Effect of Shell Closure on Neutron Multiplicity Varinderjit Singh 1 *, B.R. Behera 1 , D. Siwal 2 , S. Goyal 2 , Maninder Kaur 1 , P. Sugathan 3 , K.S. Golda 3 , A. Jhingan 3 , A. Kumar 1 , A. Saxena 4 , R.K. Bhowmik 3 , S. Kailas 4 1 Department of Physics, Panjab University, Chandigarh -160014, INDIA. 2 Department of Physics and Astrophysics, University of Delhi – 110007, INDIA. 3 Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi - 110067, INDIA. 4 Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai, - 400085, INDIA. . * email: Mangat_phy@yahoo.co.in Introduction: In the last century the development of nuclear accelerator for the production of heavy ion beams opened an opportunity to study heavy ion induced fusion-fission reactions. The importance of dissipation in fusion-fission dynamics is well established from the study of light particle emission during heavy-ion induced fusion- fission reaction. Experimental signature of large dissipation is observed through large excess in pre-fission neutrons, gamma ray multiplicities from compound nucleus giant dipole resonance (GDR), light charged particles and evaporation residue [1]. Mainly, dissipation is observed at nuclear temperature between 1 and 2 MeV, also it is found that dissipation effect increases with excitation energy. In the current scenario, one of the major objectives of nuclear reaction study is to explore the predicted Island of Super-Heavy elements and to find what will be the next Proton and Neutron magic nucleus beyond 82 and 126 respectively. However, different theoretical models predict proton and neutron shell closure at Z = 114, 120, 122, 124, 126 and N = 172, 184. Knowledge of the existing shell closure is very important in order to achieve this. Back et al. [2] reported that in order to reproduce evaporation residue cross-sections for 224 Th and 216 Th nuclei, a larger dissipation strength was required for 224 Th. It was concluded that nuclear dissipation has possible relation with neutron closed shell N C =126. To explore the effect of shell closure on nuclear dissipation, we have decided to perform a simultaneous analysis of neutron multiplicity, fission cross-section and evaporation residue cross-section for 19 F+ 194,196,198 Pt resulting in the formation of CN 213 Fr (N c =126), 215 Fr (N c =128) and 217 Fr (N c = 130) systems. Here we are presenting the results of neutron multiplicity measurement for reactions of 19 F + 194,196,198 Pt at the excitation energy range of 52-67.5 MeV. Experimental Arrangement: The experiment was performed using 15UD pelletron and National array of Neutron Detectors (NAND) at IUAC, New Delhi. Pulsed beam 19 F (Energy Range = 98 – 115 MeV) at repetition rate of 250 ns with pulse width of 1.5 ns, was bombarded on targets of 194 Pt, 196 Pt and 198 Pt of thickness 1.75 mg/cm 2 , 1.8mg/cm 2 and 2.15 mg/cm 2 respectively. Targets were located at centre of a thin walled spherical scattering chamber of 60 cm diameter. Fission fragments were detected by a pair of Multi-wire proportional counter (MWPC) (5” x 3”) kept at fission fragment folding angle at distance of 17.7 cm and 17.0 cm from target position. Two silicon surface barrier detectors were also placed inside the chamber at ±16 o to beam direction out of the reaction plane for normalization purpose. Out of 16 Neutron detectors (5” x 5”), 12 detectors were kept at 2 meter distance and remaining 4 detectors were kept at 1 meter distance from the target. These detectors were placed at different angles ranging from 30 o to 315 o around the target chamber in reaction plane. Hardware threshold of 0.5 MeV of neutron energy was applied using 137 Cs and 60 Co sources. The trigger of data acquisition was generated by Logical OR of cathode signal of two MWPC ANDed with RF of the beam pulse. Neutron gamma discrimination was performed by using both time of flight technique and IUAC made pulse shape discrimination module. Results and Discussions: The raw neutron TOF spectra were converted to energy spectra for all the detectors. Proceedings of the DAE Symp.on Nucl. Phys. 55 (2010) 320 Avilable online at www.sympnp.org/proceedings