Neutron Transfer Research Articles

Examining the correlation between multi-neutron transfer and inelastic excitations in sub-barrier fusion enhancement

Background: Heavy-ion fusion cross-sections at sub barrier energies are found to be enhanced by orders of magnitude as compared to the predictions of one-dimensional barrier penetration model (1-D BPM). The coupling of various internal degrees of freedom has been employed to decrypt the mechanism responsible for the observed sub-barrier fusion enhancement. However, the unambiguous role of multi-neutron transfer channels in the sub-barrier domain is still elusive.Purpose: We aim to explore and disentangle the effects of multi-neutron transfer channels from the inelastic excitations in the vicinity of the Coulomb barrier.Method: The fusion excitation functions for $^{28}\mathrm{Si} + ^{116,120,124}\mathrm{Sn}$ systems were measured from $\ensuremath{\approx}14%\phantom{\rule{0.222222em}{0ex}}$ below to $\ensuremath{\approx}15%\phantom{\rule{0.222222em}{0ex}}$ above the Coulomb barrier by detecting the evaporation residues (ERs) at the focal plane of the Recoil Mass Separator (RMS), Heavy Ion Reaction Analyzer (HIRA) at the Inter-University Accelerator Centre (IUAC), New Delhi.Results: The extracted fusion cross sections for the investigated systems at sub-barrier energies are significantly enhanced as compared to the predictions of 1-D BPM calculations. To probe the underlying mechanism responsible for the observed sub-barrier fusion enhancement, the coupled-channels (CC) formalism was employed. Coupled reaction channels (CRC) calculations by incorporating the one-neutron transfer channel to elucidate the significance of the transfer channel on the fusion dynamics were performed. Further, the semiempirical coupled channels (ECC) approach was explored to decipher the possible cause of the observed fusion excitation function trend. A systematic analysis of the neighboring systems available in the literature was also performed.Conclusions: CC calculations were able to reproduce the measured fusion excitation functions for $^{28}\mathrm{Si} + ^{116,120}\mathrm{Sn}$ systems to a reasonable extent. However, observed sub-barrier fusion enhancement in $^{28}\mathrm{Si}+^{124}\mathrm{Sn}$ system could not be explained using CC calculations. The influence of multi-neutron transfer channels was highlighted in the coupled-channels calculations. The interplay of collective excitations and neutron transfer was observed.

Role of neutron transfer in the reaction mechanism of 9Be+169Tm, 181Ta, 187Re and 197Au systems

The contribution of one neutron stripping cross section to the total reaction cross section has been studied for 9Be projectile incident on 169Tm, 181Ta, 187Re and 197Au targets around Coulomb barrier energy. The measured one neutron stripping cross sections for these systems have been compared with the coupled channel calculations. The recently developed global set of optical model potential parameters for 9Be projectile has been used in the present calculations. The cumulative of measured complete fusion (CF), incomplete fusion (ICF), one neutron stripping and calculated non-capture breakup (NCBU) cross sections is found to explain almost the reaction cross sections for all the targets. A very small contribution from target inelastic states and elastic breakup may contribute to the remaining part. The percentage fraction of cross section for CF, ICF, one neutron stripping, and NCBU over reaction cross section show the dominance of neutron transfer and NCBU at below barrier energies while CF and ICF processes have the major contribution at above barrier energies.

Neutron 3s1/2 occupation change across the stable tin isotopes investigated using isotopic analysis of proton scattering at 295 MeV

The isotopic systematics of Sn isotopes in the range A=116--124 were investigated by combining the nuclear structure and reaction calculations for the analysis of Sn(p,p) reactions at 295 MeV. The Sn(p,p) reactions were calculated employing relativistic impulse approximation (RIA) with theoretical densities obtained for the Sn isotopes from relativistic Hartree-Bogoliubov (RHB) calculations with the DD-ME2 interaction and nonrelativistic Skyrme Hartree-Fock-Bogoliubov (SHFB) calculations with the SKM* interaction. Calculation using the DD-ME2 density reproduced the experimental data for $^{122}$Sn(p,p) but overestimated the $^{116}$Sn(p,p) and $^{118}$Sn(p,p) cross sections at backward angles. In the isotopic analysis of the cross section ratio $R(\sigma)$ of $^{122}$Sn to $^{116}$Sn, a calculation using the SKM* density reproduced the peak amplitudes of $R(\sigma)$ obtained from the experimental cross sections, whereas a calculation using the DD-ME2 density did not. The ratio $R(\sigma)$ was found to be sensitive to the isotopic change of the neutron $3s_{1/2}$ occupation through the isotopic difference of the surface neutron density around r=4--5 fm. Isotopic analysis indicated that a rapid increase of the $3s_{1/2}$ occupation from $^{116}$Sn to $^{122}$Sn obtained by the DD-ME2 calculation is unlikely. This result derived from the 295 MeV Sn(p,p) cross section data is consistent with the direct measurement of the neutron occupations in Sn isotopes by neutron transfer and pick-up reactions.

Single neutron transfer on 23Ne and its relevance for the pathway of nucleosynthesis in astrophysical X-ray bursts

We present new experimental measurements of resonance strengths in the astrophysical 23Al(p,γ)24Si reaction, constraining the pathway of nucleosynthesis beyond 22Mg in X-ray burster scenarios. Specifically, we have performed the first measurement of the (d,p) reaction using a radioactive beam of 23Ne to explore levels in 24Ne, the mirror analog of 24Si. Four strong single-particle states were observed and corresponding neutron spectroscopic factors were extracted with a precision of ∼20%. Using these spectroscopic factors, together with mirror state identifications, we have reduced uncertainties in the strength of the key ℓ = 0 resonance at Er = 157 keV, in the astrophysical 23Al(p,γ) reaction, by a factor of 4. Our results show that the 22Mg(p,γ)23Al(p,γ) pathway dominates over the competing 22Mg(α,p) reaction in all but the most energetic X-ray burster events (T>0.85 GK), significantly affecting energy production and the preservation of hydrogen fuel.

Role of positive transfer Q values in fusion cross sections for O18+W182,184,186 reactions

Background: The relevance of including channel coupling effects in the form of target deformation and vibration in fusion reactions has been well established. Many reactions with positive $Q$ values for neutron transfer show enhancement in sub-barrier fusion cross sections. However, there are exceptions to these cases.Purpose: We aim to make a comprehensive list of factors influencing the sub-barrier fusion enhancement in systems with neutron transfer channels having positive $Q$ values.Method: Evaporation residue cross sections were measured for $^{18}\mathrm{O}+^{182,184,186}\mathrm{W}$ reactions in the energy range $68--104$ MeV in the laboratory frame, using a recoil mass spectrometer.Results: Inclusion of deformation of target and projectile low-level excitations in the coupled channels calculations explains the measured fusion excitation functions of $^{18}\mathrm{O}+^{182,184,186}\mathrm{W}$ reactions.Conclusions: Considering that all the targets have similar deformation, and comparing with $^{16}\mathrm{O}+^{182,184,186}\mathrm{W}$ reactions having negative $2n$ transfer $Q$ values, we can conclude that the positive $Q$ values of neutron transfer channels have no effect on the observed fusion cross sections of $^{18}\mathrm{O}+^{182,184,186}\mathrm{W}$ reactions.

Investigation of isotopic dependence on the O + Ni fusion cross section near barrier energies

Fusion excitation functions have been measured for $^{16}\mathrm{O}+^{61}\mathrm{Ni}$ and $^{18}\mathrm{O}+^{61,62}\mathrm{Ni}$ systems around the Coulomb barrier ($\ensuremath{\approx}0.7{V}_{B}$--$1.3{V}_{B}$) using the recoil mass spectrometer, heavy ion reaction analyzer. The ground state $Q$ value for two neutron stripping is positive for both systems with $^{18}\mathrm{O}$ as the projectile. Strong enhancement of the experimental fusion cross sections were observed below the barrier for all the systems compared to that of the predictions of the one-dimensional barrier penetration model. To understand such enhancement, a coupled-channels formalism has been used. A comparative study of these systems indicated that the coupling of two neutron transfer channels with the collective excitations could play the role behind the sub-barrier fusion enhancement for $^{18}\mathrm{O}$ induced reactions. However, the sub-barrier enhancement for $^{16}\mathrm{O} + ^{61}\mathrm{Ni}$ is found to be due to the coupling of quadrupole vibrations of both the interacting nuclei. Also after comparing these systems with other systems of different Ni isotopes, we have found that the signature of the role of coupling to neutron transfer channels due to ground state positive $Q$ value for neutron transfer is ambiguous.

Effect of Shapes Degrees of Freedom on the Fusion Cross-Sections of 16O + 154Sm and 18O + 148Nd

The fusion dynamics of 16O + 154Sm and 18O+148Nd reactions has been investigated to observe the effect of shape degree of freedom on the fusion cross-section using the coupled channel (CC) calculations. The one dimentional barrier penetration model (1-D BPM) is observed to underestimate the experimental data mainly at below barrier energies. The coupled channel calculations, including target projectile excitations with 1 phonon of vibrational state and 2+ rotational state of target, reproduce the experimental fusion cross-section for 16O + 154Sm reaction at below barrier energies, while show some discrepancy for 18O+148Nd reaction. This discrepancy for 18O+148Nd reaction can be addressed with the inclusion of low-lying states with neutron transfer channels and the neck-formation.

Interplay of Neutron Transfer in the Fusion Process of 16O + 112,116,120Sn and 18O + 116Sn Systems at Sub-barrier Energies

Interplay of Neutron Transfer in the Fusion Process of 16O + 112,116,120Sn and 18O + 116Sn Systems at Sub-barrier Energies

Experimental evidence for α production following neutron transfer in the C13 + Nb93 system

Spectra of $\ensuremath{\alpha}$ particles were measured for $^{12,13}\mathrm{C} + ^{93}\mathrm{Nb}$ systems at several angles for a bombarding energy of 65 MeV to investigate various reaction mechanisms like direct break up and cluster transfer responsible for $\ensuremath{\alpha}$ production. The $\ensuremath{\alpha}$ spectra were analyzed in detail to determine the effect of $1n$ transfer on an $\ensuremath{\alpha}$-production cross section in particular. After accounting for contributions from direct breakup and Be transfer, there is a clear signature of the $\ensuremath{\alpha}$ particles produced through the break-up process following neutron transfer in the case of a $^{13}\mathrm{C}$ induced reaction. This is an interesting result being reported for the strongly bound projectile.

The Impact of Neutron Transfer Reactions on the Heating and Cooling of Accreted Neutron Star Crusts

Abstract Nuclear reactions heat and cool the crust of accreting neutron stars and need to be understood to interpret observations of X-ray bursts and long-term cooling in transiently accreting systems. It was recently suggested that previously ignored neutron transfer reactions may play a significant role in the nuclear processes. We present results from full nuclear network calculations that now include these reactions and determine their impact on crust composition, crust impurity, heating, and cooling. We find that a large number of neutron transfer reactions indeed occur and impact crust models. In particular, we identify a new type of reaction cycle that brings a pair of nuclei across the nuclear chart into equilibrium via alternating neutron capture and neutron release, interspersed with a neutron transfer. While neutron transfer reactions lead to changes in crust model predictions and need to be considered in future studies, previous conclusions concerning heating, cooling, and compositional evolution are remarkably robust.

Occupation probabilities of valence orbitals relevant to neutrinoless double β decay of Sn124

Neutron transfer reaction measurements, relevant to the neutrinoless double $\ensuremath{\beta}$ decay candidate $^{124}\mathrm{Sn}$ and its daughter $^{124}\mathrm{Te}$, have been performed. Precise measurements of both neutron addition $[(d,p),(^{4}\mathrm{He},^{3}\mathrm{He})]$ and removal $[(p,d),(^{3}\mathrm{He},^{4}\mathrm{He})]$ cross sections have been used to determine the occupation of valence orbitals pertinent to neutrinoless double $\ensuremath{\beta}$ decay in these two nuclei. This information could be used to constrain calculations of the nuclear matrix element for the neutrinoless double $\ensuremath{\beta}$ decay of $^{124}\mathrm{Sn}$. The change in the ground-state neutron vacancies in proceeding from $^{124}\mathrm{Sn}$ to $^{124}\mathrm{Te}$ is mainly found in the ${d}_{3/2,5/2}$ and ${h}_{11/2}$ orbitals. The occupancies of states near the Fermi level are in reasonable agreement with shell model calculations.

O18 -induced single-nucleon transfer reactions on Ca40 at 15.3A MeV within a multichannel analysis

Background: Nucleon transfer reactions are selective tools for nuclear physics investigations. The theoretical and computational limits affecting in the past their data analysis could be nowadays surmounted thanks to the advent of methods with refined approximations and constraints, even when heavy-ion collisions are considered. Purpose: Modern microscopic calculations of heavy-ion-induced transfer reactions combined with precise experimental data offer the chance for accurately testing different reaction models as well as the nuclear structure description of the involved nuclear states. Method: Single proton and neutron transfer reactions were measured with the MAGNEX magnetic spectrometer for the O18+Ca40 system at 15.3A MeV. Excitation energy spectra and angular differential cross section distributions were extracted. The experimental results are compared with theoretical calculations performed in distorted wave and coupled channel Born approximation. The use of a coupled channel equivalent polarization potential to effectively describe the coupling effects affecting the initial state interaction is also considered. Spectroscopic amplitudes derived from a large-scale shell model with appropriate interactions adapted for the involved nuclei are employed. Results: Our theoretical calculations are in good agreement with experimental data, without the need for any scaling factor, validating the adopted reaction and nuclear structure parameters. Moreover, under the present experimental conditions, a weak dependence of the obtained results on the choice of the reaction models was observed. Conclusions: The good agreement between experimental and theoretical results validates the reliability of the parameter sets entering the calculations. They are extracted from or tested in complementary analyses of other reaction channels under the same experimental conditions. Such a multichannel approach represents the best option to pursue a solid, comprehensive, and model-independent description of the single-nucleon transfer reactions. The successful description of the present one-nucleon transfer data is also propaedeutic to the accurate assessment, under the same theoretical description, of higher-order transfer processes, like the sequential nucleon transfer mechanisms which are in competition with the direct charge exchange reactions.Received 14 September 2021Accepted 16 November 2021DOI:https://doi.org/10.1103/PhysRevC.104.064609©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLow & intermediate energy heavy-ion reactionsModels & methods for nuclear reactionsTransfer reactionsTechniquesOptical, coupled-channel & distorted wave modelsNuclear Physics

Investigation of neutron transfer in Li7+Sn124 system

The relative importance of neutron transfer and breakup process in a reaction around Coulomb barrier energies has been studied for the $^{7}\mathrm{Li}+^{124}\mathrm{Sn}$ system. Coupled channel calculations have been performed to understand the one neutron stripping and pickup cross sections along with the breakup in the $^{7}\mathrm{Li}+^{124}\mathrm{Sn}$ system. The systematics of one and two neutron stripping and pickup cross sections with a $^{7}\mathrm{Li}$ projectile on several targets show an approximate universal behavior, which has been explained by a simple model based on barrier penetration. Complete reaction mechanism have been studied by comparing the reaction cross sections with the measured total fusion and one neutron transfer cross sections.

Neutron transfer in $$^{9}$$Be + $$^{159}$$Tb system

One neutron stripping cross sections ($\sigma_{-1n}$) are measured in $^{9}$Be+$^{159}$Tb system in the energy range E$_{cm}$/V$_{B}$ $\sim$ 0.79 - 1.24 using offline gamma counting technique. The CRC model calculations including the ground state and the 2$^{+}$ resonance state of $^{8}$Be, carried out using the FRESCO code, give a reasonable description of the measured data. In addition, comparisons of reduced 1n-stripping cross sections- $\sigma_{red}$ with $^{9}$Be for different target nuclei (A $\sim$150-200), and $\sigma_{red}$ for $^{9}$Be, $^{6}$Li with $^{159}$Tb target are presented. While no strong target dependence is observed with $^{9}$Be projectile, $\sigma_{red}$($^{9}$Be) is significantly larger than that for $^{6}$Li, which is consistent with the Q-value for transfer reactions and breakup threshold energy of projectiles.

Study of Flow and Heat Transfer for the Supercritical Hydrogen in Spallation-Type Cylindrical Neutron Moderator

Pipe height in cylindrical neutron moderator is an important factor to flow pattern, temperature distribution and even the neutron characters. In this paper, the steady-state thermal analysis of cold neutron moderator is carrying out with different heights, conjugated heat transfer method and one-way coupled with a neutron transfer software. The different pipe heights, which is the jet-to-surface distances (H/D = 0.5~6), were compared using a 2D moderator model. The results show that vortex size and velocity gradient from container wall to vortex center vary with H/D, the center of recirculation zone nearly remain constant, and heat transfer effect is weakened on the target bottom surface. With H/D increasing, the velocity at bottom target surface is progressively decreased, and cooling effect is poor, leading to the rise in temperature. The optimal range cooling performance is (H/D) = 0.5~1 at Re = 1.7 × 105, and the enhancement of beam power further strengthens the thermal deposition difference between container and liquid hydrogen. The results can be applied to moderator component design and optimization in the future spallation neutron source.

Theoretical study of dynamic effects on fusion cross sections for reactions S32,34,36+Pb204,206,208

The universal fusion function prescription and the empirical coupled-channel (ECC) model are used to analyze the dynamic effects on fusion cross sections for reactions $^{32,34,36}\mathrm{S}+^{204,206,208}\mathrm{Pb}$. An examination of the reduced fusion functions shows that the total dynamic effects on fusion cross sections in $^{36}\mathrm{S}+^{204,206,208}\mathrm{Pb}$ are almost the same as those in $^{34}\mathrm{S}+^{204,206,208}\mathrm{Pb}$. Furthermore, at sub-barrier energies of the reactions $^{32,34,36}\mathrm{S}+^{208}\mathrm{Pb}$, the largest enhancement of reduced fusion function is observed in the reaction $^{32}\mathrm{S}+^{208}\mathrm{Pb}$. Then the enhancement of fusion cross sections due to couplings to inelastic excitation channels and neutron transfer channels with positive $Q$ values is investigated by using the ECC model. The results show that the experimental data of the reactions $^{34,36}\mathrm{S}+^{204,206,208}\mathrm{Pb}$ and $^{32}\mathrm{S}+^{208}\mathrm{Pb}$ are reproduced well by the ECC model. In addition, it can be found that the effect of coupling to positive $Q$-value neutron transfer channels is necessary to reproduce the sub-barrier fusion data of the reaction $^{32}\mathrm{S}+^{208}\mathrm{Pb}$. Furthermore, for the two reactions $^{32}\mathrm{S}+^{204}\mathrm{Pb}$ and $^{32}\mathrm{S}+^{206}\mathrm{Pb}$, the effects of coupling to positive $Q$-value neutron transfer channels are predicted to be significant. The fusion cross sections for these two reactions are also predicted.

Dynamics of the Neutron Transfer Process in the Reaction $${}^{{181}}$$Ta($${}^{{18}}$$O, $${}^{{19}}$$O) at an Energy of 10 MeV per Nucleon

Dynamics of the Neutron Transfer Process in the Reaction $${}^{{181}}$$Ta($${}^{{18}}$$O, $${}^{{19}}$$O) at an Energy of 10 MeV per Nucleon

Influence of entrance channel on multinucleon transfer cross sections

Multinucleon transfer cross sections around targetlike fragments of the $^{58}\mathrm{Ni}+^{208}\mathrm{Pb}$ reaction and the $^{64}\mathrm{Ni}+^{207,208}\mathrm{Pb}$ reactions were calculated by the improved dinuclear system $\mathrm{model}+\mathrm{GEMINI}++$. One finds that the $^{64}\mathrm{Ni}+^{208}\mathrm{Pb}$ reaction is more favorable to obtain neutron-rich nuclei as compared to the $^{58}\mathrm{Ni}+^{208}\mathrm{Pb}$ and $^{64}\mathrm{Ni}+^{207}\mathrm{Pb}$ reactions. That is because the difference of injection point position on the potential energy surface has a significant influence on the transfer of protons and neutrons in multinucleon transfer reaction. The effect of the entrance channel on transfer cross sections was predicted and analyzed by $^{58,64,70}\mathrm{Ni}+^{198}\mathrm{Pt}$ and $^{86,91,102}\mathrm{Kr}+^{208}\mathrm{Pb}$ reactions.

Ground state and excited state multinucleon transfer channels in interactions of 28Si with 90,94Zr in near barrier region

Multinucleon transfer reactions were measured in interactions of 28Si with 90,94Zr at a near barrier energy using recoil mass separator, Heavy Ion Reaction Analyzer (HIRA) at Inter University Accelerator Centre (IUAC), New Delhi. The effects of shell closure, positive Q-value transfer channels on the relative strengths of ground state and excited state transfer in multinucleon transfer processes were investigated. The kinematic coincidence technique was employed to reduce the background. The m/q ambiguity was resolved by time of flight (TOF) technique. Based on the Q-value considerations, it was observed that pickup channels were predominantly of neutron transfer, whereas, stripping channels were mainly of proton transfer. A 14-element BGO array was used to extract the relative strength of excited state transfer. In interactions of 28Si with 90Zr, it was observed that the ground state and excited states competes for populating all the transfer channels. In reactions of 28Si with 94Zr, nearly similar behavior was observed except for two-neutron pickup channel, where excited state transfer was dominant and four-neutron pickup channel where ground state transfer was dominant.

Influence of halo single-neutron transfer on near barrier ^{15}C + ^{12}C total fusion

A recent comparison of the average fusion cross section, leftlangle sigma _mathrm {F}rightrangle , for energies just above the Coulomb barrier for the ^{12-15}C + ^{12}C systems found that the behaviour as a function of projectile neutron excess could not be satisfactorily explained by static barrier penetration model calculations and suggested that the neutron dynamics plays an important rôle. In this work we demonstrate that the (^{15}C,^{14}C) single neutron transfer has a significant influence on the above barrier ^{15}C + ^{12}C total fusion, although not quite in the way expected since it leads to a reduction in the cross section, contrary to the trend in the measured leftlangle sigma _mathrm {F}rightrangle . However, this result underlines the danger of ignoring the effect of neutron transfer reactions on fusion in systems involving neutron halo nuclei.