For the first time, we have simultaneously measured fusion and quasielastic excitation functions for an intermediate mass system for angular momentum, ℓ∼ 0 using a recoil mass spectrometer. We have extracted barrier distributions using three different sets of data recorded simultaneously for the reaction ^16 O+ ^142 Ce: (a) fusion excitation function from measurement of evaporation residues at angle, θ _lab = 0^∘ , (b) quasielastic excitation function at center of mass scattering angle, θ _c.m. = π via measurement of scattered target-like ions at θ _lab = 0^∘ and (c) quasielastic excitation functions from measurement of scattered projectile-like ions at two large angles. We show that the four barrier distributions yield nearly identical results with a single peak. However, the centroids of the barriers extracted from quasielastic data at large angles are lower by ∼ 600 keV compared to the same of the barriers extracted from fusion and quasielastic data for ℓ∼ 0 . This is the first experimental verification of the validity of scaling property with respect to ℓ and iso-centrifugal approximation in extracting fusion barrier distribution from quasielastic scattering. This work also points to the importance of extracting barrier distribution from quasielastic measurements at θ _c.m. = π for systems for which measuring fusion excitation function with high precision is not feasible.

Evaporation residue (ER) cross-sections were measured for 30Si+142Ce reaction at laboratory energies in the range of 105 - 132 MeV using the HYbrid Recoil mass Analyzer (HYRA) at IUAC, New Delhi. The transmission efficiency of the HYRA was estimated using the calibration reaction 28Si+142Ce. Sub-barrier fusion enhancement was observed when compared with the one-dimensional barrier penetration model predictions for the present system. To explore the phenomena responsible for sub-barrier fusion enhancement, coupled-channels calculations were performed using the CCFULL code. The effect of couplings and the role of positive Q-value neutron transfer (PQNT) channels on sub-barrier fusion cross-sections were investigated. Coupled-channels calculations were able to reproduce the data by including the coupling in both projectile and target nucleus along with two neutron-transfer channels having positive Q-value. The measured ER cross-sections were compared with the nearby systems to explore the role of deformation of the colliding nuclei.

Enhancement of fusion cross sections below the interaction barrier has been quite successfully explained by coupled-channels methods. However, extending the measurements to deep sub-barrier energies revealed a steeper descent of the excitation function which could not be explained by standard coupled-channels calculations. Though a large number of heavy-ion and light-ion induced reactions have been investigated to understand the dynamics of fusion deep below the barrier, the phenomenon of fusion hindrance has been studied only for a handful of asymmetric systems. We report new measurements of fusion excitation functions for the systems O-16+Cd-116 and O-16+Ce-142. We also present comparisons of the same with the data for existing symmetric systems having nearly similar values of the zeta parameter, characterizing the size of the colliding system. We extracted the logarithmic derivatives of the energy-weighted cross sections and the astrophysical S-factors. Experimental results were reproduced well by coupled-channels calculations. We extrapolated our results, following the systematics, beyond the threshold energy for fusion hindrance for both the systems. From our investigation, we conclude that the present asymmetric systems, as well as the corresponding symmetric systems, show fusion hindrance and this feature is independent of the entrance channel mass asymmetry.

In heavy-ion collision experiments, the fusion cross section in the sub-barrier energy region is found to be enhanced by several orders of magnitude in comparison to the prediction of the one-dimensional barrier penetration model (1D-BPM) that involves the quantum mechanical tunneling effect during fusion. So far, the coupling-aided tunneling due to participating nuclei’s intrinsic degrees of freedom continues to be identified as an accountable factor. We intend to probe the role of structural properties and low-lying inelastic excitations of the colliding nuclei in driving the fusion phenomenon for energies in the near and sub-barrier regions. In the study, the fusion excitation function has been measured for ^30Si+^140Ce reaction for energies ≈ 11

The state-of-the-art technologies made it possible to reach the realms of fusion between two complex nuclei that offer an opportunity to explore a diverse spectrum of fusion phenomena. In this article, we have reported the influence of positive Q-value neutron transfer (PQNT) channels in fusion dynamics. Significant enhancement in sub-barrier fusion cross section has been observed for the Si-28+Gd-158 as compared to Si-30+Ce-140 and other similar systems with nearby mass asymmetry. Current findings include a comparison of the fusion excitation function of multiple systems on a reduced scale. The study sheds light on the influence of PQNT channels and deformation in nuclei on the sub-barrier fusion phenomenon. Further, fusion barrier parameters (barrier height and radius) extracted from the measured data demonstrate a good agreement with the empirical parameterization and proximity potential models.

The challenging task in heavy-ion collisions is unambiguously identifying the true fusion events in the deep sub-barrier region. Considering the primary challenge, we have measured fusion excitation functions for Si-28+ Gd-158 reaction at energies above to deep sub-barrier region to decipher the role of multineutron transfer with positive Q value and fusion hindrance in an asymmetric system. A comparison has been made with our previous measurement for Si-30+ Gd-156 system where only one transfer channel with Q > 0 exists and populates the same compound nucleus Pt-186 *. The enhancement in fusion cross sections is observed on a reduced scale in the Si-28+ Gd-158 reaction over Si-30+ Gd-156 system at sub-barrier energies. The measured fusion data and extracted barrier distribution have been analyzed within the framework of coupled-channels (CC) programs, CCFULL and empirical channel coupling. Coupling to rotational excitations in projectile and target along with up to 2n transfer channel with positive Q value is found to be promising to explain the fusion excitation functions except for the lowest energy point. However, the influence of more than two neutrons transfer is insignificant in Si-28+ Gd-158 system. At the lowest energy (approximate to 14% down the Coulomb barrier), a deviation from standard CC has been found, which may indicate the threshold for fusion hindrance, but additional lower-energy data are needed to prove this. The experimental threshold energy (E-S) for fusion hindrance is in good agreement with the empirical formula, and it is consistent with the observed pattern of E-S as a function of the entrance channel parameter (zeta) for other nearly symmetric and asymmetric systems.

The phenomenon of enhanced fusion cross-section as compared to the theoretical predictions of 1-D BPM has been extensively studied over the past few decades. However, the unambiguous role of neutron transfer channels on the sub-barrier fusion enhancement is still elusive in most cases. Fusion cross-section measurements ≈ 15% below and above the Coulomb barrier were performed to elucidate the mechanisms responsible for the experimentally observed sub-barrier fusion enhancement. Coupled-channels calculations using CCFULL were used to decipher the reaction dynamics. CC calculations explained the fusion excitation function for 28Si + 116,120Sn systems after the inclusion of inelastic excitations along with a pair transfer. However, the observed behavior for the 28Si + 124Sn system could not be explained. The fusion barrier distribution has been extracted from the experimental data to unveil the various channels coupled in the concerned rection. A single uncoupled barrier was transformed into a distribution of barriers depicting the presence of different channels coupled in the reaction. The results indicate a significant effect from multi-neutron transfer channels on the fusion dynamics.

Coupled reaction channel approach has been quite successful in describing the mechanism of multi-nucleon transfer in heavy ion-induced reactions. However, considerable ambiguities exist in the choice of potential parameters and the states of participating nuclides that should be coupled for a given reaction channel. Here we report simultaneous analysis of both angular distributions and excitation functions for one- and two-nucleon transfer in the systems Si-28+Zr-90,Zr-94 within the coupled reaction channel formalism. Spectroscopic amplitudes are obtained from the literature and large-scale shell model calculations. The uncertainties in the cross sections, introduced by the choice of effective interactions in shell model, are also investigated. While one-nucleon transfer in the system Si-28+Zr-94 have been well reproduced by inclusion of the ground and the first excited states of projectile-likes in the exit channel, more states of the same are to be coupled for the system Si-28+Zr-90. Observed bell-shaped angular distribution of one-proton stripping channel in( 28)Si+Zr-94 is found to be caused by a large contribution of direct transfer from the ground and the first excited states of projectile-likes. In contrast, a flat angular distribution of one-proton stripping channel in Si-28+Zr-90 appears to have been caused by a large number of indirect transitions. For two-nucleon transfer, both one-step and two-step processes have been considered. Reasonable reproduction of measured cross sections has been achieved by application of the extreme cluster model for the transfer of a pair of nucleons. No arbitrary scaling of the theoretical results has been necessary and only a minor variation of the binding radius has been allowed in our calculations. More such studies are warranted for mitigating the ambiguities in coupled reaction channel description of multi-nucleon transfer.

We report the first direct measurement of differential transfer cross sections using a Recoil Mass Spectrometer. Absolute differential 1 p - and 2 p -stripping cross sections at θ _c.m.=180^∘ have been determined for the system ^16 O+ ^142 Ce by detecting the heavier target-like ions at the focal plane of the Heavy Ion Reaction Analyzer. Focal plane spectra have been compared with the results of a semi-microscopic Monte-Carlo simulation to unambiguously identify the transfer channels. The methodology adopted in this work can be applied to measure multi-nucleon transfer cross sections using other similar recoil separators. The experimental excitation functions for the reactions ^142Ce(^16O,^15N)^143Pr and ^142Ce(^16O,^14C)^144Nd have been compared with coupled reaction channels calculations. Shell model calculations have been performed to extract spectroscopic information for the target-like nuclei. An excellent matching between measurement and theory has been obtained for 1 p -stripping. For 2 p -stripping, cluster transfer of two protons has been found to have dominant contribution. Measured transfer probabilities for 1 p - and 2 p -stripping channels have been compared with Time-Dependent Hartree–Fock calculations. Proton stripping channels are found to be more favourable compared to neutron pick-up channels. However, the theory overpredicts the measurement hinting at the need for extended approaches with explicit treatment of pairing correlations in the calculations.

Coupled reaction channel approach has been quite successful in describing the mechanism of multi-nucleon transfer in heavy ion-induced reactions. However, considerable ambiguities exist in the choice of potential parameters and the states of participating nuclides that should be coupled for a given reaction channel. Here we report simultaneous analysis of both angular distributions and excitation functions for one- and two-nucleon transfer in the systems ^28 Si+ ^90,94 Zr within the coupled reaction channel formalism. Spectroscopic amplitudes are obtained from the literature and large-scale shell model calculations. The uncertainties in the cross sections, introduced by the choice of effective interactions in shell model, are also investigated. While one-nucleon transfer in the system ^28 Si+ ^94 Zr have been well reproduced by inclusion of the ground and the first excited states of projectile-likes in the exit channel, more states of the same are to be coupled for the system ^28 Si+ ^90 Zr. Observed bell-shaped angular distribution of one-proton stripping channel in ^28 Si+ ^94 Zr is found to be caused by a large contribution of direct transfer from the ground and the first excited states of projectile-likes. In contrast, a flat angular distribution of one-proton stripping channel in ^28 Si+ ^90 Zr appears to have been caused by a large number of indirect transitions. For two-nucleon transfer, both one-step and two-step processes have been considered. Reasonable reproduction of measured cross sections has been achieved by application of the extreme cluster model for the transfer of a pair of nucleons. No arbitrary scaling of the theoretical results has been necessary and only a minor variation of the binding radius has been allowed in our calculations. More such studies are warranted for mitigating the ambiguities in coupled reaction channel description of multi-nucleon transfer.

We investigate the influence of neutron emission in the fission of Pa-227 populated by complete fusion of F-19 with Pb-208 at various excitation energies (E*). Mass gated pre-scission neutron multiplicities (.pre) were determined by fragment-neutron angular correlation and time of flight of fission fragments and neutrons using the National Array of Neutron Detectors facility. Obtained Mass - nu(pre) correlation showed that, at lower E* = 24.2 and 32.4 MeV, larger nu(pre) is correlated with asymmetric mass division. On the other hand, at higher E* = 46.1 and 59.6 MeV, larger nu(pr)e is correlated with symmetric mass division. The results were analyzed within the framework of the general description of fission observables GEneral description of Fission observables (GEF) model with multichance fission included. The analysis of fragment mass-total kinetic energy correlation for different chance fission clearly indicates a revival of shell effects at E* = 24.2 and 32.4 MeV as a consequence of sequential fission decay. At these energies, higher chance fission decreases the saddle point excitation energy considerably where shell effects are prominent. The interplay of shell mediated mass asymmetric fission and symmetric fission gives rise to an energy dependent Mass - nu(pre) correlation. We have compared the experimental results with Mass -nu(pre) correlation predicted by the GEF model. It is concluded that the correlation of larger nu(pre) with asymmetric mass at lower excitation energies is a signature of shell effects reinstated by sequential fission decay. At the two higher excitation energies, despite the multichance fission and consequent decrease in saddle point energy, the available excitation energy appears sufficient for the attenuation of shell effects.

Background: Advancement in accelerator facilities has opened the door to dig deep to understand the interplay between nuclear reactions and structures. Although the influence of inelastic excitations on nuclear scattering and sub-barrier fusion is somewhat established, a clear understanding of nucleon transfer with a positive-Q value is yet to achieve. Purpose: The objective of this paper is to examine the role of the 2n-transfer channel with a positive Q value on sub-barrier fusion and back-angle quasielastic (QE) scattering in the 30Si + 156Gd reaction. Furthermore, extraction of barrier distributions (BDs) from fusion and QE scattering to infer their shapes is also a prime goal. Method: The excitation functions (EFs) of fusion and back-angle QE scattering have been measured over a wide range of incident beam energy around the Coulomb barrier using a recoil mass spectrometer. Furthermore, BDs have been extracted using the measured fusion and back-scattered QE data. The underlying findings have been analyzed within the framework of coupled-channel (CC) formalism using CCFULL and ECC programs. Results: Fusion enhancement has been observed compared to those predicted from the one-dimensional barrier penetration model at sub-barrier energies. Fusion enhancement and QE EFs are explained by CC predictions considering the collective excitations among the colliding nuclei. The inclusion of 2n transfer and collective excitations in CCFULL improves the fit to the experimental fusion data in a short span of energy window around the Coulomb barrier, whereas no significant effect has been observed at the sub-barrier region. However, no such effect of 2n-pickup transfer has been observed from ECC model calculations. Thus, no firm conclusion can be made on the role of 2n-pickup transfer with a positive Q value in present measurements. Conclusion: Fusion EFs have been successfully explained by the CC calculations using CCFULL and ECC model codes. No significant effect of the 2n-pickup channel with a positive Q value was observed on sub-barrier fusion enhancement. However, QE EFs are reproduced by considering the collective excitations and 2n-transfer channel couplings. Fusion and QE BDs are similar in shape within the experimental uncertainty. One-dimensional barrier parameters extracted from the measured data agree with the different theoretical models. Also, the present system obeys the systematic based on deformation values after transfer at the exit.

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 28Si + 116,120,124Sn systems were measured from ???14% below to ???15% 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 respon-sible 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 28Si + 116,120Sn systems to a reasonable extent. However, observed sub-barrier fusion enhancement in 28Si + 124Sn 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.

Background: Advancement in accelerator facilities has opened the door to dig deep to understand the interplay between nuclear reactions and structures. Although the influence of inelastic excitations on nuclear scattering and sub-barrier fusion is somewhat established, a clear understanding of nucleon transfer with a positive-$Q$ value is yet to achieve.Purpose: The objective of this paper is to examine the role of the $2n$-transfer channel with a positive $Q$ value on sub-barrier fusion and back-angle quasielastic (QE) scattering in the $^{30}\mathrm{Si}+^{156}\mathrm{Gd}$ reaction. Furthermore, extraction of barrier distributions (BDs) from fusion and QE scattering to infer their shapes is also a prime goal.Method: The excitation functions (EFs) of fusion and back-angle QE scattering have been measured over a wide range of incident beam energy around the Coulomb barrier using a recoil mass spectrometer. Furthermore, BDs have been extracted using the measured fusion and back-scattered QE data. The underlying findings have been analyzed within the framework of coupled-channel (CC) formalism using ccfull and ecc programs.Results: Fusion enhancement has been observed compared to those predicted from the one-dimensional barrier penetration model at sub-barrier energies. Fusion enhancement and QE EFs are explained by CC predictions considering the collective excitations among the colliding nuclei. The inclusion of $2n$ transfer and collective excitations in ccfull improves the fit to the experimental fusion data in a short span of energy window around the Coulomb barrier, whereas no significant effect has been observed at the sub-barrier region. However, no such effect of $2n$-pickup transfer has been observed from ecc model calculations. Thus, no firm conclusion can be made on the role of $2n$-pickup transfer with a positive $Q$ value in present measurements.Conclusion: Fusion EFs have been successfully explained by the CC calculations using ccfull and ecc model codes. No significant effect of the $2n$-pickup channel with a positive $Q$ value was observed on sub-barrier fusion enhancement. However, QE EFs are reproduced by considering the collective excitations and $2n$-transfer channel couplings. Fusion and QE BDs are similar in shape within the experimental uncertainty. One-dimensional barrier parameters extracted from the measured data agree with the different theoretical models. Also, the present system obeys the systematic based on deformation values after transfer at the exit.

The role of nucleon transfer channel coupling on the sub-barrier fusion excitation function has been elusive. Many studies have attributed a significant sub-barrier fusion cross-section enhancement over one-dimensional barrier penetration model (1d-BPM) calculation to nucleon transfer couplings. However, several systems exhibit no such enhancement besides having positive Q-value nucleon transfer channels.The objective is to delve into the role of coupling to various internal degrees of freedom on the fusion excitation functions in interactions of ^19 F with ^64,68 Zn. Fusion cross-section measurements are performed at energies ∼ 20

The role of nucleon transfer channel coupling on the sub-barrier fusion excitation function has been elusive. Many studies have attributed a significant sub-barrier fusion cross-section enhancement over one-dimensional barrier penetration model (1d-BPM) calculation to nucleon transfer couplings. However, several systems exhibit no such enhancement besides having positive Q-value nucleon transfer channels.The objective is to delve into the role of coupling to various internal degrees of freedom on the fusion excitation functions in interactions of $$^{19}$$ F with $$^{64,68}$$ Zn. Fusion cross-section measurements are performed at energies $$\sim $$ 20 % above to $$\sim $$ 15 % below the Coulomb barrier using Heavy Ion Reaction Analyzer (HIRA) at Inter-University Accelerator Center (IUAC), New Delhi. Coupled-channel (CC) calculations, including coupling to vibrational states of $${^{64,68}}$$ Zn, rotational states of $${^{19}}$$ F, and nucleon transfer channels are performed. Cross-sections for various transfer channels are calculated to speculate their coupling effects on the fusion excitation functions. The results are compared on a reduced scale with neighboring systems involving $$^{19}$$ F and $$^{18}$$ O as the projectile. The sub-barrier fusion cross-sections of $${^{19}}$$ F $$+$$ $${^{64,68}}$$ Zn systems are enhanced by orders of magnitude compared to the corresponding 1d-BPM calculations. CC calculations performed with CCFULL, including coupling to collective excitations of reactants and pair transfer channel, failed to reproduce the experimental fusion cross-sections. GRAZING calculations show large one nucleon transfer cross-sections relative to other channels for the two systems. CCDEF calculation show that inclusion of coupling to one proton transfer channel is necessary to reproduce the experimental data for both systems.