【作者】 刘弓冶；

【导师】 马英君；

【作者基本信息】 吉林大学 ， 粒子物理与原子核物理， 2012， 博士

【摘要】 质量数A-120是原子核高自旋态研究的热点核区。本论文在这个核区选择了一个核作为主要研究对象,以期达到对原子核高自旋态谱学更为全面的认识。我们在中国原子能科学研究院HI-13串列加速器上进行了融合蒸发反应116Cd(11B,5n)E=68 MeV布居了122I的高自旋态并进行了研究。奇奇核122I位于A～120过渡区,即是从原子序数Z=50的50Sn近球形核向大形变稀土区核La(Z=57)和Ce(Z=58)过渡的核素。处在这一核区的原子核,具有γ形变软的特点。其质子费米面位于h11/2子壳层的下部,而中子费米面一般位于h11/2子壳层的中部。势能面(potential-energy surface or PES)和推转壳模型的计算表明,占据高j值的πh11/2和vh11/2单粒子轨道的价核子对核芯的形变自由度有明显的极化效应。位于h11/2子壳层下部的价核子对核具有长椭形变(γ≈0°)的形变驱动作用,而位于h11/2子壳层中上部的价核子对核具有扁椭形变(γ≈-60°)的形变驱动作用。两种相反的形变驱动作用形成了十分复杂的集体带结构,使原子核展现出了丰富的高自旋态谱学。如在与122I邻近的奇A核113-123I同位素核中都观测到了基于low-Kπh11/2组态带的带终结现象；在119-125I中观测到不同组态的长椭和扁椭的形状共存现象。此外,基于相同组态(πh11/2组态)而K值不同(K=1/2和K=9/2)的转动带分别以不同形状(低K长椭,高K扁椭)共存于同一原子核中的现象在119,121I等核中也有所报道。相对于奇A碘核而言,Z=53的奇奇核中也同样蕴含着大量热门的物理现象,如在118I发现的基于空穴-粒子πg9/2(?)vh11/2组态的候选手征双重带就是目前高自旋态研究的一个热点话题,这也是首次在I的同位素中报道πg9/2(?)vh11/2组态的手征双重带现象。在奇奇核116-120I中也观测到了带终结现象,这为实验中观测到带终结后的全顺排态乃至核芯激发现象提供了基础,并且也为验证和进一步完善原子核壳模型起到重要的作用。此外,奇奇核116-124I中还存在着一个显著的特点,其晕带的组态为准质子、准中子均位于h11/2这个高j独特宇称(Unique-parity)子壳层的πh11/2(?)vh11/2组态带。但对于122I晕带组态的指定却一直存在着很大的分歧,早先的研究将122I晕带的组态指定为负宇称的πg7/2/d5/2(?)vh11/2组态,而在最近的研究中将晕带的组态指定为正宇称πh11/2(?)vh11/2组态,如此大相径庭的结论更体现出我们有必要对122I进行研究。此外,在该区原子核存在大量同质异能态,比如奇奇核120-124I中系统性的存在7-,8-高K同质异能态。对同质异能态和建立在其上的能态或转动带的研究为我们认识原子核结构也提供了一个重要途径。总的来说,A～120区是一个蕴含丰富信息的核区。本工作主要获得以下研究结果：1.对奇奇核122I的高自旋态进行了在束γ谱学实验研究,利用符合谱拉门方法,新发现了30多条γ跃迁,建立了迄今为止最为丰富的122I能级纲图,其中包含基于不同组态的8条带结构。在确认了前人原有的研究成果之上将晕带的自旋推高到31h,次晕带的自旋推高到30h,其它各个带也被不同程度提高。2.结合推转壳模型,借助B(M1)/B(E2)比值的实验结果与几何模型理论预期的比较,以及顺排角动量的提取,激发能系统学的研究和旋称劈裂的特点对各带结构的组态进行了指定。研究结果否定了之前E.S.Paul以及H.Kaur等人对晕带指定的π(g7/2/d5/2)(?)vh11/2组态,而支持了C.B.-Moon等人给出的πh11/2(?)vh11/2组态。对于其它各带,本工作首次进行了它们的组态的讨论和指定。将次晕带带2指定为πh11/2[550]1/2(?)vd5(?)2[402]5/2-组态；指定带3的组态为πd5/2[420]1/2+(?)vh11/2[523]7/2-；带4和带6组态为πg7/2[422]3/2+(?)vh11/2[523]7/2-；带5组态为πh11/2[550]1/2-(?)vg7/2[404]7/2+；带7带8为一对基于πg9/2[404]9/2+(?)vh11/2[523]7/2-组态的伙伴带3.在122I晕带(πh11/2(?)vh11/2组态带)和带2(πh11/2(?)vd5/2组态带)之间建立了多条连接跃迁,经过对多极性的分析,确定了其△I=1的性质,又因为带1和带2拥有相反的宇称,因此我们判断这几条连接跃迁为E1跃迁。这个发现为122I存在的八极关联现象提供了实验证据。4.对122I晕带与带2存在的带终结现象进行了讨论和研究,并指定了发生带终结后各能级的内禀组态。当Iπ=(20+)时,晕带的内禀组态为π[h11/2(g7/2)2]23/2-(?)v[(h11/2)3]17/2-。满壳外的3个质子和半满壳外的3个中子发生顺排,但这3个中子并没有提供最大角动量。当Iπ=(25+)时,这3个vh11/2价中子发生顺排并提供了最大角动量27/2-h,因此,此时的内禀组态应为π[h11/2(g7/2)2]23/2-(?)v[(h11/2)3]27/2-。随着自旋的继续增加,当Iπ达到(31+)时,满壳外的3个质子和半满壳外的全部5个中子都发生顺排,即全顺排现象,并且均提供了最大的角动量,相应的内禀组态为π[h11/2(g7/2)2]23/2-(?)v[(h11/2)3(g7/2)2]39/2-通过针对另一带结构πh11/2(?)vd5/2组态带的分析,我们指定该带在Iπ=(26-)的位置发生了带终结现象,并尝试性指定Iπ(26-)和Iπ(30-)时内禀组态分别为π[h11/2(g7/2)2]23/2-(?)v[(h11/2)4d5/2]29/2+和π[h11/2(g7/2)62]23/2-(?)v[(h11/2)164d5/2]37/2+。两者虽拥有相同的准粒子组态,但是各准粒子提供的角动量不同。在Iπ=(26-)时,其中中子(不排除是质子)尚未充分顺排,未提供最大可能的单粒子角动量。而当Iπ=(30-)时8个价核子均提供了最大的角动量,发生全顺排现象。5.对122I中存在的八极关联现象以及该核区其它核素出现的这种现象进行了对比研究和讨论,并提取了122I中连接跃迁的B(E1)实验值,发现E1连接跃迁的强度增强了100倍,达到了10-4W.u.的数量级,从而支持了πh11/2(?)vh11/2带与πh11/2(?)vd5/2带间存在八极关联的阐述。6.对120核区奇奇核πh11/2(?)vh11/2组态带进行了能级系统学的比较研究,发现其在能级结构上存在的一些规律特征,指出其能级间隔的变化是因为原子核形变导致转动惯量变化而造成的。此外还对I奇奇核πh11/2(?)vh11/2组态带发生的旋称反转现象进行了系统地研究,发现由于随着转动频率的增加,其内部结构发生剧烈变化从而导致其并不符合Z>53核区普遍存在的旋称反转规律。我们还通过与邻近的奇质量核Te和I的h11/2组态带进行对比,将双奇核120-126Iπgh11/2(?)vh11/2组态带中16+(及15+)态的异常特性(激发能显著低于转动谱预期)阐述为一对πg7/2质子激发的结果。并计算了πh11/2(?)vh11/2(?)π(g7/2)2组态下的B(M1)/B(E2)理论值,计算结果与实验取得满意符合。也恰恰是由于π(g7/2)2质子激发,使得双奇I同位素核πh11/2(?)vh11/2带表现出与Z>53区域不同的旋称劈裂特性。7.对I奇奇核中πg9/2(?)vh11/2组态带进行了系统学研究。πg9/2(?)vh11/2组态带是一个高K强耦合带,并表现出典型的转动谱特征。价质子πg9/2处在高j轨道的上半壳层表现出空穴性,而价中子vh11/2处在高j轨道的下半壳层表现出粒子性,这为手征对称性破缺的发生提供了一项必要条件。这使得他们具有相反的形状驱动效应,这对原子核的形状与能级结构有很大的影响。并为原子核中产生良好的三轴形变提供了可能性。因此我们着重对I奇奇核的πg9/2(?)vh11/2组态带存在的手征对称性进行了系统的比较研究。从能级简并度、旋称劈裂、B(M1)/B(E2)以及计算三轴形变等多角度进行了讨论,初步判定122I中带7及其伴带带8很可能为一对基于πg9/2(?)vh11/2组态的手征双重带。8.基于总位能面模型(TRS, total Routhian Surface)对122I中各种不同组态进行了理论计算,从而展示了各种不同组态的核形变及其随转动频率的变化。总体上各组态在高自旋区都被预期发生y=+60°的形变,并将因此而发生带终结效应。这样的理论预期与前述πh11/2(?)vh11/2和πh11/2(?)vd5/2组态带的实验观测取得了符合。9.在推转壳模型框架下(Cranked Shell Model)对所建立各带进行了顺排行为的分析,并从理论上进行了带交叉频率的计算。基于二者的比较,对各带的顺排行为及带交叉机制进行了阐述。更多还原

【Abstract】 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-Kπh_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 basedonπg_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 theπh_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 parityπg7/2/d5/2（?）vn11/2 configuration in the previous work, while in the recent research is assigned to positive parityπg11/2（?）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 isπh11/2)[550]1/2（?）vh11/2（α=±1/2）[523]7/2-.In the mean time, we assigned the second populated negative-parity band asπh11/2[550]1/2-（?）vd5/2[402]5/2- configuration, and rotation band 3 asπd5/2[420]1/2+（?）vh/2[523]7/2- and band 4 and 6 asπg7/2[422]3/2+（?）vh11/2[523]7/2-configuration, and we also assigned the band 5 asπg11/2[550]1/2-（?）g7/2[404]7/2+ configurations. The configuration of band 7 and 8 had been assigned toπg9/2[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 Iπ=（21+） is based on the [πh11/2（πg7/2）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 Iπ=（25+） state can be generated from [πh11/2（πg7/2）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 Iπ=31+) state can be generated from the [πh11/2（πg7/2）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 Iπ=（26-）, through the TRS calculation and experimental informations. We tentatively assgined the configuration of IπI=（26-） and Iπ（30-） states asπ[h11/2（g7/2）2]23/2-（?）v[h11/2)4d5/2]29/2 andπ[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 Iπ=（26-） state, while the spin of the Iπ=（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 andπh11/2（?）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 ofπh11/2（?）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 onπh11/2（?）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 theπh11/2 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 theπh11/2（?）vh11/2 configuration. We also calculated the B（M1）/B（E2） theory prediction value ofπh11/2（?）vh11/2（?）π（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 ofπ（g7/2）2 protons.7. The energy systemics ofπg9/2（?）vh11/2 bands in odd-odd I were studied. The bands based on theπg9/2（?）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=1πg9/2（?）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 theπg9/2（?）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 ofπh11/2（?）vh11/2 andπg11/2（?）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.更多还原