Geant4 simulation of pulse-height response function of liquid scintillator based neutron detector M. Gohil 1 , * K. Banerjee 1 , C. Bhattacharya 1 , S. Kundu 1 , T. K. Rana 1 , G. Mukherjee 1 , J. K.Meena 1 , R. Pandey 1 , H. Pai 1 , M. Biswas 1 , A. Dey 1 , T. Bandhopadhyay 2 , and S. Bhattacharya 1 1 Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata - 700064, INDIA and 2 HS&E Group, BARC,VECC,1/AF, Bidhan Nagar, Kolkata -700064, INDIA Organic liquid scintillator based detectors are widely used for neutron detection in a mixed field of neutron and gamma rays due to their properties viz. high light output, good detection efficiency, fast decay time and excellent neutron-gamma discrimination. A BC501A liquid scintillator based time of flight (TOF) neutron detector array is be- ing developed for neutron spectroscopic stud- ies at the Variable Energy Cyclotron Centre, Kolkata [1]. A Geant4 based simulation code has been developed to estimate the response function of these neutron detectors. Here, we report our measurement of response func- tions of these detectors for neutron using neu- tron emitted from 252 Cf source as well as α(35 MeV) + 181 Ta reaction, using TOF technique and presents the comparison with the results obtained from Geant4 simulation. The mono-energetic neutron response func- tion was simulated using the Geant4 toolkit version 4.9.2 [2]. The code takes into consid- eration the full complexity of the light produc- tion inside the detector, wall effect and non- linear light response for secondary charged particles. All relevant physical processes for γ - rays and neutrons in the energy range in ques- tion were included in the simulation. For γ -ray interactions, the data file G4EMLOW version 6.2, containing cross-sections for low energy electromagnetic processes, was used. For neu- tron, cross section data library G4NDL3.13 as well as Geisha routine based data library, in which models of Elastic, Inelastic, Capture * Electronic address: mrgohil@vecc.gov.in and Fission process are incorporated, have been used. G4NDL3.13 is the recently up- dated high precision data library for neutron while Geisha routine based data library is de- fault data library for all hadronic process in Geant4. Beam experiment has been performed us- ing α(35 MeV) beam from K130 Cyclotron at VECC on 181 Ta target. The experi- mental neutron response was measured us- ing the neutron detector of size 7” × 5” placed at a distance 150 cm from the target. For measurement with 252 Cf source, source was kept at the target position. Neutron events were separated from γ events by Pulse- shape-discrimination method (PSD). Mono- energetic neutron response was extracted from continuous neutron spectrum by selecting a particular window in (TOF) spectrum. Win- dow in TOF spectrum for a particular neutron energy E was selected by calculating energy resolution of TOF technique given by equa- tion,  ΔE E  2 =  2Δτ t  2 +  2Δl l  2 (1) where Δτ is the time resolution, l is flight path of neutron, Δl is the flight path spread due to the length of detector. It is seen that the shape of neutron response function is strongly influenced by small variation of neutron energy; hence, to compare the sim- ulated spectrum with spectrum measured us- ing TOF technique, energy resolution of TOF was also included in the simulation by gener- ating a neutron pencil beam of energy having a Gaussian distribution with centroid at E and