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The pre- and post-scission neutron multiplicities are measured for the fission of 208Rn, populated with the reactions 30Si + 178 Hf and 48Ti + 160 Gd in the excitation energy range of 54.5 - 80.8 MeV. We found significant differences in the pre-scission neutron multiplicities of these two systems. Also, both the reactions exhibit an enhancement in the pre-scission multiplicity with respect to the existing data for the 16O + 194 Pt reaction populating 210Rn. The measured multiplicities are compared with the theoretical predictions obtained from the Langevin dynamical model for fission. Dynamical simulations are also performed to estimate the capture cross-sections. Contribution of the neutrons, emitted during the fusion process, is shown to be crucial in determining the neutron multiplicities for the present systems. Moreover, in case of the 48Ti + 160 Gd reaction, rapid quasi-fission dynamics prevents neutron emission from the thermalized target-projectile composite. The present measurement helps us to understand the relative importance of fusion and quasi-fission processes and their timescales depending on the entrance channel mass asymmetry.

In the present work, three distinct and complementary experiments viz. (i) measurement of excitation functions, (ii) measurement of recoil range distributions, and (iii) spin distribution measurements using particle-gamma technique, have been carried out to understand the dynamics of complete fusion and incomplete fusion reactions at energies ≈ 4–7 MeV/nucleon. The analysis done indicates significant ICF contribution in ^16O + ^159Tb system at these energies. The three complimentary experiments confirm the observation of ICF and the relative contributions obtained are found to agree within 5%.

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.

The pre-scission and post-scission neutron multiplicities are measured for the 18O + 184W reaction in the excitation energy range of 67.23–76.37 MeV. Langevin dynamical calculations are performed to infer the energy dependence of fission decay time in compliance with the measured neutron multiplicities. Different models for nuclear dissipation are employed for this purpose. Fission process is usually expected to be faster at a higher beam energy. However, we found an enhancement in the average fission time as the incident beam energy increases. It happens because a higher excitation energy helps more neutrons to evaporate that eventually stabilizes the system against fission. The competition between fission and neutron evaporation delicately depends on the available excitation energy and it is explained here with the help of the partial fission yields contributed by the different isotopes of the primary compound nucleus.

The light output response and efficiency of 5” × 5” BC501A liquid scintillator neutron detectors composing the NAND detector array have been measured by time of flight technique using 252 Cf source. Energy dependent detection efficiency was calculated using FLUKA Monte Carlo code and validated by experimental measurement. FLUKA simulation incorporated fission neutron correlation and kinematics of neutron emission from fission source determining the energy distribution of neutrons in the laboratory frame. The intrinsic features of the scintillator were studied by including particle dependent quenching and light output resolution. Comparison of FLUKA simulation of scintillation light produced by neutrons and gamma rays with measurements provided realistic results. The calculated neutron detection efficiency at ∼ 0.5 MeV threshold reproduces the experimental data reasonably well in the studied energy range, 0 . 5 E n 12 MeV . Neutron scattering from target chamber material in NAND was investigated using FLUKA concluding that the loss in neutron flux due to scattering from the chamber wall is ∼ 12%. The scattering amounts to background neutron counts in neighbouring detectors comparable to cross-talk events. The absolute efficiency of the NAND array was estimated to be ∼ 1.40% after taking into consideration the intrinsic efficiency of liquid cells, geometrical efficiency and flux loss due to scattering.

Asymmetric low-energy fission of neutron-deficient nuclei around lead is addressed with the measurement of fragment mass and total kinetic energy properties for the fissioning system $^{198}\mathrm{Po}$ produced in heavy-ion fusion. Interpretation of the measurement for such a transitional nucleus at finite excitation energy is challenging. The presence of asymmetric partitions is suggested by the observed weak dependence of the total kinetic energy on mass and by the nonmonotonic evolution of the fragment-mass distribution width with excitation energy. The interpretation is supported by microscopic model calculations as well as by the results of an advanced semiempirical code. Combined with previous experiments in the region, the present measurement contributes to establish the evolution of the fragment-mass distribution as a function of the fissioning system. The connection between the ``new'' and the ``old'' islands of asymmetric fission is discussed.

Fission fragment mass distributions have been measured for relatively neutron-deficient compound nuclei, $^{188}\mathrm{Pt}$ and $^{190}\mathrm{Pt}$, formed in the fusion reactions, $^{28}\mathrm{Si}+^{160}\mathrm{Gd}$ and $^{12}\mathrm{C}+^{178}\mathrm{Hf}$, respectively. The data were obtained for a similar initial excitation energy range of 49--68 MeV for both the systems. The fragment mass distributions for both the Pt isotopes were found to be single peaked and no appreciable change in the mass symmetry was observed throughout the measured excitation energy range, for both the reactions. However, relatively broader mass distributions observed in the fission of $^{188}\mathrm{Pt}$ in the studied energy domain indicates the presence of fission events originating from a nonequilibrated source as well. This signifies that the mass equilibrium has not been fully achieved in the $^{28}\mathrm{Si}+^{160}\mathrm{Gd}$ system as compared to the $^{12}\mathrm{C}+^{178}\mathrm{Hf}$ system, indicating the presence of quasifission events in the former reaction.

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.

Characteristics and performance of a time of flight (TOF) spectrometer developed for performing fission mass distribution studies are presented. The spectrometer contains two TOF arms based on multi-wire proportional counters (MWPCs). Each arm has two MWPCs to form a start–stop detection system for TOF measurements. The start detector has an active area of 4 × 4 cm2. The stop detector is a two-dimensional position sensitive MWPC with an active area of 16 × 11 cm2. Salient features of the MWPCs are the use of reduced sub-millimeter wire pitches of 0.635 and 0.317 mm in the electrodes along with the use of gold plated tungsten wires of diameters 10 and 20 µm. A delay line for position electrodes is prepared using chip inductors and capacitors. Ten different configurations of MWPC were investigated for the start detector, which involved the use of three and four electrode geometries, use of different wire pitches, and use of aluminized mylar for timing electrodes. Performance results close to micro-channel plate detectors have been observed with some designs of MWPC, displaying rise times better than 2 ns with an estimated inherent time resolution of ∼100 ps FWHM. A position resolution of ∼1 mm (FWHM) has been observed. Design features of the MWPCs and their test performance results are described in this article.

The 100 element neutron detector array facility, National Array of Neutron Detectors (NAND) has been installed at Inter University Accelerator Centre (IUAC), New Delhi for experiments in heavy-ion reaction studies. The detector array consists of 100 organic liquid scintillators, each of size 5” × 5” coupled to 5 in. diameter photomultiplier. The liquid cells are mounted at 175 cm from the target with full array covering 3.3% of 4 π solid angle. We present an overview of the design and implementation of various subsystems of the array. Description of the mechanical structure, development of detectors and nuclear electronics for signal processing, data acquisition, characterization of detectors and test results are presented. Results on the performance characteristics of the array are given based on the offline tests using radioactive sources and commissioning test by in-beam experiments. The system demonstrates good n − γ discrimination and time-of-flight performance.

Thin 160 Gd targets (enrichment 98.12%) in the thickness range of 45–55 μg/cm 2 have been prepared on carbon backing in the high vacuum environment using thermal evaporation technique. During the fabrication, a few of the challenges faced were the oxidizing tendency of Gd in the atmosphere, limited amount of enriched material and the peeling off problem during floating of the films. Several trials were first conducted with natural Gd to optimize different parameters of the fabrication process. By utilizing only 30 mg of enriched isotopic material, more than 25 targets of 160 Gd were fabricated successfully. The purity check and thickness measurement of the prepared targets were performed using Energy Dispersive X-Ray Fluorescence and Rutherford Back Scattering techniques. Absence of any heavy ions and presence of negligible amount of oxygen in the prepared films revealed the purity of the targets fabricated using thermal evaporation technique.

An attempt has been made in the present work to provide an ample opportunity to explore the information about the influence of incomplete fusion (ICf) reaction dynamics on complete fusion in heavy ion induced nuclear reactions. excitation functions for several evaporation residues produced in the interaction of projectile O with target lu have been measured over the wide projectile energy range ≈ 70-100 MeV. the recoil-catcher activation technique followed by the offline γ-ray spectroscopy has been used for the present measurements. In case of precursor decay, we have made use of Cavinato et al. formulation to calculate the independent cross-section of the identified residues. the measured efs are compared with theoretical predictions of statistical model code PACe-2 and any enhancement in the measured cross-section from theoretical prediction may be due to ICf reaction process. An attempt has been made to estimate the ICf contribution of the cross-section from the measured excitation function data and the dependence of ICf cross-section on projectile energy.

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.

The characteristics and performance of the newly commissioned neutron detector array at IUAC are described. The array consists of 100 BC501 liquid scintillators mounted in a semi-spherical geometry and are kept at a distance of 175 cm from the reaction point. Each detector is a 5″ × 5″ cylindrical cell coupled to 5″ diameter photomultiplier tube (PMT). Signal processing is realized using custom-designed home-made integrated electronic modules which perform neutron – gamma discrimination using zero cross timing and time-of-flight (TOF) technique. Compact custom built high voltage power supply developed using DC–DC converters are used to bias the detector. The neutrons are recorded in coincidence with fission fragments which are detected using multi-wire proportional counters mounted inside a 1 m diameter SS target chamber. The detectors and electronics have been tested off-line using radioactive sources and the results are presented.

The first phase of the National Array of Neutron Detectors (NAND) consisting of 26 neutron detectors has been commissioned at the Inter University Accelerator Centre (IUAC), New Delhi. The motivation behind setting up of such a detector system is the need for more accurate and efficient study of reaction mechanisms in the projectile energy range of 5–8 MeV/n using heavy ion beams from a 15 UD Pelletron and an upgraded LINAC booster facility at IUAC. The above detector array can be used for inclusive as well as exclusive measurements of reaction products of which at least one product is a neutron. While inclusive measurements can be made using only the neutron detectors along with the time of flight technique and a pulsed beam, exclusive measurements can be performed by detecting neutrons in coincidence with charged particles and/or fission fragments detected with ancillary detectors. The array can also be used for neutron tagged gamma-ray spectroscopy in (HI, xn) reactions by detecting gamma-rays in coincidence with the neutrons in a compact geometrical configuration. The various features and the performance of the different aspects of the array are described in the present paper.

The characteristics and performance of a new detector system developed for the study of low energy heavy ion binary reactions using the kinematic coincidence technique are presented. The detector system has been developed to carry out experiments such as multi-nucleon transfer reactions using the General Purpose Scattering Chamber (GPSC) facility at IUAC [1,2]. The detector system consists of a pair of two-dimensional position sensitive multi wire proportional counter (MWPC) and a ΔE−E gas ionization chamber. Both MWPC have an active area of 5×5 cm2, and provide position signals in horizontal (X) and vertical (Y) plane, and timing signal for time of flight measurements. The main design feature of MWPC is the reduced wire pitch of 0.025 in. (0.635 mm) in all electrodes, giving uniform field and faster charge collection, and usage of 10μm diameter in anode frame which gives higher gains. The position resolution of the detectors was determined to be 0.45 mm FWHM and time resolution was estimated to be 400 ps FWHM. The detector could handle heavy ion count rates exceeding 100 kHz without any break down. The timing and position signals of the detectors are used for kinematic coincidence measurements and subsequent extraction of their mass and angular distributions. The ionization chamber has a conventional transverse field geometry with segmented anode providing multiple ΔE signals for nuclear charge (Z) identification. This article describes systematic study of these detectors in terms of efficiency, count rate handling capability, time, position and energy resolution.

Pre-scission neutron multiplicities have been measured for 12C + 194, 198Pt systems at matching excitation energies at near Coulomb barrier region. Statistical model analysis with a modified fission barrier and level density prescription have been carried out to fit the measured pre-scission neutron multiplicities and the available evaporation residue and fission cross sections simultaneously to constrain statistical model parameters. Simultaneous fitting of the pre-scission neutron multiplicities and cross section data requires shell correction at the saddle point.

Fission fragment angular distributions have been measured for 16O + 194Pt reaction forming the compound system 210Rn, in the laboratory energy range from 79 to 90 MeV. The measured fission fragment anisotropies as a function of E c . m . / V B are compared with the predictions of standard saddle point statistical model ( sspm ). Anisotropies calculated using the average excitation energy and angular momentum values could not reasonably fit the experimental data. Statistical model calculations were performed using the pace with modified fission barrier and level density parameters. Fission probability, evaporation residue cross section and neutron multiplicity were simultaneously used to fix the statistical parameters. Model calculations incorporating the chance nature of fission decay and scaled values of the rotating finite range model (RFRM) moment of inertia could reasonably fit the fragment angular anisotropies.

The experiments were performed to study excitation functions (EFs) of evaporation residues (ERs), i.e. 103,102,101Ag, 101,100,99Pd, 101,100Rh, 97Ru, 96Tc, 95Tc, 94Tc, 93Mo m , 92Nb m populated in the reactions induced by 12C on 93Nb for exploring the reaction dynamics involved at energies ≈ 47–75 MeV. The activation technique followed by offline γ-ray spectrometry has been employed to measure EFs. These measurements were simulated with other reported values available in literature as well as with theoretical predictions based on computer code PACE-2. The effect of variation of level density parameter involved in this code has also been studied. An excellent agreement was found between theoretical and experimental values in some of the fusion evaporation channels. However, significant enhancement of cross-section as observed in α-emission channels may be due to incomplete fusion (ICF) process and/or direct reaction process. To confirm the aforesaid reaction mechanism, Recoil Range Distributions (RRDs) of various ERs have been measured at ≈ 80 MeV. Moreover, an attempt is made to separate the percentage relative contributions of complete and incomplete fusion components from the analysis of the measured RRDs data. Further, the relative percentage ICF fraction, also estimated from EFs data, was found to be sensitive with the projectile energy.

We present γ-ray multiplicity distributions for the formation of evaporation residues in the fusion reaction 19F + 184W → 20383Bi120 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 → 19480Hg114 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.

A 4 π multi-detector gamma-ray spectrometer named the Indian National Gamma Array (INGA) has been set up at the Inter University Accelerator Centre, New Delhi, for nuclear structure studies. The array is designed to incorporate twenty four Compton-suppressed Clover germanium detectors with a total photopeak efficiency ∼ 5 % . The spectrometer along with sub-systems developed in-house like, mechanical support structure, high voltage power supplies, automatic liquid nitrogen filling system, front-end electronics and data acquisition system are described. The mechanical support structure facilitates the use of the Clover Germanium array with a recoil mass separator. The array has been used in a number of nuclear spectroscopic investigations. The in-beam and off-beam performance of the array are reported.

Hybrid recoil mass analyzer (HYRA) is a unique, dual-mode spectrometer designed to carry out nuclear reaction and structure studies in heavy and medium-mass nuclei using gas-filled and vacuum modes, respectively and has the potential to address newer domains in nuclear physics accessible using high energy, heavy-ion beams from superconducting LINAC accelerator (being commissioned) and ECR-based high current injector system (planned) at IUAC. The first stage of HYRA is operational and initial experiments have been carried out using gas-filled mode for the detection of heavy evaporation residues and heavy quasielastic recoils in the direction of primary beam. Excellent primary beam rejection and transmission efficiency (comparable with other gas-filled separators) have been achieved using a smaller focal plane detection system. There are plans to couple HYRA to other detector arrays such as Indian national gamma array (INGA) and 4π spin spectrometer for ER tagged spectroscopic/spin distribution studies and for focal plane decay measurements.

Quasielastic scattering angular distributions have been measured for the 7Be + 9Be system at E lab = 17 , 19 and 21MeV in the angular range $ \theta_{{cm}}^{}$ = 24° - 57° . An optical model (OM) analysis of these data has been carried out in order to extract optical potential parameters and reaction cross-sections. One-proton stripping cross-sections were also measured for this system at E lab = 19 and 21MeV. These transfer angular-distribution data were compared with the finite-range distorted-wave Born approximation (FRDWBA) calculations. For the 7Li + 9Be system quasielastic scattering angular distributions were measured and emitted light charged particles were detected at E lab = 15.75 , 24.00 and 30.00MeV in the angular range $ \theta_{{cm}}^{}$ = 7° - 70° . Fusion cross-sections were obtained by reproducing the measured $ \alpha$ -evaporation spectra from the compound nucleus at backward angles with the statistical model calculations. The ratios of the experimental fusion cross-sections to the total reaction cross-sections (obtained from OM analysis) were found to be small. This result suggests that the break-up process has a strong influence on the fusion process leading to a reduction in the fusion cross-section.

Fission fragment mass angle correlation and mass ratio distributions have been measured for 16 O + 194 Pt reaction forming the composite system 210 Rn in the energy range 75 MeV to 102 MeV in laboratory frame. No mass angle correlation was observed and the mass ratio distributions were compared with theoretical calculations assuming compound nucleus formation. No evidence of quasi-fission process is observed in the reaction.

A cost-effective high-performance pulse shape discriminator module has been developed to process signals from organic liquid scintillator-based neutron detectors. This module is especially designed for the large neutron detector array used for studies of nuclear reaction dynamics at the Inter University Accelerator Center (IUAC). It incorporates all the necessary pulse processing circuits required for neutron spectroscopy in a novel fashion by adopting the zero crossover technique for neutron-gamma (n-γ) pulse shape discrimination. The detailed layout of the circuit and different features of the module are described in the present paper. The quality of n-γ separation obtained with this electronics is much better than that of commercial modules especially in the low-energy region. The results obtained with our module are compared with similar setups available in other laboratories.

The charge comparison method for n–γ discrimination has been in practice for more than two decades; technically it can be implemented in variety of ways. We present explicitly, in detail, two simple ways of implementing this technique. The first one is the conventional way of comparing the fractional or slow charge in the tail with the full charge of photomultiplier tubes (PMTs) anode pulse fed to a charge integrating ADC or QDC, but has been electronically implemented in a new fashion. The second one is a novel technique of comparing shaped-dynode pulse, fed to a peak-sensing ADC, with the fractional slow charge integrated in QDC. This technique reduces the complexity of individual gate generation, and thus reduces the number of gate and delay generators, compared with the first case. To implement these techniques, we have designed and fabricated a six-channel, single-width NIM module for use with QDC for n–γ discrimination. The module fabrication is very simple and inexpensive, and can be easily integrated with commercially available high-density NIM and CAMAC modules in experimental setups involving a large number of neutron detectors. Commercially available passive analog delay line chips have been used for delaying analog signals fed to the QDC. To simplify the setup for high-density applications we have replaced the constant fraction discriminator (CFD) by a fast leading-edge discriminator. The quality of n–γ discrimination obtained is comparable with that reported earlier in the literature.

Elastic scattering angular distributions have been measured for 7Be + 9Be system at Elab = 17, 19 and 21 MeV in the angular range θcm=26○–58°, and for 7Li + 9Be system at Elab= 15.75, 24 and 30 MeV. An optical model (OM) analysis of these data have been carried out. For the 7Li + 9Be system fusion cross sections were obtained at Elab = 15.75, 24 and 30 MeV by measuring the α-evaporation spectra from the compound nucleus at backward angles. The measured α-evaporation spectra were reproduced by the statistical model calculations and fusion cross sections were extracted therefrom. The ratios of the experimental fusion cross sections to the total reaction cross sections (obtained form OM analysis) were found to be rather small. This result suggests that break-up process has a strong influence on fusion process leading to a reduction in fusion cross section.

With a view to study complete- and incomplete-fusion components in 159Tb , 169Tm ( 16O , x reactions, experiments have been carried out at the Inter-University Accelerator Center, New Delhi, India using the 15UD Pelletron accelerator facilities. The forward mean recoil ranges for some radio-nuclides; 168m Lu , 167Lu , 167Yb , 166Tm produced in the 16O$ + $159Tb system at ≈ 90 MeV, and 179Re , 177Re , 177W , 178Ta and 177Hf produced in the 16O$ + $169Tm system at ≈ 87 MeV have been measured. The recoil-catcher activation technique followed by off-line γ -spectrometry has been employed in the present work. The analysis of forward mean ranges for different radio-nuclides has been done in the framework of the degree of linear momentum transfer from projectile to target nucleus by adopting break-up fusion model considerations. Different complete- and incomplete-fusion components, which may be attributed to the fusion of 16O and/or 12C and 8Be transfer from the 16O projectile to the target nucleus have been observed. An attempt has also been made to separate out the relative percentage contributions of complete- and incomplete-fusion components using experimentally measured forward recoil ranges. The complete-fusion contributions deduced from recoil range distribution are found to be consistent with the prediction of the theoretical model code PACE. The analysis of data indicates the complete- and incomplete-fusion competition for both the systems at the given energies.

Background: Complete fusion between two massive nuclei after capture inside the potential barrier is inhibited by competing fission-like processes. The target-projectile composite system may reseparate after capture without proceeding towards formation of the compound nucleus (CN), which is equilibrated in all degrees of freedom. The nature of these non-CN fission (NCNF) processes and factors that affect them are not completely known yet.Purpose: The nuclear mass regions from where NCNF processes begin to manifest themselves are not clearly demarcated. This work aims to study the onset of NCNF, if any, in the mass region $\ensuremath{\sim}200$.Methods: Fission fragment (FF) mass and angular distribution (MAD) and pre-scission and post-scission neutron multiplicities were measured for the reaction $^{19}\text{F}+^{184}\text{W}$ at a laboratory energy $({E}_{\mathrm{lab}})$ range of 84--125 MeV. The measurements were carried out using two multiwire proportional counters (MWPC) to detect the FFs in coincidence and four neutron detectors to measure neutron time of flight (TOF). Statistical model (SM) calculation was performed.Results: No significant mass-angle correlation was observed in the MAD plots. Extracted mass ratio distributions were single-peaked and of Gaussian shape. Measured pre-scission neutron multiplicity values indicated dissipative nature of CN decay for this reaction.Conclusions: No clear signatures of NCNF were observed in the studied reaction, indicating that the target-projectile composite system predominantly proceeds towards formation of the CN after capture.

Evaporation residue (ER) cross sections for the $^{16,18}\mathrm{O}+^{198}\mathrm{Pt}$ reactions are measured in order to investigate fission hindrance. Compound nuclei ($^{214,216}\mathrm{Rn}$) are formed in the above fusion reactions at excitation energies in the range of 40--68 MeV. The experimental ER cross sections are compared with predictions from the statistical model calculations of compound nuclear decay where Kramers' fission width is used. The strength of nuclear dissipation is treated as a free parameter in the statistical model calculations in order to fit the experimental data.

The pre- and post-scission neutron multiplicities have been extracted for the 220,222,224Th nuclei for the excitation energy range of 40 MeV to 64 MeV using the National Array of Neutron Detectors (NAND). The Th isotopes are populated from the fusion reaction of 16O+204,206,208Pb systems in order to investigate the dynamics of fusion-fission reactions using the neutron multiplicity as a probe. The theoretical calculations were performed using the Bohr-Wheeler fission width as well as the dissipative dynamical fission width from Kramers prescription. It is observed that the Bohr-Wheeler fission width underestimates the pre-scission yields to a large extent. A large amount of dissipation is required in the Kramers width to fit the observed pre-scission neutron multiplicities

The systematic study of the properties of hot nuclei by detecting the emitted charged particles and neutrons in coincidence with residual nuclei provides very critical information about its nuclear level density. These emitted particles capable to explain the behavior of the nucleus at various stages of the de-excitation cascade process. So, we have studied, a set of four compound nuclei, which were populated by mass-symmetric and mass-asymmetric channels, leading to the same compound nuclei, namely 80 Sr* , 79 Se* , 76 Kr* and 58 Ni* at same excitation energies, respectively and found that the experimental neutron and charged particle spectra for symmetric channel show deviations at higher energies in comparison to the statistical model calculations.

In order to study complete and incomplete fusion in heavy ion reactions, the excitation functions for several residues produced in the system 16O + 159Tb have been measured in the energy range ≈ 70 – 95 MeV , employing activation technique. The measured excitation functions have been compared with those calculated using computer codes CASCADE, PACE2 and ALICE-91. Comparison of measured and theoretically calculated excitation functions has indicated significant contributions from incomplete fusion in some α-emission channels. In the present experiment, the recoil range distributions of several residues at ≈ 90 MeV incident beam energy have also been measured using recoil catcher technique and off-line gamma ray spectrometry. Analysis of the recoil range distributions has further confirmed the presence of contributions from incomplete fusion reactions. An attempt has been made to separate out the relative contributions of complete and incomplete fusion channels.

High-spin states in 79Rb were populated in the reaction $\mathrm{^{63}Cu \ensuremath(^{19}F, p2n) ^{79}Rb}$ at E(beam) = 60 MeV. The lifetimes of the excited states of the $\pi \ensuremath g_{\frac{9}{2}}$ positive-parity yrast band and of the $\pi \ensuremath p_{\frac{3}{2}}$ negative-parity band in 79Rb were measured by the Doppler Shift Attenuation Method. The deduced transition quadrupole moments Qt are found to have a decreasing trend with rotational frequency for both the bands, consistent with those found experimentally in neighbouring nuclei.

High-spin states in 78Kr have been studied via the 63Cu (19F, 2p2n)78Kr reaction at a beam energy of 60 MeV using the Indian National Gamma Array (INGA). In this nucleus, lifetimes have been measured upto the Iπ=22+ level in the yrast positive-parity band and upto the Iπ=15- level in the negative-parity band using the Doppler Shift Attenuation Method (DSAM). The deduced transition quadrupole moments Qt's are found to decrease with rotational frequency for both the bands.

The performance of a dual time of flight (TOF) spectrometer using two 24 cm×10 cm area X – Y position sensitive Multi-Wire Proportional Counters (MWPC) is reported. Good separation of compound nuclear fission channel from elastic, quasi-elastic and non-compound fission channels have been achieved. Time calibration of the spectrometer using a technique, dependent on the detector independence of the mass or kinetic energy distributions gave a three times better mass resolutions compared to the conventional calibration technique using elastically scattered projectiles in a heavy ion induced fission reaction. Detail experimental methods in determining fission fragment mass and angular distribution have been presented and discussed.

Background: Detailed studies on the effect of the breakup of weakly bound projectile on fission are scarce. Distinguishing the events of compound nuclear (CN) fission from the breakup or transfer induced fission to understand the properties of measured fission fragments is difficult but desirable.Purpose: To investigate the effect of projectile breakup and its breakup threshold energy on fission-fragment (FF) mass distributions and folding angle distributions for $^{6,7}\mathrm{Li} + ^{238}\mathrm{U}$ reactions and find out the differences in the properties of the fission events produced by complete fusion (CF) from the total fusion (TF).Methods: The FF mass and folding angle distributions have been measured at energies around the Coulomb barrier using gas detectors by time-of-flight technique. The results are compared with the ones involving tightly bound projectiles as well as predictions from systematics to bring out the effect of the breakup.Results: A sharp increase in the peak to valley (P:V) ratio of FF mass distribution with the decrease in bombarding energy for $^{6,7}\mathrm{Li} + ^{238}\mathrm{U}$ reactions is observed when all events are assumed to be CN fission. As the beam energy falls through the fusion barrier, the full width half maximum (FWHM) of the FF folding angle distribution is found to increase at sub-barrier energies, unlike the reactions involving tightly bound projectiles where a linear decrease in FWHM is expected. By selecting pure CN events from the scatter plot of the velocity components of the composite nuclei, the energy dependence of the deduced FWHM is found to be consistent with the ones involving tightly bound projectiles. Similarly, the P:V ratio obtained for the selected CN events is consistent with the theoretical calculations as well as the experimental data for the proton induced reaction forming similar CN.Conclusions: The presence of projectile breakup induced fission and a relatively low breakup threshold for $^{6}\mathrm{Li}$ compared to $^{7}\mathrm{Li}$ explains the observed differences in the energy dependence of the P:V ratio and the FWHM of FF folding angle distributions for CF and TF fission in the present reactions.

Background: The ${}^{12}\mathrm{C}{(}^{6}\mathrm{Li},d)$ reaction has been used as an indirect method to calculate the astrophysical $S$ factor for the $^{12}\mathrm{C}$($\ensuremath{\alpha},\ensuremath{\gamma})$ reaction at Gamow energy (300 keV).Purpose: The ${}^{12}\mathrm{C}{(}^{6}\mathrm{Li},d)$ reaction is usually interpreted in terms of direct transfer. In this work we investigate the reaction mechanism and determine the effects of breakup on transfer and therefore on the extracted spectroscopic amplitudes.Method: The deuteron angular distributions for the ${}^{12}\mathrm{C}{{(}^{6}\mathrm{Li},d)}^{16}{\mathrm{O}}^{*}$has been measured at 20 MeV, populating discrete states of $^{16}\mathrm{O}$. continuum discretized coupled channel-coupled reaction channel (CDCC-CRC) calculations have been used to analyze the data.Results: Results show a new reaction mechanism, where transfer occurs after the breakup of the loosely bound $^{6}\mathrm{Li}$ in the population of some bound states of $^{16}\mathrm{O}$. A comparison of the CDCC-CRC calculations with respect to the measured data were used to determine the $\ensuremath{\alpha}$ spectroscopic amplitudes and factors for the different states of $^{16}\mathrm{O}$. Using the spectroscopic amplitudes obtained in this work, the $E2$ $S$ factor for the $^{12}\mathrm{C}$($\ensuremath{\alpha}$,$\ensuremath{\gamma}$) reaction has been calculated in the framework of a two-body potential model and compared to measurements.Conclusions: The present study very clearly shows that the breakup and transfer coupling effects are strong in the ${}^{12}\mathrm{C}{(}^{6}\mathrm{Li},d)$ reaction. The present work extracts, in the framework of a coupled reaction channel theory, the spectroscopic amplitudes of the bound and unbound states of $^{16}\mathrm{O}$. All previous analysis and new measurements should therefore be reexamined from this viewpoint to extract the astrophysical observables correctly.

Background: Several inclusive and few exclusive [evaporation residue (ER) gated] measurements for symmetric systems reported in the literature describe the anomalous deviations of light particle evaporation spectra from the statistical model predictions. However, no consistent description exists for these deviations.Purpose: To establish the consistent interpretation of reported anomalous deviations.Method: The inclusive and exclusive measurements of $\ensuremath{\alpha}$-particle, proton, and neutron spectra were carried out for the fusion of two relatively symmetric systems ${}^{28}$Si + ${}^{45}$Sc (E${}_{\mathrm{lab}}$ = 125 MeV and l${}_{\mathrm{max}}$ = 47$\ensuremath{\hbar}$) and ${}^{32}$S + ${}^{45}$Sc (E${}_{\mathrm{lab}}$ = 125 MeV and l${}_{\mathrm{max}}$ = 43$\ensuremath{\hbar}$) leading to the compound nuclei ${}^{73}$Br and ${}^{77}$Rb with excitation energies 78 MeV and 71 MeV, respectively.Results: The experimental light particle spectra for both the reactions show anomalous deviations from the statistical model predictions. The charged particles spectra are found to be suppressed, whereas the neutron spectra exhibit a bump at the higher energy tail. These spectra are investigated in terms of the modification of the important ingredients of the statistical model viz. the level density parameter, rescaling of the yrast line, and the use of $l$ value suggested by the dynamical models.Conclusions: It is conjectured that the higher partial waves do not fuse, but result in the formation of a deformed dinuclear system. The binding energy of neutrons is reduced while those for protons and $\ensuremath{\alpha}$ particles are enhanced in the deformed dinuclear system as compared to those in the shape-equilibrated system. Hence, the higher partial waves inhibit their contributions to the $\ensuremath{\alpha}$-particle spectra and lead to the preshape equilibrium of neutrons and protons, with the neutron emission dominating over the proton emission.

The information of ICF reaction has been obtained from the measurement of excitation function (EF) of ERs populated in the interaction of 20Ne & 16O on 55Mn, 159Tb & 156Gd targets. Sizable enhancement in the measured cross-sections has been observed in α-emitting channels over theoretical predictions, which has been attributed to ICF of the projectile. In order to confirm the findings of the measurements and analysis of EFs, the forward recoil range distributions of ERs populated in 20Ne+159Tb (E ∼165MeV) and 16O+156Gd (E ∼ 72, 82 & 93MeV) systems, have been measured. It has been observed that peaks appearing at different cumulative thicknesses in the stopping medium are related with different degree of linear momentum transfer from projectile to target nucleus by adopting the break-up fusion model consideration. In order to deduce the angular momentum involved in various CF and / or ICF reaction products, spin distribution and side-feeding intensity profiles of radio-nuclides populated via CF and ICF cha...

Pre- and post-scission neutron multiplicities are measured for the three isotopes of Fr (${}^{217}$Fr, ${}^{215}$Fr, and ${}^{213}$Fr) in the excitation energy range of 48\char21{}91.8 MeV. Out of these three isotopes, ${}^{213}$Fr has shell closure (${N}_{C}=126$) while the other two are non-closed-shell nuclei. Statistical model calculations using Kramers' fission width are performed to investigate shell effects on the dissipation strength which fit the experimental data. It is observed that shell correction to the binding energies of the evaporated particles strongly affects the fitted values of the dissipation strength. However, the best-fit dissipation strength is only weakly influenced by the inclusion of shell correction in fission barrier.

To study the heavy-ion-induced reactions, particularly complete and incomplete fusion, the excitation functions for ${}^{16}$O$+{}^{165}$Ho interactions at energies $\ensuremath{\approx}$73--105 MeV have been measured using a well-established activation technique. The excitation functions have been compared with the predictions of the statistical model code pace4. The comparison advises that the formation of $xn$ and $pxn$ channels is governed only via complete fusion processes. Even after correction for precursor contribution, a significant enhancement in the production of $\ensuremath{\alpha}$-emitting channels may be attributed to incomplete fusion contribution. Furthermore, in order to understand incomplete fusion reactions in a more conclusive way, the incomplete fusion fraction has been analyzed with the existing data for ${}^{12}$C and ${}^{20}$Ne beams on the same target (${}^{165}$Ho) where a strong projectile structure effect has been observed, which correlates incomplete fusion reaction dynamics with the $\ensuremath{\alpha}$-$Q$ value of the projectile.

The compound nucleus 76Kr* is formed in the heavy-ion fusion reactions by an asymmetric entrance channel 12C+64Zn and the symmetric entrance channel 31P+45Sc at the excitation energy of 75 MeV and angular momentum of 39 ℏ. Neutron energy spectra of the asymmetric system (12C+64Zn) at different angles are well described by the statistical model predictions using the normal value of the level density parameter a = A/8 MeV −1. However, in the case of the symmetric system (31P+45Sc), the statistical model interpretation of the data requires the change in the value of a = A/10 MeV−1. The delayed evolution of the compound system in case of the symmetric 31P+45Sc system may lead to the formation of a temperature equilibrated dinuclear complex, which may be responsible for the neutron emission at higher temperature, while the protons and alpha particles are evaporated after neutron emission when the system is sufficiently cooled down and the higher l-values do not contribute in the formation of the compound nucle...