GEANT4 simulation of a new 4π sum-spin spectrometer at TIFR

A 4π sum-spin spectrometer has been designed, constructed and commissioned at the Tata Institute of Fundamental Research (TIFR), Mumbai. The spectrometer is an array of 32 conical NaI detectors of pentagonal and hexagonal cross sections. The primary role of the array is to measure the multiplicity and sum-energy of the discrete low energy gamma rays emitted in heavy-ion induced fusion-evaporation reactions. The determination of the multiplicity and sum-energy of the low energy discrete gamma rays from evaporation residues is crucial for angular momentum gating or selection of narrow angular momentum windows in the phase space. This spectrometer is primarily to be used in conjunction with a high energy gamma ray spectrometer for detection of phase space selected Giant Dipole Resonance (GDR) gamma rays. Since its commissioning the array has been successfully used in in-beam experiments to study GDR decay from hot-rotating ¹⁹²Pt nucleus. We have carried out extensive simulations for calculating the detection efficiencies of the individual detectors and the entire array including the scattering chamber. The total intrinsic and photo peak efficiencies have been measured accurately with calibrated low energy gamma ray sources and are found to be in very good agreement with the values obtained from GEANT4 simulations. We have also carried out efficiency measurements and GEANT4 simulations for a smaller array of 14 straight NaI detectors of hexagonal cross sections packed in castle geometry and the results have been compared with the 4π array. The fold distributions have also been calculated for different gamma multiplicities for both the 14 elements and the 4π array. The detailed results of the simulations and measurements for the 4π array and the results of the comparative studies with the 14 elements array in castle geometry are presented.