Improvements Of The TOF Detector And Mass Measurements Of T_z=-2 Proton-rich Nuclides

Masses of short-lived nuclides can be measured using Isochronous Mass Spectrometry?IMS?at HIRFL-CSR complex.The work of this paper can be divided into two section: improvements of the TOF detector in IMS,and mass measurements of T_z=-2 proton-rich nuclides.In the isochronous mass measurement,TOF detector is installed in the experimental Cooler Storage Ring?CSRe?to measure the revolution times of ions.The time resolution of TOF detector had a big influence on the resolving power of spectrometry,the detection efficiency and amplitude of signals play an important role in identifying species of ions of secondary beam.Analysis and research for detector can help us extend and improve the performance of spectrometry,and provide necessary mechanical parameters for data analysis.In order to solve the systematic deviation in mass calibration and improve resolving power of spectrometry,IMS using Double TOFs was developed by installing Double TOF detectors at the straight section of CSRe to measure the velocities of ions.Here timing performance of TOF detector play a crucial role.To achieve a better timing performance,structure of TOF detector was improved and off-line test was performed.Meanwhile several followed improvements were conducted for the IMS using double TOFs after on-line test.Short-lived T_z=-2 proton-rich nuclides are produced via projectile fragmentation of ⁵8Ni¹⁹⁺ primary beam.As there are less ions in each injection and low yields of nuclei of interest,strict conditions were improved to increase the number of ions used in data analysis.The events of accidental coincidence are excluded and identified by the method of association between amplitude and detection efficiency,and weighting shift method was adopted in correcting magnetic field.Masses of ⁴⁴Cr?⁴⁶Mn?⁴⁸Fe?⁵⁰Co?⁵²Ni have been directly measured for the first time and the precisions of masses for ²⁹S?⁴⁰Ti have been improved.The validations of IMME and FRDM models were tested with the results of this measurement.Some points are worth noting in the following:Under the simplified assumptions of an uniformly charged sphere and Coulomb interaction as the only source of the breaking of the isospin-symmetry,the Coulomb radius parameters deduced from linear and quadratic IMME coefficients respectively are not equal,which can not be explained by theory up to now.That nuclear force may be also a source of isospin-symmetry breaking was suspected.The mass of 29 S in this work further confirmed it in sd-shell.Masses of other crucial sd-shell exotic nuclides were highly suggested to be re-measured here.The mass of ⁴⁴Crcan predict the mass of ⁴⁴V^IAS using IMME local mass relation,and the IAS in ⁴⁴Tican be distinguished from the double T=2,T=0 isospin mixing states.Using a recent experiment of ? delayed proton emitter of ⁴⁴Cr,we established new decay scheme of ⁴⁴Cr.This can be confirmed compared with the E_x of mirror nuclei.According to another independent mass measurement of ⁵²Co^g.s.and ⁵²Co^isomer,it was found that the highest intense proton group are not from the IAS state via the super-allowed ? decay of ⁵²Ni.A new decay scheme of ⁵²Ni was proposed and the mass of ⁵²Co^IAS were determined.Using the mass of ⁵²Ni in this experiment,we confirmed the assignments of ⁵²Co^IAS and ⁵²Mn^IAS with IMME.Up to now,all the masses of T=2 quints obtained in experiment only exist at A=8,12,20,24,28,32,36 multiplets.The whole T_z=-2 member in these multiplets are even-even and sd-shell nuclei.IMME in fp-shell was tested using masses of T_z=-2 nuclei determined in this experiment.