In this review the main advances in heavy-ion fusion research that have taken place over the last decade are addressed. During this period, experimental studies have been extended to deep subbarrier energies to reveal the unexpected phenomenon of fusion hindrance. The coupled-channels descriptions have been refined to include the effects of nucleon transfer and to account for the fusion hindrance in terms of the ion-ion potential in the strongly overlapping region. Substantial progress has been made in time-dependent Hartree-Fock theory to the point that this approach now can make parameter-free predictions of heavy-ion fusion excitation functions. As several heavy-ion fusion reactions are of crucial importance in late-stage giant-star evolution, these reactions continue to be studied with better experimental and theoretical tools in order to provide improved input to astrophysical models. The effects of loosely bound valence nucleons on the fusion cross sections are the focus of a number of experimental studies involving radioactive beams, which have only recently become available. And finally, as the active field of synthesizing superheavy elements relies on heavy-ion fusion to reach the nuclei of interest, it is important to understand the fusion dynamics that plays a crucial role in both the “cold-fusion” and “hot-fusion” approaches to the superheavy island of stability. Also this area has seen significant progress in several different approaches to the problem of predicting the cross sections for formation and survival of these rare nuclei.

Recent developments in heavy-ion fusion reactions

Fusion excitation functions with high precision have been measured for the ${}^{16}$O+${}^{76}$Ge and ${}^{18}$O+${}^{74}$Ge systems at energies near and below the Coulomb barrier. The barrier distributions have been extracted from the corresponding excitation functions. The coupling effect of the positive $Q$-value neutron transfer channel in the heavy-ion fusion reaction was examined by a comparison of the two systems. The fusion excitation functions are well reproduced by coupled-channels calculations including the 2${}^{+}$ and 3${}^{\ensuremath{-}}$ vibrational states of the targets and the 2${}^{+}$ rotational state of ${}^{18}$O projectile. No obvious fusion enhancement due to the positive $Q$-value two-neutron transfer for the ${}^{18}$O+${}^{74}$Ge system is observed as compared with the reference system of ${}^{16}$O+${}^{76}$Ge. Analogous systems which are available in the literature are also discussed to realize the role of one- and/or two-neutron transfers with positive $Q$ values in the fusion process.

Fusion of the 16 O+ 76 Ge and 18 O+ 74 Ge systems and the role of positive Q-value neutron transfers

The effect of the positive $Q$-value neutron transfer (PQNT) channels on the subbarrier fusion process is still far from a clear understanding and hence is worth further investigation. To this end, we have measured the fusion excitation function for $^{32}\mathrm{S}+^{94}\mathrm{Zr}$ at energies near and below the Coulomb barrier with rather good accuracy, and deduced the experimental fusion barrier distribution. The reaction system $^{32}\mathrm{S}+^{94}\mathrm{Zr}$ is a good candidate for the evaluation of PQNT effect because it has multineutron transfer channels with positive ground-state ${Q}_{xn}$ values as well as the relatively weak low-lying ${3}^{\ensuremath{-}}$ vibrational state in $^{94}\mathrm{Zr}$. Our results show that the fusion cross sections of $^{32}\mathrm{S}+^{94}\mathrm{Zr}$ are strongly enhanced at the lower-energy region. By means of a cross comparison of the $^{32}\mathrm{S}$,$^{40}\mathrm{Ca}+^{90,94,96}\mathrm{Zr}$ systems, further evidence is given that the positive ${Q}_{xn}$-value neutron transfer channels, rather than the strong low-lying ${3}^{\ensuremath{-}}$ vibrational state, bring about the additional subbarrier fusion enhancement. Relative to the coupled-channels results with couplings to the low-lying inelastic collective excitations, an unexpectedly large enhancement for the subbarrier fusion cross sections is observed in $^{32}\mathrm{S}+^{94}\mathrm{Zr}$ compared to $^{32}\mathrm{S}+^{96}\mathrm{Zr}$. Some existing theoretical models have been discussed, but a good explanation cannot be achieved. Exact coupled-channels theory and more experiments on transfer reactions themselves are strongly desired to comprehensively understand the effect of transfer on fusion.

Fusion of S 32 + Zr 94 : Further exploration of the effect of the positive Q x n value neutron transfer channels

The fusion excitation function of 60
 Ni + 100
 Mo has been measured from above the Coulomb barrier down to a cross section around 2 μb, looking for coupling and hindrance effects in this soft medium-mass system with positive Q-values for several neutron transfer channels. A comparison is made with previous results for 64
 Ni + 100
 Mo where no Q > 0 transfer channels exist and the hindrance effect is quite clear. The two excitation functions are very similar, as well as the corresponding logarithmic derivatives showing analogous saturations below the barrier. It appears that transfer couplings to Q > 0 channels seem to play a marginal role near and below the barrier for 60
 Ni + 100
 Mo , even if measurements of cross sections lower than 1 μb would be needed also for this system. Coupled-channels calculations confirm these observations and indicate that multi-phonon excitations dominate the fusion dynamics in the whole measured energy range.

Fusion of 60 Ni + 100 Mo near and below the Coulomb barrier

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 tofindtheotherrequirements(besidepositive Qvalues)whichfavor(orprevent)sub-barrierfusionenhancement. 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. DOI: 10.1103/PhysRevC.90.014614

http://nrv.jinr.ru/pdf_file/PhysRevC.90.014614.pdf

Examining the enhancement of sub-barrier fusion cross sections by neutron transfer with positive Q values

The fusion excitation functions for radioactive (132)Sn + (58)Ni and stable (130)Te + (58,64)Ni were measured at energies near the Coulomb barrier. The coupling of transfer channels in heavy-ion fusion was examined through a comparison of Sn + Ni and Te + Ni systems, which have large variations in the number of positive Q-value nucleon transfer channels. In contrast with previous experimental comparisons, where increased sub-barrier fusion cross sections were observed in systems with positive Q-value neutron transfer channels, the reduced excitation functions were equivalent for the different Sn + Ni and Te + Ni systems. The present results suggest a dramatically different influence of positive Q-value transfer channels on the fusion process for the Sn + Ni and Te + Ni systems.

/pdf/near-barrier-fusion-of-sn-ni-and-te-ni-systems-examining-the-5f24531tm0.pdf

Near-barrier fusion of Sn + Ni and Te + Ni systems: examining the correlation between nucleon transfer and fusion enhancement

En este trabajo se presenta un detector centellador en el que se determina la posicion del destello luminoso producido por el paso de particulas cargadas mediante la logica Anger. El material sensible del detector es una placa centelladora (PILOT-U) de 170 × 170 × 10 mm 3 . La luz producida es recibida por un arreglo de nueve fotomultiplicadores acoplados a una de las caras de la placa. Los datos obtenidos utilizando una fuente radioactiva emisora alfa, en comparacion con simulaciones Monte Carlo, sugieren una resolucion intrinseca para la determinacion de la posicion de 6 mm para este sistema.

http://www.scielo.org.mx/pdf/rmf/v58n3/v58n3a6.pdf

Position sensitive detection system for charged particles

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Near-barrier fusion of Sn + Ni and Te + Ni systems: examining the correlation between nucleon transfer and fusion enhancement

Position sensitive detection system for charged particles