Study Of High Spin Spectroscopy In Odd-Odd¹²⁰I Nucleus

The nucleus, as an important composition of substance, contains alot of interesting phenomenon and scientific knowledge. Based onin-beam gamma-ray spectroscopy, more and more important nuclearphenomenon have been investigating, such as triaxial deformation、octupole deformation、 band termination、 core breaking、 signatureinversion and chiral symmetry spontaneous breaking. Nuclear isotopes ofiodine, which lie in the transitional region between the primarily sphericaland well-deformed nuclei. Their proton Femi surface lies near the bottomof high j subshell, whereas the neutron Femi surface lies from the bottomto the top of high j subshell with the increasing neutron number.Researches show that, the valence nucleon lying near the bottom of thehigh j subshell provides the prolate（γ=0°） driving force to the core, whilethe valence nucleon lying near the top of the the high j subshell proviesthe oblate（γ=±60°） driving force. The cores of the iodine isotopes inA¹30region which are soft to γ-deformation, can be drastically affectedby the different deformation-driving effects between the quasi-proton andquasi-neutron. Thus, many typical characteristic of transitional region canexpected in iodine isotope. Such as prolate-oblate shape coexistence andband termination.Although many researches were performed for high spin states ofiodine isotopes, there are still some issues which can not be clarified. Forinstance:â‘ The study of¹¹⁹I by Tormanen et al denied the conclusionfrom Liang et al that high K oblate rotational bands exist in^117,119,121I andsuggested those bands were generated by γ-vibration.â‘¡There arewidespread controversies about the prolate and oblate rotational bandswhen the quasi-nucleon lies in g_7/2and d_5/2obitals.â‘¢Whether oblaterotational bands existing in odd-A nuclei also exist in odd-odd nucleus is still up in the air.â‘£The chiral twin bands reported in¹¹⁸I need furtherinvestigation.The odd-odd iodine nuclei were chosen in this thesis as mainresearch object, in order to investigate the characters and structure usingin-beam gamma-ray spectroscopy. Excited states of the doubly odd¹²⁰Ihave been investigated via three different reactions, including the¹¹⁰Pd(¹⁴N,4n) E=64MeV、¹¹⁴Cd(¹⁰B,4n) E=48MeV and¹¹⁴Cd(¹¹B,5n) E=70MeV,with beams produced from the HI-13tandem accelerator of ChinaInstitute of Atomic Energy（CIAE） in Beijing in the year of2008and2010. By using these cross reactions, different high spin states wereenhanced in our experiment.The experimental results include: confirming the previously knownstructure、extracting the intensity and ADO value of the γ-rays、extracting the B（M1）/B（E2） value of the coupling bands. Moreover, newresults have been obtained as listed below:1) We extend the four previously known bands to higher spins. Thefavored branch of yrast band was extended up with four gammatransitions; while the unfavored branch was extended to23, which is thehighest observed state of this unfavored branch in odd-odd iodineisotopes;2was added to a weakly populated band which may be thechiral partner of the yrast band; more than3was added to the coupledband based on Ï€g_9/2 νh_11/2configuration; we modified the structure ofone previously known decoupled band and extended this band to higherspin state.2) We establish a branch path between the （14⁺）and（22⁺） parallel to themajor path of the yrast band.3) More than four bands were newly established, and moreover, a lot ofinterband transitions were identified. 4) Based on the γ-γ concidence analysis, a new band was established. Notransition was identified between this band and other known bands.Utilizing the data from the three different reactions and previous¹¹⁶Cd+¹¹B reaction, we verified this new band belongs to¹²⁰I.5) The bandhead based on Ï€g_9/2 νh_11/2configuration was previouslyknown as the T1/2=53min isomeric state, which directly decays intodaughter nucleus¹²⁰Te via β+decay. However the excitation energy of theisomeric state was previously uncertain. In this work, we identifiedseveral transitions between the high spin states of this band and otherbands with known excitation energy, and specified exactly the excitationenergy of the T_1/2=53min isomeric state, which is72keV.6) A new isomeric state in the high excited states of yrast band wasidentified.The experimental results have been discussed as listed below:1) We analyzed and summarized the characters of odd-A nucleus in highspin states（such as the back bending and electromagnetic property etc）.2) Based on Relativity mean field（RMF） theory, we calculated the totalenergies Etot、 triaxial deformation parameters as well as theircorresponding valence nucleon configurations near the proton andneutron femi surface in¹²⁰I.3) Based on total routhian surface（TRS）, we calculated the variation ofnucleus shape with rotational frequencies corresponding to differentconfigurations.4) Based on CSM, we calculated the variation of energies related todifferent quasi-particles along with rotational frequencies, which impliedthe theoretical rotational frequency of band-crossing.5) Based on geometrical model, we calculated the variation ofB（M1）/B（E2） values along with the spins corresponding to differentconfigurations. 6) There has been significant discrepancy in the assignment of the yrastband in¹²⁰I: the yrast band was assigned to negative parity Ï€g_7/2 νh_11/2configuration by Kaul et al, and later was re-assigned to positive parityÏ€h_11/2 νh_11/2configuration by Moon et al. Based on CSM, the comparisonbetween experimental and theoretical B（M1）/B（E2） values, extraction ofalignment angular momentum, we support the Ï€h_11/2 νh_11/2assignment ofyrast band by Moon.7) In previous work, the side band of yrast band was assigned to thecandidate of chiral partner of yrast band without detailed analysis. Bycomparing the alignment character and B（M1）/B（E2） values of the yrastband and side band, as well as the triaxial deformation based on TRScalculation, we suggest the yrast band and side band as the chiral twinbands. The chiral twin bands based on Ï€h_11/2 νh_11/2configuration inneighboring odd-odd nucleus such as Cs、La and Pr have been widelyreported.8) The configuration of each band has been assigned, respectively. Twoâ–³I=2band structures present regular linking mode with weak I=1interband transitions at low spin. Considering this two bands as signaturepartner, the characters are highly similar to oblate bands suggested inodd-A nucleus, which implies the existence of the rare oblate bandcorresponding to the Ï€g_7/2(d_5/2)[413]5/2⁺νh_11/2[514]9/2^-configuration.However, it is not inevitable for the two bands to be the signature partnerssolely because of the weak I=1interband transitions. There might be asecond explanation, namely, the two bands is based onÏ€g_7/2[422]3/2⁺νh_11/2[523]7/2^-and Ï€d_5/2[420]1/2⁺νh_11/2[523]7/2^-configuration at prolate shape, respectively. The assignment ofconfiguration is coincidence with the theoretical CSM calculation, andthe weakâ–³I=1interband transitions due to the highly mixed Ï€g_7/2and Ï€d5/2orbits. Meanwhile, other two bands are assigned to Ï€h_11/2 νd_5/2andÏ€h_11/2 νd_3/2configurations, respectively.9) The new band which couldnot be linked to other bands is assigned toÏ€g_9/2 νd_5/2configuration, which is rare in neighboring doubly oddnucleus. While in¹²⁰I, the Ï€g_9/2and νd_5/2orbits are both close to the Femisurface, which migh result in the rotational band of Ï€g_9/2 νd_5/2configuration.10) Our work approve the previous assignment of the negative-parityband, which is Ï€g_9/2 νh_11/2configuration. The excited energy of thebandhead is just72keV due to the Ï€g_9/2and νh_11/2orbits which lie close tothe Femi surface. On the other hand, the chiral bands based onÏ€g_9/2 νh_11/2configuration in A¹00nuclear region have been widelyreported. Hence, it is significant to investigate the chiral bands based onthe same configuration in A¹20nuclear region. As expected, the sideband was established, which presents the similar characters with theÏ€g_9/2 νh_11/2band. For instance, the tiny energy signature splitting, theregular rotational energy level, similar behavior of alignment angularmomentum and high B（M1）/B（E2） values etc. Although the side band andyrast band present the high B（M1）/B（E2） values, the B（M1）/B（E2） valuesof side band is larger by one order of magnitude. Therefore, we do notconsider this side band is be the chiral partner of the Ï€g_9/2 νh_11/2band,neither is it generated by γ-vibration of the core coupled to Ï€g_9/2 νh_11/2configuration. The Ï€g_9/2 νh_11/2band and the side band are suggested tobe based on the Ï€g_9/2 νh_11/2configuration with markedly differentdeformation or even shape. The RMF calculation manifests that theminimum of potential energy surface is observably distinguishing withthe corresponding energies closed to ground state for Ï€g_9/2 νh_11/2configuration, while the valence nucleons in-core occupy different orbits from one sub shell. In experiment, two bands can be observed withdifferent deformation based on same configuration. Therefore, theexplanation of the twin bands based on Ï€g_9/2 νh_11/2configuration is inaccordance with the RMF theoretical calculation qualitatively.11) A vice-deexcitation path was established between the （14⁺）and（22⁺）parallel to the major path of the yrast band. In major deexcitation path, apair of g_7/2protons and another pair of h_11/2neutrons have been separatedsequentially, while in vice path the sequence of separating is opposite.12) The life of the isomeric state observed in the yrast band is estimatedto be about200ns, which are proposed to be caused by the yrast trap. Thespin and parity of this isomeric state are25and positive parity and theγ transition below the state is suggested to be M3transition.13) In the research the yrast band basedon Ï€h_11/2 νh_11/2configuration andnear-yrast band based on Ï€h_11/2 νd_5/2configuration were observed tohigh spin state, respectively. The TRS calculations present the variationof nucleus shape from prolate（γ=0°） shape to uncollective oblate（γ=+60°）shape, which indicate the occurrence of band termination and fullalignment in theory. The phenomenon has been observed in experimentand the configurations after band termination have been discussed,including the fully aligned states Ï€[h_11/2(g_7/2)²]_23/2 ν[(h_11/2)³]_27/2 andÏ€[h_11/2(g_7/2)²]_23/2 ν[d_5/2(h_11/2)²]_25/2 configuration. Additional excitedstates have been observed after all valance nucleons outside the closed orsemi-closed shell aligned owing to the core breaking which had beenreported in^122,123I.