Neutron Transfer Channels Research Articles

Sub-barrier fusion enhancement due to neutron transfer

From the analysis of appropriate experimental data within a simple theoretical model, it is shown that the intermediate neutron transfer channels with positive Q values really enhance the fusion cross section at sub-barrier energies. The effect is found to be very large, especially for fusion of weakly bound nuclei. New experiments are proposed, which may shed additional light on the effect of neutron transfer in fusion processes.

Probing fusion dynamics with scattering experiments

The fusion of heavy nuclei at energies spanning the Coulomb barrier involves the coupling of their relative motion to the internal degrees of freedom of the binary system formed by them. The fusion process is thus a realisation of the fundamental problem of surmounting a potential barrier in an environment of many other degrees of freedom. In the nuclear binary system the number, strength, and nature of the coupling interactions can be widely varied by selecting specific projectile-target combinations. Strong coupling interactions result in a distribution of fusion barriers. These strong couplings can often be clearly identified by extracting representations of this fusion barrier distribution from precision measurements of the excitation function. Alternative representations of the fusion barrier distribution can be obtained from scattering experiments, which generally involve less experimental effort. Since they are not sensitive to the fusion barrier distribution at energies above the average fusion barrier, these alternative representations can only reveal coupling interactions with signatures in the low energy part of the distribution, such as coupling to positive Q-value neutron transfer channels.

Isotopic dependence and channel coupling effects in the fusion of16O+112,116Snand32S+112,116,120Snat energies around the barrier

We report here measurements of fusion cross-sections for the ${}^{16}\mathrm{O}{+}^{112,116}\mathrm{Sn}$ and ${}^{32}\mathrm{S}{+}^{112,116,120}\mathrm{Sn}$ systems. The set of systems studied with widely varying ground state Q values for the transfer of neutrons is a good case to understand the influence of single and multineutron transfer on the fusion process. The evaporation residue cross-sections were measured in small energy steps covering the region around the Coulomb barrier $(\ensuremath{\approx}{0.8V}_{b}$ to ${1.2V}_{b}).$ The data have been analyzed in the framework of a coupled-channels formalism using the coupled-channels code CCFULL. A comparative study of the ${}^{16}\mathrm{O}{+}^{A}\mathrm{Sn}$ and ${}^{32}\mathrm{S}{+}^{A}\mathrm{Sn}$ systems suggests that the fusion process is predominantly influenced by coupling to collective excitations with coupling to multiphonon states (of target and projectile) becoming increasingly important for heavier systems. From the present analysis an unambiguous signature of the role of coupling to neutron transfer channels could not be inferred.

Influence of higher-order deformations in the34S+168Erfusion reaction

The shape of the measured barrier distribution for the ^{34}S + ^{168}Er reaction is analysed using the coupled-channels description. The ^{168}Er nucleus is a good candidate to test current fusion models description of deformation since it has a large quadrupole deformation with an insignificant hexadecapole deformation. Coupling to weaker channels, the 2^+_1 state in ^{34}S, the 3^-_1 state in ^{168}Er, and the pair neutron transfer channel, all were found to have little influence on the barrier distribution. A successful description of the barrier distribution was only obtained after the hexacontatetrapole deformation term in ^{168}Er (beta_6) was included in the coupling scheme. However, a positive value for beta_6 was needed where the macroscopic-microscopic model predicts a negative one.

Angular momentum in the6He+209Bireaction deduced from isomer ratio measurements

The angular momentum distribution of the compound nucleus is a fundamental characteristic of the reaction dynamics and can provide insight into reactions involving neutron- or proton-rich projectiles. Specifically, following the fusion of ${}^{6}\mathrm{He}$ with ${}^{209}\mathrm{Bi}$ (at center-of-mass energies of 18 to 27 MeV), ${}^{212}\mathrm{At}$ is formed by the evaporation of three neutrons from the compound nucleus. The decay process leaves the residual ${}^{212}\mathrm{At}$ in either the ground state ${(J}^{\ensuremath{\pi}}{=1}^{\ensuremath{-}},$ ${T}_{1/2}=314$ ms) or a metastable state ${(J}^{\ensuremath{\pi}}{=9}^{\ensuremath{-}},$ ${T}_{1/2}=119$ ms). The ratio of the number of residual ${}^{212m}\mathrm{At}$ to the total number of ${}^{212}\mathrm{At}$ residual nuclei is sensitive to the original momentum distribution of the compound nucleus. The measured isomer ratio is consistent with that predicted by standard models. This agreement is observed even at the lower energies where the measured three neutron evaporation cross section is greatly enhanced compared to model calculations. While the inclusion of coupling to the neutron-transfer channels improves the agreement with the observed cross-section data somewhat, the predicted isomer ratio then diverges from the measured ratio.

Dynamics of low-energy heavy-ion fusion

Preliminary data on the fusion of 36S+96Zr are reported; the excitation function near the barrier is intermediate between those of 40Ca+90,96Zr. The peculiar role of the strong 3− octupole vibration of 96Zr is pointed out, in addition to the couplings to neutron transfer channels with positive Q-values. Recent data on 40Ca+124Sn are also shown; for that system the fusion barrier distribution is wide without separated peaks, similar to the case of 40Ca+96Zr. Simplified coupled-channel calculations have been performed, including surface vibrations and sequential neutron pick-up channels, with form factors that fit the single- and multi-nucleon transfer data for 40Ca+124Sn. A good agreement with the data is found.

Sub-barrier Fusion ofH6ewithB209i

The fusion of ${}^{6}\mathrm{He}$ with a ${}^{209}\mathrm{Bi}$ target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in ${}^{6}\mathrm{He}$, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive $Q$ value neutron transfer channels, resulting in ``neutron flow'' between the projectile and the target.

Fusion of40Ca+ 124Sn at subbarrier energies

Fusion-evaporation cross-sections of40Ca +124Sn have been measured, following a recent experiment on multinucleon transfer processes in the same system. The40Ca XTU Tandem beam was used (~5 pnA), and the evaporation residues were detected at 0° and at small angles by an Energy-TOF telescope following an electrostatic beam deflector. High statistical accuracy was achieved (~1% near the barrier). A few energy points were measured for40Ca +116Sn, too. The two fusion excitation functions are similar to each other and are greatly enhanced at low energies over the simple one-dimensional barrier penetration estimate. The fusion barrier distribution of40Ca +124Sn was extracted from the data as the second derivative ofE × σ with respect to the energy. Its shape is wide without separated peaks, similar to the recent result for40Ca +96Zr, where it was attributed to a dominant effect of couplings to neutron transfer channels with positiveQ-values. Such channels are available in40Ca +124Sn as well and they are known to be very strong from the multinucleon transfer experiment. Simplified coupled-channel calculations including surface vibrations and sequential neutron pick-up channels are unable to account for the data. The peculiar role of the strong 3 octupole vibration of96Zr is pointed out.

Coupled-channels analysis of58Ni+124Snreactions

Measurements of multineutron transfer reactions in ${}^{58}\mathrm{Ni}{+}^{124}\mathrm{Sn}$ collisions are analyzed in a simplified coupled-channels model. Successive one-neutron and direct two-neutron transfer couplings are included explicitly, together with ${2}^{+}$ and ${3}^{\ensuremath{-}}$ excitations of projectile and target. Capture reactions, i.e., fusion and deep-inelastic reactions, are described by ingoing-wave boundary conditions. Quasielastic charged-particle transfer reactions are not treated explicitly but are simulated by a weak imaginary potential. The model provides a comprehensive description of the measured reaction and elastic scattering data, from far below to well above the Coulomb barrier. We find that subbarrier capture rates are enhanced by couplings to neutron transfer channels but the enhancement is not as large as caused by couplings to low-lying surface modes.

Experimental investigation on the connection between nucleon transfer channels and near-barrier fusion in 32S+64Ni and 28Si+68Zn systems

Abstract One- and two-neutron transfer reactions were studied at bombarding energies near the barrier in 32 S+ 64 Ni and 28 Si+ 68 Zn systems using scattered ion-γ-ray coincidence. Transfer probabilities were obtained in coincidence with selected γ-rays of the product nuclei. The missing probabilities due to transfer leading to the ground state of the product nuclei were estimated using known spectroscopic strengths from the light ion transfer reaction data. The form factors for the neutron transfer channels were obtained from these measurements using a semiclassical theory. The known inelastic couplings, together with the experimentally determined transfer form factors were utilized in a simplified coupled channel calculation to obtain fusion cross sections and the average angular momenta. A comparison of these values with the existing data shows that inclusion of the transfer channels with known coupling strengths is essential to achieve a good agreement with the data.

Neutron flow and necking in heavy-ion fusion reactions

The “barrier distributions” arising from neutron transfer channels are discussed. It is shown that sequential transfers can lead to the broad distributions characteristic of many experimental fusion cross sections. Finite Q-value effects can lead to neutron flow and a build up of a neck between the colliding nuclei. “Anti-necking” may also occur.

Cross section measurements for the 16O+ 13C reaction at subbarrier energies

The cross sections for the different exit channels of the 16O + 13C reaction have been determined for incident energies in the range 4.8 ⩽ E c.m. ⩽ 9.8 MeV by measurement of the characteristic γ-rays emitted by the residual nuclei. The total fusion cross section, obtained as a sum of partial cross sections, agrees with that measured by the total γ-ray yield method and is well described by the global proximity model calculation. The cross section for the 16O( 13C, 12C) 17O ∗ (0.871 MeV) neutron transfer channel remains small over the whole energy interval investigated.

Influence of neutron-transfer channels on Coulomb scattering of polarized 7Li

The coupling of the ( 7Li, 6Li) and ( 7Li, 8Li) one-neutron transfer channels into the elastic channel is considered. The contributions of these channels to the tensor analyzing power T 20 for elastic scattering of aligned 7Li ions at energies well below the Coulomb barrier are calculated. They are negligible in many cases, in the 1% region at the worst for the energies of interest.

Neutron and proton transfer in 32,36S+ 58,64Ni around the Coulomb barrier

Quasi-elastic transfer cross sections have been measured for the systems 32,36S+ 58,64Ni at three energies around the Coulomb barrier. Q-value distributions have been extracted for each of the neutron and charged particle transfer channels. The influence of the transfer channels on the sub-barrier fusion cross section is discussed, and the importance of the proton stripping in 32S + 64Ni is pointed out.

Sub-coulomb barrier fusion of O+Sn

Evaporation residue excitation functions have been measured for from 30 MeV above to 6 MeV below the Coulomb barrier. Reduced excitation functions, scaled to remove the effects of smoothly varying barrier parameters, do not show any strong dependence on the target or projectile isotope. The relative changes are less than a factor two, and are not correlated with positive Q-values of neutron transfer channels.

On the spin-dependence of transfer channel coupling in sub-Coulomb deuteron scattering

It is shown that, for sub-Coulomb incident deuteron energies, coupling to weakly bound neutron transfer channels produces large changes in the calculated elastic channel vector analysing powers, i T 11 as compared with those obtained using a folding model spin-orbit interaction. Coupled channels calculations for deuteron scattering from 208Pb at MeV and 9 MeV are in quantitative agreement with the experimental i T 11 data.