Accurate Mass Measurements Of Short-lived Nuclides At The HIRFL-CSR Facility

Nuclear mass is one of the fundamental quantity of a nucleus, and the total binding energy, which can be calculated through masses, reflects all the interactions among the nucleons. High-accuracy mass values for atoms play an important role not only in nuclear physics, but also in nuclear astrophysics and other branches of physics.In the last decades, a number of new masses have been measured in Penning traps or storage rings. The isochronous mass spectrometry (IMS) in storage rings, which has ultimate sensitivity for individual ion and need no beam cooling, is an ideal tool to perform accurate mass measurements for short-lived exotic nuclides.Recently, several masses of neutron-deficient nuclides have been measured by us-ing isochronous mass spectrometry at the HIRFL-CSR facilities in Lanzhou. The mass measurements of 78Kr and 58Ni projectile fragments, which aimed at the masses of nu-clei with isospin projection Tz= -1/2 and Tz=-3/2 respectively, have been per-formed consecutively in 2009 and 2011. The primary beam was accelerated by the main ring (CSRm) and bombard a beryllium target placed at the entrance of RIBLL2. The fragments were separated in-flight in RIBLL2, and the cocktail beam of exotic nuclei within the Bp-acceptance, including the nuclei of interested and other reference nuclei, was injected into the experimental storage ring (CSRe). The revolution periods of stored ions were measured by using a time-of-flight detector in CSRe.Instabilities of the magnetic fields in storage rings are one of the major factors limiting the achievable resolving power of revolution periods of stored ions, which is directly related to the precision of the obtained mass values. A new data analysis method is proposed allowing one to minimize the effect of such instabilities. The details of the new method is described specifically in this thesis, and is applied to re-analyze the data of the 78Kr and 58Ni experiments. A largest improvement of the mass precision by a factor of about 1.7 has been achieved in the isochronous region, compared to the previous results. Furthermore, the method can be used as an on-line tool for checking the isochronous conditions of the storage ring in future experiments.In the 58Ni experiment, the revolution periods of 51Co27+ and 34Ar18+ ions are unsolvable, due to their very close mass-over-charge ratios. However, we found that, the average pulse heights of the stored ions obtained by the TOF detector are strongly related to the energy loss of the corresponding ion penetrating through the carbon foil of the TOF detector, which scales in first order with the square of the ion’s charges. By using the additional pulse-height information of the ions, as well as the revolution periods, we can resolve 51Co27+ and 34Ar18+ ions very well, and the mass excess of 51Co is determined experimentally for the first time.Using the masses of nuclei in fp-shell with Tz=-1/2, -1, - 3/2 determined in the 78Kr and 58Ni experiments, Coulomb displacement energy (CDE) of the mirror nuclei and triplet displacement energy (TDE) of isobaric analog states are investigated, respectively. Shell-model calculations in the fp-shell nuclei compared to the new data indicate the need to include isospin-nonconserving forces.The masses of Tz= -3/2 nuclei in fp-shell, which are determined in the 58Ni ex-periment for the first time, are used to test the isobaric multiplet mass equation (IMME). A significant deviation of cubic coefficient from zero has been discovered in the A= 53, T= 3/2 quartet, which cannot be explained by any theory up to now. However, in-depth analysis suggests that, this violation against the second-order IMME should be confirmed by checking the mass of the isobaric analog state of 53Co.The newly determined mass excess of 56Cu in the 78Kr experiment, as well as the mass of 55Cu in the 58Ni experiment, are applied to investigate the proton-neutron interaction strength of the odd-odd nuclei around the doubly magic 56Ni. A dramatic variation of the p - n interaction strength for Copper isotopes has been found when crossing the Z= N= 28 shell closure, which can be empirically explained by the overlap of the wave functions of the last valence neutron and proton.Since the problem of instability of the magnetic field in CSRe could be solved statistically, the momentum spread of stored ion become the major contribution to the spread of the revolution periods for the stored ions. Moreover, large systematic de-viation, which is probably due to the asymmetric momentum distribution of different ion species, has been found between different series of nuclides in mass calibration. In order to solve the problem and achieve higher mass precision, we propose a new isochronous mass spectrometry with double TOF detectors. The velocity of stored ions could be measured by using the two TOF detectors, and applied to correct the momen-tum towards the specific magnetic rigidity corresponding to the specific orbit length. In this thesis, we study the relationship among the revolution period, the velocity and the circumference of individual ion, and propose a feasible algorithm to reduce the mo-mentum spread of all the stored ion. The isochronous mass spectrometry with double TOF detectors still need to be tested in on-line experiments.