~(114)In能级结构和~(174)Os形状演化的研究
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摘要
本论文主要介绍了三方面的研究结果。第一部分是114In能级纲图的建立和磁转动带的研究；第二部分是利用延迟符合技术测量174Os的第一个2+态寿命，并对其异常高的跃迁几率和低自旋的形状相变进行了研究讨论；第三部分是利用多普勒线移衰减法测量174Os的高自旋态的寿命，研究讨论了高自旋离心力和科里奥利力对原子核形状的影响。
一、奇奇核~(114)In能级结构的研究：
A～110核区丰富的核结构现象一直得到核结构专家的广泛关注和研究。尤其近些年，人们倾入了大量的精力和时间寻找这个核区新奇转动机制的带结构“磁转动带”（包括反磁转动带）。这种新的转动机制已经通过倾斜轴推转壳模型（TAC）得出合理的解释。形变核的转动是大量配对的核子参与的集体转动，科里奥利力的作用是拆散核子对，使核子对角动量朝向着转动角动量方向靠拢。现在的磁转动参与的是几个未配对的高角动量的粒子或空穴，他们不是一步达到全顺排，而是逐步增加，直到带终结。由于未配对的高角动量的粒子或空穴逐步靠拢，极其像一把剪刀，因此又被称为“剪刀带”。在此核区In同位素核的中子、质子费米面分别位于高Ω轨道的g_(9/2)和低Ω轨道的h_(11/2)。在奇奇核的~（106-112）In中已有大量磁转动带的报道；在奇A的In核中，稳定的~（113）In核也已经发现了若干磁转动带。因此，在中子数N=65接近半满壳处的丰中子核~（114）In中会不会依然有磁转动带出现，或更加丰富的核结构现象，这极大的引起了我们的兴趣。另外，到目前为止还没有关于~（114）In高自旋态的有关报道。基于上述原因我们选择~（114）In做为研究的目标核。
~（114）In的高自旋态是通过~（110）Pd(~7Li,3n)~（114）In布居的，`7Li束流是由中国原子能科学研究院HI-13串列加速器提供的。靶为厚度为2.4mg/cm2的金属110Pd，为了消除多普勒效应的影响，靶镀在了厚度为10.55mg/cm~2的金属Au衬底上。探测系统是由12台带反康的高纯锗探测器和2台小平面探测器组成。其中5个探测器放置在与束流夹角分别为900的方向，三个在150°，42°和140°方向各有两个，34°和127°方向各一个。实验前我们用PACE程序计算了反应的产物和截面。基于(?))ACE的计算结果,我们选择了22、24、26、28、32MeV五个能量点做实验激发函数,发现26MeV时114In的反应截面最大。综合考虑,我们最终选择26和28MeV两个能量点各进行了40个小时实验。在束实验前我们利用133Ba和152Eu的混合标准源测量了两个小时左右的能谱,对γ射线能量以及探测器的效率进行了刻度。实验总共记录了3.5×107个二重符合事件。离线数据处理反演了一个对称化的矩阵和一个非对称化的DCO矩阵,其中DCO矩阵是由42°的探测器和90°的探测器组成,来区分γ射线的跃迁极性。
利用RADWARE和GASWARE软件对上述两维矩阵进行开窗谱分析,建立了114In新的能级纲图。共找到约50条新γ跃迁和30多个新的能级,自旋和激发能分别推高至约16h和5MeV。首次发现114In的高自旋态信息,建立了三条新的转动带,极大的拓展了114In的能级纲图。对新发现的能级和带进行了组态指定。初步认定低自旋10+态为,G9/2-1(?) v(d5/2/g7/2)(h11/2)2的多粒子组态。其他的三个新发现的磁偶极转动带2、3和4也分别尝试性地给出了组态结构：πg9/2-1(?)(d5/2/g7/2)(h11/2)2、πg9/2-1(?) v(h11/2)3、πg9/2-1(?)v(d5/2/g7/2)(h11/2)2。在晕带中发现了回弯现象,经系统学比较和TAC-RMF理论计算认定为一对97/2/d5/2中子顺排造成的。其中正宇称带2是由非常强的规则的M1跃迁、缺失的带内E2跃迁、没有旋称劈裂等实验现象,显示明显的磁转动带特征。TAC-RMF理论计算表明带内跃迁强度B(M1)随着自旋的增加而减小,带内跃迁B(E2)随着自旋增加呈上升趋势,这与磁转动带特征符合的很好。此外,相互作用的半经验的剪刀机制模型计算表明114In核中带2的价质子与中子之间的相互作用值约750keV,这与此核区的剪刀带的系统学非常一致。
二、偶偶核174Os的21+态的异常高的跃迁几率和临界点对称性的研究
原子核能级的跃迁机率最能反映核内禀属性,因此是核结构研究最重要的内容之一。偶-偶核的第一个21+激发态的寿命为核形状的四极形变提供重要信息。根据实验测定的B(E2;21+→01+)跃迁几率值可以用来与各种不同模型的理论计算进行比较。系统学研究偶偶核从球形振子到轴对称转子的特征可以对了解核结构及其随价核子的演化具有重要意义,因此A～160-190核区原子核的21+态的寿命测量一直吸引着核物理学家的广泛关注。IBA模型预期当中子数从N=82闭壳趋向N=82和126两闭壳的中间值时(N=104),跃迁几率B(E2;21+→01+)随之增大,并预言在中间壳时具有最大值。当从中间壳趋向N=126闭壳时,B(E2;21+→01+)值逐渐减小。然而,必须指出的是,Os同位素的跃迁几率B(E2;21+→01+)并不遵循这一简洁明确的系统性趋势,违背了当中子数朝向N=82闭壳时B(E2;21+→01+)值缓慢减小这一预言,而最大值出现在了中子数N=98处。这种异常现象究竟是源自真正的结构效应,还是仅仅因为实验本身的问题所引起,至今尚不清楚。不过对于174Os来说,B(E2;21+→01+)值的实验误差与相邻Os核的这些量的不确定度相比较,还是相当大的。另一方面,最近的实验证明,176,178Os低位态具有上佳的X(5)临界点对称性。因此,有必要对174Os进行能谱和跃迁几率的精确测量,以了解174Os是否延续了X(5)对称性,以致X(5)结构效应引起174Os形状演化趋势的异常。
最近,科隆大学和Agone实验室发展了一项利用新型闪烁体探测器LaBr3(Ce)测量核能级寿命的实验方法。这种方法是结合能量分辨超好的HPGe探测器与时间和能量分辨都较好的LaBr3(Ce)探测器对在束γ谱学实验中原子核能级寿命的测量。这种方法是测量HPGe和LaBr3(Ce)之间的三重γ符合事件,能量分辨超好的HPGe探测器选择预测能级所在的能级分支,然后用能量和时间分辨都比较好的LaBr3(Ce)探测器开门提取时间谱。这种方法能够测量几十个皮秒以上的寿命。我们用这种新的快时间的延迟符合技术测量了174Os的2+态的能级寿命。由于174Os的2+态的能级寿命接近反冲符合方法的测量上限值,前人用反冲法(RDM)测量得到的寿命值的误差较大。我们利用LaBr3(Ce)探测器的延迟符合技术是一种直接测量寿命的技术手段,对于长寿命的核能级可以得到精确的值。
174Os的高自旋态是通过150Sm(28Si,4n)174OS布居的,28Si束流是由中国原子能科学研究院HI-13串列加速器提供的。靶为厚度为0.9mg/cm2的金属150Sm,同时为了测量高自旋态能级的寿命,靶镀在12.5mg/cm2的金属Pb衬底上。整个探测阵列是由11台带反康的高纯锗探测器和7台LaBr3(Ce)探测器组成的。7台LaBr3(Ce)环绕着靶室垂直于束流摆放。在束实验之前我们用标准的放射源(22Na,60Co,133Ba, and152Eu)进行了测量,以使整个探测系统的能量和时间特征达到最好。
最后得到的174Os的21+态寿命结果为τ=513(20)ps,与前人测量的结果值基本一致,但测量精度大大提高。通过比较Os同位素跃迁几率B(E2;21+→01+)值随着中子的变化趋势,发现174Os的21+态的B(E2;21+→01+)值依然最大,甚至比半壳180Os的21+态的B(E2;21+→01+)值还大。通过实验测定的寿命提取的跃迁几率B(E2;21+→01+)值与各种不同模型的理论计算进行比较,发现目前没有一个模型能够完全预测这种发展的趋势。通过与邻近的核也做了系统学比较,发现w同位素与Os同位素具有相同的趋势,但在更轻的Yb和Hf同位素却不同,最大跃迁几率值出现在中子半满壳附近。因此,我们初步建议Os和w同位素B(E2;21+→01+)值随着中子演化异常的原因为N=98的中子壳隙和大形变的质子h9/2闯入轨道共同的影响。
通过174Os的实验信息与X(5)模型预测比较,发现174Os核相比于临界点对称性X(5)核更靠近球形振子核区。而最近发展的描述振子与X(5)核过渡的X(5)-β2模型能够合理的描述174Os核的能级特征,但是X(5)-β2模型预测跃迁几率B(E2)值与实验的B(E2)值相差却比较大。两参数的IBA-1模型计算能够很好的再现实验的能谱和B(E2)值。因为在IBA-1模型中包含了参数χ值(γ自由度),因此本部分的工作建议174Os核并非完全的轴对称的转子核,而是包含γ软的形变核。
三、多普勒位移衰减法测量174Os高自旋态的寿命
测量偶偶核集体转动带高自旋态的能级寿命(达到I=20h以上)能够为我们研究由于科里奥利力和离心力对原子核形状变化的影响。在一些中子数N=104、106和108的核的高自旋态发现了集体性突然下降的趋势,比如在182-186Pt晕带中,自旋I≥10h时电四极矩突然下降。这主要是因为这些核的中子费米面位于i13/2壳的上部,在I-10h时中子发生拆对顺排,此时的原子核的形状由低自旋的近似轴对称长椭核到三轴形变核(正的γ形变值)。
在第二部分的实验当中,我们同时利用多普勒线移衰减法(DSAM)测量了174Os晕带12+—22+态的能级寿命。理论预测N-96,98的原子核的中子费米面位于i13/2壳的中部,这些中子拆对顺排会造成负的γ形变值,因此集体性不会大幅度下降,而是几乎保持不变。然而,推转HFB计算显示其四级形变β2值在高自旋也下降了很多,但是负值的γ形变值抵消了上述四级形变的减小导致四极矩的下降。我们测量的~（174）Os高自旋态的四极矩在回弯后并没有发生下降，而是几乎保持一个不变的趋势，与上述预测的结果一致。
The main content of this thesis could be divided to three sections as follows: Thefirst part introduces the level scheme and magnetic rotation in doubly odd nucleus~(114)In; the second part investigates the anomalously high transition strength for the first2+state and critical-point symmetry in174Os through delayed coincidence technique;The last part concerns in nuclear shape changes that occur from Coriolis andcentrifugal forces through lifetime measurements of high spin states in the yrast bandof174Os using Doppler-shift attenuation method.
1. The level structure of doubly odd nucleus~(114)In
In recent years, a variety of nuclear structure phenomena in the A～110massregion have attracted great interest. Particularly, a great deal of effort has been madeto study newly observed quantized modes of rotation characterized as “magneticrotation”. In this case, sequences of M1transitions are observed with weak or absentcrossover E2transitions, indicating small deformation of the structures involved.Lifetime measurements show that the M1transitions are strongly enhanced whereasthe E2transition probabilities are very small as expected for almost spherical nuclei.These so-called magnetic rotation bands were described in the tilted axis cranking(TAC) model and represent a novel rotational mechanism called “shears mechanism”.In the case of these bands, the angular momentum is generated by gradual alignmentof spins of the deformation aligned and the rotation-aligned quasiparticles and theangular momentum is not along any of the principal axes of the nucleus. The M1bands in the indium isotopes have been observed for configurations where the protonholes are in high Ω orbitals of g9/2while the neutron particles occupy the low Ωorbitals of h11/2. In even-In isotopes the shears mechanism has been observed in thelight indium isotopes~(106–112)In, whereas in odd-In isotopes it has been observed up tostable~(113)In. It is of particular interest to investigate the shears mechanism along an isotopic chain moving away from the shell closure at the same time coming close tothe N=66neutron mid-shell. For the~(114)In nucleus the previous information onhigh-spin states was very scarce. The motivation of the present experiment was toextend the level scheme of~(114)In for the systematic search of ΔI=1dipole bands andanti-magnetic (ΔI=2) rotational bands.
High-spin states in~(114)In were populated using the reaction~(110)Pd (7Li,3n)~(114)In.The target consisted of a2.45mg/cm~2~(110)Pd (enrichment97.2%) rolled onto a10.55mg/cm~2Au backing. The experiment was performed at the HI-13tandemaccelerator of the China Institute of Atomic Energy. The γ-rays were detected with anarray consisting of12Compton-suppressed HPGe detectors and2planar-type HPGedetectors. Five of these detectors were placed at90, three at150, two at42and140,and one each at34and127with respect to the beam direction. Estimation ofcross-sections and fusion products were done using the code PACE. Based on thesecalculations, relative excitation functions for the emitted γ-rays were measured atbeam energies of22,24,26,28and32MeV. The maximum of~(114)In productionoccurred at26MeV, and thus this energy was selected for all in beam experiments.The detectors were calibrated for γ-ray energies and efficiencies using the~(113)Ba and~(152)Eu radioactive sources. A total of35million γ-γ coincidence events were recorded.The data were sorted into symmetrized and angular correlation γ-γ matrices foroff-line analysis. An asymmetric angular correlation matrix between the detectors at42and at90was constructed and used for the directional correlation of orientedstates (DCO) ratio analysis to distinguish between quadrupole and dipole transitions.menaejrogTryhit y～e o l5ef vtMelhee s Vchob saeenmrdev es odpf isn1e1q～4Iu ne1n6cc oe ns s wisinitti1hn1g4tIh onef four bands is established up to excitation haadvdei tbioenen o sfu agbgoeustt e5d0c noenwfi gturarnatsiiotinosn. sT. hTrheeesequences of dipole bands and two bands of E2γ-ray transitions have been placed inthe level scheme for the first time. The I (ω) values of yrast band are reasonablyreproduced by TAC-RMF calculations on the basis of the proposed configurationsbefore and after backbend. The experimental results of positive-parity dipole bandhave been discussed and show the characteristic features of magnetic rotation. The comparison of the characteristics of experimental results in the band2with thepredictions of the TAC-RMF calculations shows that the band can be described bythis tilted configuration that implies a strong magnetic component. It is an example ofrAme ag～gnio etic rotation that microscopic TAC-RMF model can describe the shears band inn1,10e mspaescsi arlelyg ioanf.t eInr tihnec lfuudtuinreg, itth ies npeacierisnsgar yc toor reexlaatmioinn e iont hethr iiss omtoopdese li. n Tthhieseffective-interaction model was also successfully applied to interpret shears bandfrom a phenomenological perspective. Both models predict that the majority of theangular momentum in band2is generated by alignment of proton–hole and neutronblades. However, lifetime measurements will be useful to understand the structure ofthese closely spaced dipole bands. It is to be noted that the positive dipole band seemsto indicate the onset of further alignment at the highest observable frequency. Thus,this band needs to be extended further in order to investigate the nature of alignment.
2. Investigation of anomalously high transition strength for the first2+state and critical-point symmetry in174Os through lifetimemeasurement
Measurements of excitation transition probabilities are useful for studyingnuclear structure. Reduced transition probabilities of first2+states B(E2), foreveneven nuclei are important in evaluating nuclear collectivity. Among thesemeasurements, nuclei in the transitional region of isotopes having a mass number ofA～160-190still attract attention because they provide an opportunity to study thetransition from spherical structures to deformed axially symmetric rotors.
Very recently, a new experimental method with LaBr3(Ce) detectors incombination with the power of theGe array, employing triple-γ coincidencemeasurements, has been developed to deduce state lifetimes longer than tens ofpicoseconds. The method is especially suitable for in-beam γ-ray spectroscopymeasurements. In this newmethod, triple-γ coincidences are measured with an arraycontaining both HPGe and LaBr3(Ce) detectors. The high-energy resolution of theGedetectors is used to select the desired γ-ray cascade, and the array of fast LaBr3(Ce) to build the delayed coincidence time spectra for selected levels. We used this newtriple-γ coincidences method to remeasure the lifetime of the2+1state in~(174)Os. Thismethod applied in a delayed coincidence technique provides a more reliable valuemvcoaelmausepu arroeefd with earlier measurements with the plunger method because the measuredm e～n1t onf st hiiss realta ttihvee lyli lmonitg oliff etthime e.technique, causing a large error in the
The excited states in~(174)Os were populated using the~(150)Sm (~(28)Si,4n)~(174)Os)reaction at a beam energy of140MeV. The target consisted of a0.9mg/cm2~(150)Smrolled onto a12.5mg/cm2Pb backing. The experiment was performed at the HI-13tandem accelerator of the China Institute of Atomic Energy. The lifetimes of the statesof interest were determined by using a fast timing setup consisting of four LaBr3(Ce)scintillator detectors working in coincidence with11Compton-suppressed HPGedetectors. The seven LaBr3(Ce) detectors were mounted below the target chamber ona ring of approximately90with respect to the beam axis. Four of LaBr3(Ce) detectorshave crystals with a size of30mm in height and a diameter of40mm and they werecoupled to an XP2020Q photomultiplier tube. At first, we performed off-line testswith standard radioactive sources (~(22)Na,60Co,133Ba, and152Eu) to optimize theperformance of our fast timing setup.
The lifetime of the2+state in~(174)Os has been remeasured in a fast timingexperiment and determined with better precision to be τ=513(20) ps. The small errormakes this value sufficiently precise to serve as a normalization parameter formeaningful tests of relevant models. The systematics of B(E2) indicate that thedeformation reaches a maximum at N=98for W and Os isotopes, which can beattributed to both the effect of the deformed subshell at N=98and the proton“intruder” h9/2orbital.
An initial comparison of~(174)Os to the predictions of the X(5) critical-point modelpoints to~(174)Os lying slightly to the spherical-vibrator side of the phase transition. TheX(5)-β2model, which describes the structure between a spherical vibrator and X(5),well describes the energies in~(174)Os but shows large discrepancies with the relativeB(E2) transition strengths. These disagreements perhaps can be attributed to~(174)Os having a potential in the γ degree of freedom which is softer than the axiallysymmetric potential assumed in the X(5) and X(5)-βnmodels. This idea was tested bytrying to describe~(174)Os using two-parameter IBA-1calculations. The agreement,overall, is reasonable and the resulting χ value points to a structure that is intermediatebetween a γ-soft and axially symmetric potential in the γ degree of freedom. While thepresent work provides some evidence that the γ degree of freedom may need to beconsidered in this mass region, more extensive data are needed on the off-yraststructures of other potential X(5) candidates in this region.
3. Lifetime measurements probing shape changes in~(174)Os
Transition quadrupole moments from lifetime mearurements of states with spinup to I=20in collective bands near the yrast line have proved to br a value tool fortracing nuclear shape changes that occur from Coriolis and centrifugal forces. Areduction of collectivity at high spin has been observed in several N=104、106and108muclei. The effect in~(182-186)Pt is qualitatively caused by the deformation drivingforce of the aliged neutrons when the Feimi level lies high in the i13/2shell. Thenucleus changes from prolate shape at low spin to a triaxial with reduced collectivity,i.e. γ>0at high spin.
The calculated systemmatics of triaxial-shape driving orbitals suggests that thecollectivity should not decrease at higher neutron number around N=96and98wherethe aliged i13/2orbitals are situated near the middle of the shell and negative γ valueare energetically favored. However, Cranked HFB calculations indicate that there alsooccurs a reduction of β2in collectivity. The latter effect approximately cancels out theincrease in collectivity caused by the negative γ values. In line with these predictions,our mearsurement show no significant reduction of the transition quadrupole momentsfor spin I≥10.
一、奇奇核~(114)In能级结构的研究：
A～110核区丰富的核结构现象一直得到核结构专家的广泛关注和研究。尤其近些年，人们倾入了大量的精力和时间寻找这个核区新奇转动机制的带结构“磁转动带”（包括反磁转动带）。这种新的转动机制已经通过倾斜轴推转壳模型（TAC）得出合理的解释。形变核的转动是大量配对的核子参与的集体转动，科里奥利力的作用是拆散核子对，使核子对角动量朝向着转动角动量方向靠拢。现在的磁转动参与的是几个未配对的高角动量的粒子或空穴，他们不是一步达到全顺排，而是逐步增加，直到带终结。由于未配对的高角动量的粒子或空穴逐步靠拢，极其像一把剪刀，因此又被称为“剪刀带”。在此核区In同位素核的中子、质子费米面分别位于高Ω轨道的g_(9/2)和低Ω轨道的h_(11/2)。在奇奇核的~（106-112）In中已有大量磁转动带的报道；在奇A的In核中，稳定的~（113）In核也已经发现了若干磁转动带。因此，在中子数N=65接近半满壳处的丰中子核~（114）In中会不会依然有磁转动带出现，或更加丰富的核结构现象，这极大的引起了我们的兴趣。另外，到目前为止还没有关于~（114）In高自旋态的有关报道。基于上述原因我们选择~（114）In做为研究的目标核。
~（114）In的高自旋态是通过~（110）Pd(~7Li,3n)~（114）In布居的，`7Li束流是由中国原子能科学研究院HI-13串列加速器提供的。靶为厚度为2.4mg/cm2的金属110Pd，为了消除多普勒效应的影响，靶镀在了厚度为10.55mg/cm~2的金属Au衬底上。探测系统是由12台带反康的高纯锗探测器和2台小平面探测器组成。其中5个探测器放置在与束流夹角分别为900的方向，三个在150°，42°和140°方向各有两个，34°和127°方向各一个。实验前我们用PACE程序计算了反应的产物和截面。基于(?))ACE的计算结果,我们选择了22、24、26、28、32MeV五个能量点做实验激发函数,发现26MeV时114In的反应截面最大。综合考虑,我们最终选择26和28MeV两个能量点各进行了40个小时实验。在束实验前我们利用133Ba和152Eu的混合标准源测量了两个小时左右的能谱,对γ射线能量以及探测器的效率进行了刻度。实验总共记录了3.5×107个二重符合事件。离线数据处理反演了一个对称化的矩阵和一个非对称化的DCO矩阵,其中DCO矩阵是由42°的探测器和90°的探测器组成,来区分γ射线的跃迁极性。
利用RADWARE和GASWARE软件对上述两维矩阵进行开窗谱分析,建立了114In新的能级纲图。共找到约50条新γ跃迁和30多个新的能级,自旋和激发能分别推高至约16h和5MeV。首次发现114In的高自旋态信息,建立了三条新的转动带,极大的拓展了114In的能级纲图。对新发现的能级和带进行了组态指定。初步认定低自旋10+态为,G9/2-1(?) v(d5/2/g7/2)(h11/2)2的多粒子组态。其他的三个新发现的磁偶极转动带2、3和4也分别尝试性地给出了组态结构：πg9/2-1(?)(d5/2/g7/2)(h11/2)2、πg9/2-1(?) v(h11/2)3、πg9/2-1(?)v(d5/2/g7/2)(h11/2)2。在晕带中发现了回弯现象,经系统学比较和TAC-RMF理论计算认定为一对97/2/d5/2中子顺排造成的。其中正宇称带2是由非常强的规则的M1跃迁、缺失的带内E2跃迁、没有旋称劈裂等实验现象,显示明显的磁转动带特征。TAC-RMF理论计算表明带内跃迁强度B(M1)随着自旋的增加而减小,带内跃迁B(E2)随着自旋增加呈上升趋势,这与磁转动带特征符合的很好。此外,相互作用的半经验的剪刀机制模型计算表明114In核中带2的价质子与中子之间的相互作用值约750keV,这与此核区的剪刀带的系统学非常一致。
二、偶偶核174Os的21+态的异常高的跃迁几率和临界点对称性的研究
原子核能级的跃迁机率最能反映核内禀属性,因此是核结构研究最重要的内容之一。偶-偶核的第一个21+激发态的寿命为核形状的四极形变提供重要信息。根据实验测定的B(E2;21+→01+)跃迁几率值可以用来与各种不同模型的理论计算进行比较。系统学研究偶偶核从球形振子到轴对称转子的特征可以对了解核结构及其随价核子的演化具有重要意义,因此A～160-190核区原子核的21+态的寿命测量一直吸引着核物理学家的广泛关注。IBA模型预期当中子数从N=82闭壳趋向N=82和126两闭壳的中间值时(N=104),跃迁几率B(E2;21+→01+)随之增大,并预言在中间壳时具有最大值。当从中间壳趋向N=126闭壳时,B(E2;21+→01+)值逐渐减小。然而,必须指出的是,Os同位素的跃迁几率B(E2;21+→01+)并不遵循这一简洁明确的系统性趋势,违背了当中子数朝向N=82闭壳时B(E2;21+→01+)值缓慢减小这一预言,而最大值出现在了中子数N=98处。这种异常现象究竟是源自真正的结构效应,还是仅仅因为实验本身的问题所引起,至今尚不清楚。不过对于174Os来说,B(E2;21+→01+)值的实验误差与相邻Os核的这些量的不确定度相比较,还是相当大的。另一方面,最近的实验证明,176,178Os低位态具有上佳的X(5)临界点对称性。因此,有必要对174Os进行能谱和跃迁几率的精确测量,以了解174Os是否延续了X(5)对称性,以致X(5)结构效应引起174Os形状演化趋势的异常。
最近,科隆大学和Agone实验室发展了一项利用新型闪烁体探测器LaBr3(Ce)测量核能级寿命的实验方法。这种方法是结合能量分辨超好的HPGe探测器与时间和能量分辨都较好的LaBr3(Ce)探测器对在束γ谱学实验中原子核能级寿命的测量。这种方法是测量HPGe和LaBr3(Ce)之间的三重γ符合事件,能量分辨超好的HPGe探测器选择预测能级所在的能级分支,然后用能量和时间分辨都比较好的LaBr3(Ce)探测器开门提取时间谱。这种方法能够测量几十个皮秒以上的寿命。我们用这种新的快时间的延迟符合技术测量了174Os的2+态的能级寿命。由于174Os的2+态的能级寿命接近反冲符合方法的测量上限值,前人用反冲法(RDM)测量得到的寿命值的误差较大。我们利用LaBr3(Ce)探测器的延迟符合技术是一种直接测量寿命的技术手段,对于长寿命的核能级可以得到精确的值。
174Os的高自旋态是通过150Sm(28Si,4n)174OS布居的,28Si束流是由中国原子能科学研究院HI-13串列加速器提供的。靶为厚度为0.9mg/cm2的金属150Sm,同时为了测量高自旋态能级的寿命,靶镀在12.5mg/cm2的金属Pb衬底上。整个探测阵列是由11台带反康的高纯锗探测器和7台LaBr3(Ce)探测器组成的。7台LaBr3(Ce)环绕着靶室垂直于束流摆放。在束实验之前我们用标准的放射源(22Na,60Co,133Ba, and152Eu)进行了测量,以使整个探测系统的能量和时间特征达到最好。
最后得到的174Os的21+态寿命结果为τ=513(20)ps,与前人测量的结果值基本一致,但测量精度大大提高。通过比较Os同位素跃迁几率B(E2;21+→01+)值随着中子的变化趋势,发现174Os的21+态的B(E2;21+→01+)值依然最大,甚至比半壳180Os的21+态的B(E2;21+→01+)值还大。通过实验测定的寿命提取的跃迁几率B(E2;21+→01+)值与各种不同模型的理论计算进行比较,发现目前没有一个模型能够完全预测这种发展的趋势。通过与邻近的核也做了系统学比较,发现w同位素与Os同位素具有相同的趋势,但在更轻的Yb和Hf同位素却不同,最大跃迁几率值出现在中子半满壳附近。因此,我们初步建议Os和w同位素B(E2;21+→01+)值随着中子演化异常的原因为N=98的中子壳隙和大形变的质子h9/2闯入轨道共同的影响。
通过174Os的实验信息与X(5)模型预测比较,发现174Os核相比于临界点对称性X(5)核更靠近球形振子核区。而最近发展的描述振子与X(5)核过渡的X(5)-β2模型能够合理的描述174Os核的能级特征,但是X(5)-β2模型预测跃迁几率B(E2)值与实验的B(E2)值相差却比较大。两参数的IBA-1模型计算能够很好的再现实验的能谱和B(E2)值。因为在IBA-1模型中包含了参数χ值(γ自由度),因此本部分的工作建议174Os核并非完全的轴对称的转子核,而是包含γ软的形变核。
三、多普勒位移衰减法测量174Os高自旋态的寿命
测量偶偶核集体转动带高自旋态的能级寿命(达到I=20h以上)能够为我们研究由于科里奥利力和离心力对原子核形状变化的影响。在一些中子数N=104、106和108的核的高自旋态发现了集体性突然下降的趋势,比如在182-186Pt晕带中,自旋I≥10h时电四极矩突然下降。这主要是因为这些核的中子费米面位于i13/2壳的上部,在I-10h时中子发生拆对顺排,此时的原子核的形状由低自旋的近似轴对称长椭核到三轴形变核(正的γ形变值)。
在第二部分的实验当中,我们同时利用多普勒线移衰减法(DSAM)测量了174Os晕带12+—22+态的能级寿命。理论预测N-96,98的原子核的中子费米面位于i13/2壳的中部,这些中子拆对顺排会造成负的γ形变值,因此集体性不会大幅度下降,而是几乎保持不变。然而,推转HFB计算显示其四级形变β2值在高自旋也下降了很多,但是负值的γ形变值抵消了上述四级形变的减小导致四极矩的下降。我们测量的~（174）Os高自旋态的四极矩在回弯后并没有发生下降，而是几乎保持一个不变的趋势，与上述预测的结果一致。
The main content of this thesis could be divided to three sections as follows: Thefirst part introduces the level scheme and magnetic rotation in doubly odd nucleus~(114)In; the second part investigates the anomalously high transition strength for the first2+state and critical-point symmetry in174Os through delayed coincidence technique;The last part concerns in nuclear shape changes that occur from Coriolis andcentrifugal forces through lifetime measurements of high spin states in the yrast bandof174Os using Doppler-shift attenuation method.
1. The level structure of doubly odd nucleus~(114)In
In recent years, a variety of nuclear structure phenomena in the A～110massregion have attracted great interest. Particularly, a great deal of effort has been madeto study newly observed quantized modes of rotation characterized as “magneticrotation”. In this case, sequences of M1transitions are observed with weak or absentcrossover E2transitions, indicating small deformation of the structures involved.Lifetime measurements show that the M1transitions are strongly enhanced whereasthe E2transition probabilities are very small as expected for almost spherical nuclei.These so-called magnetic rotation bands were described in the tilted axis cranking(TAC) model and represent a novel rotational mechanism called “shears mechanism”.In the case of these bands, the angular momentum is generated by gradual alignmentof spins of the deformation aligned and the rotation-aligned quasiparticles and theangular momentum is not along any of the principal axes of the nucleus. The M1bands in the indium isotopes have been observed for configurations where the protonholes are in high Ω orbitals of g9/2while the neutron particles occupy the low Ωorbitals of h11/2. In even-In isotopes the shears mechanism has been observed in thelight indium isotopes~(106–112)In, whereas in odd-In isotopes it has been observed up tostable~(113)In. It is of particular interest to investigate the shears mechanism along an isotopic chain moving away from the shell closure at the same time coming close tothe N=66neutron mid-shell. For the~(114)In nucleus the previous information onhigh-spin states was very scarce. The motivation of the present experiment was toextend the level scheme of~(114)In for the systematic search of ΔI=1dipole bands andanti-magnetic (ΔI=2) rotational bands.
High-spin states in~(114)In were populated using the reaction~(110)Pd (7Li,3n)~(114)In.The target consisted of a2.45mg/cm~2~(110)Pd (enrichment97.2%) rolled onto a10.55mg/cm~2Au backing. The experiment was performed at the HI-13tandemaccelerator of the China Institute of Atomic Energy. The γ-rays were detected with anarray consisting of12Compton-suppressed HPGe detectors and2planar-type HPGedetectors. Five of these detectors were placed at90, three at150, two at42and140,and one each at34and127with respect to the beam direction. Estimation ofcross-sections and fusion products were done using the code PACE. Based on thesecalculations, relative excitation functions for the emitted γ-rays were measured atbeam energies of22,24,26,28and32MeV. The maximum of~(114)In productionoccurred at26MeV, and thus this energy was selected for all in beam experiments.The detectors were calibrated for γ-ray energies and efficiencies using the~(113)Ba and~(152)Eu radioactive sources. A total of35million γ-γ coincidence events were recorded.The data were sorted into symmetrized and angular correlation γ-γ matrices foroff-line analysis. An asymmetric angular correlation matrix between the detectors at42and at90was constructed and used for the directional correlation of orientedstates (DCO) ratio analysis to distinguish between quadrupole and dipole transitions.menaejrogTryhit y～e o l5ef vtMelhee s Vchob saeenmrdev es odpf isn1e1q～4Iu ne1n6cc oe ns s wisinitti1hn1g4tIh onef four bands is established up to excitation haadvdei tbioenen o sfu agbgoeustt e5d0c noenwfi gturarnatsiiotinosn. sT. hTrheeesequences of dipole bands and two bands of E2γ-ray transitions have been placed inthe level scheme for the first time. The I (ω) values of yrast band are reasonablyreproduced by TAC-RMF calculations on the basis of the proposed configurationsbefore and after backbend. The experimental results of positive-parity dipole bandhave been discussed and show the characteristic features of magnetic rotation. The comparison of the characteristics of experimental results in the band2with thepredictions of the TAC-RMF calculations shows that the band can be described bythis tilted configuration that implies a strong magnetic component. It is an example ofrAme ag～gnio etic rotation that microscopic TAC-RMF model can describe the shears band inn1,10e mspaescsi arlelyg ioanf.t eInr tihnec lfuudtuinreg, itth ies npeacierisnsgar yc toor reexlaatmioinn e iont hethr iiss omtoopdese li. n Tthhieseffective-interaction model was also successfully applied to interpret shears bandfrom a phenomenological perspective. Both models predict that the majority of theangular momentum in band2is generated by alignment of proton–hole and neutronblades. However, lifetime measurements will be useful to understand the structure ofthese closely spaced dipole bands. It is to be noted that the positive dipole band seemsto indicate the onset of further alignment at the highest observable frequency. Thus,this band needs to be extended further in order to investigate the nature of alignment.
2. Investigation of anomalously high transition strength for the first2+state and critical-point symmetry in174Os through lifetimemeasurement
Measurements of excitation transition probabilities are useful for studyingnuclear structure. Reduced transition probabilities of first2+states B(E2), foreveneven nuclei are important in evaluating nuclear collectivity. Among thesemeasurements, nuclei in the transitional region of isotopes having a mass number ofA～160-190still attract attention because they provide an opportunity to study thetransition from spherical structures to deformed axially symmetric rotors.
Very recently, a new experimental method with LaBr3(Ce) detectors incombination with the power of theGe array, employing triple-γ coincidencemeasurements, has been developed to deduce state lifetimes longer than tens ofpicoseconds. The method is especially suitable for in-beam γ-ray spectroscopymeasurements. In this newmethod, triple-γ coincidences are measured with an arraycontaining both HPGe and LaBr3(Ce) detectors. The high-energy resolution of theGedetectors is used to select the desired γ-ray cascade, and the array of fast LaBr3(Ce) to build the delayed coincidence time spectra for selected levels. We used this newtriple-γ coincidences method to remeasure the lifetime of the2+1state in~(174)Os. Thismethod applied in a delayed coincidence technique provides a more reliable valuemvcoaelmausepu arroeefd with earlier measurements with the plunger method because the measuredm e～n1t onf st hiiss realta ttihvee lyli lmonitg oliff etthime e.technique, causing a large error in the
The excited states in~(174)Os were populated using the~(150)Sm (~(28)Si,4n)~(174)Os)reaction at a beam energy of140MeV. The target consisted of a0.9mg/cm2~(150)Smrolled onto a12.5mg/cm2Pb backing. The experiment was performed at the HI-13tandem accelerator of the China Institute of Atomic Energy. The lifetimes of the statesof interest were determined by using a fast timing setup consisting of four LaBr3(Ce)scintillator detectors working in coincidence with11Compton-suppressed HPGedetectors. The seven LaBr3(Ce) detectors were mounted below the target chamber ona ring of approximately90with respect to the beam axis. Four of LaBr3(Ce) detectorshave crystals with a size of30mm in height and a diameter of40mm and they werecoupled to an XP2020Q photomultiplier tube. At first, we performed off-line testswith standard radioactive sources (~(22)Na,60Co,133Ba, and152Eu) to optimize theperformance of our fast timing setup.
The lifetime of the2+state in~(174)Os has been remeasured in a fast timingexperiment and determined with better precision to be τ=513(20) ps. The small errormakes this value sufficiently precise to serve as a normalization parameter formeaningful tests of relevant models. The systematics of B(E2) indicate that thedeformation reaches a maximum at N=98for W and Os isotopes, which can beattributed to both the effect of the deformed subshell at N=98and the proton“intruder” h9/2orbital.
An initial comparison of~(174)Os to the predictions of the X(5) critical-point modelpoints to~(174)Os lying slightly to the spherical-vibrator side of the phase transition. TheX(5)-β2model, which describes the structure between a spherical vibrator and X(5),well describes the energies in~(174)Os but shows large discrepancies with the relativeB(E2) transition strengths. These disagreements perhaps can be attributed to~(174)Os having a potential in the γ degree of freedom which is softer than the axiallysymmetric potential assumed in the X(5) and X(5)-βnmodels. This idea was tested bytrying to describe~(174)Os using two-parameter IBA-1calculations. The agreement,overall, is reasonable and the resulting χ value points to a structure that is intermediatebetween a γ-soft and axially symmetric potential in the γ degree of freedom. While thepresent work provides some evidence that the γ degree of freedom may need to beconsidered in this mass region, more extensive data are needed on the off-yraststructures of other potential X(5) candidates in this region.
3. Lifetime measurements probing shape changes in~(174)Os
Transition quadrupole moments from lifetime mearurements of states with spinup to I=20in collective bands near the yrast line have proved to br a value tool fortracing nuclear shape changes that occur from Coriolis and centrifugal forces. Areduction of collectivity at high spin has been observed in several N=104、106and108muclei. The effect in~(182-186)Pt is qualitatively caused by the deformation drivingforce of the aliged neutrons when the Feimi level lies high in the i13/2shell. Thenucleus changes from prolate shape at low spin to a triaxial with reduced collectivity,i.e. γ>0at high spin.
The calculated systemmatics of triaxial-shape driving orbitals suggests that thecollectivity should not decrease at higher neutron number around N=96and98wherethe aliged i13/2orbitals are situated near the middle of the shell and negative γ valueare energetically favored. However, Cranked HFB calculations indicate that there alsooccurs a reduction of β2in collectivity. The latter effect approximately cancels out theincrease in collectivity caused by the negative γ values. In line with these predictions,our mearsurement show no significant reduction of the transition quadrupole momentsfor spin I≥10.
引文
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