Neutron Transfer Research Articles

Special Features of Nuclear Reactions Induced by Loosely Bound Nuclei in the Vicinity of the Coulomb Barrier Height

Excitation functions are measured for complete-fusion and transfer reactions induced by the interaction of 6He and 6Li with 206Pb, 209Bi, and Pt. Data obtained for fusion reactions induced by 6He ions deviate from the predictions of the statistical model of compound-nucleus decay at projectile energies in the vicinity of the Coulomb barrier height. A strong enhancement of cross sections for fusion reactions induced by the interaction of 6He with target nuclei is observed. The cross sections for reactions of cluster transfer, neutron transfer from 6He, and deuteron transfer from 6Li at deep-subbarrier energies, are also found to be enhanced. These results are discussed from the point of view of the effect of the cluster structure of nuclei on the interaction probability at energies in the vicinity of the Coulomb barrier height.

Unexpected results in neutron-rich radioactive beams induced fusion

The fission-fragment beams at HRIBF provide a unique opportunity for studying the mechanisms of fusion involving nuclei with large neutron excess. To explore the role of transfer couplings, fusion excitation functions have been measured using neutron-rich radioactive 132Sn beams incident on 40Ca and 58Ni targets. The sub-barrier fusion enhancement for 132 Sn+40Ca is larger than that for 132Sn+58Ni although the neutron transfer Q-values are similar for the two reactions. The fusion excitation function for 46Ti+124Sn has been measured in an attempt to resolve the differences observed in 132Sn+40Ca and 132Sn+58Ni.

Near-barrier fusion of 32S+90,96Zr, 16O+76Ge and 18O + 74Ge

This contribution presents the recent experimental results on heavy-ion near-barrier fusion, mainly aiming at the effect of positive Q-value neutron transfer on the fusion process. Fusion excitation functions of 32S+90'96Zr, 16O+76Ge and 18O+74Ge have been measured with an electrostatic deflector setup. The fusion barrier distributions have been extracted from the measured fusion excitation functions. Different conclusions are obtained from the analysis of both the fusion excitation functions and the barrier distributions, that is, 32S+96Zr shows obvious fusion enhancement related to the positive Q-value neutron transfer channels and was reproduced with Zagrebaev's semiclassical model, but 18O+74Ge shows no such effect. More comprehensive reaction dynamics need to be considered than the up-to-date coupled-channels calculations.

Exploring the breakup and transfer coupling effects in9Be elastic scattering

Two cluster structures of 9Be namely, 5He+4He and 8Be+n have been considered for its breakup to describe the available data on elastic scattering for 9Be projectile with 28Si, 64Zn and 144Sm targets. The results of these calculations suggest that the breakup coupling effects are significant for 5He+4He cluster model at above the barrier energies, while they are dominant at relatively lower energies for 8Be+n model. The addition of one neutron stripping channel in the 8Be+n model gives an overall good description of the elastic data for all the systems considered. The couplings generated by breakup in 8Be+n model have different behaviour than the coupling effects obtained within the 5He+4He model, the former being more prominent at lower energies and for the heavier target systems. The behaviour of extracted dynamic polarisation potentials (DPP) generated due to breakup and one neutron transfer couplings have been investigated.

Nuclear Data Sheets for A = 89

The evaluated experimental data are presented for 14 known nuclides of mass 89 (Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh). New data on high–spin excitations are available for 89Kr, 89Rb, 89Sr, 89Tc and 89Ru, including a superdeformed structure in 89Tc. Fluorescence studies in 89Y have revealed a large number of new levels in 7–11 MeV range. New two–neutron transfer data are available for 89Zr. New direct and precise measurement of atomic masses of 89Se, 89Br, 89Kr, 89Rb, 89Mo and 89Tc have greatly improved the Q value landscape in this mass region. This work supersedes previous A=89 evaluation published in 1998Si31, and also the older ones 1989Si20, 1975Ko21, and M.W. Johns et al., Nuclear Data Tables A 8, 373 (1970).In spite of extensive experimental work on the isobaric nuclei of this mass chain several deficiencies remain. The identification of 89Rh isotope remains unconfirmed. The half–lives of ground states of 89Ge, 89As and 89Rh have not been measured, only the lower limits are estimated from time–of–flight in a reaction and experimental arrangement, where produced and identified. The decay schemes of 89Ge, 89As, 89Se, 89Ru and 89Rh are not known, and those for 89Tc and 89Mo are incomplete. High–lying (neutron unbound) levels in 89Kr, expected to be populated in the decay of 89Br, have not been investigated. The spins and parities for levels in 89Kr are largely unassigned, while for 89Tc and 89Ru, only few low–spin excitations are established. For 89Rb, little information is available for high–spin structures. Detailed gamma–ray data are available for 88Sr(n,γ)89Sr reaction, but most of these γ rays remain unassigned in a level scheme. Excited state data are nonexistent for 89Ge, 89As, 89Se, 89Br and 89Rh. The 89Y and 89Zr are the most extensively studied nuclei in A=89.

Study of Isotopic Effects in Capture Process

Recently, many experimental and theoretical efforts have been devoted to the investigation of fusion, fission, and capture processes at sub-barrier energies [1–4]. Measurements of excitation functions down to the extreme sub-barrier energies are important for studying the long range behavior of the nucleus–nucleus interaction. The experimental data obtained are also of interest for solving astrophysical problems related to nuclear synthesis. The main objective of the present work is to study the isotopic dependencies of the capture cross section and the mean-square angular momentum in the reactions 48Ca + 144,150,154Sm and 40Ca + 154Sm. The quantum diffusion approach [3, 4] is employed. The collisions of nuclei are treated in terms of a single dynamic collective coordinate, the relative distance between the colliding nuclei. Our approach takes into consideration the fluctuation and dissipation effects in collisions of heavy ions that model the coupling with various channels [3, 4]. The deformation and neutron transfer effects are taken into account through the dependence of the nucleus–nucleus potential on the deformations and orientations of interacting deformed nuclei.

Fusion of the16O+76Ge and18O+74Ge systems and the role of positiveQ-value neutron transfers

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.

Complete γ-spectroscopy of n-rich nuclei around 48Ca with Multi-Nucleon Transfer reactions

The gamma decay of neutron-rich nuclei around 48Ca was measured at Legnaro National Laboratory with the PRISMA-CLARA setup, using the multi-nucleon transfer reaction 48Ca on 64Ni at 282 MeV. Evidence is found for a large spin alignment which allows to use angular distributions and polarizations of gamma rays to firmly establish, for the first time, spin and parities of several excited states. In the one neutron transfer channels, 49Ca and 47Ca, states arising by coupling a single particle to the 3− phonon of 48Ca are observed, showing the robustness of nuclear collectivity in rather light systems. The work demonstrates the feasibility of complete in-beam γ-spectroscopy with heavy-ion transfer reactions and provides a method that can be further exploited in the future with heavy targets and radioactive beams.

Missing mass spectroscopy on oxygen isotopes beyond the proton-drip line: mirror symmetry of nuclear shell evolution

Missing mass spectroscopy of the proton-rich 12O and 13O nuclei was performed via the neutron transfer (p, t and (p, d reactions, respectively, using a 14O beam of 51 MeV/u at GANIL (Grand Accelerateur National d’Ions Lourds). In addition to the ground states, an excited state of 12O at 1.8(4) MeV and two excited states of 13O at 2.8(3) and 4.2(3) MeV were observed. Spin-parity and/or spectroscopic factors were obtained from the comparison of the differential cross-sections to distorted-wave calculations. The excited state of 12O with a suggested spin-parity of 0+ or 2+ has a significantly low excitation energy, indicating that the proton shell closure at Z = 8 vanishes in 12O. The spin-parity of 1/2+ was suggested for the 2.8MeV state, which implies that the proton shell gap is weak in 13O, whereas the large spectroscopic factor extracted from the 14O(p , d reaction indicates that the ground 3/2− state remains dominated by normal p-shell configurations. These features of 12O and 13O have marked similarities with their neutron-rich mirror partners 12Be and 13B, respectively, demonstrating mirror symmetry in the fading of the shell closure at magic numbers 8 near the drip lines.

Sub-barrier interaction between deuterons and 58, 62Ni nuclei

The interaction between deuterons and 58, 62Ni nuclei at energies of E d = 3.5, 4.5 and 5.16 MeV is investigated. The discrepancy between measured scattering elastic cross section and the Rutherford ones is higher than the value calculated theoretically by considering deuterons polarization and Coulomb breakup. Analysis of measured cross section of 58, 62Ni(d, p) reaction and the results of calculation of Coulomb breakup cross section integrated over neutron emission angles shows that that the dominant mechanism of proton formation is the reaction of neutron transfer to the target nucleus.

Influence of neutron transfer in reactions with weakly and strongly bound nuclei on the sub-barrier capture process

The sub-barrier capture reactions ${}^{13,14,15}\mathrm{C}+{}^{232}\mathrm{Th}$, ${}^{4,6}\mathrm{He}+{}^{208,206}\mathrm{Pb}$, ${}^{32}\mathrm{S}+{}^{94,96,98,100}\mathrm{Mo}$, ${}^{100,102,104}$Ru, ${}^{104,106,108,110}$Pd, ${}^{112,116}$Sn, ${}^{36}\mathrm{S}+{}^{92}\mathrm{Mo}$, and ${}^{18}\mathrm{O}+{}^{112,118,124}\mathrm{Sn}$ with light weakly and strongly bound nuclei are studied within the quantum diffusion approach. Based on our analysis a weak influence of halo nuclei on the capture is demonstrated in the reactions mentioned. It is shown that in some cases the neutron transfer with positive $Q$ value can weakly influence or suppress the sub-barrier capture through the change of the deformations of the colliding nuclei. Capture cross sections for the reactions ${}^{13,14,15}\mathrm{C}+{}^{208}\mathrm{Pb}$, ${}^{32,36}\mathrm{S}+{}^{132}\mathrm{Sn}$, and ${}^{40,48}\mathrm{Ca}+{}^{126}\mathrm{Sn}$ are predicted.

Erratum: Role of neutron transfer in capture processes at sub-barrier energies [Phys. Rev. C85, 024616 (2012)

Erratum: Role of neutron transfer in capture processes at sub-barrier energies [Phys. Rev. C<b>85</b>, 024616 (2012)

Complete γ-spectroscopy of neutron-rich nuclei around 48Ca by Heavy-Ion Transfer reactions

The gamma decay of neutron-rich nuclei around 48Ca was measured at Legnaro National Laboratory with the PRISMA-CLARA setup, using the heavy-ion transfer reaction 48Ca on 64Ni at ~6MeV/A. The analysis shows the feasibility to perform complete in-beam γ-spectroscopy with heavy-ion transfer reactions (in terms of angular distributions, polarizations of the γ rays and lifetimes of the nuclear states), providing a method that can be further exploited in the future with heavy targets and radioactive beams. In the one neutron transfer channels, 49Ca and 47Ca, states arising by coupling a single particle to the 3 phonon of 48Ca are observed, showing the robustness of nuclear collectivity in rather light systems.

Microscopic sub-barrier fusion calculations for the neutron star crust

Fusion of very neutron-rich nuclei may be important to determine the composition and heating of the crust of accreting neutron stars. Fusion cross sections are calculated using time-dependent Hartree-Fock theory coupled with density-constrained Hartree-Fock calculations to deduce an effective potential. Systems studied include ${}^{16}$O+${}^{16}$O, ${}^{16}$O+${}^{24}$O, ${}^{24}$O+${}^{24}$O, ${}^{12}$C+${}^{16}$O, and ${}^{12}$C+${}^{24}$O. We find remarkable agreement with experimental cross sections for the fusion of stable nuclei. Our simulations use the SLy4 Skyrme force that has been previously fit to the properties of stable nuclei, and no parameters have been fit to fusion data. We compare our results to the simple S\~ao Paulo static barrier penetration model. For the asymmetric systems ${}^{12}$C+${}^{24}$O or ${}^{16}$O+${}^{24}$O we predict an order of magnitude larger cross section than those predicted by the S\~ao Paulo model. This is likely due to the transfer of neutrons from the very neutron rich nucleus to the stable nucleus and dynamical rearrangements of the nuclear densities during the collision process. These effects are not included in potential models. This enhancement of fusion cross sections, for very neutron rich nuclei, can be tested in the laboratory with radioactive beams.

Fusion of124,132Sn with40,48Ca

Fusion excitation functions for the reactions of {sup 124,132}Sn with {sup 40,48}Ca were measured at energies near and below the Coulomb barrier. Weak sub-barrier fusion enhancement accounted for by coupling to the first 2{sup +} and 3{sup -} states in the target and projectile was observed for {sup 124,132}Sn+{sup 48}Ca. However, the {sup 124,132}Sn+{sup 40}Ca fusion cross sections are very strongly enhanced below the barrier and this difference survives even after correcting for trivial size effects. Although the enhancement appears to be related to the existence of large positive Q values for neutron transfer reactions, it is not proportional to the magnitudes of those Q values, which are much larger for {sup 132}Sn+{sup 40}Ca than for {sup 124}Sn+{sup 40}Ca.

Evidence Concerning the Mechanism of the Nuclear Reaction between Deuterium and Tritium

Both hydrogen and deuterium in their electrolysis on palladium gave linear Tafel lines but each had a kink in it at about 0.35 V overpotential (η). At this value damage begins to be seen in the SEM photographs of the interior of the electrodes. Tritium formation became detectable at an overpotential (η) of about 0.4 V. The reaction occurs in two consecutive steps: the first is a Volmer–Tafel reaction which occurs when the overpotential is less than 0.3 V. But as the measured coverage θD approaches unity, the deuterons begin to discharge onto the adsorbed deuterium (Heyrovsky mechanism). The second (consecutive) step leads to the tritium formation via a neutron transfer from the electrode to the Pd–D bond which at this stage (near to full occupancy by D) is relatively weak. The ready availability of neutrons in the surface region of the metal catalyst is rationalized in terms of the high electron concentration associated with the surface and he presence of D arriving from D2O in the solution: e− + D+ = 2 neutrons + a neutrino. The model is applied to two anomalies which have troubled this reaction. One is irreproducibility and also the long time (100 h or more) before any nuclear activity is seen.

Dynamic tunnel effect at low-energy nuclear reactions with neutron-rich nuclei

The probability of neutron transfer between nuclei at close low-energy collisions has been investigated as a function of the energy of the neutron separation, the energy in a center-of-mass system E and the collision impact parameter. The energy dependence of the neutron transfer cross section in the 6He + 197Au reaction has been explained.

Role of neutron transfer in capture processes at sub-barrier energies

The role of neutron transfer with positive $Q$-value in sub-barrier capture reactions ${}^{132}$Sn,${}^{130}$Te+${}^{58,64}$Ni, and ${}^{60}$Ni+${}^{100}$Mo, measured recently by Z. Kohley et al. [Phys. Rev. Lett. 107, 202701 (2011)] and F. Scarlassara et al. [EPJ Web Conf. 17, 05002 (2011)], is studied and explained within the quantum diffusion approach. It is shown that the transfer of two neutrons influences the sub-barrier capture through the change of the deformations of the colliding nuclei. Based on our analysis, one can understand why the influence of the neutron transfer is strong in some capture reactions, but is weak in others.

Low-lying neutronfp-shell intruder states in27Ne

The quenching of the N=20 shell gap in neutron-rich nuclei is investigated by studying the single-particle structure of 27Ne via neutron transfer using a 26Ne beam. Two low-lying negative-parity intruder states have been observed, the lowest of which is identified as Jπ=3/2−, confirming earlier speculations. A level identified as 7/2− is observed higher in energy than the 3/2−, contrary to the ordering at β-stability and at an energy significantly different from the predictions of previous shell-model calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-ℏω 0s-0p-0d-1s-0f-1p calculations in which there is a significant ad hoc reduction (∼0.7 MeV) in the N=20 shell gap.

Effect of neutron rearrangement on subbarrier fusion reactions

Abstract 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.