made to develop suitable theoretical tools (Schunck and Robledo 2016 Rep. Prog. Phys. 79 116301) for modeling the non-equilibrium, large-amplitude collective motion leading to fission. Stochastic models provide a fully developed technical framework. The main features of the fission-fragment mass distribution have been well reproduced from mercury to fermium and beyond (Moller and Randrup 2015 Phys. Rev. C 91 044316). However, limited computer resources still impose restrictions, for example, on the number of collective coordinates and on an elaborate description of the fission dynamics. In an alternative semi-empirical approach (Schmidt et al 2016 Nucl. Data Sheets 131 107), considerable progress in describing the fission observables has been achieved by combining several theoretical ideas, which are essentially well known. This approach exploits (i) the topological properties of a continuous function in multidimensional space, (ii) the separability of the influence of fragment shells and the macroscopic properties of the compound nucleus, (iii) the properties of a quantum oscillator coupled to a heat bath of other nuclear degrees of freedom, (iv) an early freeze-out of collective motion, and (v) the application of statistical mechanics for describing the thermalization of intrinsic excitations in the nascent fragments. This new approach reveals a high degree of regularity and allows the calculation of high-quality data that is relevant to nuclear technology without specifically adjusting the empirical data of individual systems.

https://iopscience.iop.org/article/10.1088/1361-6633/aacfa7/pdf

Review on the progress in nuclear fission-experimental methods and theoretical descriptions.

https://cyberleninka.org/article/n/675134.pdf

Fission fragments mass distributions of nuclei populated by the multinucleon transfer channels of the 18O+232Th reaction

The effect of the production mechanism on the decay of a compound nucleus is investigated. The nucleus $^{90}\mathrm{Zr}$ was produced by three different reactions, namely $^{90}\mathrm{Zr}(p,{p}^{\ensuremath{'}})^{90}\mathrm{Zr}$, $^{91}\mathrm{Zr}(p,d)^{90}\mathrm{Zr}$, and $^{92}\mathrm{Zr}(p,t)^{90}\mathrm{Zr}$, which served as surrogate reactions for $^{89}\mathrm{Zr}(n,\ensuremath{\gamma})$. The spin-parity $({J}^{\ensuremath{\pi}})$ distributions of the states populated by these reactions were studied to investigate the surrogate reaction approach, which aims at indirectly determining cross sections for compound-nuclear reactions involving unstable targets such as $^{89}\mathrm{Zr}$. Discrete $\ensuremath{\gamma}$ rays, associated with transitions in $^{90}\mathrm{Zr}$ and $^{89}\mathrm{Zr}$, were measured in coincidence with light ions for scattering angles of ${25}^{\ensuremath{\circ}}--{60}^{\ensuremath{\circ}}$ and $^{90}\mathrm{Zr}$ excitation energies extending above the neutron separation energy. The measured transition systematics were used to gain insights into the ${J}^{\ensuremath{\pi}}$ distributions of $^{90}\mathrm{Zr}$. The $^{90}\mathrm{Zr}(p,{p}^{\ensuremath{'}})$ reaction was found to produce fewer $\ensuremath{\gamma}$ rays associated with transitions involving high spin states $(J=6--8\phantom{\rule{4.pt}{0ex}}\ensuremath{\hbar})$ than the other two reactions, suggesting that inelastic scattering preferentially populates states in $^{90}\mathrm{Zr}$ that have lower spins than those populated in the transfer reactions investigated. The $\ensuremath{\gamma}$-ray production was also observed to vary by factors of 2--3 with the angle at which the outgoing particle was detected. These findings are relevant to the application of the surrogate reaction approach.

/pdf/spin-differences-in-the-zr-90-compound-nucleus-induced-by-p-23q4nv26jc.pdf

Spin differences in the Zr 90 compound nucleus induced by ( p , p ′ ) , ( p , d ) , and ( p , t ) surrogate reactions

NeutronSTARS ( Neutron - S ilicon T elescope A rray for R eaction S tudies) consists of 2.2-tons of gadolinium-doped liquid scintillator for neutron detection and large area silicon detectors for charged particle identification. This detector array is intended for low-energy-nuclear-reaction measurements that result in the emission of neutrons such as ( n , x n ) and fission. This paper describes the NeutronSTARS experimental setup, calibration, and the array’s response to neutral and charged particles.

NeutronSTARS: A segmented neutron and charged particle detector for low-energy reaction studies

The $^{240}\mathrm{Am}(n,f$) cross section has been measured for the first time above 4 MeV, using the surrogate-ratio method over the neutron energy range of 200 keV to 14 MeV. The reactions $^{243}\mathrm{Am}(p,tf$) and $^{238}\mathrm{U}(p,tf$), which proceed through the fissioning excited nuclei $^{241}\mathrm{Am}$* and $^{236}\mathrm{U}$*, were used as surrogates for the desired $^{240}\mathrm{Am}(n,f$) and $^{235}\mathrm{U}(n,f$) reactions. The experiment was fielded using the STARLiTeR detector system with a recently commissioned VME-based data acquisition system. The 38.4-MeV proton beam used in these measurements was provided by the K150 cyclotron at the Texas A Cyclotron Institute. The measured $^{240}\mathrm{Am}(n,f$) cross section disagrees with many of the most recent evaluations, and a reevaluation is recommended.

/pdf/measurement-of-the-am-240-n-f-cross-section-using-the-3mcpoy2e3m.pdf

Measurement of the Am 240 ( n , f ) cross section using the surrogate-ratio method

We performed the 174 Yb(p,d) reaction in order to measure the gamma-emission probability of 173 Yb. The identification of the ejectiles — allowing us to tag the production of 173 Yb nuclei — was performed using the STARLiTeR detector system. Unusually, the "statistical" γ-rays were used to determined the gamma- emission probability and a spin distribution was extracted from it. A comparison with the spin distribution from the 174 Yb( 3 He,α) reaction shows that the transferred angular momentum is similar in both reactions.

/pdf/measurement-of-the-g-emission-probability-of-173-yb-using-4nn32v0xnw.pdf

Measurement of the γ emission probability of 173 Yb using surrogate reactions

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Measurement of the Am 240 ( n , f ) cross section using the surrogate-ratio method

Measurement of the γ emission probability of 173 Yb using surrogate reactions