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The fusion evaporation residue (ER) excitation function has been measured for $^{35,37}\mathrm{Cl}+^{181}\mathrm{Ta}$ reactions at energies above the Coulomb barrier. The measurements were performed using the HYbrid Recoil mass Analyzer at IUAC, New Delhi. Comparable ER cross sections have been observed in both reactions and there is no isotopic dependence. Measured ER cross sections were compared with theoretical calculations employing the dinuclear system model at projectile and target nuclei interaction and statistical model for the deexcitation of the formed compound nucleus. Larger ER cross sections at the complete deexcitation cascade of the formed compound nucleus are noticed in both reactions at higher excitation energies $({E}^{*}g80 \mathrm{MeV})$ over the calculated results. Fusion probability varies from $95%$ to $40%$ in the excitation energy range of the study. No appreciable difference in the fusion probability is noticed in the two reactions. Comparison of our results with other reactions populating $^{216}\mathrm{Th}$ shows a very strong entrance channel dependence.

138 Ba enriched isotopic targets, of thickness ranging from 200 μg/cm 2 - 240 μg/cm 2 , with carbon sandwich have been fabricated from barium metal, by high vacuum evaporation technique at Inter-University Accelerator Centre (IUAC), New Delhi. Resistive heating and electron beam evaporation methods were used for the deposition of barium and carbon respectively. 138 Ba being a highly reactive metal, target foils with carbon sandwich is fabricated in a continuous process without breaking the vacuum. The eight targets so prepared were further subjected to RBS analysis to check their purity. The thicknesses of these targets were verified by alpha energy loss method and DektakXT surface profilometer.

Evaporation residue cross sections are measured for the reaction $^{48}\mathrm{Ti}+^{138}\mathrm{Ba}$ which forms the compound nucleus ${}^{186}{\mathrm{Pt}}^{*}$. The cross sections are measured at beam energies in the range of 189.3--234.4 MeV. The experimental evaporation residue cross sections are compared with the dynamical model which employs one-dimensional Langevin dynamical calculations. The dissipation strength of the Langevin equation is calculated using both chaos weighted wall formula and a constant reduced dissipation function. The measured ER cross sections are found to be much less than the theoretical predictions. Further, the measured ER cross sections for the system $^{48}\mathrm{Ti}+^{138}\mathrm{Ba}$ are compared with those of $^{32}\mathrm{S}+^{154}\mathrm{Sm}$ forming the same compound nucleus. The suppression in the evaporation residue cross sections of the former reaction may be attributed to the increasing competition from quasifission. The quasifission reaction is found to have superseded any effect of neutron shell closure $(N=82)$ of the target in the present study.

Evaporation residue cross sections for the $^{16}\mathrm{O}+^{203,205}\mathrm{Tl}$ reactions were measured at laboratory beam energies in the range of 82--113 MeV using a gas-filled separator. Transmission efficiency of the separator was estimated using a calibration reaction $^{16}\mathrm{O}+^{197}\mathrm{Au}$ and by simulating the evaporation residues angular distributions. Statistical model calculations were performed for both the measured systems. These calculations overestimate the experimental evaporation residue cross sections. This could be attributed to the presence of noncompound nuclear fission. An estimation of noncompound nuclear fission contribution was carried out. Comparison with neighboring systems shows that a slight change in the entrance channel or the compound nucleus properties makes a large difference in evaporation residue cross sections.

Neutron multiplicity excitation function has been measured for the $^{30}\mathrm{Si}+^{197}\mathrm{Au}$ reaction populating the $^{227}\mathrm{Np}$ compound nucleus at excitation energies in the range 44.1--78.8 MeV using the National Array of Neutron Detector facility of Inter University Accelerator Centre, New Delhi. Measured pre-scission neutron multiplicity values are analyzed using a statistical model incorporating Krammer's fission width due to the dissipative drag in nuclear fission, shell corrections in fission barrier and level density, collective enhancement of level density, and $K$-orientation effect. The present work demonstrates that a strong fission hindrance is essential to reproduce the experimental pre-scission neutrons, whereas the temperature dependent dissipation coefficient as observed in a few recent measurements is not required to reproduce the experimental ${\ensuremath{\nu}}_{\mathrm{pre}}$ data. No substantial effect of collective enhancement of nuclear level density and tilting away effect of compound nucleus spin on neutron emission prior to the scission configuration was observed unlike fission of preactinides.

Fission fragment mass–angle and mass ratio distributions have been measured for the 30Si + 180Hf reaction in the beam energy range 128–148 MeV. Quasifission signature is observed in this reaction, forming the compound system 210Rn. The results are compared with a very asymmetric reaction 16O + 194Pt, forming the same compound nucleus. Calculations assuming saddle point, scission point and DNS models have been performed to interpret the experimental results. The results strongly suggest the entrance channel dependence of quasifission in heavy ion collisions.

Exchange biased systems consisting of ferromagnetic (FM)–antiferromagnetic (AFM) interfaces are increasingly being investigated because of their application potential in spin valves and tunnel junctions. In bilayer systems, ion irradiation is capable of modifying the interface and thereby offers unique opportunities to tailor exchange field. In the present study, irradiation with 100 MeV Ag 8+ ions is utilized to alter the exchange bias field in zinc ferrite–FeNiMoB bilayer system. The thin films which were deposited by RF sputtering technique and annealed at 600 °C were irradiated at various fluences. Structural and magnetic studies were carried out by using Glancing X Ray Diffractometer (GXRD) and Superconducting Quantum Interference Device Vibrating Sample Magnetometer (SQUID VSM) respectively. It was observed that the as deposited films exhibited exchange bias and on ion irradiation, bias field could be enhanced at certain fluences. The enhancement in bias field is attributed to defects created in the antiferromagnet as a result of ion irradiation. The experimental result is fitted in accordance with the diluted antiferromagnet model. Coercivity was also found to vary with ion fluence. Ion fluence was thus effectively used to enhance bias field as well as coercivity in the bilayer consisting of zinc ferrite–FeNiMoB.

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.

Fission fragment angular distributions for three reactions, $^{19}\mathrm{F}+^{182}\mathrm{W},^{19}\mathrm{F}+^{187}\mathrm{Re}$, and $^{19}\mathrm{F}+^{193}\mathrm{Ir}$, are measured in the laboratory energy range of 82--120 MeV. Extracted fission cross sections of the present systems as well as those of three others from literature $(^{19}\mathrm{F}+^{192}\mathrm{Os},^{19}\mathrm{F}+^{194}\mathrm{Pt}$, and $^{19}\mathrm{F}+^{197}\mathrm{Au})$ are compared with the predictions of a statistical model which takes into account the effects of shell, orientation degree of freedom, and collective enhancement in level density (CELD). In all the cases, the standard statistical model predictions overestimate the measured fission cross section, indicating the presence of some amount of dynamical effects in the exit channel. A dissipation strength of $2\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ is found to be sufficient to reproduce the data of all the reactions. No scaling of fission barrier height to fit the data is required.

We studied the competition between fusion and quasifission by populating the compound nucleus (CN) $^{210}$Rn through different entrance channels - $^{16}$O+$^{194}$Pt and $^{30}$Si+$^{180}$Hf, at energies around the Coulomb barrier. The larger widths of the fission fragment mass distribution in the $^{30}$Si+$^{180}$Hf reaction which could not be explained by using transition state models, indicated the onset of quasifission in this reaction. Further, the evaporation residues (ERs) produced in these reactions were measured using the gas-filled recoil mass separator HYRA at IUAC. The measurements showed reduced ER cross sections for the $^{30}$Si+$^{180}$Hf reaction when compared with that of $^{16}$O+$^{194}$Pt at similar excitation energies, confirming the presence of quasifission in the $^{30}$Si+$^{180}$Hf reaction. The experimental results are analysed using the dinuclear system (DNS) and statistical models to understand the possible influence of potential energy surfaces (PES) and different entrance channel conditions in heavy ion fusion reactions.

Background: Formation of the compound nucleus (CN) is highly suppressed by quasifission in heavy-ion collisions involving massive nuclei. Though considerable progress has been made in the understanding of fusion-fission and quasifission, the exact dependence of fusion probability on various entrance channel variables is not completely clear, which is very important for the synthesis of new heavy and superheavy elements.Purpose: To study the interplay between fusion and quasifission in reactions forming CN in the boundary region where the fusion probability starts to deviate from unity.Methods: Fusion evaporation residue cross sections were measured for the $^{28,30}\mathrm{Si}+^{180}\mathrm{Hf}$ reactions using the Hybrid Recoil Mass Analyser at IUAC, New Delhi. Experimental data were compared with data from other reactions forming the same CN or isotopes of the CN. Theoretical calculations were performed using the dinuclear system and statistical models.Results: Reduced evaporation residue cross sections were observed for the reactions studied compared with the asymmetric reaction forming the same CN, indicating fusion suppression in more symmetric systems. The observations are consistent with fission fragment measurements performed in the same or similar systems. Larger ER cross sections are observed with increase in mass in the isotopic chain of the CN.Conclusions: Fusion probability varies significantly with the entrance channels in reactions forming the same CN. While complete fusion occurs for the $^{16}\mathrm{O}+^{194}\mathrm{Pt}$ reaction, the fusion probability drops to approximately $60--70%$ for the $^{30}\mathrm{Si}+^{180}\mathrm{Hf}$ and less than $20%$ for the $^{50}\mathrm{Ti}+^{160}\mathrm{Gd}$ reactions, respectively, forming the same CN at similar excitation energies.

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability $({P}_{\mathrm{CN}})$ starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified.Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections $({\ensuremath{\sigma}}_{\mathrm{fis}})$ in a number of nuclear reactions carried out at and above the Coulomb barrier $({V}_{\mathrm{B}})$.Method: Fission fragment angular distributions were measured for six $^{28}\mathrm{Si}\text{-induced}$ reactions involving isotopically enriched targets of $^{169}\mathrm{Tm},\phantom{\rule{0.16em}{0ex}}^{176}\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}^{175}\mathrm{Lu},\phantom{\rule{0.16em}{0ex}}^{180}\mathrm{Hf},\phantom{\rule{0.16em}{0ex}}^{181}\mathrm{Ta}$, and $^{182}\mathrm{W}$ leading to probable formation of CN in the pre-actinide region, at a laboratory energy $({E}_{\mathrm{lab}})$ range of 129--146 MeV. Measurements were performed with large angular coverage $({\ensuremath{\theta}}_{\mathrm{lab}}={41}^{\ensuremath{\circ}}$--${170}^{\ensuremath{\circ}})$ in which fission fragments (FFs) were detected by nine hybrid telescope $(E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E)$ detectors. Extracted fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ were compared with statistical model (SM) predictions.Results: Barring two reactions involving targets with large non-zero ground state spin $(\mathcal{J})$, viz., $^{175}\mathrm{Lu}\left({\frac{7}{2}}^{+}\right)$ and $^{181}\mathrm{Ta}\phantom{\rule{4pt}{0ex}}\left({\frac{7}{2}}^{+}\right)$, experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at ${E}_{\mathrm{c}.\mathrm{m}.}$ near ${V}_{\mathrm{B}}$. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero $\mathcal{J}$, this deviation was subdued. Comparison between measured ${\ensuremath{\sigma}}_{\mathrm{fis}}$ and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation.Conclusions: Systematic SM analysis of measured fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding.

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.

Evaporation residue (ER) and fission cross sections were measured for the reaction O+ forming the compound nucleus 212Rn. Fission fragment angular distributions and anisotropies are consistent with the saddle point model predictions. Measured ER cross sections were fitted with statistical model calculations assuming shell-corrected free energy fission barrier height. The non-zero dissipation strength required to fit the ER cross section supports the dissipative nature of fission dynamics in heavy ion fusion. In the present study shell closure effects are not observed in the dissipation strength.