# Sub-barrier few-nucleon transfer reaction and channel coupling effects in heavy ion fusion

Bangalore University

^{1}, University of Calcutta^{2}, Saurashtra University^{3}, Andhra University^{4}, University of Calicut^{5}, University of Delhi^{6}TL;DR: In this paper, the surface vibration coupling is treated up to two phonon states with second-order coupling terms, and simplified coupled-channels calculations are carried out for these systems with transfer form factors extracted from the measured transfer probabilities.

Abstract: Measurements of sub-barrier transfer reactions are reported for the systems , and using the recoil mass spectrometer HIRA in kinematic coincidence mode. The problem related to M/Q-ambiguity in measurements with mass separators has been resolved. Excellent mass resultion with a large solid angle is obtained by correction of the aberrations. Simplified coupled-channels calculations are carried out for these systems with transfer form factors extracted from the measured transfer probabilities. The surface vibration coupling is treated up to two phonon states with second-order coupling terms.

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TL;DR: In this article, large-angle quasielastic scattering measurements were carried out over a wide range of incident beam energies around the Coulomb barriers and the experimental results were compared with the predictions from coupled-channels calculations carried out using different coupling schemes.

Abstract: Barrier distributions for the $^{28}\mathrm{Si}+^{142,150}\mathrm{Nd}$ systems were extracted from large-angle quasielastic scattering measurements. The measurements were carried out over a wide range of incident beam energies around the Coulomb barriers. The experimental results were compared with the predictions from coupled-channels calculations carried out using different coupling schemes. Reasonable agreement between the experimental and theoretical results was obtained. The role of coupling effects of the various excitation modes of the projectile and target on the observed barrier distributions is discussed. The sensitivity of the quasielastic scattering process on the mode of projectile excitation is clearly been seen from the use of two different types of targets, $^{142}\mathrm{Nd}$ and $^{150}\mathrm{Nd}$, having spherical and deformed shapes at the ground state, respectively.

4 citations

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TL;DR: In this article , the role of the $2n$-transfer channel with a positive $Q$ value on sub-barrier fusion and back-angle quasielastic (QE) scattering in the $^{30}\mathrm{Si}+^{156}\mathm{Gd}$ reaction is examined.

Abstract: 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.

2 citations

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TL;DR: In this paper, the authors present simulation of a recoil mass spectrometer for measurement of multi-nucleon transfer reactions, based on Monte Carlo techniques, where target-like ions are generated event by event and transported to the focal plane of the spectrometers by the method of transfer matrices, and the measured focal plane spectra for the elastic and six transfer channels of the reaction are reproduced at two projectile energies near the Coulomb barrier.

Abstract: We present simulation of a recoil mass spectrometer for measurement of multi-nucleon transfer reactions, based on Monte Carlo techniques. Target-like ions are generated event by event and transported to the focal plane of the spectrometer by the method of transfer matrices. Measured focal plane spectra for the elastic and six transfer channels of the reaction $$^{28}$$
Si+
$$^{94}$$
Zr are reproduced at two projectile energies near the Coulomb barrier. Excellent reproduction is achieved even with first-order ion-optical calculations, indicating not so significant role of higher order aberrations in conventional recoil separators with smaller acceptance. This is in striking contrast with the large acceptance magnetic separators used for studying transfer reactions, for which complex trajectory reconstruction algorithms involving higher order aberrations are essential. Transmission efficiency of the spectrometer, calculated by the reported code, is crucial to convert measured transfer probabilities to differential and integral transfer cross sections for each channel. We show effectiveness of the proposed methodology by extracting differential elastic scattering cross sections from measured yields for the reaction $$^{28}$$
Si+
$$^{94}$$
Zr. Besides being useful for other recoil mass spectrometers, our methodology can be employed for measurement of quasi-elastic reaction cross sections in velocity filters and gas-filled separators.

2 citations

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TL;DR: In this article , the first direct measurement of differential transfer cross sections using a recoil mass spectrometer was reported by detecting the heavier target-like ions at the focal plane of the Heavy Ion Reaction Analyzer.

Abstract: We report the first direct measurement of differential transfer cross sections using a Recoil Mass Spectrometer. Absolute differential 1p- and 2p-stripping cross sections at $$\theta _\mathrm {c.m.}=180^\circ $$ 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 $$^{142}\mathrm {Ce(}^{16}\textrm{O,}^{15}\mathrm {N)}^{143}\textrm{Pr}$$ and $$^{142}\mathrm {Ce(}^{16}\textrm{O,}^{14}\mathrm {C)}^{144}\textrm{Nd}$$ 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 1p-stripping. For 2p-stripping, cluster transfer of two protons has been found to have dominant contribution. Measured transfer probabilities for 1p- and 2p-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.

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TL;DR: In this article, the spin distributions for deformed, interacting nuclei are constructed in the sudden limit by averaging the coupled-channel results over all relative orientations of the nuclear shapes.

Abstract: Cross sections for heavy-ion fusion and the corresponding spin-distributions for the compound nuclei can be strongly affected by the presence of static deformations, as well as by couplings to collective vibrations and other reaction channels. Fusion cross sections and spin distributions for deformed, interacting nuclei are constructed in the sudden limit by averaging the coupled-channel results over all relative orientations of the nuclear shapes.

93 citations

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01 Feb 1994-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment

TL;DR: The heavy ion reaction analyzer (HIRA) as mentioned in this paper is a large solid angle online mass separator for the reaction products preserving the kinematic correlation, which is designed to effectively separate the nuclear reaction products of interest from the elastically scattered beam, dispersing them with good mass resolution at its focal plane with energy and space focusing.

Abstract: An inter-university research facility [1] using a 16 MV tandem ion accelerator has been set up at Nuclear Science Centre (NSC), New Delhi. The heavy ion reaction analyzer (HIRA), a major experimental facility at NSC, is a large solid angle online mass separator for the reaction products preserving the kinematic correlation [2]. It is designed to effectively separate the nuclear reaction products of interest from the elastically scattered beam, dispersing them with good mass resolution at its focal plane with energy and space focusing. HIRA transports the ions within a short time (approximate to mu s) without losing the time correlation with the instant of reaction. The transit time spread (for a given energy) due to different flight paths is very small. With capability of operation over an angular range of -15 degrees to +40 degrees, HIRA is planned as a versatile reaction analyzer for heavy ion studies. The HIRA facility is operational and initial reports on its performance are presented [3]. In section 1 we give details of ion optical considerations and design goals. Section 2 gives some hardware details and results of field mapping of various magnetic components. The last section gives some of the results obtained in the preliminary measurements. A detailed description of the performance results of HIRA will be presented in a following publication.

71 citations

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TL;DR: In this paper, an analysis of 32 S + 64 Ni fusion excitation data has been performed to derive an empirical barrier distribution in order to investigate the relationship between the simplified coupled-channel model and the neutron-flow model.

Abstract: Results of the measurements of fusion excitation functions and average angular momenta are presented for 37 Cl + 59 Co, 28 Si + 68 Zn, 32 S + 64 Ni and 45 Sc + 51 V, for centre-of-mass energies ranging from ∼5 MeV below to ∼10 MeV above the Coulomb barrier. These results are discussed in terms of the simplified coupled-channel model and the neutron-flow model due to Stelson. It is pointed out that simplified coupled-channel calculations including collective inelastic excitations alone do not reproduce the observed sub-barrier fusion cross section in most of the cases and that there is a need to include couplings to other channels, like transfer, in order to explain the observations. An analysis of 32 S + 64 Ni fusion excitation data has been performed to derive an empirical barrier distribution in order to investigate the relationship between the simplified coupled-channel model and the neutron-flow model. A simultaneous analysis of the fusion excitation function and average angular momentum seems to indicate that these two quantities are related to each other, as was pointed out by Balantekin and Reimer.

56 citations

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TL;DR: A systematic analysis of the data indicates lack of isotopic dependence in the sub-barrier fusion for these three systems, as seen in the cross section and mean angular momentum data.

Abstract: Fusion process in the near and sub-barrier region has been investigated for the systems $^{48}\mathrm{Ti}$${+}^{58,60,64}$Ni using the heavy-ion reaction analyzer (HIRA). Fusion excitation functions and the mean angular momenta are obtained from the measured evaporation residue cross sections. Significant enhancements both in the cross section and mean angular momentum data are seen with respect to the predictions of the one-dimensional barrier penetration model. Simplified coupled channel calculations incorporating linear coupling to the inelastic channels (lowest ${2}^{+}$ and ${3}^{\mathrm{\ensuremath{-}}}$ states of both the projectile and the target) are not able to explain the observed enhancements. A systematic analysis of the data indicates lack of isotopic dependence in the sub-barrier fusion for these three systems. \textcopyright{} 1996 The American Physical Society.

34 citations