Study Of High Spin Spectroscopy In Odd-Odd ¹²²I Nucleus

The study of high spin states in A¹20 nuclear region is very hot. A nuclide inthis region was chosen in this thesis as main research object, in order to achieve thecomprehensive understanding for the nuclear spectroscopy of high spin states. Excitedstates of the odd-odd nucleus¹²²I were populated via the fusion-evaporation reaction¹¹⁶Cd(¹¹B,5n)at a beam energy of 68 MeV using the HI-13 tandem accelerator ofChina Institute of Atomic Energy (CIAE) in Beijing.The odd-odd nucleus¹²²I, which lies in the transitional region between theprimarily spherical50Sn nuclei and well-deformed57La and58Ce nuclei. The nuclei ofthis region haveγ-soft characteristic. The proton Fermi level lies near the low-Ωh_11/2obitals, prolate (γ=0) driving, whereas the neutron Fermi level lies in the mid-Ωh_11/2obitals, oblate (γ=60°) driving. Nuclear potential energy surface (PES) calculationshows that the nuclei in A～120 mass region are characterized with aγ-soft deformedcore and the orbitals occupied by the protons and the neutrons with competing shapeddriving tendency. A number of interesting phenomena have been exhibited in thisregion. Such as band termination phenomenon in the bands of odd-A^113-123I base onthe low-KÏ€h_11/2configuration, and the shape coexistence between oblate and prolatealso have been observed in^119,121I.For the odd-A iodine nuclei, a number of interesting phenomena have beenexhibited in odd-odd Z=53 nuclei. For instance, the candidate of chiral bands basedonÏ€g_9/2νh_11/2configuration in118I, which is the first time observed in iodine isotope,is a hot topic in the study of high spin states. Band terminations have been observed in^116-120I, which are the basis of the observation of the fully aligned even the coreexcited states above the band termination, and furthermore, the study of thesephenomenons play an important role in improving the shell model of nuclei. Besides,one more remarkable characteristic is worth of pointing out in^116-124I: the yrast bandsare based on theÏ€h_11/2νh_11/2configuration and the quasi-proton and quasi-neutron both lie in the high-K unique-parity h11/2 obital. There has been significant discrepancy in the assignment of the yrast band in 122I:the yrast band was assigned to negative parityÏ€g7/2/d5/2(?)vn11/2 configuration in the previous work, while in the recent research is assigned to positive parityÏ€g11/2(?)h11/2 configuration. Such an opposite assignment is necessary for us to do a further research in 122I. In addition, a number of isomeric states are observed in the nuclei of this region, such as the high-K 7- and 8- states in 120-124I. The study of isomeric states and the bands above is an important method to investigate the nuclear structure. In summary, nuclei in A-120 region provide a number of interesting informations of the nuclear structure. The research results in this work blew:1. The excited states of odd-odd 122I have been studied using in-beam gamma-ray spectroscopy. From the y-y coincidence analysis, almost 30 gamma transitions have been identified and the most complete level scheme has been established, including eight bands based on different configuration. The previously known structure has been confirmed, and we extended the yrast band to 31h, second populated negative-parity band was extended to 30h, other rotation bands also been extended to the higher spin, respectively.2. Based on CSM, the comparison between experimental and theoretical B(M1)/B(E2) value, extraction of alignment angular momentum, the study of exciting energy of systematics and the behavior of signature splitting, we assign the configuration of each band. Our investigation approved the previous assignment of the positive-parity yrast band, which isÏ€h11/2)[550]1/2(?)vh11/2(α=±1/2)[523]7/2-.In the mean time, we assigned the second populated negative-parity band asÏ€h11/2[550]1/2-(?)vd5/2[402]5/2- configuration, and rotation band 3 asÏ€d5/2[420]1/2+(?)vh/2[523]7/2- and band 4 and 6 asÏ€g7/2[422]3/2+(?)vh11/2[523]7/2-configuration, and we also assigned the band 5 asÏ€g11/2[550]1/2-(?)g7/2[404]7/2+ configurations. The configuration of band 7 and 8 had been assigned toÏ€g9/2[404]9/2(?)vh11/2[523]7/2- as twin bands.3. A number of transitions between the yrast band and band 2 have been identified. We assigned these transitions as E1 via the analysis of multipolarity and the opposite parity of the two bands, which is as an evidence of octupole correlation phenomenon in 122I. 4. Band termination phenomenon in the yrast band 1 also has been studied and discussed in this thesis, and the configuration of noncollective oblate states also have been assigned. The lowest-spin oblate state at IÏ€=(21+) is based on the [Ï€h11/2(Ï€g7/2)2]23/2-(?)[vh11/2)3]17/2structure. At this state, three quasi-protons outside the closed shell(Z=50) and three quasi-neutrons outside the semi-closed shell(N=64) aligned and provided part of the single-particle angular momenta. The spin of the IÏ€=(25+) state can be generated from [Ï€h11/2(Ï€g7/2)2]23/2-(?)[vh11/2)3]27/2-configuration, which is provided the maxium angular momenta by the three quasi-neutrons outside the semi-closed shell. With the spin increasing, the full alignment of all valance nucleons is expected and the nuclear spin arises only from the single-particle angular momenta of these valence nucleons since the nuclear collective motion disappears at a non-collective oblate shape (γ≈60°). The nuclear spin of the IÏ€=31+) state can be generated from the [Ï€h11/2(Ï€g7/2)2]23/2-(?)[vh11/2)3(v7/2)2]39/2- configuration. Such a configuration corresponds to a state where all valance nucleons outside the closed or semi-closed shell align their single-particle angular momenta along the symmetric axis of the nucleus. We also pointed out band 2 occur band termination phenomenon at IÏ€=(26-), through the TRS calculation and experimental informations. We tentatively assgined the configuration of IÏ€I=(26-) and IÏ€(30-) states asÏ€[h11/2(g7/2)2]23/2-(?)v[h11/2)4d5/2]29/2 andÏ€[h11/2(g7/2)62]23/2-(?)]V[(h11/2)164d5/2]37/2+ respectively. It is worth pointing that different angular momenta are provided by the same quasi-particles at the two states. Only part of angular momenta are provided by the quasi-neutrons at the IÏ€=(26-) state, while the spin of the IÏ€=(30-) state is generated by all the quasi-nucleons outside the closed or semi-closed shell aligned their single-particle angular momenta along the symmetric axis of the nucleus.5. Octupole correlation in the 122I and other nuclei in this mass region was discussed and studied systematacially. The E1 transitions between yrast andÏ€h11/2(?)vd5/2 bands confirmed in this work infers the reflection symmetry break in 122I. The measured B(E1) shows the E1 transitions are largely enhanced, which is the experimental fingerprint of the octupole correlation.6. The energy systemics ofÏ€h11/2(?)vh11/2 bands in odd-odd nuclei A-130 region were studied comparatively, and we found the energy level structure exist some rules of features. The systematic study of signature inversion in odd-odd iodine isotopes based onÏ€h11/2(?)vh11/2 configuration has been performed. With the increasing rotation frequency, the internal structure of odd-odd iodine isotopes change dramatically, which result in the inconsistency of signature inversion regulation between odd-odd iodine isotopes and Z>53 region. As we already mentioned, the systematics of the levels built on theÏ€h11/2 orbital in odd-mass 121-125I isotopes follow the energy trends of the ground-state band in the neighboring Te core nuclei. Accordingly, the 16+level (or the 15+ level) in 120-126I might be attributed to the excitation of a pair of protons in the g7/2 orbital coupled to theÏ€h11/2(?)vh11/2 configuration. We also calculated the B(M1)/B(E2) theory prediction value ofÏ€h11/2(?)vh11/2(?)Ï€(g7/2)2 configuration. The discrepancy of signature inversion between the nuclei in Z>53 region and the odd-odd iodine isotopes can be interpreted as the excitation ofÏ€(g7/2)2 protons.7. The energy systemics ofÏ€g9/2(?)vh11/2 bands in odd-odd I were studied. The bands based on theÏ€g9/2(?)vh11/2 configuration with high-K value were strongly coupled bands. The proton Fermi level lies near the high-j g9/2 obitals show a proton-hole property, whereas the neutron Fermi level lies above the mid-Ωh11/2 obitals show a corpuscular property, and this provides a necessary condition for chiral symmetry breaking. Protons in the higher part of the g9/2 subshell favor a noncollectively oblate shape (γ=60°) while neutrons in the middle part of the h11/2 subshell favor a collectively rotating triaxial shapo30(γ°). A systematic comparison of the candidate chiral band in odd-odd 118-122I has been performed. The near degenerateΔI=1Ï€g9/2(?)vh11/2 bands in odd-odd 118-122I isotopes have been studied. The comparisons of the excited energy, signature splitting, B(M1)/B(E2) ratios, and TRS calculations show the partner bands in the odd-odd 118-122I fulfill the picture of chiral bands. We tentatively pointed out that band 7 and 8 were candidate chiral twin bands based on theÏ€g9/2(?)vh11/2 configration.8. Theoretical calculations based on the total Routhian Surface model display the deformation parameter of nuclei changing with rotational frequence of each bands in 122I. Totally,γ=+60°deformations were expected at high spin states in each bands, and occurred with the bands termination effect. These theoretical calculations correspond to the experimental observation ofÏ€h11/2(?)vh11/2 andÏ€g11/2(?)vd5/2 bands very well.9. Cranked Shell Model calculations for the quasineutron Routhian of all bandshave been done in order to study the alignment. Based on the experimental results andtheoretical calculations, we discussed the mechanism of alignment and band crossingin each bands.