After a brief introduction of the role of shell effects in determining the limiting nuclear masses, the experimental investigation of the decay properties of the heaviest nuclei is presented. For the production of superheavy nuclides fusion, reactions of heavy actinide nuclei with 48Ca-projectiles have been used. The properties of the new nuclei, the isotopes of elements 112–118, as well as of their decay products, together with the known data for the light isotopes with Z ≤ 113, give evidence of the significant increase of the stability with the neutron number of the heavy nucleus. The obtained results are discussed in the context of the theoretical predictions about the 'island of stability' of the hypothetical superheavy elements.

https://iopscience.iop.org/article/10.1088/0954-3899/34/4/R01/pdf

Heaviest nuclei from 48Ca-induced reactions

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.

Fusion and fission of heavy and superheavy nuclei (experiment)

The CORSET time-of-flight spectrometer has been developed at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research (Dubna, Russia) for investigating binary products of nuclear reactions. The spectrometer has been used to study the dynamics of fusion-fission and quasi-fission of superheavy elements. The design and the main characteristics of the spectrometer, as well as the algorithms for deducing the mass-energy distributions of fragments and the cross sections of nuclear reactions, are presented. The spectrometer contains two time-of-flight arms based on microchannel-plate detectors and three telescopes, each of which is composed of two microchannel-plate detectors and one semiconductor detector. A system of four semiconductor detectors is used to obtain the absolute value of a cross section. The time resolution of the time-of-flight arms is 150 ps, which allows the time-of-flight distances to be set at 10–20 cm, thus providing a mass resolution of 3 amu and an angular resolution of 0.3°. Owing to these characteristics, the spectrometer can be used as a trigger in multidetector setups for measuring light charged particles, neutrons, and γ rays in coincidence with reaction fragments.

The CORSET time-of-flight spectrometer for measuring binary products of nuclear reactions

https://www-win.gsi.de/charms/Preprints/FisSecBeams2001/NPA_RIB13.pdf

Fission of nuclei far from stability

Results of fission fragment mass-energy distributions of the compound ${}^{226}\mathrm{Th}$ nucleus formed in the sub-barrier fusion reaction ${}^{18}\mathrm{O}{+}^{208}\mathrm{Pb}$ at an energy of ${}^{18}\mathrm{O}$ ions ${E}_{\mathrm{lab}}=78$ MeV are reported. The reaction has been studied twice using two different accelerators, and both sets of experimental data agree quite well. Performed analysis of the experimental data with the use of a new multicomponent method has shown that alongside the well-known modes, i.e., the symmetric (S) and two asymmetric modes standard-one and standard-two, a high-energy mode standard-three has manifested itself. The last named mode appears due to the influence of the close-to-sphere neutron shell with $N\ensuremath{\approx}50$ in the light fission fragment group. Theoretical calculations of the prescission shapes of the fissioning nuclei ${}^{224,226}\mathrm{Th}$ confirm this conclusion.

Fission modes in the reaction 208 Pb( 18 O, f)

The fission of the systems 220,224,226Th was investigated by measuring the mass-energy distributions of the fission fragments. The corresponding excitation energies at the saddle point, Esp*, ranged from 16 to 40 MeV. As Esp* decreases, an asymmetric mass component becomes visible on the predominately symmetric mass distribution. The contribution of the asymmetric mode is characterized by the total yield ratio Ys/Ya, which decreases rapidly for the heavier isotopes of thorium. This behavior of Ys/Ya is in qualitative agreement with theoretical calculations. For all isotopes studied, the subtracted asymmetric fission component, Ya=Y1−Ys, exhibits a complex structure, actually showing two components, Ya=Ya1+Ya0, which have average masses Ma1=132 and Ma0=140.

Observation of fission modes in heavy ion induced reactions

The γ-rays from the multimodal fission of the 226Th formed in 18O+208Pb was investigated at the near- and sub-barrier energies The corresponding excitation energies at the saddle point, Esp*, ranged from 23 to 26 MeV The average γ-ray multiplicities and relative γ-ray energies as a function of the mass of the fission fragments exhibits a complex structure and strong variations Such strong variations have never been previously observed in heavy ion-induced fusion-fission reactions Obtained results may be explained with the influence of shell effects on the properties of the fission fragments Present work is the one in series of investigation of the multimodal fission phenomena in At-Th region

K. Lukashin

Papers

Fission modes in the reaction 208 Pb( 18 O, f)

Observation of fission modes in heavy ion induced reactions

Angular momentum effects in multimodal fission of 226Th