摘要
本论文利用在束核谱学的实验手段,重点开展了关于A~130区的四个奇A核的高自旋态谱学的实验研究,包括奇质子核123I和129Cs,奇中子核129Xe和131Xe。这些核普遍具有γ-软的特点,原子核的形状深受占据不同Nilsson轨道的价核子影响,并因此展现出丰富的各具特色核结构现象。通过对这些现象的研究,可以加深对这些核自身的核谱学的认识,丰富整个A~130区高自旋态谱学的系统学规律,并有助于核理论的进一步深入和完善。
利用在丹麦玻尔所获得的116Cd(14N,α3n)融合蒸发反应的实验数据,开展了对123I高自旋态的实验研究,拓展了其能级结构信息,并新建立了一条位于高自旋区域的三准粒子集体转动带。这条转动带与原来已知的πg7/2?(νh11/2)2带具有诸多相似的实验谱学特征并共同馈入到低自旋区的扁椭πg7/2转动带中。通过从多角度考察两条三准粒子带的各种实验特征,本论文论证了二者最可能是建立在相同组态即πg7/2?(νh11/2)2上的一对手征双重带。对于手征双重带的研究是目前核结构领域中的前沿热点课题。如果本工作关于123I中新建立的三准粒子带的阐述成立,那么这将是第五例在奇A核中发现手征双重带的存在,与手征双重带在双奇核中的大量发现形成呼应,并对手征双重带的深层理论机制的揭示具有重要的意义。
利用中国原子能院HI-13串列加速器,通过122Sn (11B, 4n)布居了129Cs的高自旋态。细致测量了此核中各种基本实验信息,例如γ射线相对强度和多极性,耦合带的带内B(M1)/B(E2)比值等,并建立了比较丰富的能级纲图。与以往工作相比,拓展了基于πg7/2,πd5/2,πh11/2组态的转动带的能级结构,修正并拓展了一条偶极跃迁带的带结构,并建立了2条新的转动带。首次观测到了πh11/2组态带中非优惠分支即α=+1/2分支的回弯,并将其阐述为h11/2中子的拆对顺排。以往工作把πg7/2转动带中观测到的回弯阐述为一对h11/2中子的拆对顺排;在本工作中,基于推转壳模型关于带交叉的计算、实验B(M1)/B(E2)值与几何模型理论预言值的比较、以及邻近核的系统学规律,将该现象阐述为一对h11/2质子的拆对顺排。基于相似的理由,将偶极跃迁带阐述为基于πg7/2/πd5/2?(νh11/2)2组态的转动带。结合临近核的系统学规律,将新带之中的一条阐述为基于πh11/2组态的γ振动带。对于另外一条新带,分析了其内禀组态的各种可能,包括πg7/2?νg7/2?νh11/2组态和以及与晕带共存但具有不同形变的πh11/2?(νh11/2)2组态等。对奇A的Cs同位素核中πh11/2组态带的旋称劈裂行为进行了系统的总结和比较,观测到了旋称劈裂幅度在h11/2中子带交叉后的系统性减小,指出这种系统学规律的内在机制可能在于拆对h11/2中子的负γ形变扁椭驱动效应,而这样的阐述与总位能面(TRS)模型的理论预言取得了一致。
利用中国原子能院串列加速器以及日本筑波大学物理研究所加速器装置,通过核反应124Sn(10B,p4n)、122Sn(11B,p3n)和124Sn(11B, p3n)生成了奇中子核129Xe和131Xe的激发态。这两个核靠近β稳定线,因而很难通过A>10的入射粒子诱发的融合蒸发反应来布居。以往工作采用A<5的入射粒子对这两个核进行了研究,观测到了一些自旋较低的能态。本工作采用了相对较重的入射粒子,并因此观测到了更高自旋的能态。虽然由于反应截面的限制而未能如愿地观测到次晕的νh11/2组态带,但是已在νh11/2晕带中观测到了带交叉。这一结果把关于这一核区奇中子核带交叉行为的系统学研究从缺中子区延伸到了稳定线附近。结合推转壳模型的理论计算以及临近核的系统学规律,把在两个核中观测到的带交叉阐述为h11/2中子拆对顺排的结果。尽管最低频率的h11/2中子拆对顺排在νh11/2晕带中被阻塞,但是其次频率的h11/2中子拆对顺排依然先于一对h11/2质子的拆对顺排。129,131Xe都非常接近球形核区,四极形变较小,这导致了质子费米面位于h11/2壳层以外的下方,并进一步导致了h11/2质子拆对顺排频率的相对延迟。
此外,本论文中还进行了以下两方面的工作。第一,结合普通物理学和量子力学的基本原理,论证了核物理领域中手征对称性理论的不足,并提出了一些质疑。第二,改进了本课题组的数据分析方法,特别是把matlab工具引入到符合开窗谱的分析中,大大提高了数据分析的效率。
Nuclei in the mass A~130 region are predicted to be soft with respect toγdeformation. In this region, the proton Fermi level lies near the bottom of the h11/2 shell favoring prolate shapes (γ=0°, Lund convention), while the neutron Fermi surface is towards the top of the h11/2 shell, driving the nucleus towards a negativeγshape. The deformation-driving properties of h11/2 neutrons and protons are therefore conflicting and a given nucleus may have either a prolate, oblate or triaxial shape depending on the configuration. It is a hot spot for the studies of the high-spin states which shows rich structural characteristics with the above features. In this thesis, by means of in-beam nuclear spectroscopy experiments, we focus on the studies of four odd-A nuclei, involving the odd-Z 129Cs, 123I and the odd-N 129Xe, 131Xe. All these nuclei areγsoft. They will show rich structural phenomena of various nuclear characteristics, since the shapes of these nuclei would be strongly influenced by the quasi-particles in various Nilsson orbitals. Through the study of these phenomena, you can deepen the understanding of these nuclides, enrich the systematics of nuclear spectroscopy of the whole A ~ 130 high-spin states and contribute to the nuclear theory and improve further.
a). Observation of 3-quasiparticle doublet bands in 123I: possible evidence of chirality
A triaxial rotating nucleus may attain a chiral character, which manifests itself as nearly degenerate twinΔI = 1 bands with the same configuration. Experimental evidence of chirality was first found in a number of A~130, 104 and 190 odd-odd nuclei , where a high-j valence proton particle and high-j valence neutron hole (or vice versa) are coupled to the triaxially deformed core, resulting in the so-called aplanar solutions : the particlelike and holelike orbitals align their angular momenta along the short and long axes, respectively, and the collective angular momentum of the core orients along the intermediate axis. Thus the vector sum, i.e. the total angular momentum lies outside any of the three principal planes determined by the triaxial core deformation, and reversing the direction of the component of the angular momentum on the intermediate axis results in a change of the chirality from a left-handed system to a right-handed system (or vice versa). In a similar manner, it can be expected that chirality may also occur in multi-qusiparticle configurations of triaxially deformed even-even or odd-A nuclei if, with increasing spin, the additional rotationally aligned nucleons have opposite particle/hole character in comparison to the original valence nucleon(s). Indeed, candidate chiral twin bands do have been proposed in the even-even 136Nd and in several odd-A nuclei, such as 135Nd , 103,105Rh and 125Cs.
High spin states of the odd-proton nucleus 123I have been investigated via the 116Cd (14N,α3n) reaction at 65 MeV and a newΔI = 1 three-quasiparticle band has been identified (marked as A for the convenience of discussion).
Band A comprises of two strongly coupled decay sequences and decays into the previously knownπg7/2[404]7/2+ band (marked B for short), to which we assigned the oblateπg7/2[404]7/2+ configuration in our previous study. Several links between band A and theπg7/2? (νh11/2)2 band (marked C for short). To assign the spin/parity of levels in band A, ADO ratios have been extracted for the sufficiently intense transitions linking band A to bands C and B and transitions in band A. These ratios enable us make the spin/parity assignment for band A. Further, the assignment is also strongly supported by the observed decay pattern from band A to bands C and B.
To discuss the possible configurations of band A, its alignment and routhian have been extracted according to the standard Cranked Shell Model. It is seen that bands C and A possess almost identical alignment (~ 9h) and cross band B at almost identical frequency (0.35 MeV for bands C-B crossing and 0.36 MeV for bands A-B crossing), which implies that band A is a 3-quasiparticle band like band C. Previous extensive experimental and theoretical investigations have reveal that the (νh11/2)2,νh11/2?νg7/2 and (πh11/2)2 excitations are three typical mechanisms for observed band crossings in 123I region. Within an isotonic chain, with decreasing proton number, the (νh11/2)2 and (πh11/2)2 crossing frequencies show decreasing and increasing trends, respectively. For the Z=53 iodine isotopes, because the proton Fermi surface lies well below the lowest orbital of theπh11/2 multiplet, extra energy is needed for the excitation of a pair of h11/2 protons as compared with Z≥56 nuclei, for which the proton Fermi surface lies on or above the lowestπh11/2 orbital. Therefore the (πh11/2)2 alignment is expected to occur at a notably higher frequency in iodine nuclei. Indeed, CSM calculations predict the (πh11/2)2 alignment in 119I to occur at hω≥0.5 MeV. In contrast, the (νh11/2)2 alignment is predicted to occur at hω≈0.36 MeV. We have also performed CSM calculations for 123I, and arrived at the same conclusion as made by Tomanen et al. The observed crossing frequency of 0.36 MeV for band 3 is significantly lower than what is expected for the (πh11/2)2 alignment, we therefore refrain from bringing the (πh11/2)2 excitation into the composition of the possible configurations of band A. If, instead, theνh11/2?νg7/2 configuration is involved in band A, the h11/2 quasiproton has to be introduced to act as the third valence nucleon to ensure a positive parity for band A. Under this assumption, band A has no valence nucleon in common with bands B and C, which seems inconsistent with the observation that band A feeds strongly on bands B and C and the non-observation of any connections between band A and the previously knownπh11/2 band. In addition, band A shows properties that are quite different from what is observed in theπh11/2?νh11/2?νg7/2 bands reported in some nearby nuclei, such as in 125I, where theπh11/2?νh11/2?νg7/2 band decays into theπh11/2 band and consists of only dipole transitions without quadrupole transitions observed. Therefore, we discard theπh11/2?νh11/2?νg7/2 assignment for band A. It seems tempting to assign the oblateπd5/2[413]5/2+?(νh11/2)2 configuration to band A because theπd5/2[413]5/2+ andπg7/2[404]7/2+ orbitals lie very close in energy and the admixture of the two orbitals may explain the observed strong feeding of band A on bands C and B. However, this scenario can not explain the non-observation of band A to theπd5/2[413]5/2+ band itself. It is noted that theπd5/2[413]5/2+ band had been observed up to 23/2+ and ~3.3 MeV, sufficiently high for observing the possible decays from band A to theπd5/2[413]5/2+ band. Furthermore, theπd5/2?(νh11/2)2 assignment for band A is not supported by a comparison of measured B(M1)/B(E2) ratios with predictions from the geometrical model. The measured ratios, are about 2μN2e?2b?2, significantly lower than the predictions ( ~9μN2e?2b?2) for theπd5/2?(νh11/2)2 configuration. Therefore, theπd5/2?(νh11/2)2 assignment is not considered to be reasonable for band A. Based on similar considerations, theπg9/2?(νh11/2)2 assignment has also been excluded.
The recently proposed concept of chiral bands provide another possible interpretation for band A. Band A decays into bands C and B through several transitions, which suggests these bands may have similar configurations with a large overlap in their wave functions. Also bands C and A cross band B almost at an identical rotational frequency and with identical alignment gain, which may suggest bands 1 and 3 contain the same pair of aligned quasiparticles. Therefore, bands A and C are probably a pair of chiral bands based on the same configuration, namely, theπg7/2?(νh11/2)2 configuration. To further check this idea, we show a comparison of the measured B(M1)/B(E2) ratios, excitation energy and energy staggering of band 3 with those of band C as a functions of spin. Three features are remarkable: 1) the magnitudes of measured B(M1)/B(E2) ratios of the two bands are similar; 2) the energy separation between sates with the same spin in the two bands is about only 180 keV and remains nearly constant in the observed spin range; and 3) the two bands show similar energy staggering amplitude and the same staggering phase with the variation of spin. All these features are consistent with the expectations for chiral doublet bands. It is seen that a better agreement between the theory and experiment can be obtained atγ= ?30°than atγ= ?60°, which may implies an increase of the triaxial deformation after the (νh11/2)2 alignment. At near oblate deformation, the proton Fermi surface lies at the bottom of the g7/2 subshell while the neutron Fermi surface lies at the middle of the h11/2 subshell. Thus, it is possible that the important conditions, i.e. triaxial deformation nearγ≈±30°and the existence of high-j particle(s) and high-j hole(s), for the occurrence of chirality are met in 123I.
b). Band structures of the nucleus 129Cs
Excited states in 129Cs were populated via the 122Sn (11B, 4n) fusion-evaporation reaction at beam energies of 55 and 60 MeV. The 11B beam was provided by the HI-13 tandem accelerator at CIAE in Beijing. In order to obtain information on the multipolarity ofγ-transitions, angular correlations of theγ-rays were evaluated by using the directional correlation of orientation (DCO) methodology.
Most of the previously known bands have been extended to higher spins, and two new bands have been identified. The ordering of theγ-rays has been determined by coincidence relationships and relative intensities. Spins and parities of the levels were either adopted from previous work or assigned on the basis of the present measurements of DCO ratios together with the systematics from neighboring nuclei. In comparison to the earlier work, two new structures have been extracted from the present data; the previously observed unfavoredπg7/2 band and the favored and unfavoredπh11/2 bands have been pushed up by 6h, 4h and 4h, respectively; two new states at excitation energies of 2980 and 3158 keV deexciting into both signature ofπd5/2 (through the 312.5 keV transition to the unfavored signature and the 206.5 keV transition to the favored signature band) have been established; the 653 keV transition tentatively established at the top of favoredπg7/2 band is confirmed. Furthermore, due to observations of several new cross-over as well as cascade transitions, the structure of band 1 has been changed and pushed up by 4h. To facilitate discussions, experimental alignments and Routhians have been extracted for the observed bands according to the prescription of Cranked Shell Model (CSM) .
With the extension established from the present work, the unfavoredπh11/2 band is observed to experience an upbend at hω= 0.42 MeV, slightly lower than the band-crossing frequency of 0.45 MeV observed in favored signature band. Since the first h11/2 proton crossing is blocked both in the two bands, the upbends at similar frequency must be due to the alignment of an h11/2 neutron pair. We have also performed CSM calculations to investigate the possible band crossings in 129Cs. Such calculations indicate that the first h11/2 proton and h11/2 neutron crossings occur at similar rotational frequency, viz. 0.35～0.45 MeV. Thus the interpretation for the upbends in the two h11/2 band is compatible with CSM calculations. However, it should be noted that the calculated crossing frequency depends to some extent on the choice of parameters, such as quadrupole deformation (ε2) and pairing gap (Δ). In cases that two different crossings occur at close frequency, other arguments, especially reliable systematics of neighboring nuclei, become more important for finding out the origin of observed band crossings. In the present case, the proton origin for upbends in the two signature partners ofπh11/2 band can be excluded in the light of the blocking argument.
An onset of a band crossing was observed in the two bands of the previously establishedπg7/2 band. Based on comparison with CSM calculations, which predict the neutron alignment to come earlier than the proton alignment, the authors attributed the upbends in the two bands to the h11/2 neutron alignment. The twoπg7/2 bands show decoupled structure at high spins, indicating low B(M1)/B(E2) values. If the two are of aπg7/2?(νh11/2)2 character at high spins, B(M1)/B(E2) values predicted from the geometrical model are at least～2μN2/e2b2. Such bands are expected to show coupled structure with relatively strongΔI =1 cascade transitions linking the twoΔI =2 sequences. Indeed,πg7/2?(νh11/2)2 bands in the neighboring 127Cs and 131Cs both show strongly coupled structure consistent with the above expectation. Therefore, the h11/2 neutron alignment might not be a proper interpretation for the upbends in the two signature branches. Instead, the h11/2 proton alignment is considered to be the most reasonable interpretation. Bands with the same configuration, i.e. theπg7/2?(πh11/2)2 configuration, and with similar decoupled structure have also been reported in 131Cs. B(M1)/B(E2) values predicted from the geometrical model for this configuration are relatively low (～0.3μN2/e2b2), consistent with the un-observation ofΔI =1 cascade transitions in the present as well as previous work. The observed crossing frequency of～0.39 MeV is also in agreement with CSM calculations since the (πh11/2)2 alignment is not blocked in theπg7/2 configuration.
Because only four states were unambiguously established in band 1, no definite conclusion was drawn for its configuration in the previous work. With the extension established in the present work, band 1 is seen to exhibit a typical collective structure which decays into theπd5/2 andπg7/2 bands. The alignment of band 1 is about 9 h and remains nearly constant up to the highest observed state, suggesting that band 1 may be a three-quasiparticle band arising from a crossing of the admixedπg7/2/πd5/2 configuration with two rotationally aligned quasiparticles. Band 1 crosses theπg7/2 andπd5/2 bands at a frequency of ~0.35 MeV, which is slightly lower than the (πh11/2)2 crossing frequency observed inπg7/2 bands. Meanwhile, band 1 comprises strongΔI = 1 dipole transitions along with weakΔI = 2 transitions, indicating high B(M1)/B(E2) values. All these characters are consistent with the expectation as mentioned in the preceding passage for the h11/2 neutron alignment. We thus assign band 1 to theπd5/2/πg7/2?(νh11/2)2 configuration.
A new band established from the present work comprises aΔI = 2 sequence and decays into theπh11/2 band. Very similar structures have been systematically observed in the other odd-A Cs isotopes and interpreted as aπh11/2 quasiparticle coupled toγvibration of the core.γvibrational band has also been observed in the even-even neighbors. The systematic occurrence of suchγvibrational band is further evidence of theγ-softeness in nuclei of the A~130 region.
The other rotation-like band newly established from the present work comprises only transitions ofΔI = 1 dipole character. This band decays into the favored signature of theπh11/2 configuration. Both the linking pattern and the measured DCO ratios for the linking transitions from the new band to favored signature of band 7 determine negative parity for this band. Large initial alignment and high excitation energy of the bandhead indicate that the band must be a three-quasiparticle band. A band with similar features has also been observed in the neighboring 131Cs by two different research groups, and theπg7/2?νg7/2?νh11/2 andπh11/2?νd3/2?νg7/2 configurations were assigned by Kumar et al and by Sihotra et al., respectively. The observation that the new band decays into theπh11/2 rather than theπg7/2 band seems to favor the latter assignment. However, the involvement of theνd3/2 orbital is not plausible due to its low-j character. An alternative interpretation for the new band might be aπh11/2?(νh11/2)2 band coexisting with the yrastπh11/2?(νh11/2)2 band but having different deformation. Further investigation is necessary for a better understanding of this band. The nucleus 129Cs lies in theγ-soft A~130 region. At low spins, the nuclear shape is mainly influenced by the valence quasiproton, in particular the high-j h11/2 proton. At higher frequencies, one needs to consider also the shape driving effects of additional rotationally aligned quasiparticels. The shape driving properties of the high-j neutrons and protons are well known and have been described in the first paragraph of this paper. To investigate the possible shape evolution, the signature splitting, which is known to be sensitive to theγdeformation, has been extracted for theπh11/2 configuration in 129Cs. A systematic decrease of signature splitting with increasing spin is seen. Total-Routhian-Surface calculations have been performed previously for theπh11/2 configuration of 129Cs, which predictγ≈22°and≈-40°before and after the (νh11/2)2 alignment, respectively. Thus, the observed decrease of signature splitting is consistent with the predicted picture that a change from near-prolate to near-oblate shape is induced due to the h11/2 neutron alignment.
c). The observation of the neutron alignment in 129Xe and 131Xe
The high spin states of the 129Xe and 131Xe have been populated through 124Sn(10B,p4n)129Xe,124Sn(11B,p4n) 131Xe,122Sn(11B,p3n) 129Xe reactions. Both the two nuclei are difficult to populate through the fusion evaporation reactions with A>10 particles induced, as they are near theβstability. The A<5 beam had been used to study the low spin states of the two nuclei previously. A heavier particle have been used to populate the high-spin states in present work. Though we failed to observe the yrare bands which we expected to observe in both theνh11/2 band of the two nuclei due to the restriction of the cross section, we have observed the band crossings in the yrastνh11/2 band. This result extends the systematical studies of the band crossings in the odd N nuclei from the lack of neutron nuclide to theβstability region. The band crossings in both nucleus are assigned to (νh11/2)2 origin based on the Cranked Shell Model calculations and the systematics in neighbor nuclei. Though the (νh11/2)2 band crossing frequencies are delayed in theνh11/2 yrast band due to the blocking effect, the (νh11/2)2 still aligns prior to the (πh11/2)2. The mechanism of the relatively delayed (πh11/2)2 alignment may mainly due to the small quadruple deformation of the two nuclei which leads that the proton Fermi surface lies under the lowest orbital of the h11/2 subshell, and the alignment needs extra energy, hence delaying the band crossing frequency.
d). The improving of the methodology of the data analysis
Two improvements have been made for the method of the nuclear spectrum analysis: the first, a matlab program have been designed to realize the 3-D analysis of the experimental spectrum. This improvement leads to a more visual and compact analysis of the experimental spectrum. The second, the detector efficiency have been directly added to the experimental matrix. This makes the intensity balance analysis and the measurements of the intensities in coincidence spectrum more easily and accurately.
e). Symmetry principle and the chiral phenomena
In the course of symmetry analysis for the nuclear structure, we found that there are some confusions in the description of the symmetry in many nuclear and quantum mechanics text books. A more explicit classification of the definition has been made in present work.
One of the most interesting phenamena, the chirality have been analyzed in the framework of basic symmetry. Through the systematic studies for the chiral doublets and the interpretations of various theories, three conclutions can be drawn: first, because of the insufficiency for the interpretations for the non-degeneracy and the linking transitions of the chiral doublet bands in the experimental observations, the traditional chiral picture may be not reasonable explanation for the so called chiral doublet bands observed in experiment; second, compared to the traditional chiral picture, the atypical configuration composed of one atypical particle (or hole) and typical hole (or particle ) coupled to the triaxial core could make a chiral doublet band more easily; third, any existing arguments could not interpret the degenerate doublet bands in the 128Cs nucleus.
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