Heavy-ion Induced Fusion Reactions Research Articles

Investigating the influence of input angular momentum on independence hypothesis in heavy-ion induced fusion reactions

The experimental verification of compound nucleus theory has successfully been performed for light ion induced reactions using proton and α-particle beams on different targets. However, such studies using the heavy ion beams are scarce due to the complex nature of heavy ion interaction process. The present work is an attempt to validate the Bohr's independent hypothesis using heavy ion beam of 18O beam on 159Tb targets and 12C on 165Ho targets, forming the same compound nucleus 177Ta. A channel wise analysis of cross section data for evaporation residues produced in 18O+159Tb and 12C+165Ho systems has been performed within the framework of statistical model code PACE4 and found to reasonably agree with the experimental data. Further, a comparison of reaction cross section data for the same evaporation residues produced in the two different systems has been found to follow Bohr's compound nucleus hypothesis at relatively higher excitation energies. However, at relatively lower excitation energies, a significant deviation in the measured cross section data of the same residues in the above mentioned systems have been observed and is attributed to the discrepancies in angular momentum values. Such discrepancies are well explained on the basis of input angular momentum involved in the reactions.

Fabrication and characterization of thin [formula omitted]Gd targets for nuclear fusion reaction studies

Thin isotopically enriched 156,158Gd target foils of thickness between ≈100–150μg/cm2 have been fabricated. The target foils were prepared on carbon backing to provide extra support using the physical vapor deposition (PVD) technique. The fabricated target foils have been used to study the dynamics of heavy-ion induced fusion reactions using the Heavy Ion Reaction Analyser (HIRA) facility at IUAC New-Delhi, India. Due to the high melting point, the e-gun method was adopted to deposition carbon and gadolinium. During fabrication, the challenges were (i) minimal isotopic material, (ii) selection of the parting agent, and (iii) the floating process for separating the gadolinium film from the glass slides. In view of this, the first several attempts have been made with natural gadolinium oxide material to optimize specific parameters. Finally, using only ≈38 mg isotopically enriched material for each isotope, eight 156Gd and twenty 158Gd targets were prepared successfully. The nobleness of this work is that the gadolinium material was in oxide form, which provides extra stability to the fabricated targets even in the atmospheric environment. Further, various characterization techniques such as Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffractometry (XRD), and Rutherford Back-scattering Spectroscopy (RBS), have been explored to investigate surface morphology, elemental purity, chemical composition, and thickness of the target foils. The characterizations and analysis of experimental data manifested that no such impurities were present in the fabricated target foils. Essentially, it has been observed that the fabricated thin 156,158Gd target foils survived against the heavy-ion (28,30Si) irradiation throughout the online fusion experiment.

Capture and fission analysis of various superheavy isotopes with ACN = 258–280

The capture and fission analysis of heavy ion induced fusion reactions leading to the formation of Z = 107–111 superheavy nuclei has been carried out. Attempts have been made to analyze the synthesis traits, such as excitation functions, formation probabilities, barrier characteristics etc. The ℓ-summed Wong model provides a decent description of available data on capture (σCap) and fusion-fission (σff) cross-sections and hence is exploited to make relevant predictions for future experiments. The capture and fusion-fission excitation functions are predicted for the least explored region of superheavy nuclei (SHN) i.e. Z = 107–111. The role of mass-asymmetry (η), Coulomb factor (ZPZT), deformation and orientations, Businari-Gallone mass-asymmetry (αBG), fission barrier (Bf) etc is duly explored. The present study concludes that the mass-asymmetric reactions involving 24Mg, 30Si, and 36S projectiles are preferred for the synthesis of unknown isotopes of Z = 107–111. Alternatively, the doubly magic 48Ca-projectile also provides a competing alternative to produce neutron-rich isotopes of the above-mentioned SHN.

Investigating the fusion enhancement for neutron-rich mid-mass nuclei using the dynamical cluster-decay model

The mechanism involved in compound nucleus formation through heavy ion induced fusion reactions at near- and sub-barrier energies has been stimulating the interest of both experimentalists and theoreticians. The study of fusion reactions is important not only for the production of superheavy elements but also to unfold the mysteries involved in the evolution of neutron stars and nucleosynthesis of elements in astrophysical scenarios. The detection of gravitational waves originating from the merging of two black holes, and the possibility of observing similar events originating from black-hole--neutron-star mergers, highlighted the necessity of understanding the behavior of neutron-rich nuclear matter. One of the potential methods to understand the character of neutron-rich matter is through the study of fusion of an isotopic chain of reactions. Recently, the first experimental evidence of fusion enhancement at near-barrier energies for neutron-rich nuclei was reported for the light and mid-mass regime. To understand the dynamics involved in such reactions, the investigation of K-induced reactions has been performed using the quantum mechanical fragmentation based dynamical cluster-decay model (DCM). The experimental fusion cross sections of $^{39,47}\mathrm{K}+^{28}\mathrm{Si}$ are reproduced using DCM and, further, the fusion cross sections are predicted for other isotopes of K beam on $^{28}\mathrm{Si}$ to investigate the fusion enhancement as a function of neutron number. It has been observed that fusion enhancement is significant at near-barrier energies for neutron-rich nuclei. The fusion enhancement observed is larger for odd neutron number nuclei as compared to adjacent neutron nuclei. This indicates the influence of unpaired neutrons on fusion cross sections. Also, the fusion cross sections predicted in the present work will act as input for planning more experimental measurements.

Evaporation residue cross-section measurements for Ti48 -induced reactions

Background: A significant research effort is currently aimed at understanding the synthesis of heavy elements. For this purpose, heavy ion induced fusion reactions are used and various experimental observations have indicated the influence of shell and deformation effects in the compound nucleus (CN) formation. There is a need to understand these two effects.Purpose: To investigate the effect of proton shell closure and deformation through the comparison of evaporation residue (ER) cross sections for the systems involving heavy compound nuclei around the ${Z}_{\mathrm{CN}}=82$ region.Methods: A systematic study of ER cross-section measurements was carried out for the $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}, ^{144}\mathrm{Sm}$ systems in the energy range of $140--205\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. The measurement has been performed using the gas-filled mode of the hybrid recoil mass analyzer present at the Inter University Accelerator Centre (IUAC), New Delhi. Theoretical calculations based on a statistical model were carried out incorporating an adjustable barrier scaling factor to fit the experimental ER cross section. Coupled-channel calculations were also performed using the ccfull code to obtain the spin distribution of the CN, which was used as an input in the calculations.Results: Experimental ER cross sections for $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}$ were found to be considerably smaller than the statistical model predictions whereas experimental and statistical model predictions for $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ were of comparable magnitudes.Conclusion: Though comparison of experimental ER cross sections with statistical model predictions indicate considerable non-compound-nuclear processes for $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}$ reactions, no such evidence is found for the $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ system. Further investigations are required to understand the difference in fusion probabilities of $^{48}\mathrm{Ti}+^{142}\mathrm{Nd}$ and $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ systems.

The Coincident Fission Fragment Detector (CFFD)

A Parallel Plate Avalanche Counter (PPAC) based fission detector system, called the Coincident Fission Fragment Detector (CFFD), has been developed for the ReA3 re-accelerator facility of the National Superconducting Cyclotron Laboratory (NSCL). Binary reaction kinematics are reconstructed based on position and time-of-flight measurements of fission fragments. Large area PPACs provide 1ns level time resolution and mm level position resolution. The detectors allow measurements of fission product angular and mass distributions of heavy-ion induced fusion reactions. The 30cm by 40cm active area of each PPAC provides large solid angle coverage well suited for measurements of low intensity rare-isotope beams (RIBs).

Fission barrier, damping of shell correction, and neutron emission in the fission ofA∼200

Decay of $^{210}$Po compound nucleus formed in light and heavy-ion induced fusion reactions has been analyzed simultaneously using a consistent prescription for fission barrier and nuclear level density incorporating shell correction and its damping with excitation energy. Good description of all the excitation functions have been achieved with a fission barrier of 21.9 $\pm$ 0.2 MeV. For this barrier height, the predicted statistical pre-fission neutrons in heavy-ion fusion-fission are much smaller than the experimental values, implying the presence of dynamical neutrons due to dissipation even at these low excitation energies ($\sim$ 50~MeV) in the mass region A $\sim$ 200. When only heavy-ion induced fission excitation functions and the pre-fission neutron multiplicities are included in the fits, the deduced best fit fission barrier depends on the assumed fission delay time during which dynamical neutrons can be emitted. A fission delay of (0.8 $\pm$ 0.1 )$\times 10^{-19}$ s has been estimated corresponding to the above fission barrier height assuming that the entire excess neutrons over and above the statistical model predictions are due to the dynamics. The present observation has implication on the study of fission time scale/ nuclear viscosity using neutron emission as a probe.

Study of Fission and Evaporation Residue Cross Sections and Pre-Scission Neutron Multiplicities for ^(168)Yb

A Kramers-modified statistical model was applied to the study of the fission properties of the compound nucleus ^(168)Yb formed in heavy ion-induced fusion reactions in an intermediate range of excitation energies. The evaporation residue cross section, fission cross section, and the average pre-fission neutron multiplicities were calculated for ^(168)Yb formed in ^(18)O + ^(150)Sm reactions and the results compared with the experimental data. To calculate these quantities, the effects of temperature and the spin K about the symmetry axis have been considered in the calculations of the potential energy surfaces and the fission widths. It was shown that the results of the calculations are in good agreement with the experimental data by using values of the temperature coefficient of the effective potential and scaling factor of the fission-barrier height equal to k = 0.007 ± 0.002 MeV^(-2) and r_s = 0.994 ± 0.002, respectively.

Study of fission dynamics in fusion–fission reactions

One-dimensional Langevin equations were applied to study the fission dynamics of compound nucleus 200Pb formed in heavy ion-induced fusion reactions in an intermediate range of excitation energies. A modified wall and window dissipation with a reduction coefficient, ks, has been used in the Langevin equations. The average pre-fission multiplicities of neutrons, light charged particles and fission probabilities were calculated for 200Pb and results compared with the experimental data. It was shown that the results of the calculations are in good agreement with the experimental data of neutron and charged particles (p and alphas) multiplicities and fission probabilities by using values of ks in the range 0.27⩽ks⩽0.48.

SUPERHEAVY RESEARCH AT RIKEN

At RIKEN (The Institute of Physical and Chemical Research) in Japan, we have performed experiments to study the productions and decays of the heaviest elements produced by one neutron emission channels of 208 Pb and 209 Bi based heavy-ion induced fusion reactions. A gas-filled type recoil separator has been used for collecting evaporation residues of the reactions separating them from high intensity beam particles. The reactions studied were 208 Pb (58 Fe , n )265 Hs , 208 Pb (64 Ni , n ) 271 Ds , 209 Bi (64 Ni , n ) 272 Rg , 208 Pb (70 Zn , n ) 277112, and 209 Bi (70 Zn , n ) 278113. In studies of the first four reactions we have provided the independent confirmations of the productions and their decays of the isotopes, 265 Hs , 271 Ds , 272 Rg , and 277112, as well as the decay properties of their decay daughters, previously studied by Hofmann et al., a group of Gesellschaft für Schwerionenforschung (GSI), Germany. In the last reaction, we observed two decay chains originated from the isotope 278113, assigned firstly by generic correlation of the alpha decay chains connected into the previously known decay of 266 Bh and 262 Db via previously unknown alpha decays of 278113, 274 Rg and 270 Mt .

Gas chemical investigation of hafnium and zirconium complexes with hexafluoroacetylacetone using preseparated short-lived radioisotopes

Volatile metal complexes of the group 4 elements Zr and Hf with hexafluoroacetylacetonate (hfa) have been studied using short-lived radioisotopes of the metals. The new technique of physical preseparation has been employed where reaction products from heavy-ion induced fusion reactions are isolated in a physical recoil separator – the Berkeley Gas-filled Separator in our work – and made available for chemistry experiments. Formation and decomposition of M(hfa)4 (M=Zr, Hf) has been observed and the interaction strength with a fluorinated ethylene propylene (FEP) Teflon surface has been studied. From the results of isothermal chromatography experiments, an adsorption enthalpy of -ΔHa=(57±3) kJ/mol was deduced. In optimization experiments, the time for formation of the complex and its transport to a counting setup installed outside of the irradiation cave was minimized and values of roughly one minute have been reached. The half-life of 165Hf, for which conflicting values appear in the literature, was measured to be (73.9±0.8) s. Provided that samples suitable for α-spectroscopy can be prepared, the investigation of rutherfordium (Rf), the transactinide member of group 4, appears possible. In the future, based on the studies presented here, it appears possible to investigate short-lived single atoms produced with low rates ( e.g. , transactinide isotopes) in completely new chemical systems, e.g. , as metal complexes with organic ligands as used here or as organometallic compounds.

Comparison of complementary reactions for the production ofBh261,262

Two heavy-ion induced fusion reactions producing $^{261,262}\mathrm{Bh}$ were studied using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. An excitation function for the production of $^{262}\mathrm{Bh}$ via the reaction ${}^{209}\mathrm{Bi}({}^{54}\mathrm{Cr},n){}^{262}\mathrm{Bh}$ was measured with a maximum cross section from a fit to the data of 430 \ifmmode\pm\else\textpm\fi{} 110 pb observed at a compound nucleus excitation energy of 15.7 MeV. New data have been measured for the 1$n$ exit channel of the ${}^{208}\mathrm{Pb}({}^{55}\mathrm{Mn}, n){}^{262}\mathrm{Bh}$ reaction. We present an updated excitation function with an observed maximum cross section of $530\ifmmode\pm\else\textpm\fi{}100$ pb at a compound nucleus excitation energy of 14.1 MeV. Events corresponding to the 2$n$ exit channel for the ${}^{209}\mathrm{Bi}({}^{54}\mathrm{Cr},2n){}^{261}\mathrm{Bh}$ and ${}^{208}\mathrm{Pb}({}^{55}\mathrm{Mn},2n){}^{261}\mathrm{Bh}$ reactions were also observed and are presented as partial excitation functions. The measured decay properties correspond well with existing experimental data. We compare these experimental results to cross section predictions from a model by \ifmmode \acute{S}\else \'{S}\fi{}wia\ifmmode \mbox{\k{}}\else \k{}\fi{}tecki et al. and discuss entrance channel effects on the magnitude of 1$n$ cross sections.

Systematics of heavy-ion fusion hindrance at extreme sub-barrier energies

The recent discovery of hindrance in heavy-ion induced fusion reactions at extreme sub-barrier energies represents a challenge for theoretical models. Previously, it has been shown that in medium-heavy systems, the onset of fusion hindrance depends strongly on the ``stiffness'' of the nuclei in the entrance channel. In this work, we explore its dependence on the total mass and the Q-value of the fusing systems and find that the fusion hindrance depends in a systematic way on the entrance channel properties over a wide range of systems.

Search for entrance channel effects in heavy ion induced fusion reactions via neutron evaporation

Search for entrance channel effects in heavy ion induced fusion reactions via neutron evaporation

Search for entrance channel effects in heavy ion induced fusion reactions through the compound system79Rb*

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The Leuven isotope separator on-line laser ion source

An element-selective laser ion source has been used to produce beams of exotic radioactive nuclei and to study their decay properties. The operational principle of the ion source is based on selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a noble gas at high pressure. The ion source has been installed at the Leuven Isotope Separator On-Line (LISOL), which is coupled on-line to the cyclotron accelerator at Louvain-la-Neuve. 54,55Ni and 54,55Co isotopes were produced in light-ion-induced fusion reactions. Exotic nickel, cobalt and copper nuclei were produced in proton-induced fission of 238U. The b decay of the 68–74Ni, 67–70Co, 70–75Cu and 110–114Rh isotopes has been studied by means of β– γ and γ– γ spectroscopy. Recently, the laser ion source has been used to produce neutron-deficient rhodium and ruthenium isotopes ( 91–95Rh, 98Rh, 90,91Ru) near the N= Z line in heavy ion-induced fusion reactions.

Alpha particle evaporation as a probe for dynamical deformations

Evaporation of alpha particles in heavy ion reactions followed by fusion has proved to be a powerful probe for the properties of emitting nuclei. Detailed experimental data and different model calculations allow us to probe whether the foundation of the statistical model holds for the compound nucleus populated in these reactions. It has been observed that in the case of composite nuclei at moderate energies and angular momenta, evaporation spectra are well explained in terms of the standard statistical model CASCADE code employing optical model transmission coefficients in the description of particle evaporation. However, it has been observed that experimental particle spectra from heavy ion induced fusion reactions are no longer consistent with the predictions of such models. It has been predicted by some authors that in these systems the emission barriers are lower than those expected from optical model transmission coefficients calculated for the respective inverse absorption channels. Some authors claim that these spectra may be well explained in terms of the statistical model incorporating only spin dependent level density and without lowering the emission barriers. The field is not yet free from the controversies. Furthermore, the assumption of the very short formation time in statistical model analysis is one extreme of the general evolution process which in fact is a continuous relaxation process, leading to the composite system from the entrance channel to the equilibrated configuration. Recent dynamical description of heavy ion collisions do not support this assumption in many cases. In symmetric entrance channels and for collisions where centre of mass energy is well above the Coulomb barrier, formation time can be even larger than decay time of the resulting composite system. In such cases realistic approach will be to couple the dynamical evolution of the intrinsic excitation of the composite system to a time dependent statistical model calculation. The above question has been addressed in the light of the alpha particle spectra taken in coincidence with the evaporation residues for the asymmetric 28Si+51V and the symmetric 28Si+27Al systems. The experimental data have been interpreted in the framework of dynamical trajectory model calculations.

Measurements of heavy-ion induced fusion cross sections with the gas-filled-magnet technique

A gas-filled magnet has been used to measure excitation functions of heavy-ion induced fusion reactions. Equipped with a large x − y-position sensitive parallel-grid avalanche detector in the focal plane the setup allows for a very efficient measurement of fusion cross sections at energies above and below the Coulomb barrier.

How well do we understand the synthesis of heavy elements by heavy-ion induced fusion?

Experimental cross sections to synthesize heavy ion induced fusion reactions are confronted with calculations using the conventional picture of complete fusion after passing the barrier plus statistical deexcitation after complete equilibration. The sensitivity of the calculations to some of the parameters is demonstrated. Despite this sensitivity, a single parameter set is found that reproduces well the large body of data from experiments using actinide targets. By comparison, for the more symmetric entrance channels using targets around208Pb, we find a fusion hindrance that increases steeply with increasing compound nuclear charge. Predictions for a few reactions of future interest for the synthesis of heavy elements are also presented.

Yields of evaporation residues and average angular momentum in heavy ion induced fusion reactions leading to compound nucleus96Ru

Cross-sections for production of evaporation residues from the compound nucleus96Ru* formed by fusion reactions28Si+68Zn,32S+64Ni,37Cl+59Co and45Sc+51V have been obtained from the yields of their characteristicγ-rays. The measurements span an excitation energy range of 55 MeV to 70 MeV of the compound nucleus. The evaporation residue (ER) cross-sections have been analysed in terms of statistical model for the decay of the compound nucleus. A good agreement is found between statistical model calculation and the experimental evaporation residue cross-sections in all the four cases. It is shown that the average angular momentum\(\bar \ell \) of the compound nucleus can be deduced from a comparison of the experimentally measured and the statistical model predictions for the ER cross-sections. The validity of this method of deriving\(\bar \ell \) has been discussed for the case of16O+154Sm system.