Quasielastic Barrier Distributions Research Articles

Validity of scaling property and iso-centrifugal approximation in quasielastic barrier distribution: the first experimental verification

Validity of scaling property and iso-centrifugal approximation in quasielastic barrier distribution: the first experimental verification

Investigating the Effect of Transfer Channels on Reaction Dynamics Using the Quasi-elastic Barrier Distribution

Investigating the Effect of Transfer Channels on Reaction Dynamics Using the Quasi-elastic Barrier Distribution

Dissipation by transfer and its influence on fusion

A new version of the CCQEL code has been developed by upgrading the method of coupling of transfer channels during fusion and backscattering processes. In particular, the number of transfer reactions included has been increased and the dependence of the strength of transfer coupling on the transferred particle and experimental Q-value distribution was introduced. The upgraded code was employed for the investigation of the influence of transfer on the smoothing of the measured quasielastic barrier distribution (Dqe) of the 24Mg + 92Zr and 20Ne + 208Pb systems and found interesting discrepancies with respect to the standard approximations. The study with the upgraded code indicates the transfer responsible for generating strongly excited targets as the leading cause of the smearing of the barrier distribution, even in the case of negative ground state to ground state Q value (Qgg). The smoothing observed in the barrier distribution is dominated rather by one neutron transfer, despite the negative Qgg value for this reaction and the positive Qgg value for two-neutron transfer. Of particular interest is the case of the 20Ne + 208Pb, where the smoothing of the Dqe is mainly influenced by the one neutron pick-up at the beam energy above the barrier, while the one neutron pick-up and one proton stripping transfers are dominant for lower beam energy. These results highlight the importance of the transfer coupling dependence on the experimental Q-value distribution and, consequently, on the projectile kinetic energy.

Fusion and back-angle quasielastic measurements in Si30+Gd156 near the Coulomb barrier

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.

Quasielastic backscattering and barrier distribution for the weakly bound projectile Li6 on Tb159

The excitation function for quasielastic scattering of the weakly bound projectile $^{6}\mathrm{Li}$ on a $^{159}\mathrm{Tb}$ target, at large backward angle, has been measured at energies around the Coulomb barrier. The corresponding quasielastic barrier distribution has been extracted from the experimental cross sections, both including and excluding the $\ensuremath{\alpha}$ particles produced in the reaction. The quasielastic scattering cross sections, excluding the $\ensuremath{\alpha}$ particles, have been analyzed in the framework of coupled channels calculations. The centroid of the quasielastic barrier distribution, including the $\ensuremath{\alpha}$ particles, is found to shift towards higher energy relative to the centroid of the fusion barrier distribution for the system. This has been attributed to the low $\ensuremath{\alpha}$-breakup threshold of the nucleus $^{6}\mathrm{Li}$.

Large back-angle quasielastic scattering for Li7+Tb159

Quasielastic scattering excitation function at large backward angle has been measured for the weakly bound system, $^{7}$Li+$^{159}$Tb at energies around the Coulomb barrier. The corresponding quasielastic barrier distribution has been derived from the excitation function, both including and excluding the $\alpha$-particles produced in the reaction. The centroid of the barrier distribution obtained after inclusion of $\alpha$-particles was found to be shifted higher in energy, compared to the distribution excluding the $\alpha $-particles. The quasielastic data, excluding the $\alpha$-particles, have been analyzed in the framework of continuum discretized coupled channel calculations. The quasielastic barrier distribution for $^{7}$Li+$^{159}$Tb, has also been compared with the fusion barrier distribution for the system.

Study of Quasielastic Barrier Distributions as a Step towards the Synthesis of Superheavy Elements with Hot Fusion Reactions.

The excitation functions for quasielastic scattering of ^{22}Ne+^{248}Cm, ^{26}Mg+^{248}Cm, and ^{48}Ca+^{238}U are measured using a gas-filled recoil ion separator. The quasielastic barrier distributions are extracted for these systems and are compared with coupled-channel calculations. The results indicate that the barrier distribution is affected dominantly by deformation of the actinide target nuclei, but also by vibrational or rotational excitations of the projectile nuclei, as well as neutron transfer processes before capture. From a comparison between the experimental barrier distributions and the evaporation residue cross sections for Sg (Z=106), Hs (108), Cn (112), and Lv (116), it is suggested that the hot fusion reactions take advantage of a compact collision, where the projectile approaches along the short axis of a prolately deformed nucleus. A new method is proposed to estimate the optimum incident energy to synthesize unknown superheavy nuclei using the barrier distribution.

THE LARGE ANGLE QUASI-ELASTIC SCATTERING CROSS SECTIONS AND THE EFFECTIVE WEIGHT FUNCTION BASED ON THE BARRIER DISTRIBUTION FOR 32S+92,94,96,98,100Mo REACTIONS

We studied quasi-elastic scattering and the effective weight function for the barrier distribution for 32S+92,94,96,98,100Mo reactions have been calculated simultaneously in a wide range of bombarding energies around the Coulomb barrier. The quasi-elastic angular distribution data are analyzed using the optical model code with Woods-Saxon potentials. It is shown that the calculations taken into account, for these reactions can explain structures of the quasi-elastic cross-section and the quasi-elastic barrier distribution. These results indicate that the coupled-channels formalism can still valid even for the various mass systems. The large-angle quasi-elastic scattering reactions were scrutinized with the identical core-core potential recommended for designating fusion reactions. Given the effect of a barrier distribution and nucleon transfer on the Woods-Saxon potential, the calculated semi-elastic scattering cross-sections of a series of reactions are in good harmony each other.

Strong neutron-transfer coupling effects in the reaction mechanism of the O18+Zn64 system at energies near the Coulomb barrier

The precise quasielastic excitation function for the $^{18}\mathrm{O}+^{64}\mathrm{Zn}$ system was measured at energies near and below the Coulomb barrier at ${\ensuremath{\theta}}_{\mathrm{lab}}={161}^{\ensuremath{\circ}}$, from which its correlated quasielastic barrier distribution was derived. The excitation functions for the two-neutron, $\ensuremath{\alpha}$, and tritium transfer reactions have also been measured at the same conditions. These data were well described by the coupled channel and coupled reaction channel calculations. The comparison of the data with theoretical calculations shows the strong influence of three inelastic channels in the coupling process: the quadrupole and octupole vibrational states of $^{64}\mathrm{Zn}$, ${2}_{1}^{+}$ and ${3}_{1}^{\ensuremath{-}}$, and the quadrupole vibrational state of $^{18}\mathrm{O}$, ${2}_{1}^{+}$. In addition, theoretical calculations indicate that the two-neutron stripping is mostly due to a one-step process, which has a striking effect on the reaction mechanism of this system.

Effects of Surface Diffuseness and Coupling Radius Parameters on The Fusion and Quasi-Elastic Barrier Distributions

We study the heavy-ion reaction at sub-barrier energies for <sup>16</sup>O+<sup>144,154</sup>Smsystems using full order coupled-channels formalism. We especially investigate the effect of fusion and quasi- elastic barrier distributions on the surface diffuseness and the coupling radius parameters of the nuclear potential for these systems. We found that the structure of fusion and quasi-elastic barrier distributions is more sensitive to the surface diffuseness and coupling radius parameters for the reaction with spherical target, <sup>16</sup>O+<sup>144</sup>Sm systemcompared to the reaction that involves the deformed target, i.e., <sup>16</sup>O+<sup>154</sup>Sm system. In more detail, the results of coupled-channels calculations for the fusion and the quasi-elastic barrier distributions for deformed target are not sensitive to the choice of the coupling radius and surface diffuseness parameters. In mark contrast, the structure of the fusion and the quasi-elastic barrier distributions for spherical target are very sensitive to the coupling radius and surface diffuseness parameters. We found that the small surface diffuseness parameter smeared out the fusion barrier distributions and the larger coupling radius smoothed the high energy peak of the quasi-elastic barrier distributions. We also found that the larger coupling radius, , is required by the experimental quasi-elastic barrier distribution for the <sup>16</sup>O+<sup>144</sup>Sm system whereas the experimental fusion barrier distribution compulsory the small value, i.e., .

Reaction mechanisms of the O18+Cu63 system at near-barrier energies

A precise quasielastic excitation function for the $^{18}\mathrm{O}+^{63}\mathrm{Cu}$ system has been measured at energies around the Coulomb barrier at ${\ensuremath{\theta}}_{\mathrm{LAB}}={161}^{\ensuremath{\circ}}$. The corresponding quasielastic barrier distribution has been derived. Two-neutron-, one-proton-, and \ensuremath{\alpha}-transfer-excitation functions have also been measured at the same energies and angle. Coupled reaction channels calculations were performed to describe the experimental data. Large-scale shell-model calculations were performed to derive most of the spectroscopic amplitudes. No surface imaginary potential was necessary for the interaction potential because almost all relevant reaction channels were explicitly included in the calculation. The theoretical results were compared to the experimental quasielastic barrier distribution and a very good agreement was achieved. The comparison of the coupled reaction channel calculations and data has put in evidence several important details of the reaction mechanism of the $^{18}\mathrm{O}+^{63}\mathrm{Cu}$ system. The collectivity of the $^{63}\mathrm{Cu}$ nucleus has important contribution to the reaction mechanism of this system, mainly due to its first $5/{2}^{+}$ and $7/{2}^{+}$ states. It was also observed a striking influence on the reaction dynamics of the $^{18}\mathrm{O}$(${2}^{+}$) state, the two-neutron transfer and the reorientation of the target ground-state spin. The best agreement to data was achieved when the nuclear matter diffuseness for the $^{18}\mathrm{O}$ was assumed equal to 0.60 fm, value that we have derived in a previous paper and that is 10% greater than the $^{16}\mathrm{O}$ diffuseness. Another significant result was that the two-neutron transfer process is much more relevant than the one-neutron-transfer process, which suggests that the pairing correlation could play an important role in the transfer process of this system.

Determination of Fusion Barrier Distributions from Quasielastic Scattering Cross Sections towards Superheavy Nuclei Synthesis

In order to study the nucleus–nucleus interactions for syntheses of superheavy nuclei, we measured excitation functions for the quasielastic scattering of 48Ca+208Pb, 50Ti+208Pb, and 48Ca+248Cm using the gas-filled-type recoil ion separator GARIS. The quasielastic scattering events were clearly separated from deep-inelastic events by using GARIS and its focal plan detectors, except for high-incident-energy points. The quasielastic barrier distributions were successfully extracted for these systems, and compared with coupled-channels calculations. The results of the calculations indicate that vibrational and rotational excitations of the colliding nuclei, as well as neutron transfers before contact, strongly affect the structure of the barrier distribution. For the reactions of 48Ca+208Pb and 50Ti+208Pb, a local maximum of the barrier distribution occurred at the same energy as the peak of the 2n evaporation cross section of the system. On the other hand, for the hot fusion reaction of 48Ca+248Cm, the 4n e...

Study of Quasielastic scattering for 7Li+159Tb at around- barrier energies

Quasielastic scattering cross sections for the reaction 7 Li+159 Tb have been measured at large backangles, at energies around the Coulomb barrier. The quasielastic barrier distribution has been extracted from the measured quasielastic scattering excitation function, including and excluding α particle contribution. The peak of the quasielastic barrier distribution including α particle contribution shows a shift towards higher energy compared to the peak of the distribution without α particles. The quasielastic barrier distribution when compared to the calculated fusion barrier distribution, appears to show reasonable agreement for the system.

Effect of coupling in theSi28+Sm154reaction studied by quasi-elastic scattering

The study of the coupling to collective states of the $^{28}\mathrm{Si}$ projectile and $^{154}\mathrm{Sm}$ target in fusion mechanism is reported. Understanding such couplings is important as they influence the barrier height and the formation probability of the compound nuclei, which in turn may be related to the synthesis of superheavy elements in heavier systems. In the present work, before performing the coupled-channel calculations, we wish to obtain an experimental signature of coupling to projectile and target excitation through barrier distribution (BD) study. To this end, the BDs of the $^{28}\mathrm{Si}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ and $^{16}\mathrm{O}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ systems have been compared using existing fusion data, scaled to compensate for the differences between the nominal Coulomb barriers and the respective coupling strengths. However, the large error bars on the high-energy side of the fusion BD prevent any definite identification of such signatures. We have, therefore, performed a quasi-elastic (QE) scattering experiment for the heavier $^{28}\mathrm{Si}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ system and compared its results with existing QE data for the $^{16}\mathrm{O}$ projectile. Since QE BDs are precise at higher energies, the comparison has shown that the BD of $^{28}\mathrm{Si}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ is similar to that of $^{16}\mathrm{O}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ to a large extent except for a peaklike structure on the higher energy side. The similarity shows that the $^{154}\mathrm{Sm}$ deformation plays a major role in the fusion mechanism of $^{28}\mathrm{Si}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ system. The peaklike structure is attributed to $^{28}\mathrm{Si}$ excitation. In contrast with previous studies, it is found that a coupled-channel calculation with vibrational coupling to the first ${2}^{+}$ state of $^{28}\mathrm{Si}$ reproduces this structure rather well. However, an almost identical result is found with the rotational coupling scheme if one considers the large positive hexadecapole deformation of the projectile. A value around that given by M\oller and Nix $({\ensuremath{\beta}}_{4}\ensuremath{\approx}0.25)$ leads to a strong cancellation in the re-orientation term that couples the ${2}^{+}$ state back to itself, making that state look vibrational in this process. Thus, unlike the existing fusion data, our new QE results contain subtle details about the fusion mechanism of $^{28}\mathrm{Si}+\phantom{\rule{0.16em}{0ex}}^{154}\mathrm{Sm}$ system. They even show a sensitivity to the $^{28}\mathrm{Si}$ hexadecapole deformation and hence may be capable of giving a physically reasonable estimate for ${\ensuremath{\beta}}_{4}$ in an indirect way.

Role of triple phonon excitations in large angle quasi-elastic scattering of very heavy mass systems

We study the effect of multi-phonon excitations on large-angle quasi-elastic scattering of massive systems using the full order coupled-channels formalism. We especially investigate the role of triple phonon excitations of the target and projectile nuclei on the quasi-elastic scattering cross-section as well as the barrier distribution for [Formula: see text]Cr, [Formula: see text]Fe, [Formula: see text]Ni and [Formula: see text]Zn + [Formula: see text]Pb systems. It is shown that the calculations taken into account, the triple octupole phonon excitations of the target and triple quadrupole phonon excitations of the projectile for these systems can explain the experimental data of the quasi-elastic cross-section and the quasi-elastic barrier distribution. These results indicate that the coupled-channels formalism is still valid even for the very heavy mass systems.

Measurement of Quasi-elastic Scattering: to Probe $^{28}$Si+$^{154}$Sm Reaction

We discuss the role of channel coupling of 28Si on fusion mechanism with permanently deformed target 154Sm. To this end, we analyze the experimental quasi-elastic cross sections at a large backward angle and quasi-elastic barrier distribution for 28Si+154Sm system using the coupled-channels approach. While earlier studies have reported the rotational excitation of 28Si playing role on fusion with spherical and near spherical target nuclei, we find its vibrational excitation as origin of observed barrier distribution for 28Si+154Sm system. Our study also reveals significant influence of channel couplings on the surface diffuseness parameter of an inter-nuclear potential supporting the earlier observations.

Mapping from quasi-elastic scattering to fusion reactions

The fusion barrier distribution has provided a nice representation for the channel coupling effects on heavy-ion fusion reactions at energies around the Coulomb barrier. Here we discuss how one can extract the same representation using the so called sum-of-differences (SOD) method with quasi-elastic scattering cross sections. In contrast to the conventional quasi-elastic barrier distribution, the SOD barrier distribution has an advantage in that it can be applied both to non-symmetric and symmetric systems. It is also the case that the correspondence to the fusion barrier distribution is much better than the quasi-elastic barrier distribution. We demonstrate its usefulness by studying $^{16}$O+$^{144}$Sm, $^{58}$Ni+$^{58}$Ni, and $^{12}$C+$^{12}$C systems.

Quasi-elastic scattering in the20Ne+90,92Zr reactions: Role of noncollective excitations

Conventional coupled-channels analyses, that take account of only the collective excitations of the colliding nuclei, have failed to reproduce the different behavior of the experimental quasi-elastic barrier distributions for the ${}^{20}$Ne+${}^{90,92}$Zr systems. To clarify the origins of this difference, we investigate the effect of noncollective excitations of the Zr isotopes. Describing these excitations in a random-matrix model, we explicitly take them into account in our coupled-channels calculations. The noncollective excitations are capable of reproducing the observed smearing of the peak structure in the barrier distribution for ${}^{20}$Ne+${}^{92}$Zr, while not significantly altering the structure observed in the ${}^{20}$Ne+${}^{90}$Zr system. The difference is essentially related to the closed neutron shell in ${}^{90}$Zr.

Quasi-elastic processes of the48Ca + 120Sn system and the48Ca nuclear matter density

We present the results of a high-precision quasi-elastic excitation function measurement for the ${}^{48}$Ca + ${}^{120}$Sn system at ${\ensuremath{\theta}}_{\mathrm{LAB}}$ = 160\ifmmode^\circ\else\textdegree\fi{} at near-barrier energies in steps of 1.0 MeV. The corresponding quasi-elastic barrier distribution is derived. A large-scale coupled-channel calculation was performed to investigate the role of several reaction channels in the reaction mechanism. An excellent agreement between theory and data was obtained for the barrier distribution. The first quadrupole vibrations of the ${}^{48}$Ca and ${}^{120}$Sn, the 2$n$, and the ${}^{4}$He transfers have a strong influence on the reaction mechanism and are responsible for the good agreement achieved. The 1$n$ transfer has a minor importance in the result when compared with the 2$n$ transfer, which suggests that the pairing correlation might play an important role in the 2$n$-neutron transfer process. However, if the octupole vibration of the projectile is included in the coupling scheme, the agreement with the data gets worse. The comparison of the coupled-channel calculations with experimental data leads to the conclusion that the nuclear matter diffuseness of the ${}^{48}$Ca nucleus is 0.56 fm in agreement with most of the double-magic nuclei.

Threshold energy for sub-barrier fusion hindrance phenomenon

The relationship between the threshold energy for a deep sub-barrier fusion hindrance phenomenon and the energy at which the regime of interaction changes (the turning-off of the nuclear forces and friction) in the sub-barrier capture process is studied within the quantum diffusion approach. The quasielastic barrier distribution is shown to be a useful tool to clarify whether the slope of capture cross section changes at sub-barrier energies.