The NAND is a neutron time-of-flight spectrometer consisting of 100 neutron detectors (Fig. 1) installed in beam hall II of IUAC. The first phase of NAND came into existence in year 2006 with 26 neutron detectors [12] which was later upgraded to a larger array in year 2015 with 100 detectors [13, 14]. Each neutron detector contains BC501 organic liquid scintillator in a \(5^{\prime \prime } \times 5^{\prime \prime }\) cylindrical cell coupled to \(5^{\prime \prime }\) diameter photo-multiplier tube (PMT). The neutron detectors are mounted on eight rings in a dome shaped mechanical structure at a fixed distance of 175 cm from target position covering \(\sim\) 3.3 \(\%\) of 4\(\pi\) solid angle. The mechanical structure is made of hollow metal pipes (mild steel) to minimize material usage, thereby reducing chances of neutron scattering. A 100-cm-diameter spherical vacuum chamber (stainless steel, 4 mm thickness) houses the target ladder and charged particle detectors.

Neutron detector signals are processed using custom-built electronic modules which contain circuits for pulse shaping amplifier, pulse shape discrimination (PSD), constant fraction discriminator (CFD) and time-to-amplitude converter (TAC) [20]. The standard zero-cross time based PSD is used for neutron-\(\gamma\) discrimination. A single-width NIM module processes signals from two neutron detectors, with each channel having a prompt and a delayed CFD outputs for ToF and a TAC output corresponding to zero-cross time distribution. Signals from processing units are digitized and recorded using a VME based data acquisition system and software running on Linux operating system [21].

Complementary fission fragments are detected in two large area position sensitive Multi-Wire Proportional Counters (MWPC). The MWPCs with active area of 16 cm \(\times\) 11 cm are kept at a distance of \(\approx\) 17–25 cm from target position as per the experimental requirement. MPWCs are built using three-electrodes, a central electrode made of double-sided aluminized mylar foil as cathode and two position sensing electrodes made of wire-frames as anodes [22]. The position encoding is achieved based on the readout of LC delay-lines connected to MWPC anodes. The MWPC achieved a position resolution of \(\sim\) 1.2 mm and time resolution of \(\sim\)600 ps [22].

For fission fragment angular distribution measurement, an array of 16 hybrid detector telescopes (HYTAR) is used [23]. Each detector telescope consist of an ionization chamber (IC) followed by a 300 \(\mu m\) Passive Implanted Planar Silicon (PIPS) detector. The IC is made of three wire frames with an active area of 40 mm² and 18 mm depth and filled with Isobutane gas at typical pressure of 70-100 mbar (depending on energy of the particle).

The elastically scattered beam particles are detected by two PIPS detectors of thickness 300 \(\mu\)m, placed at ± 13° with respect to beam direction. Light-charged particles such as protons and alpha particles are detected using an array of 16 CsI(Tl) detectors coupled to photodiodes [24]. CsI(Tl) detector having active area of 20 mm \(\times\) 20 mm and 3 mm thickness is coupled to 10 mm \(\times\) 10 mm photodiode. Particle identification is achieved using pulse shape discrimination based on ballistic deficit method.

2.1 Performance of the system

The response characteristics of neutron detectors were found by scanning the linearity of detector pulse height distribution using mono-energetic \(\gamma\)-rays from standard radioactive sources of ²²Na, ¹³⁷Cs, ⁶⁰Co, and ²⁴¹Am-⁹Be. As efficiency of liquid scintillator depends on incident neutron energy and the detection threshold at which it is operated, it is important to determine intrinsic efficiency of neutron detectors as a function of neutron energy. The detection efficiency of liquid scintillator was determined from energy spectra of prompt neutrons emitted from spontaneous fission ²⁵²Cf source [14]. The associated fission fragments were detected using a large area Parallel Plate Avalanche Counter (PPAC). The source ²⁵²Cf was mounted on PPAC electrode to ensure 2\(\pi\) solid angle coverage for fission fragments emitted from the source. Energy distribution of neutrons were determined from ToF registered between PPAC (start) and liquid scintillator (stop) detectors. The measurement was performed with liquid scintillator threshold set to \(\sim\) 120 keVee (keV electron equivalent) that corresponds to equivalent neutron energy of \(\sim\) 0.5 MeV. The measured neutron energy distribution was compared with standard ²⁵²Cf neutron spectrum available in literature [25] and energy dependent efficiencies were deduced. Figure 2 shows the detection efficiency data for incident neutron energy up to 10 MeV. A Monte-Carlo simulation of detection efficiency was carried out using FLUKA [26, 27] and the results found to be in good agreement with experimental data as shown in Fig. 2. For NAND detectors, good neutron-\(\gamma\) discrimination is achieved using PSD based on zero-cross time distribution [20]. Figure 3 displays a typical neutron-\(\gamma\) discrimination spectrum, where zero-cross time distribution is shown as a function of light output, illustrating clear separation between neutron and \(\gamma\)-ray events.

The cross-talk due to neutron scattering is a major concern for large detector arrays. The neutrons scattered from neighboring detectors may hit a particular detector and produce signals within detectable limit. Background scattering can distort neutron measurement and it is important to estimate neutron cross-talk probability for NAND array. The cross-talk probability was determined from offline measurements [14] using a reference detector for detecting neutrons from ²⁴¹Am-⁹Be source which decays by emitting one neutron per event. Identifying neutron-neutron coincidences observed in neighboring detectors as cross-talk events, the cross-talk probability for two detectors in near vicinity was found to be \(\sim\) 4.6 ± 0.28 \(\times\)10⁻⁴. When neutron multiplicity is more than one, cross-talk would be higher. Considering reactions emitting average three neutrons per fragment, the maximum cross-talk probability in this case would be \(\sim\) 1.38 ± 0.08 \(\times\)10⁻³ [14].

Fission is a complex process where dynamical evolution of the fissioning systems depends on many reaction parameters. Since neutron emission is possible at all stages of shape evolution of the di-nuclear system, neutron ToF measurement is a preferred experimental probe to explore the fission mechanism. The NAND facility has been extensively used to study different aspects of fission dynamics, especially the role of nuclear dissipation, entrance channel dynamics, nuclear shell effects and non-compound nuclear processes in reactions producing nuclei in very heavy or near super-heavy mass region. Research highlights of some of these studies carried out at NAND facility are discussed below.

3.1 Role of nuclear dissipation

The observation of excess pre-scission neutrons over theoretical predictions of statistical model assuming the Bohr-Wheeler fission formalism was attributed to the hindrance of fission due to nuclear dissipation effects [28]. Nuclear dissipation slows down fission process thereby increasing the likelihood of more neutrons and light-charged particles being emitted during the pre-fission phase. Measurement of \(n_{pre}\) provides important information on dissipation in fission reactions. In statistical model calculations, the dissipation strength is usually extracted as a free parameter (reduced friction coefficient \(\beta\)) that is adjusted to reproduce the experimental \(n_{pre}\) data using Kramer’s modified fission width formula, which incorporates dissipation effects [29].

A series of experiments were performed using NAND to understand the role of nuclear dissipation in fission. These experiments extracted \(n_{pre}\) from neutron ToF measurements and studied their evolution with excitation energy for a range of fissioning nuclei near mass A = 200. [30,31,32]. For example, the pre-scission neutron multiplicities were extracted for fission of ^212,214,216Ra produced in reactions using ³⁰Si projectiles bombarding ^182,184,186W targets forming CN in the excitation energy range of 45 to 95 MeV [32]. Experimental \(n_{pre}\) were obtained from least square fit to neutron angular correlation data using multiple moving sources [33]. Extracted \(n_{pre}\) were compared with standard statistical model calculations [34, 35] incorporating Kramer’s fission width. Measured neutron multiplicities for these nuclei compared with statistical model calculations using different dissipation strengths are shown in Fig. 4.

Experimental \(n_{pre}\) vary monotonically with excitation energy and show a weak dependence on fissioning isotopes. The statistical model underpredict the experimental \(n_{pre}\) in the absence of nuclear dissipation (\(\beta\) = 0). With a best fit value of \(\beta\) = 8 zs⁻¹ for all three fissioning isotopes, the theory reproduces the experimental \(n_{pre}\) and its variation with excitation energy quite well. The lower panels of Fig. 4 display two components of \(n_{pre}\), the average number of neutrons emitted during saddle-to-scission transition (post-saddle neutron multiplicity \(n_{sad-sci}\)) and the average number of neutrons emitted prior to reaching the saddle point (pre-saddle neutron multiplicity \(n_{pre-sad}\)) which are estimated from model calculations. The post-saddle neutron multiplicity \(n_{sad-sci}\) contributes more to \(n_{pre}\) and increases more quickly with excitation energy than the pre-saddle neutron multiplicity \(n_{pre-sad}\). It is explained that, with increasing beam energy, CN spin increases as well, lowering the fission barrier height and creating a more compact saddle configuration, which in turn increases the saddle to scission path. As a result, \(n_{sad-sci}\) receives more neutron contributions. Since saddle-to-scission transition time increases with \(\beta\), the post-saddle neutron multiplicity \(n_{sad-sci}\) increases with the dissipation strength parameter \(\beta\).

Similar results were observed in the excitation function of \(n_{pre}\) extracted for fission of ²²⁷Np formed in the reaction ³⁰Si + ¹⁹⁷Au \(\rightarrow\) ²²⁷Np at excitation energies in the range 44-78 MeV [31]. In order to explain the \(n_{pre}\) data, the model required to incorporate nuclear dissipation with \(\beta\) = 10 zs⁻¹ which indicates substantial dissipation in highly fissile ²²⁷Np. In this case as well, the post-saddle stage contributes significantly to \(n_{pre}\) highlighting the crucial role that post-saddle dynamics play in heavy nuclei fission. It must be noted that, although experimental \(n_{pre}\) for aforementioned reactions is described by excitation energy (temperature) independent \(\beta\), previous statistical model analysis of \(n_{pre}\) for Fr [36] and Rn [37] isotopes reported \(\beta\) varying with excitation energy. Although, nuclear dissipation in the fission of ²¹³Fr revealed a weak signature of neutron magic number N = 126 shell closure effects [36], no such effects are observed in ²¹⁴Ra which likewise has N = 126 neutron closed shell [32]. A major difference is that the statistical model analysis in Ref. [31, 32] took into account the tilting of CN spin orientation (K degree of freedom) and collective enhancement of level density (CELD) as extra input parameters in model computations. The inclusion of CELD results in suppression of the ratio of neutron and fission width, \(\Gamma _n\)/\(\Gamma _f\) and also its dependence on excitation energy. The combined effect results in almost energy independent \(\beta\) as reported in Ref. [31, 32]. The significance of selecting different input parameters for statistical model analysis of experimental data for fission of extremely fissile nuclei is further illustrated by this work.

3.2 Role of entrance channel mass asymmetry

It is well recognized that, for reactions forming the same CN through symmetric and asymmetric entrance channels, entrance channel dynamics play crucial role in the reaction dynamics. The mass asymmetry in the entrance channel is defined by parameter \(\alpha\) = \((A_t-A_p)/(A_t+A_p)\), where \(A_t\) and \(A_p\) are masses of target and projectile nuclei, respectively. Mass asymmetry \(\alpha\) and its correlation with Bussinaro Gallone critical mass asymmetry, \(\alpha _{BG}\) [38] found to have large influence on the mass and angular distributions of fission fragments [39, 40]. For mass asymmetric systems (large \(\alpha\)), the di-nuclear system rapidly drives toward fully equilibrated CN leading to fusion-fission. For mass symmetric systems (lower \(\alpha\)), the di-nuclear system may re-separate without achieving full equilibration leading to quasi-fission, a non-compound decay. Quasi-fission occurs faster than fusion-fission, suppressing the overall neutron yield. Therefore pre-scission neutron multiplicity measurement is often used as a sensitive tool to probe the entrance channel dynamics. To investigate the impact of entrance channel dynamics on neutron emission, average \(n_{pre}\) were extracted for the fission of ²⁰⁸Rn formed in two distinct reactions [41]. The neutron ToF experiments were performed using NAND facility detecting neutrons in coincidence with fission fragments from reactions ³⁰Si+¹⁷⁸Hf and ⁴⁸Ti+¹⁶⁰Gd populating ²⁰⁸Rn in the excitation energy range of 54–80 MeV.

Figure 5 shows extracted pre-scission multiplicities and theoretical predictions for studied reactions. The results are compared with previously reported \(n_{pre}\) data from reaction ¹⁶O+¹⁹⁴Pt populating nearby ²¹⁰Rn [37]. Reactions corresponding to different entrance channel mass asymmetry show distinct \(n_{pre}\). The measured \(n_{pre}\) for systems ³⁰Si+¹⁷⁸Hf and ⁴⁸Ti+¹⁶⁰Gd was found to be higher than that of ¹⁶O+¹⁹⁴Pt. The very mass asymmetric system ¹⁶O+¹⁹⁴Pt results in fully equilibrated CN without any considerable fusion delay (CN formation time). Hence neutron evaporation during CN formation is negligible in this reaction. More \(n_{pre}\) observed in reactions ³⁰Si+¹⁷⁸Hf and ⁴⁸Ti+¹⁶⁰Gd suggest significant delays in CN formation leading to neutron emission during the fusion time. The entrance channel potential energy surface calculations estimate long CN formation time for ³⁰Si induced reactions [41]. A significant part of the measured \(n_{pre}\) for system ³⁰Si+¹⁷⁸Hf, where the quasi-fission probability is low, was accounted by neutrons emitted during fusion delay time of \(\tau =65 \times 10^{-1} s\), according to Langevin dynamical calculations [42] that included CN formation delay time. On the other hand, the lower \(n_{pre}\) for system ⁴⁸Ti+¹⁶⁰Gd as compared to ³⁰Si+¹⁷⁸Hf is attributed to fast quasi-fission dominating the entrance channel ⁴⁸Ti+¹⁶⁰Gd. Presence of quasi-fission prohibits formation of CN and overall neutron number decreases considerably. The significance of including entry channel dynamics in theoretical models for a more accurate description of fission observables is highlighted by this experimental investigation.

3.3 Studies of quasi-fission reactions

The presence of non-compound nuclear process such as quasi-fission is a major hindrance to formation of superheavy nuclei using heavy ion reactions. As optimized reaction parameters are often required for synthesis of superheavy elements, it is important to understand the factors that influence the occurrence of quasi-fission processes. Exploring reactions forming super heavy nuclei is more challenging due to significant presence of quasi-fission. Attempts have been made to study fission of very heavy nuclei using neutron multiplicity as a probe to disentangle quasi-fission process. In one such experiment, near superheavy nuclei ²⁵⁶Rf was formed in reaction ⁴⁸Ti + ²⁰⁸Pb \(\rightarrow\) ²⁵⁶Rf. Fission fragment mass-\(n_{pre}\) and mass-TKE correlations were extracted at 57.4 MeV excitation energy [43]. Figure 6 displays the mass-TKE correlation and TKE distribution of fragments produced in this reaction. In Fig. 6a, fusion-fission and quasi-fission events are spread between two distinct peaks corresponding to events arising from elastic, quasi-elastic process etc.

It was suggested in Ref. [44] that fragment mass-\(n_{pre}\) and neutron-TKE correlations could be a useful tool to disentangle quasi-fission from fusion fission events. Following similar approach, \(n_{pre}\) corresponding to symmetric mass split (\(A_{FF}=\frac{A_{CN}}{2}\pm 20\)) and asymmetric mass split (\(68 \le A_{FF} \le 108\)) were extracted separately (\(A_{FF}\) and \(A_{CN}\) represent mass of fission fragments and CN respectively). The \(n_{pre}\) correlated with asymmetric mass split was found to be smaller than \(n_{pre}\) associated with symmetric mass division. The variation in \(n_{pre}\) from asymmetric mass split to symmetric mass split may arise due to different time-scales for quasi-fission and fusion-fission process. To get further insight on the dynamics of fusion-fission and quasi-fission, the dependence of \(n_{pre}\) on TKE was investigated. In Fig. 6a cuts applied in the mass-TKE matrix are shown. In order to maintain a similar mass distribution in each gate, a defined relative TKE (RTKE) was used [44]. The lower panel of Fig. 6 show the TKE projection and extracted neutron multiplicities (pre-scission \(n_{pre}\), post-scission \(n_{post}\) and total \(n_{total}\)) corresponding to four RTKE cuts.

It is observed that extracted \(n_{pre}\) increases when TKE is smaller which further supports the strong \(n_{pre}\)-TKE correlation observed in fast quasi-fission reaction ⁶⁴Ni + ¹⁵⁴Sm reported by Hinde et al. [44]. Low TKE correlated with high \(n_{pre}\) signify more neutrons are emitted near to scission from accelerating fragments soon after scission. The moving three-source fit used to extract \(n_{pre}\) account neutrons emitted during acceleration of the fragments as part of pre-scission component thus overestimating \(n_{pre}\). This strong \(n_{pre}\)-TKE dependence is explained by the shorter time-scale of quasi-fission leaving more excitation energy available for neutron emission near scission [6, 44]. Important information on the dynamics of fission process are revealed by the measured neutron multiplicity dependency on mass-split and TKE, which supports the strong evidence of quasi-fission.

3.4 Multi-chance fission

It is well accepted that, fission data is used to test various reaction models which are often sensitive to excitation energy available at the time of scission. When initial excitation energy is high, fission can occur after the evaporation of one or more neutrons and this multi-chance fission can influence the characteristics of fission observables. Multi-chance fission has been a recognized phenomenon in fission for a long time [45]. Pre-scission neutron yields and shape of fragment mass distribution extracted from experimental data may vary depending upon on the relative contribution of various fission chances. Experimentally it is difficult to distinguish the contribution arising from each steps of sequential fission, however a better picture of the reaction dynamics can be obtained by incorporating the role of multi-chance fission in data analysis. A recent experiment using NAND facility explored fission properties of ²²⁷Pa by measuring the mass-gated pre-scission neutron multiplicity in the reaction ¹⁹F+²⁰⁸Pb \(\rightarrow\) ²²⁷Pa [46] at E*= 59.6, 46.1, 32.4 MeV. Data at 24.2 MeV from was taken from Ref. [47] for comparison.

Fragment mass-TKE correlation and fragment mass-neutron correlations were extracted, and the mass-gated \(n_{pre}\) corresponding to symmetric and asymmetric mass splits were analyzed using semi-empirical model code, GEneral description of Fission observables (GEF) [48] by incorporating multi-chance fission. GEF model was used to generate mass-TKE distribution for different chance fission. The mass-TKE distribution obtained for various chance fission were summed up according to their relative probabilities and the same was found to be comparable with the experimental mass-TKE distributions at different measured excitation energies. The GEF simulations clearly showed shell induced asymmetric fission for late chance fission at lower excitation energies. The relative probabilities of late chance fission were found to be as high as \(\approx\) 43 \(\%\) and \(\approx\) 65 \(\%\) respectively at 32.4 and 24.2 MeV excitation energies. It is observed that asymmetric fission mode is dominant at low excitation energy in comparison to the high excitation energy, which is a clear indication of the persistence of shell effects at low excitation energies. Figure 7 shows the correlation between experimental \(n_{pre}\) and fission fragment mass from ²²⁷Pa fission at different excitation energies. The higher value of \(n_{pre}\) associated with asymmetric mass division as compared to symmetric mass split at lower excitation energies is a signature of delay due to the late-chance fission. Even though multi-chance fission happens at higher excitation energies of E*= 59.6 and 46.1 MeV, the shell effects fade away which lead to more symmetric fission. This work emphasizes the importance of considering multi-chance fission in interpreting fission data, especially at higher energies where multi-chance fission is more significant.

The NAND facility was designed primarily to study the dynamics of heavy-ion induced fission reactions near the Coulomb barrier by detecting neutrons in coincidence with fission fragments. However, the versatile detection system, the custom made electronics and large spherical vacuum chamber allow us to carry out experiments in other related areas of nuclear physics research. Some aspects of these works are discussed here.

Recently, the observation of asymmetric fission in low energy fission of very neutron-deficient nuclei near to Mercury (Hg) [49] has been a topic of special interest. Different theoretical approaches are used to explain this phenomenon and new experiments are being performed to explore how asymmetric mass distribution varies with excitation energy and the transition from Mercury to other heavier nuclides [5]. In this direction, few experiments have been performed in NAND facility to study the excitation-energy dependence of fission fragment mass distribution in reactions forming neutron-deficient nuclei around lead (Pb). The reaction ²⁸Si + ¹⁷⁰Yb \(\rightarrow\) ¹⁹⁸Po was studied at several excitation energies ranging from 34 to 44 MeV [50]. The NAND chamber with two MWPC detectors were used to measure fragment mass and total kinetic energy (TKE) distribution of the fissioning system ¹⁹⁸Po. At the lowest energy measured, fragment mass distribution found to be best described by sum of three Gaussian distributions, suggesting non-negligible presence of asymmetric mass splitting in fission of ¹⁹⁸Po at low energy. Moreover, the experimental indication of asymmetric partitions, up to a few tens of MeV above the fission barrier is found in agreement with GEF model predictions. Another experiment explored evolution of asymmetric mass distribution in ¹⁸⁶Pt by measuring fission fragment mass distribution in reaction ³⁰Si + ¹⁵⁶Gd\(\rightarrow\) ¹⁸⁶Pt at different excitation energies [51]. The measured mass distributions showed flat-top shape at low excitation energies. The nuclei studied in these reactions somewhat extend the region of mass-asymmetric fission earlier observed in beta delayed fission of ¹⁸⁰Hg.

Another important aspect of heavy ion induced fusion reactions is the strong coupling between relative motion and internal degrees of freedom of colliding nuclei, which leads to a distribution of fusion barriers rather than a single potential barrier. Knowledge of fusion barrier distribution is important for understanding reaction mechanism, especially for reactions selected to synthesize super heavy elements. Quantitative estimates of fusion barriers are important for producing superheavy elements, as it may differ from theoretical predictions due to the coupling of various intrinsic degrees of partner nuclei. The most commonly used method to extract barrier distribution is from fusion excitation measurement. However, for very heavy nuclear systems where fusion excitation measurement becomes difficult, complementary quasi-elastic scattering methods are used [52]. Using array of hybrid detector telescopes (HYTAR) mounted inside NAND chamber, quasi-elastic scattering measurements have been performed. In one of these experiments, the quasi-elastic scattering measurement was performed for the reaction ⁴⁸Ti + ²³²Th \(\rightarrow\) ²⁸⁰Cn\(_{112}\) (Copernicium) at backward angles [53]. The experimental quasi-elastic excitation functions could be described reasonably well by Coupled channel calculations using CCFULL code [54] when coupling between the rotational state of deformed target nucleus and the vibrational state of projectile nucleus is taken into account. This experiment demonstrates the feasibility of measuring fusion barrier through back angle quasi-elastic scattering for reactions producing nuclei close to superheavy regions.

The High Current Injector (HCI) system [55, 56] coupled to LINAC can provide new ion beams that are currently not achievable with Pelletron. With HCI facility using a high-temperature superconducting electron cyclotron resonance ion source (HTS-ECRIS), it is expected that new species of ion beams with higher charge states are available for experiments. The first beam test was completed recently by accelerating Ne beam through HCI facility. Fine tuning of various beam line components is yet to be completed to deliver accelerated beam to experimental facilities. With higher charge state of the ions from HTS-ECRIS, it is expected to achieve beam energy of \(\sim\)5 MeV/A for elements up to mass 80 amu. The availability of high energy heavy mass projectiles will make it possible to research the reaction dynamics of superheavy elements. High intensity beams will enable neutron multiplicity distribution study by collecting high fold neutron-fission coincidence data. Neutron multiplicity distribution data may help to disentangle fusion-fission and quasi-fission events for reactions using heavy projectiles [57, 58]. Experimental data with good statistics can be obtained only if the reaction rate is considerably increased for low cross-section processes.

Concerning fission fragment detectors, currently large-area MWPC are used for fragment mass and TKE distributions extracted from the ToF and position information from these detectors. A more reliable TKE information can be obtained if energy of the fragments is measured directly and/or the absolute velocity of the fission fragments are measured [59, 60]. A couple of small area fast timing detectors based on Parallel Plate Avalanche Counters (PPACs) has been developed. Using them as time zero start detector in fragment ToF measurement will enable measurement of absolute velocities of the complementary fission fragments. For direct TKE measurements, large-area position-sensitive silicon detectors are planned.

The study of transfer-induced fission opens up a wide area of new research where a large number of fissioning nuclei away from line of stability can be produced in single reaction [61]. The transfer products need to be detected along with the fission fragments to identify the fissioning nucleus. It is envisaged to use annular position-sensitive silicon detector in NAND set up for detection of transfer products emitted in the forward cone.

Another area where future experiments planned is the study of pre-equilibrium emission of particles from the di-nuclear system formed just after the nucleus-nucleus collision. The maximum beam energy that LINAC can achieve for light ion beams like ¹²C, ¹⁴N, and ¹⁶O is roughly 10 MeV/A. Pre-equilibrium emissions are characterized by forward-peaked angular distribution of emitted particles. With high granularity of the NAND array, good quality data on neutron angular distribution could be obtained. This will help in separating pre-equilibrium component from neutrons emitted from equilibrated compound system. The CsI detector array of the NAND facility can be utilized to get a better insight into pre-equilibrium particle emission by detecting light charged particles emitted in the reaction.

A brief description of the NAND facility and highlights of few selected experiments using the facility are discussed in this article. Experiments are carried out to get a better insights into the dynamics involved in fission of nuclei covering range of masses from light actinides to near superheavy nuclei. Role of nuclear dissipation, entrance channel mass asymmetry, quasi-fission and shell effects are investigated through studies on pre-scission neutron multiplicity, fission fragment mass distribution, fragment mass-neutron and neutron-TKE correlations. Experimental pre-scission neutron multiplicities could only be described after considering nuclear dissipation in theoretical models, confirming the well-established dissipative nature of nuclear matter. Multiplicity of pre-scission neutrons from highly fissile actinide nuclei reveal major de-excitation take place during post-saddle phase of the shape evolution. It is shown that statistical model calculations that incorporate collective enhancement of level density and K-orientation effects provide more reliable results, emphasizing the importance of selecting right input parameters for statistical model analysis. Neutron multiplicity measured in reactions having different entrance channel mass asymmetry clearly indicated the role of entrance channel mass asymmetry influencing the dynamics. While mass asymmetric reaction partners often lead to fusion-fission, more mass symmetric entrance channels mostly lead to quasi-fission. Comparison of neutron multiplicities observed in symmetric reactions also indicate strong interplay of entrance channel parameters leading to longer time delay for CN formation. This emphasizes the need for including entrance channel dynamics in theoretical models describing the dynamics of heavy-ion collisions. For fast quasi-fission reactions, the strong dependence of pre-scission neutron multiplicity on TKE show evidence of fast quasi-fission reactions where neutron emission near scission point constitute large part of measured pre-scission neutron multiplicity. Excitation energy too plays a crucial role and features observed at high initial excitation energy may often contain contribution from multi-chance fission as evidenced from fragment mass-neutron correlations observed in ²²⁷Pa. For future experiments, it is expected that with availability of new beam species with higher charge state and higher beam intensity from HCI accelerator, the fission study can be extended further to super heavy mass region and fresh measurements involving multi-fold neutron correlation are promising to gain better insight into reaction dynamics.

The data and figures used are derived from published materials belonging to the authors of each publication.

The authors gratefully acknowledge Prof. A. C. Pandey, the present director, and Dr. A. Roy and Dr. D. Kanjilal, the former directors, for their support and encouragement. We are thankful to N. Saneesh and Mohit Kumar for their invaluable contributions to the development of the facility and experiments. The majority of the works reviewed here are the result of ongoing study conducted by a number of research scholars from universities, working in collaboration with scientists from this Centre, national institutes, and university faculty. We appreciate their support. We are grateful for the support received from technical and engineering divisions at IUAC, accelerator group, the NAND project investigators, Dr. A Chatterjee, and Prof. M.B Chatterjee. We acknowledge Dr. S. K. Datta and Dr. R. K. Bhowmik for their leadership during the early phases of the NAND project.

The NAND project was funded by the Department of Science and Technology, Government of India (No. IR/S2/PF-02/2007).

Authors and Affiliations

Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, 110067, Delhi, India
Golda Komalan Satheedas, Akhil Jhingan & Sugathan Pullahnhiotan

Contributions

GKS, AJ, and SP contributed to the writings of the manuscript. All authors proofread and approved the final version of the manuscript.

Corresponding author

Correspondence to Sugathan Pullahnhiotan.

Consent for publication

All authors have agreed to the published version of the manuscript.

Competing interests

The authors declare that they have no competing interests.

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