Study Of The Mass Number Distribution Of Fission Fragments In Heavy-Ion Induced Fission Reaction

The mechanism of heavy-ion induced fission in the vicinity of the Coulomb barrier is closely related to nuclear fission dynamics and the synthesis of superheavy nuclei,which has been a long-term hot topic since its discovery.The fission mechanism is extremely complex and requires further in-depth study both experimentally and theoretically.The mass distribution of fission fragments of the important physical quantities in the study of heavy-ion induced fission mechanism in the vicinity of the barrier,and a deeper study of the mass distribution of fission fragments can help to better understand the fission mechanism.To further study the mass distribution of fission fragments induced by heavy-ions in the vicinity of the barrier,a detection array for triple distribution of mass number,kinetic energy,and angle was built in the Φ1.20 m target chamber at the R60° beamline of the Beijing HI-13 tandem accelerator laboratory.The array consists of eight sets of 90° deflection type microchannel plate（MCP）detector telescopes,followed by a semiconductor detector telescope composed of a Double-Sided silicon strip detector（Doublesided Silicon Strip Detector,DSSD）and a quadrant silicon detector（QSD）,aimed at realizing measurements of fission fragment mass using the double velocity method,double energy method,and energyvelocity method.The experiment is based on the time-of-flight（TOF）method,where the MCP detector is used to measure the flight time TOF_F and TOF_B of a complementary fission fragment pair that satisfies the detection criteria along their flight path.Based on momentum conservation and the relationship between the velocity and mass number of the fission fragment pair,the mass numbers A_F and A_B of the fragment pair are obtained,while the DSSD provides position and energy information.The rise time of the MCP detector is about 500 ps,and the time resolution is measured using alpha particles from ²⁴¹ Am with a time resolution（FWHM）of less than 250 ps.The energy resolution of the DSSD is about 1.2%,with an angular resolution 0.3°.The calibration experiment was conducted in the ⁷⁹Br+¹²⁰Sn,¹⁸⁴W reaction system with two beam energies of 120 MeV and 167.515 MeV.The flight time zero point of the detector array was calibrated using the elastic scattering particles in the reaction.By coincident measurement of the elastic scattering and recoil particles at an energy of 167.515 MeV,the mass and folding angle distributions were obtained through the double velocity method to determine the mass number distribution of the MCP detector and the scale factor.The lower limit σA of the Gaussian fitting of yhe mass number distribution obtained was 0.688±0.01 u.Under the same conditions,the mass number distribution and total kinetic energy distribution of the fission fragments in the ¹²C+²⁰⁹Bi system were obtained at an energy of 75 MeV.By the preliminary establised data analysis method,a good experimental result has been obtained.This work has established base for the future experimental study for fission mechanism of heavier systems.