The new neutron-deficient isotope $$^{249}$$ No was synthesized for the first time in the fusion-evaporation reaction $$^{204}$$ Pb( $$^{48}$$ Ca,3n) $$^{249}$$ No. After separation, using the kinematic separator SHELS, the new isotope was identified with the GABRIELA detection system through genetic correlations with the known daughter and granddaughter nuclei $$^{245}$$ Fm and $$^{241}$$ Cf. The alpha-decay activity of $$^{249}$$ No has an energy of 9129(22) keV and half-life 38.3(2.8) ms. An upper limit of 0.2% was measured for the fission branch of $$^{249}$$ No. Based on the present data and recent information on the decay properties of $$^{253}$$ Rf and aided by Geant4 simulations, the ground state of $$^{249}$$ No is assigned the 5/2 $$^+$$ [622] neutron configuration and a partial decay scheme from $$^{253}$$ Rf to $$^{245}$$ Fm could be established. The production cross-section was found to be $$\sigma $$ (3n)=0.47(4) nb at a mid-target beam energy of 225.4 MeV, which corresponds to the maximum of the calculated excitation function. Correlations of the $$^{249}$$ No alpha activity with subsequent alpha decays of energy 7728(20) keV and half-life $$1.2_{-0.4}^{+1.0}$$ min provided a firm measurement of the electron-capture or $$\beta ^{+}$$ branch of $$^{245}$$ Fm to $$^{245}$$ Es. The excitation function for the 1n, 2n and 3n evaporation channels was measured. In the case of the 2n-evaporation channel $$^{250}$$ No, a strong variation of the ground state and isomeric state populations as a function of bombarding energy could be evidenced.

The paper describes the NRV web knowledge base on low-energy nuclear physics developed in the Joint Institute for Nuclear Research. The NRV knowledge base working through the Internet integrates a large amount of digitized experimental data on the properties of nuclei and nuclear reaction cross sections with a wide range of computational programs for modeling of nuclear properties and nuclear dynamics. Today, the NRV becomes a powerful instrument for nuclear physics research as well as for educational applications. Advantages of the functioning scheme of the knowledge base provide the synergy of coexistence of the experimental data and computational codes within one platform.

The NRV web knowledge base on low-energy nuclear physics has been created in the Joint Institute for Nuclear Research. This knowledge base working through the Internet integrates a large amount of digitized experimental data on the properties of nuclei and nuclear reaction cross sections with a wide range of computational programs for modeling of nuclear properties and various processes of nuclear dynamics which run directly in the browser of a remote user. Today, the NRV knowledge base is a powerful instrument for nuclear physics research. The basic principles of the NRV knowledge base are covered, and a brief description of its structure is given. The practical usage of the NRV knowledge base for both scientific and educational applications is demonstrated in detail.

The formation of new isotopes of heavy and superheavy elements in the fusion of neutron-enriched projectiles with actinide targets is discussed. Cross sections for the formation of evaporation residues in fusion reactions is predicted for several combinations of colliding nuclei.

The NRV web knowledge base on low-energy nuclear physics has been created in the Joint Institute for Nuclear Research. This knowledge base working through the Internet integrates a large amount of digitized experimental data on the properties of nuclei and nuclear reaction cross sections with a wide range of computational programs for modeling of nuclear properties and various processes of nuclear dynamics which run directly in the browser of a remote user. Today, the NRV knowledge base is both a powerful tool for nuclear physics research and an educational resource. The system is widely used, as evidenced by the large number of user queries to its resources and the number of references to the knowledge base in the articles published in scientific journals. The practical usage of the NRV knowledge base for both scientific and educational applications is demonstrated.

A quantum coupled-channel approach with collective degrees of freedom (the rotation of deformed nuclei and/or their surface vibrations) is combined with an empirical coupled-channel model to add neutron rearrangement channels to vibrational and rotational excitations. The calculated fusion cross sections and the barrier distribution functions for several combinations of nuclei are in good agreement with experimental data.

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.

Background: Significant enhancement of sub-barrier fusion cross sections owing to neutron transfer with positive Q values was observed in many combinations of colliding nuclei. This degree of freedom has not yet been included into the rigorous quantum coupled-channels (QCC) approach. However, the empirical coupled-channels model with neutron rearrangement [Zagrebaev, Phys. Rev. C 67, 061601 (2003)] has already been successfully used in several papers to reproduce and predict cross sections for sub-barrier fusion reactions of stable nuclei. Purpose: The objective of this study is to combine the QCC approach and the empirical model to account for additional channels of neutron rearrangement. Method: Coupling of relative motion to collective degrees of freedom (rotation of nuclei and/or their surface vibrations) are taken into account within the QCC approach. The probability of transfer of x neutrons with a given Q value is estimated semiclassically. Results: The proposed new model was successfully tested on a few combinations of fusing nuclei Ca-40(+) Zr-90,Zr-94,Zr-96, S-32+ Zr-90,Zr-94,Zr-96, and Ni-60,Ni-64 + Mo-100. The calculated fusion cross sections and barrier distribution functions agree well with experimental data. Conclusions: The model developed in this work confirms all the conclusions previously made within the empirical coupled-channels model with neutron rearrangement [see Rachkov et al., Phys. Rev. C 90, 014614 (2014)]. Moreover, it has an advantage of a more reliable microscopic account for the coupling between relative motion and the collective degrees of freedom. The proposed model can also be used to reproduce the structure of the barrier distribution function. This is a step forward to a complete solution of the long-term problem of accounting for neutron transfer channels in the QCC model.

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

Different factors influencing the sub-barrier fusion enhancement owing to neutron rearrangement with positive Q values are studied. It was found that opposite to the previous 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. Neutron binding energy has a strong impact but only in the case of fusion of light nuclei.

The effect neutron rearrangement channels have on nuclear fusion at energies below the Coulomb barrier is considered. The latest experimental data, which reveal no enhancement of the fusion cross section at subbarrier energies despite the presence of rearrangement channels for neutrons with Q > 0 are explained using the empirical channel coupling model. The mechanism of neutron rearrangement in fusion reactions is studied in detail. The conditions required for additional enhancement (complementary to the one associated with the effect of collective excitation channels) of the subbarrier fusion cross section are formulated.

Background: Significant enhancement of sub-barrier fusion cross sections owing to neutron rearrangement with positive Q values were found for many combinations of colliding nuclei. However, several experimental results on fusion reactions were reported recently in which such enhancement has not been observed in spite of a possibility for neutron rearrangement with positive Q values.Purpose: We aim to clarify much better the mechanism of neutron rearrangement in sub-barrier fusion reactions to find the other requirements (beside positive Q values) which favor (or prevent) sub-barrier fusion enhancement. Methods: A channel coupling approach along with the semiclassical model for neutron transfer has been used for analysis of available experimental data on sub-barrier fusion of heavy ions.Results: The role and interplay of different factors determining the enhancement of sub-barrier fusion (such as Q values for neutron rearrangement, properties of collective excitations, and neutron binding energies) have been studied and clarified.Conclusions: (1) Only 1n and 2n transfers with positive Q values have a noticeable impact on sub-barrier fusion. A positive Q value for neutron rearrangement is a necessary but not sufficient requirement for additional sub-barrier fusion enhancement to take place. (2) The "rigidity" of colliding nuclei with respect to collective excitations is important for sub-barrier fusion enhancement due to neutron rearrangement with positive Q values to be clearly visible. (3) The neutron binding energy of the "donor" nucleus has a strong impact only in the case of fusion of light weakly bound nuclei.

[Background] Significant enhancement of sub-barrier fusion cross sections owing to neutron rearrangement with positive $Q$-values were found for many combinations of colliding nuclei. However several experimental results on fusion reactions were reported recently in which such enhancement has not been observed in spite of a possibility for neutron rearrangement with positive $Q$-values. [Purpose] We aim to clarify much better the mechanism of neutron rearrangement in sub-barrier fusion reactions to find the other requirements (beside positive $Q$-value) which favour (or prevent) sub-barrier fusion enhancement. [Method] Channel coupling approach along with the semi-classical model for neutron transfer have been used for analysis of available experimental data. [Results] (1) Only 1n and 2n transfers with positive $Q$-values have a noticeable impact on sub-barrier fusion. Positive $Q$-value for neutron rearrangement is necessary but not sufficient requirement for additional sub-barrier fusion enhancement takes place. (2) "Rigidity" of colliding nuclei in respect of collective excitations is important that the sub-barrier fusion enhancement due to neutron rearrangement with positive $Q$-value be clearly visible. (3) Neutron binding energy in "donor" nucleus has a strong impact only in the case of fusion of light weakly bound nuclei.

The role of neutron transfer in fusion reactions of weakly-bound nuclei at subbarrier energies is studied within the empirical model of channel coupling. The results from calculating the fusion cross sections for the 7 Li + 209 Bi, 9, 11 Li + 208, 206 Pb, 6, 7, 9, 11 Li + 152 Sm reactions are presented. Good agreement with the available experimental data is shown. Several combinations of colliding nuclei for which the strong enhancement of subbarrier fusion due to the effect of neutron transfer processes are predicted.

Reactions of radiative capture of weakly bound light nuclei are investigated in the scope of the potential model. The applicability of the approach is demonstrated with an analysis of the cross section for the 6 Li(p, γ) 7 Be reaction. Good agreement with the available experimental data is shown. The radiative capture cross section for the 6 He(p, γ) 7 Li reaction in the region of low sub-barrier energies is evaluated. It is concluded that reactions involving 6 He could also play a noteworthy part in stellar nucleosynthesis processes.

The role of neutron transfer is investigated in the fusion process near and below the Coulomb barrier within the empirical channel coupling approach. The possibility of neutron transfer with positive Q-values considerably increases the barrier penetrability. The enhancement of fusion cross sections for 58Ni+64Ni, 32S+64Ni, 40Ca+48Ca, and 40Ca+124Sn is well reproduced at subbarrier energies by the empirical channel coupling approach including the coupling to the neutron-transfer channels. The predictions of the fusion cross sections for several combinations of colliding nuclei are also proposed which may shed additional light on the effect of neutron transfer in fusion processes. A huge enhancement of deep subbarrier fusion probability was found for light neutron-rich weakly bound nuclei. This may be quite important for astrophysical primordial and supernova nucleosynthesis.