Promotion And Application Of The Reflection Asymmetric Shell Model

In this thesis, the odd mass nuclei with the intrinsic reflection-asymmetric shape and its spectroscopic fingerprints are studied by means of the reflection asymmetric shell model (RASM). As a special application of the RASM, theoretical simulation for identical bands(IBs) is made. In order to investigate the mechanism of IBs, the pairing fields of SD nuclei are also studied.The origin of intrinsic reflection-asymmetric shape in nuclei is systematically rewiewed. It is shown that this reflection-symmetry breaking effect arises from the octupole interaction between (N,l,j) intruder orbits and (N -1,l- 3,j -3)normal orbits with the opposite parity. Some theoretic models, experimental spectroscopic properties of odd mass nuclei related to the octupole-deformation are briefly introduced.The framework of the RASM is given in this thesis, and is extended to describe the octupole-deformed odd mass nuclei with the reflection asymmetric shape. This model is applied to the odd Radium and Thorium isotopes223,225,227,229Th and 221,223,225,227Ra, which are typical octupole deformed nuclei.Some characteristics of the octupole rotational bands in odd mass nuclei are reproduced in the framework of RASM, such as, the ground-state spin and parity, the ground-state band spectroscopy and some parity doublets (PD). In addition, the calculations also reproduce an indirect evidence of octupole deformation in odd mass nuclei: the decoupling parameters for K= 1/2 PD bands have opposite sign, but approach to the same absolute value. The agreement between the present calculations based on the shell model and the experimental results is generally good. This shows that the RASM can well describe the octupole deformed odd mass nuclei. In contrast, the traditional shell model can not describe octupole deformed heavy nuclei.The theoretical simulations for IBs are carried out by the RASM. The results show that the frequency of the occurrence of IBs is much higher for SD nuclei than for ND nuclei. The results of statistical analysis of incremental alignments of SDand ND IBs show that the alignments are quantized for SD IBs when the selecting criterion 5 becomes strict, and unquantized for ND IBs even if 5 goes to strict. Furthermore, the results also show that the number of SD IBs increases when the octupole deformation is considered. From the point of the view of statistics, the occurrence and the behavior of IBs revealed in the experimental observations have been generally reproduced by the present theoretical simulation. The rotational bands produced by the RASM have good parity and good angular momentum, and the octupole correlation is an universal phenomenon in the considered SD mass regions, so the present theoretical simulation for IBs has some unique merits. The dynamic moments of inertia (J(2)) and the pairing field of the different SD nuclei are calculated by means of the nonisotropic harmonic oscillator quadrupole pairing HFB model, and the different features of J(2) for the SD nuclei in the different mass regions are interpreted. The results show that SD nuclei in different mass regions have different critical frequence c for the collapse of the pairing field. For nuclei in A~190,150 and 60-80 regions, the calculated hc is 0.4,0.5 and 0.7MeV, respectively, and these results coincide with the experimental data. Both experimental analysis and theoretical calculations show that there is no, undergoing and complete collapse of the pairing fields at the measured regions in the A~190,150 and 60-80 mass regions, respectively. From the present calculations we point out that the fast rotation is essential for the collapse of the pairing field in nuclear SD states and the large deformation may lead to a significant reduction of the pairing correlation. And these results can be used to for a further understanding of the mechanism of IBs in different SD mass regions.