As already noted, at low collision energies in reactions with heavy ions with nonzero impact parameters, a binary process dominates, in which two new fragments are formed: a + A → b + B. In a narrow region of grazing collisions, it is possible to distinguish channels with a small transfer of mass, energy, and angular momentum, that is, the quasi-elastic scattering processes considered in the preceding chapter. However, in most cases, such collisions show a significant loss of kinetic energy, and sometimes a significant rearrangement of the charge and mass of the colliding nuclei. However, as we shall see below, even with a large loss of kinetic energy in the exit channel, nuclei close in mass and charge to the projectile and target are most likely to be observed. Such fragments are commonly referred to as projectile-like fragments (PLF) and target-like fragments (TLF).

Based on Valery Zagrebaev's lecture materials, this textbook offers a unique and comprehensive survey of modern research in heavy-ion nuclear physics at low energies. Designed for students in physics, it serves as a guide to understand the on-going research in nuclear physics.

It has been found experimentally that with more or less central collisions at low (but above-barrier) energy, fusion, that is, the formation of a composite mono-nucleus with ZCN = Z1 + Z2 and ACN = A1 + A2, is likely to occur. Thus the fusion cross section at above-barrier energies is close to the geometric cross section. This is true, however, only for the collision of light and medium-mass nuclei (or low-mass nuclei with heavy ones). As the mass of the colliding nuclei increases, quasi-fission processes play an increasing role (see the previous chapter) and the probability of fusion sharply decreases, making the synthesis of new superheavy nuclei difficult (see below). At energies below the height of the Coulomb barrier, the fusion cross section decreases exponentially. The probability of fusion in this case is determined essentially by the probability of tunneling through a potential barrier (the inverse process to emission of an α particle or heavy cluster). The excitation of the collective degrees of freedom (for example, surface vibrations or the rotation of deformed nuclei) strongly influence the process of sub-barrier fusion (see below).

Received 23 January 2018DOI:https://doi.org/10.1103/PhysRevC.97.019904©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionLow & intermediate energy heavy-ion reactionsNuclear fusionNuclear reactionsTransfer reactionsProperties190 ≤ A ≤ 219Nuclear Physics

Background: Low-energy multinucleon transfer reactions may be used for production of new neutron-enriched heavy nuclei. Purpose: Our aim is to investigate the influence of proton (Z = 82) and neutron (N = 82, 126) shells as well as orientation effects on the formation of reaction products in the inverse quasifission process in the reactions Gd-156,Gd-160 + W-186. Methods: Mass, energy, and angular distributions of primary binary fragments formed in the reactions Gd-156 + (186)Wat an energy of 878 MeV, and Gd-160 + (186)Wat 860 and 935 MeV, have been measured using the double-arm time-of-flight spectrometer CORSET at the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research (JINR), Dubna. Results: Enhancement in the yield of products with masses 200-215 u has been found for both reactions. The cross sections of the formation of trans-target fragments with masses around 208 u are found to be about 10 mu b at the Coulomb barrier energy and reach the level of 0.5 mb at the energy above the barrier for side-to-side collision. Conclusions: The enhanced yield of products with masses heavier than the target mass confirms the important role of the closed shells at Z = 82 and N = 82, 126 in the inverse quasifission process in low-energy damped collisions. The orientation effect caused by the strong deformation of colliding nuclei can result in a gain in the yield of heavy target-like fragments.

This is a brief report on the current status of the new GAs cell based Laser ionization Setup (GALS) at Flerov Laboratory for Nuclear Reactions (FLNR) - JINR, Dubna. GALS is planned to exploit available beams from the U-400M cyclotron in low energy multi-nucleon transfer reactions to study exotic neutron-rich nuclei located in the "north-east" region of nuclear map. Products from 4.5 to 9 MeV/nucleon heavy-ion collisions, such as Xe-136 on Pb-208, are to be captured in a gas cell and selectively laser-ionized in a sextupole (quadrupole) ion guide extraction system.

Low values of the fusion cross sections and very short half-lives of nuclei with Z > 120 put obstacles in synthesis of new elements. However the fusion reactions of medium mass projectiles (including RIB) with different actinide targets still can be used for the production of the not-yet-synthesized SH nuclei. The gap of unknown SH nuclei, located between the isotopes which were produced earlier in the cold and hot fusion reactions, could be filled in fusion reactions of Ca-48 with available lighter isotopes of Pu, Am, and Cm. The neutron-enriched isotopes of SH elements may be produced with the use of a Ca-48 beam if a Cm-250 target would be prepared. In this case we get a real chance to reach the island of stability owing to a possible electron capture in (291)Fl nucleus formed in the 3n evaporation channel of this reaction with a cross section of about 0.8 pb. Multi-nucleon transfer processes at near barrier collisions of heavy (and very heavy, U-like) ions seem to be the most realistic reaction mechanism allowing one to produce new neutron enriched heavy nuclei located in the unexplored upper part of the nuclear map. The predictions for the production of new neutron rich heavy nuclei in multinucleon transfer reactions will be given. A special attention will be paid to the "inverse" quasi-fission mechanism leading to formation of reaction fragments with masses lighter than projectile and heavier than target masses.

Principles underlying the organization and operation of the NRV web knowledge base on low-energy nuclear physics (http://nrv.jinr.ru) are described. This base includes a vast body of digitized experimental data on the properties of nuclei and on cross sections for nuclear reactions that is combined with a wide set of interconnected computer programs for simulating complex nuclear dynamics, which work directly in the browser of a remote user. Also, the current situation in the realms of application of network information technologies in nuclear physics is surveyed. The potential of the NRV knowledge base is illustrated in detail by applying it to the example of an analysis of the fusion of nuclei that is followed by the decay of the excited compound nucleus formed.

The problem of production and study of heavy neutron-rich nuclei has been intensively discussed during recent years. Many reasons arouse a great interest in this problem. In the upper part of the nuclear map mostly proton-rich nuclei were studied while the unexplored area of heavy neutron-rich nuclides (also those located along the neutron closed shell N = 126 to the right-hand side of the stability line) is extremely important for the understanding of the r process of astrophysical nucleogenesis. For elements with Z > 100 only neutron deficient isotopes (located to the left of the stability line) have been synthesized so far. The "northeast" area of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes. There are only 3 methods for the production of heavy and superheavy (SH) nuclei, namely, a sequence of neutron capture and beta(-) decay, fusion reactions, and multinucleon transfer reactions. Multinucleon transfer processes look most promising for the production and study of neutron-rich heavy nuclei located in the upper part of the nuclear map. Reactions with actinide beams and targets are of special interest for synthesis of new neutron-enriched transfermium nuclei and not-yet-known nuclei with closed neutron shell N = 126 having the largest impact on the astrophysical r-process. The estimated cross sections for the production of these nuclei allows one to plan such experiments at currently available accelerators if the problem of separation of heavy transfer reaction products would be solved.

Production and studying properties of superheavy (SH) nuclei meet significant experimental difficulties owing to extremely low cross sections of their formation in heavy ion fusion or multinucleon transfer reactions. Accurate predictions of these cross sections along with the corresponding excitation functions are quite desirable for planning and performing experiments of such kind. Study of these cross sections (their dependence on projectile–target combination and energy dependence) gives us an opportunity to investigate complicated dynamics of low-energy heavy ion collisions as well as decay properties of excited SH nuclei.

Exotic Nuclei, pp. 103-113 (2015) No AccessSUB-BARRIER FUSION: DOES REARRANGEMENT OF NEUTRONS PLAY A ROLE?A. V. Karpov, V. A. Rachkov, A. Adel, A. S. Denikin, and V. I. ZagrebaevA. V. KarpovFlerov Laboratory of Nuclear Reactions, JINR, Dubna, 141982, Russia, V. A. RachkovFlerov Laboratory of Nuclear Reactions, JINR, Dubna, 141982, Russia, A. AdelPhysics Department, Faculty of Science, Cairo University, Giza, Egypt, A. S. DenikinFlerov Laboratory of Nuclear Reactions, JINR, Dubna, 141982, RussiaInternational University Dubna, 141982, Russia, and V. I. ZagrebaevFlerov Laboratory of Nuclear Reactions, JINR, Dubna, 141982, Russiahttps://doi.org/10.1142/9789814699464_0011Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: Different factors influencing the sub-barrier fusion enhancement owing to neutron rearrangement with positive Q values are studied. It was found that opposite to existing opinion the presence of positive Q values is necessary but not sufficient to observe enhancement of the sub-barrier fusion. "Rigidity" of colliding nuclei with respect to collective excitations plays a crucial role for the sub-barrier fusion enhancement due to neutron rearrangement. A special attention is paid to the peculiarities of fusion of light exotic nuclei. Keywords: sub-barrier fusionchannel couplingneutron rearrangementlight weakly-bound nuclei FiguresReferencesRelatedDetails Exotic NucleiMetrics History Keywordssub-barrier fusionchannel couplingneutron rearrangementlight weakly-bound nucleiPDF download

Multinucleon transfer processes in low-energy heavy ion collisions open new field of research in nuclear physics, namely, production and studying properties of not-yet-explored heavy neutron-rich nuclei. Beams of very heavy U-like ions are needed to produce new long-living isotopes of transfermium and superheavy elements. Beams of medium-mass ions can be used for the production of neutron-rich nuclei located along the neutron closed shell N = 126 (the last waiting point) having the largest impact on the astrophysical r-process. Low-energy multinucleon transfer reactions is a very effective tool also for the production and spectroscopic study of light exotic nuclei. The corresponding cross sections are found to be 2 orders of magnitude larger as compared with high energy fragmentation reactions.