Heavy Ion Reaction Analyzer Research Articles

Fabrication and characterization of thin [formula omitted]Sn films for heavy-ion nuclear reaction measurements

The fabrication of thin isotopically enriched 120,124Sn targets have been presented. The thin film targets prepared have been used in nuclear physics experiments to apprehend the heavy-ion fusion and multi-nucleon transfer reaction dynamics around the Coulomb barrier. Vacuum evaporation viz. resistive heating technique has been used for the fabrication of the targets with varying areal densities in the range of 50–250 μg/cm2 backed with a carbon layer of ≈20μg/cm2 using a nominal amount of 30 mg isotopically enriched material. The fabricated targets have been characterized using multiple advanced techniques viz. Energy Dispersive X-ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM), and X-ray Diffractometry (XRD). The thickness of the targets has been measured using Rutherford Back-scattering Spectrometry (RBS). The isotopic contamination levels have been ascertained with an in-beam experiment using Heavy Ion Reaction Analyzer at IUAC, New Delhi.

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.

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.

Fusion studies in O16+Nd142,150 reactions at energies near the Coulomb barrier

Background: Enhancement in fusion cross sections over one-dimensional barrier penetration model (1D-BPM) predictions has been observed at sub-barrier energies owing to the coupling of various internal degrees of freedom to the relative motion. On the other hand, hindrance in fusion is noticed at energies well above the barrier in a few cases.Purpose: The purpose is to probe the dynamics of heavy ion fusion at energies below and well above the Coulomb barrier.Methods: Fusion excitation functions for the $^{16}\mathrm{O}+^{142,150}\mathrm{Nd}$ reactions are measured using the Heavy Ion Reaction Analyzer (HIRA) at IUAC, New Delhi. Measurements have been performed in the range $12%$ below to $50%$ above the Coulomb barrier for both systems. The measured fusion cross sections are compared with coupled channels calculations using ccfull code. The cross sections are also compared with reactions using $^{16}\mathrm{O}$ beams with other isotopes of Nd to explore the systematic behavior of fusion.Results: Compared with 1D-BPM predictions, fusion enhancement is observed in both $^{16}\mathrm{O}+^{142}\mathrm{Nd}$ and $^{16}\mathrm{O}+^{150}\mathrm{Nd}$ reactions at below-barrier energies, with the latter reaction showing a higher enhancement. Coupled channels calculations incorporating the collective excitations of the target nuclei reproduce the fusion cross sections in both reactions. The collective excitations of the projectile nucleus do not seem to contribute to the observed fusion enhancement. Calculations using Aky\"uz-Winther potential parameters fail to reproduce the fusion cross sections at energies well above the barrier.Conclusions: Fusion enhancement is observed in both reactions studied. Degree of enhancement of sub-barrier fusion cross section is larger for the reaction using $^{150}\mathrm{Nd}$ target. Fusion hindrance is observed in both reactions at very high energies. The hindrance seems to increase with increasing beam energy. Larger value of diffuseness parameter compared with the value consistent with elastic-scattering measurements is required to reproduce the fusion excitation function at energies well above the barrier. This could be a strong indication of dynamical effects in fusion at very high energies.

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.

Relationship between and effect of inelastic excitations and transfer channels on sub-barrier fusion enhancement

Background: It is a well established fact that nuclear deformation and vibration influence fusion dynamics around the Coulomb barrier. This effect was observed for several systems with the inclusion of inelastic excitations in coupled-channels calculations. Sub-barrier fusion cross sections were also observed to be affected by neutron transfer in systems carrying positive $Q$ value for transfer channels. However, recent experimental analysis with a few systems showed that inelastic excitations are enough to explain the sub-barrier fusion behavior, and no effect was noticed due to positive $Q$-value transfer channels.Purpose: The motivation behind present investigation is to explore the effects of colliding nuclei structure and the transfer channel on enhancement of sub-barrier fusion cross sections.Method: An experiment was performed with Heavy Ion Reaction Analyzer (HIRA) at New Delhi to measure the fusion cross sections for $^{28}\mathrm{Si}+^{92,96}\mathrm{Zr}$ systems. These cross sections were later compared with quantum mechanical coupled-channels calculations. In order to explore the effects of nuclear deformation on fusion cross sections, the present data were compared with those of other researchers on fusion who have used various projectiles of different structural properties on a $^{96}\mathrm{Zr}$ target.Results: Experimental fusion cross sections have been extracted around the Coulomb barrier. In the coupled-channels framework, inclusion of inelastic excitations of both projectile $(^{28}\mathrm{Si})$ and targets $(^{92,96}\mathrm{Zr})$ could reproduce the experimental cross sections around the Coulomb barrier, but they deviated substantially in the sub-barrier region. This indicates that positive $Q$-value neutron transfer channels may need to be included in the calculations to reproduce the experimental cross sections at sub-barrier energies.Conclusions: The nuclear structure of interacting nuclei has a strong influence on sub-barrier fusion enhancement. The effect of multineutron transfer channels was observed to be significant for fusion, especially for those systems where interacting nuclei are less deformed or spherical.

Influence of neutron transfer channels on fusion enhancement in sub-barrier region

Fusion cross-section measurements were performed for system 40 Ca + 70 Zn around the Coulomb barrier energies using Heavy Ion Reaction Analyzer (HIRA). The observed enhancement in experimental fusion cross-sections was investigated via coupled-channels formalism. The coupling of inelastic excitations alone could not reproduce the experimental data, however, the effect of octupole state of the projectile was observed to be significant. The multi-neutron positive Q-value transfer channels were also included in the calculations using semi-classical model. It was observed that two neutrons pick-up channel gave a major contribution to the fusion enhancement and successfully reproduce the experimental data at above as well as below barrier energies. The coupling of more than two neutrons transfer could not give any significant enhancement to subbarrier fusion.

Deformation effects on sub-barrier fusion cross sections inO16+Yb174,176

Background: Couplings with various reaction channels are known to enhance sub-barrier fusion cross sections by several orders in magnitude. However, a few open questions still remain. For example, the influence of higher order static deformations on sub-barrier fusion cross sections is yet to be comprehensively understood.Purpose: We study the role of hexadecapole nuclear deformation effect on sub-barrier fusion cross sections. Also, this work aims to extract hexadecapole deformation $({\ensuremath{\beta}}_{4})$ in nuclei in the lanthanide region.Method: The evaporation residue (ER) excitation functions for $^{16}\mathrm{O}+^{\phantom{\rule{0.16em}{0ex}}174,176}\mathrm{Yb}$ were measured at laboratory beam energies $({E}_{\mathrm{lab}})$ in the range of 64.6--103.6 MeV. Measurements were carried out by employing the recoil mass spectrometer Heavy Ion Reaction Analyzer (HIRA) at IUAC, New Delhi. Fusion barrier distributions (BDs) were extracted from data. Results from the experiment were subjected to coupled-channels analysis, in which ${\ensuremath{\beta}}_{4}$ was varied as a free parameter.Results: Experimental fusion cross sections at energies below the barrier expectedly showed strong enhancement compared to the predictions from the one-dimensional barrier penetration model. Data were satisfactorily reproduced after inclusion of negative ${\ensuremath{\beta}}_{4}$ for both the targets in the coupled-channels calculation.Conclusions: The significant role of hexadecapole deformation was observed in the sub-barrier fusion of $^{16}\mathrm{O}+^{\phantom{\rule{0.16em}{0ex}}174,176}\mathrm{Yb}$. The proposed value of ${\ensuremath{\beta}}_{4}$ reproduced the measured fusion excitation function reasonably well. The BDs from these data were also extracted but no definitive conclusions could be drawn from them.

Fusion and transfer reactions around the Coulomb barrier for 28Si+90,94Zr systems

Fusion excitation functions and transfer probabilities were measured for 28Si+90,94Zr systems around the Coulomb barrier, using the recoil mass separator, Heavy Ion Reaction Analyzer (HIRA) at Inter University Accelerator Centre (IUAC), New Delhi. The aim of these experiments was the study of coupling effects of inelastic and transfer channels on the sub-barrier fusion cross section enhancement. The experimental fusion cross sections were found to be strongly enhanced as compared to one dimensional barrier penetration model (1-d BPM) predictions in both the cases. The trend of data could be easily reproduced theoretically using coupled channels code CCFULL in the case of 28Si+90Zr but the observed enhancement could not be explained in the case of 28Si+94Zr, which may be attributed to the coupling of multinucleon transfer channels as both isotopes have similar collective strengths. The transfer probabilities in the case of 94Zr were found to be substantially higher than for 90Zr in the sub-barrier region.

Multinucleon transfer reactions for theSi28+Zr90,94systems in the region below and near the Coulomb barrier

Measurements on multinucleon transfer reactions for $^{28}\mathrm{Si}+^{90,94}\mathrm{Zr}$ systems were performed at sub- and near-barrier energies. The fact that $^{90}\mathrm{Zr}$ has a closed neutron shell ($N$ $=$ 50) and $^{94}\mathrm{Zr}$ has four neutrons outside the closed shell, allows us to investigate the effects of shell closure and pairing correlation on multinucleon transfer mechanism. The experiment was performed with pulsed $^{28}\mathrm{Si}$ beam using the Heavy Ion Reaction Analyzer (HIRA) at Inter University Accelerator Centre (IUAC), New Delhi. Based on the $Q$-value considerations, it turned out that pickup channels were neutron transfer whereas stripping channels were proton transfer. For the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system, the values of the slope parameter for two-neutron pickup turned out to be less than that for one-neutron pickup. The values of the slope parameter were almost the same for two-, three-, and four-neutron pickup channels in the case of the $^{28}\mathrm{Si}+^{94}\mathrm{Zr}$ system. The transfer probabilities in the case of the $^{28}\mathrm{Si}+^{94}\mathrm{Zr}$ system were much larger than those for the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system, further supporting the fact that there is a correlation between the transfer channels and sub-barrier fusion cross-section enhancement. An odd-even staggering was observed in the extracted transfer probabilities at the barrier radius implying the role of pairing correlation in transfer reactions.

Angular momentum distribution for the formation of evaporation residues in fusion of 19F with 184W near the Coulomb barrier

We present γ-ray multiplicity distributions for the formation of evaporation residues in the fusion reaction 19F + 184W → 203 83Bi 120 at beam energies in the range of 90–110 MeV. The measurements were carried out using a 14 element BGO detector array and the Heavy Ion Reaction Analyzer at the Inter University Accelerator Centre. The data have been unfolded to obtain angular momentum distributions with inputs from the statistical model calculation. Comparison with another neighboring system, viz. 19F + 175Lu → 194 80Hg 114 with nearly similar entrance-channel mass asymmetry, hints at the depletion of higher angular momenta after crossing of the Z = 82 shell in the compound nucleus.

Evaporation residue excitation function from complete fusion ofF19withW184

Evaporation residue cross sections for $^{19}\mathrm{F}+^{184}\mathrm{W}$ have been measured at beam energies in the range of 90--130 MeV using the Heavy Ion Reaction Analyzer at the Inter University Accelerator Centre. A new approach, based on Monte Carlo calculations, has been adopted for estimating evaporation residue (ER) transmission efficiency. The challenges in the detection of ERs caused by the presence of isomeric states with longer half-lives have been thoroughly discussed. Measured cross sections have been fitted with statistical model calculations treating the fission barrier as an adjustable parameter. Comparison with other neighboring systems with nearly similar entrance-channel mass asymmetry indicates the onset of instability in the compound nucleus ${}_{83}^{203}{\mathrm{Bi}}_{120}$ after crossing of the $Z=82$ shell.

Channel coupling effects on the fusion excitation functions forSi28+Zr90,94in sub- and near-barrier regions

Fusion excitation functions and angular distributions of evaporation residues (ERs) have been measured for $^{28}\mathrm{Si}+^{90,94}\mathrm{Zr}$ systems around the Coulomb barrier using the recoil mass spectrometer, Heavy Ion Reaction Analyzer (HIRA). For both systems, the experimental fusion cross sections are strongly enhanced compared to the predictions of the one-dimensional barrier penetration model (1-d BPM) below the barrier. Coupled channels formalism has been employed to theoretically explain the observed sub-barrier fusion cross section enhancement. The enhancement could be explained by considering the coupling of the low-lying inelastic states of the projectile and target in the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system. In the sub-barrier region, the measured fusion cross sections for $^{28}\mathrm{Si}+^{94}\mathrm{Zr}$ turned out to be about an order of magnitude higher than the ones for the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system, which could not be explained by coupling to inelastic states alone. This observation indicates the importance of multinucleon transfer reaction channels with positive $Q$ values in the sub-barrier fusion cross section enhancement, because $^{90,94}\mathrm{Zr}$ are believed to have similar collective strengths. This implies that no strong isotopic dependence of fusion cross sections is expected as far as the couplings to collective inelastic states are concerned. In addition, the role of projectile and multiphonon couplings in the enhancement has been explored.

TERS v2.0: An improved version of TERS

TERS v2.0: An improved version of TERS

A Monte Carlo C-code for calculating transmission efficiency of recoil separators and viewing residue trajectories

We present a semimicroscopic Monte Carlo code for calculating absolute transmission efficiency of recoil separators for heavy ion-induced complete fusion reactions. The code generates realistic distributions for energy, charge state and angle of evaporation residues. Residue trajectories are calculated using first order ion optical transfer matrices. Trajectory plots in the dispersive and the non-dispersive planes are generated. Using this code, we have obtained good agreement between calculated and measured transmission efficiencies for the Heavy Ion Reaction Analyzer at IUAC. The code can be adapted easily to any other electromagnetic recoil separator. Program summary Program title: TERS Catalogue identifier: AEBD_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEBD_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 6818 No. of bytes in distributed program, including test data, etc.: 1 216 097 Distribution format: tar.gz Programming language: C Computer: The code has been developed and tested on a PC with Intel Pentium IV processor Operating system: Linux RAM: About 8 Mbytes Classification: 17.7 External routines: pgplot graphics subroutine library [1] should be installed in the system for generating residue trajectory plots. Nature of problem: Recoil separators are employed to select and identify nuclei of interest, produced in a nuclear reaction, rejecting unreacted beam and other undesired reaction products. It is important to know what fraction of the selected nuclei, leaving the target, reaches the detection system. This information is crucial for determining absolute cross section of the studied reaction. Solution method:Interaction of projectiles with target nuclei is treated event by event, semimicroscopically. Position and angle (with respect to beam direction), energy and charge state of the reaction products are calculated by Monte Carlo method. Trajectory of each nuclei inside the separator is then calculated by ion optical transfer matrix method. Ratio of the number of trajectories completing their journey up to the detection system to the total number of trajectories is a direct measure of absolute transmission efficiency of the separator. Restrictions: The present version of the code is applicable to complete fusion reactions only. The code can be applied to other types of reactions (e.g., few nucleon transfer) as well, by suitably modifying energy and angular distribution of reaction products. Also, ion optical specifications and acceptance are unique for each recoil separator. Transmission efficiency calculation has been done for a specific recoil separator, viz. the Heavy Ion Reaction Analyzer [2,3] at IUAC. One has to make necessary changes in the code, while performing calculations for other recoil separators. Further, atomic number of the residual nucleus should not exceed 92, as the method used for calculating stopping power of ions [4] is valid for Z ⩽ 92 . Running time: From few seconds to several minutes depending on the reaction, number of events and separator layout.

A Monte Carlo code to calculate transmission efficiency of HIRA

A semimicroscopic Monte Carlo code is presented for calculating absolute transmission efficiency of the Heavy Ion Reaction Analyzer at IUAC. The code generates realistic distributions for energy, charge state and angle of evaporation residues (ERs) produced in heavy ion-induced complete fusion reactions. Further, individual ER trajectories are calculated using first-order transfer matrices. Trajectories in dispersive and non-dispersive planes and one as well as two-dimensional position spectra of the ERs at the focal plane are simulated by the code. Calculated efficiencies are in good agreement with measurements.

Time-dependent aspects of the semiclassical approach in the analysis of heavy ion reactions

The semiclassical approach is one of the fundamental tools used to investigate and analyze heavy ion reaction processes. We show in this contribution that the evolution of the motion that emerges from the knowledge of the semiclassical amplitudes for the reaction channels as a function of time cannot be taken as a reliable representation for an actual wave function of the intrinsic states.

General behavior of the effective nucleon-nucleon interaction as a function of the relative velocity

We have studied volume integrals of the central part of optical potentials extracted from data analyses for a variety of light and heavy systems. The data-extracted integrals present a quite simple behavior as a function of the relative velocity between target and projectile. This behavior is system independent and, therefore, it reflects a feature of the effective nucleon-nucleon interaction itself. The overall results are in good agreement with the predictions of the S\~ao Paulo potential, which is a model for the nuclear interaction that so far has been employed mostly in analyses of heavy-ion reactions.

A new detector system for radioactive ion beam experiments at the focal plane of the recoil mass spectrometer HIRA at NSC

7Be radioactive ion beam (RIB) has been produced by an in-flight separation technique using the recoil mass spectrometer Heavy Ion Reaction Analyzer (HIRA) at NSC. To carry out nuclear reaction studies using the low-intensity RIB, a compact geometry detector system has been installed at the HIRA focal plane. The new detector setup consists of a Multi-Wire Proportional Counter (MWPC) followed by a DE-E detector telescope. The MWPC is used for obtaining the RIB profile and normalization. Fast timing signal of the MWPC is also used for the time-of-flight measurements for correction of energy spread of the RIBs in event mode. The detector telescope consists of a gas ionization chamber which gives energy loss (DE), followed by a two-dimensional position-sensitive silicon detector which gives residual energy and position. The telescope provides particle identification, energy and reaction angle information. The measured position resolution of the MWPC is about 0.9 mm (FWHM). The detector setup has also been used in stable beam experiments as well where the MWPC provides mass spectrum of the recoils at the RMS focal plane according to their position.

The present and planned recoil mass spectrometers at Nuclear Science Centre, New Delhi

Recoil mass spectrometers (RMS) are ideal instruments for identifying weakly populated reaction products in the forward angle from heavy-ion reactions amidst an intense beam background. The Heavy Ion Reaction Analyzer (HIRA) at the Nuclear Science Centre (NSC), New Delhi, is one of the few operating first generation RMSs. The features of and physics studies pursued using HIRA are covered in the paper. With the augmentation of the accelerator facilities at NSC, a second generation RMS combined with a gas-filled separator is planned. The design details of the new facility, Hybrid Recoil Mass Analyzer (HYRA), are presented. HYRA will be operated either stand-alone or in conjunction with a Large Gamma Array (LGA) at its target position. Physics studies planned with these forthcoming facilities are outlined.