奇奇核~(92)Nb高自旋态的研究
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摘要
本论文主要介绍了奇奇核92Nb能级纲图的建立以及高自旋态能级结构的研究，并利用准粒子半经验壳模型与壳模型Nushellx程序对其能级进行了计算，对92Nb大部分能级结构进行了解释；另外对反冲距离多普勒线移法（RDDM）测量装置（也被称为Plunger装置）中的靶膜与阻停膜的制备、安装调试以及反冲距离多普勒线移法在束实验的操做步骤进行了介绍，本部分内容将以附录的形式进行详细介绍。
原子核高自旋态研究是目前核物理研究的前沿领域，它为人们认识和理解核形状、核结构、核子耦合、新的对称性等性质提供了重要的信息。近年来，奇奇核核区丰富的核结构现象尤其是奇奇核作为目标核来研究准质子和准中子之间的相互作用受到人们越来越多的关注。由于实验上所得到的很多奇奇核的能级纲图在一定程度上存在着不确定性，核结构特性非常丰富而复杂,例如能级的激发能位置、核子的激发机制、相应能级的组态、自旋和宇称的指定，以及奇奇核在能级纲图结构、级联跃迁系列的核素的归属等一些方面都存在一些问题。因此奇奇核相对奇A核和偶偶核，奇奇核的能级结构信息更为复杂，实验上对其高自旋态的研究也比较困难。
中子数接近N=50中子闭壳，质子数在Z=38子壳附近，关于本核区的大量研究表明，在这个核区存在着一些十分有趣的现象，如一些同位素激发态很特殊，同位素可以在该状态下维持很长时间才回到基态，该核区绝大多数核素都具有球形或近球形结构和不规则的跃迁能量等。该核区核的激发态以内禀单粒子激发为主,它们的低激发态由fp子空间（2f5/2，2p3/2，2p1/2）质子激发产生。随着激发能的增大，N=50的中子满壳被打破,形成中子[(g9/2)-1] d5/2/g7/2粒子一空穴组态,称为核芯激发。同时，在以往的研究中发现，该核区绝大多数核素的能级结构与壳模型计算结果符合的很好。在这些核的壳模型描述中，价质子主要占据f5/2、p3/2、p1/2空穴轨道和p1/2、g9/2粒子轨道。而价中子主要占据g9/2空穴和d5/2、g7/2粒子轨道，在低能部分主要是价质子结构变化引起角动量的增长，而高能部分中子跨越N＝50中子闭壳的核芯激发也开始起到一定作用。在对92Nb同中子素核进行系统学比较时发现，这些同中子素核的能级结构很相似，并且在92Nb核同中子素奇奇核进行比较，发现相同自旋态的能级，随着质子数增加，相应能级的能量降低，这种现象可能是由于质子数目的增加使得质子的激发变得更容易而引起的。另外临近核的高自旋态能级组态的价核子耦合机制具有相似性，利用某些已知原子核能级的组态能够求得相应价核子轨道间的相互作用，然后利用求得的轨道间相互作用来计算其它核素相应组态能级能量，对于确定实验测量能级的组态具有非常重要的意义。
该实验是在中国原子能科学研究院HI-13串列加速器上完成的，利用重离子熔合蒸发反应82Se (14N;4n)92Nb布居了92Nb的高自旋态。14N束流能量为54MeV。靶为质量厚度0.99mg/cm2同位素丰度为96%的82Se和质量厚度为8.27mg/cm2的Yb衬组成。符合事件由9台HPGe－BGO反康谱仪和2台小平面组成。共记录了150×106个二重及以上的符合事件。基于标准的在束谱学实验测量结果，离线将符合数据处理生成一个对称化矩阵和两个不对称的ADO矩阵，采用RADWARE开窗软件分析这些矩阵，在前人工作的基础上，发现了30多条新的射线，根据提取出的ADO系数,初步指定了新发现能级的自旋，在前人建立的能级纲图的基础上，扩展了92Nb已有的能级纲图，其中正负宇称能级自旋被推高21+(Ex=9784.9keV)、21-(10738keV)。
通过分析92Nb核的能级结构，发现其能级结构具有球形核的结构特征，奇奇核92Nb在88Sr核芯外有三个价核子，分别位于质子g9/2轨道和中子p1/2，d5/2轨道。在B.A.Brown有关92Nb的较低能级结构的研究工作中发现，壳模型的计算结果能够很好的重现实验测量值。对于92Nb高自旋态，我们利用准粒子半经验壳模型理论和壳模型Nushellx程序分别计算了92Nb能级能量及相应的组态，分析了92Nb高自旋态能级的价核子耦合机制。并分别把准粒子半经验壳模型与Nushellx程序的计算结果与实验测得值进行比较分析，发现大部分能级的理论计算能够较好的重现实验测量值。
在Plunger的研制过程中，掌握了靶膜展平技术、靶膜与阻停膜的安装、用视频显微镜检测两膜平行度的方法，提高了距离测量精度进而提高寿命测量精度，增加了测量系统的稳定性。由于该装置的建立，使能够测量的能级寿命范围得到扩展，可以获得更为丰富的核结构信息。
This paper describes the establishment of odd-odd nucleus92Nb level scheme andthe study of high spin states. Scmi-Empcrical Shell Model (SESM) and shell modelcalculations (NUSHELLX) have been per formed to interpret the experimental levelscheme of92Nb. A series of methods such as capacitance method, the method to preparevery flat target foil and so on were mastered in the process of building the Plunger.
The research on the high-spin states in the nuclei was regarded as one of thefrontiers of fields in nuclear physics, which can provide us with valuable informationabout nuclear shape,nuclear structure,nucleon coupling and so on.
In recent years, a variety of nuclear structure phenomena in the A～90massregion have attracted great interest. The study of odd-odd nuclei is most important fordetermining the effective proton-neutron residual interaction between quasi-proton andquasi-neutron, have attracted more and more attention. The level structures of odd–oddnuclei more complicated than even–even and odd A nuclei. It is more difficult toinvestigate the high-spin states of odd–odd nuclei due to their complicated levelstructure are uncertain.
In the vicinity of Z=38subshell closure and N=50shell closure，many studiesshowed that the nucleus in this mass region, there are some very interestingphenomena, such as some isotope excited state, the isotope can be maintained for along time before de-excitation to the ground state, the vast majority of the nucleusshow the spherical or near spherical structure and irregular transition energy. Theexcited states are intrinsic single-particle excitations within the fp subspace(2f5/2,2p3/2,2p1/2) proton excitation generated. As the excitation energy increases, N=50neutron closed shell is broken, forming neutrons [(g9/2)-1]/d5/2/g7/2particles-holeconfiguration, called the core excitation. In previous studies previous, the majority oflevel energy are good agree with shell model calculations. In the description of shell model, For the low spin states, the valence protons excitations mainly described theangular momentum of growth and the high spin states attributed to theneutron-particle-hole excitation of the N=50inert core-specifically,the excitation of asingle g9/2neutron across the N=50shell closure into the d5/2/g7/2orbits coupled to thevalence proton states in the π(f5/2, p3/2, p1/2, g9/2) valence space. As the excitationenergy increases, the high spin states might be concern the proton excitation across theZ=38shell gap π(f5/2, p3/2)→g9/2and the neutron the neutron excitation within (d5/2,h11/2) shell model orbitals.
According to systematic analysis and comparison with neighboring isotonesnuclei, their energy level structures were very similarly. And with the number ofprotons increasing,the corresponding energy of levels decreased,this may due to theincrease in the number of protons, it makes that the excitation energy of protonsdecreased.
Systematic comparison in the isotones of N=51region, will help us betterunderstand that the valence nucleon filling mechanisms effect on the energy levelstructure of the nucleus.
Due to the similarity of high spin level configuration adjacent valence nucleonfilled orbits,it may be exactly similar to the corresponding the same spin energy levelstructure. According to known nucleus level configuration,we can extract thecorresponding interactions of the valence nucleon orbits,and calculate the other nucleicorresponding configuration energy level, it is important to determine the energy levelof configurations.
In present work, excited levels in92Nb were populated using a82Se(14N;4n)92Nbreaction at a beam energy of54MeV. The14N beam was provided by the HI-13tandem accelerator in the China Institute of Atomic Energy(CIAE). The targetconsisted of a0.99mg/cm2foil of82Se(isotopically enriched to96%)evaporated onto a8.27mg/cm2natural Yb backing. The de-excitation rays from the reaction residueswere detected with an array consisting of9Compton-suppressed HPGe detectors and2 planar HPGe detectors. A total of about150×106coincidence events were accumulated.Measurement results of standards in-beam spectroscopy experiments were sortedoffine into a two-dimensional symmetrized E-E coincidence matrix and twoasymmetrized ADO matrices. A total of36new transitions have been identified andplaced in this scheme. positive and negative parity level which have been substantiallyextended up to21+(Ex=9784.9keV) and21-(10738keV).
As a conclusion，the level scheme of92Nb has been substantially extended on theprevious level scheme, The Shell Model calculations have been performed performedin the model spaceπ(0f5/2,1p3/2,1p1/2,0g9/2) ν(1p1/2,0g9/2,1d5/2,0g7/2) and the results werein line with the experimental results. The systemical analyzes for the isotones of N=51showed that there were some rules for the high spins level structure of nuclei in thisregion.
The Recoil Distance Doppler Shift (RDDS) method has been well developed. Aseries of methods such as installation of target and the stopper foil and the delicatemeasurement of the distance between the target and the stopper foil and the steps of inbeam experiment were mastered in the process of building the Plunger. And theapparatus will have a good application for ground. The lifetimes of the excited nuclearstates can be measured and more information of the nuclear structure can be gotten byPlunger.
原子核高自旋态研究是目前核物理研究的前沿领域，它为人们认识和理解核形状、核结构、核子耦合、新的对称性等性质提供了重要的信息。近年来，奇奇核核区丰富的核结构现象尤其是奇奇核作为目标核来研究准质子和准中子之间的相互作用受到人们越来越多的关注。由于实验上所得到的很多奇奇核的能级纲图在一定程度上存在着不确定性，核结构特性非常丰富而复杂,例如能级的激发能位置、核子的激发机制、相应能级的组态、自旋和宇称的指定，以及奇奇核在能级纲图结构、级联跃迁系列的核素的归属等一些方面都存在一些问题。因此奇奇核相对奇A核和偶偶核，奇奇核的能级结构信息更为复杂，实验上对其高自旋态的研究也比较困难。
中子数接近N=50中子闭壳，质子数在Z=38子壳附近，关于本核区的大量研究表明，在这个核区存在着一些十分有趣的现象，如一些同位素激发态很特殊，同位素可以在该状态下维持很长时间才回到基态，该核区绝大多数核素都具有球形或近球形结构和不规则的跃迁能量等。该核区核的激发态以内禀单粒子激发为主,它们的低激发态由fp子空间（2f5/2，2p3/2，2p1/2）质子激发产生。随着激发能的增大，N=50的中子满壳被打破,形成中子[(g9/2)-1] d5/2/g7/2粒子一空穴组态,称为核芯激发。同时，在以往的研究中发现，该核区绝大多数核素的能级结构与壳模型计算结果符合的很好。在这些核的壳模型描述中，价质子主要占据f5/2、p3/2、p1/2空穴轨道和p1/2、g9/2粒子轨道。而价中子主要占据g9/2空穴和d5/2、g7/2粒子轨道，在低能部分主要是价质子结构变化引起角动量的增长，而高能部分中子跨越N＝50中子闭壳的核芯激发也开始起到一定作用。在对92Nb同中子素核进行系统学比较时发现，这些同中子素核的能级结构很相似，并且在92Nb核同中子素奇奇核进行比较，发现相同自旋态的能级，随着质子数增加，相应能级的能量降低，这种现象可能是由于质子数目的增加使得质子的激发变得更容易而引起的。另外临近核的高自旋态能级组态的价核子耦合机制具有相似性，利用某些已知原子核能级的组态能够求得相应价核子轨道间的相互作用，然后利用求得的轨道间相互作用来计算其它核素相应组态能级能量，对于确定实验测量能级的组态具有非常重要的意义。
该实验是在中国原子能科学研究院HI-13串列加速器上完成的，利用重离子熔合蒸发反应82Se (14N;4n)92Nb布居了92Nb的高自旋态。14N束流能量为54MeV。靶为质量厚度0.99mg/cm2同位素丰度为96%的82Se和质量厚度为8.27mg/cm2的Yb衬组成。符合事件由9台HPGe－BGO反康谱仪和2台小平面组成。共记录了150×106个二重及以上的符合事件。基于标准的在束谱学实验测量结果，离线将符合数据处理生成一个对称化矩阵和两个不对称的ADO矩阵，采用RADWARE开窗软件分析这些矩阵，在前人工作的基础上，发现了30多条新的射线，根据提取出的ADO系数,初步指定了新发现能级的自旋，在前人建立的能级纲图的基础上，扩展了92Nb已有的能级纲图，其中正负宇称能级自旋被推高21+(Ex=9784.9keV)、21-(10738keV)。
通过分析92Nb核的能级结构，发现其能级结构具有球形核的结构特征，奇奇核92Nb在88Sr核芯外有三个价核子，分别位于质子g9/2轨道和中子p1/2，d5/2轨道。在B.A.Brown有关92Nb的较低能级结构的研究工作中发现，壳模型的计算结果能够很好的重现实验测量值。对于92Nb高自旋态，我们利用准粒子半经验壳模型理论和壳模型Nushellx程序分别计算了92Nb能级能量及相应的组态，分析了92Nb高自旋态能级的价核子耦合机制。并分别把准粒子半经验壳模型与Nushellx程序的计算结果与实验测得值进行比较分析，发现大部分能级的理论计算能够较好的重现实验测量值。
在Plunger的研制过程中，掌握了靶膜展平技术、靶膜与阻停膜的安装、用视频显微镜检测两膜平行度的方法，提高了距离测量精度进而提高寿命测量精度，增加了测量系统的稳定性。由于该装置的建立，使能够测量的能级寿命范围得到扩展，可以获得更为丰富的核结构信息。
This paper describes the establishment of odd-odd nucleus92Nb level scheme andthe study of high spin states. Scmi-Empcrical Shell Model (SESM) and shell modelcalculations (NUSHELLX) have been per formed to interpret the experimental levelscheme of92Nb. A series of methods such as capacitance method, the method to preparevery flat target foil and so on were mastered in the process of building the Plunger.
The research on the high-spin states in the nuclei was regarded as one of thefrontiers of fields in nuclear physics, which can provide us with valuable informationabout nuclear shape,nuclear structure,nucleon coupling and so on.
In recent years, a variety of nuclear structure phenomena in the A～90massregion have attracted great interest. The study of odd-odd nuclei is most important fordetermining the effective proton-neutron residual interaction between quasi-proton andquasi-neutron, have attracted more and more attention. The level structures of odd–oddnuclei more complicated than even–even and odd A nuclei. It is more difficult toinvestigate the high-spin states of odd–odd nuclei due to their complicated levelstructure are uncertain.
In the vicinity of Z=38subshell closure and N=50shell closure，many studiesshowed that the nucleus in this mass region, there are some very interestingphenomena, such as some isotope excited state, the isotope can be maintained for along time before de-excitation to the ground state, the vast majority of the nucleusshow the spherical or near spherical structure and irregular transition energy. Theexcited states are intrinsic single-particle excitations within the fp subspace(2f5/2,2p3/2,2p1/2) proton excitation generated. As the excitation energy increases, N=50neutron closed shell is broken, forming neutrons [(g9/2)-1]/d5/2/g7/2particles-holeconfiguration, called the core excitation. In previous studies previous, the majority oflevel energy are good agree with shell model calculations. In the description of shell model, For the low spin states, the valence protons excitations mainly described theangular momentum of growth and the high spin states attributed to theneutron-particle-hole excitation of the N=50inert core-specifically,the excitation of asingle g9/2neutron across the N=50shell closure into the d5/2/g7/2orbits coupled to thevalence proton states in the π(f5/2, p3/2, p1/2, g9/2) valence space. As the excitationenergy increases, the high spin states might be concern the proton excitation across theZ=38shell gap π(f5/2, p3/2)→g9/2and the neutron the neutron excitation within (d5/2,h11/2) shell model orbitals.
According to systematic analysis and comparison with neighboring isotonesnuclei, their energy level structures were very similarly. And with the number ofprotons increasing,the corresponding energy of levels decreased,this may due to theincrease in the number of protons, it makes that the excitation energy of protonsdecreased.
Systematic comparison in the isotones of N=51region, will help us betterunderstand that the valence nucleon filling mechanisms effect on the energy levelstructure of the nucleus.
Due to the similarity of high spin level configuration adjacent valence nucleonfilled orbits,it may be exactly similar to the corresponding the same spin energy levelstructure. According to known nucleus level configuration,we can extract thecorresponding interactions of the valence nucleon orbits,and calculate the other nucleicorresponding configuration energy level, it is important to determine the energy levelof configurations.
In present work, excited levels in92Nb were populated using a82Se(14N;4n)92Nbreaction at a beam energy of54MeV. The14N beam was provided by the HI-13tandem accelerator in the China Institute of Atomic Energy(CIAE). The targetconsisted of a0.99mg/cm2foil of82Se(isotopically enriched to96%)evaporated onto a8.27mg/cm2natural Yb backing. The de-excitation rays from the reaction residueswere detected with an array consisting of9Compton-suppressed HPGe detectors and2 planar HPGe detectors. A total of about150×106coincidence events were accumulated.Measurement results of standards in-beam spectroscopy experiments were sortedoffine into a two-dimensional symmetrized E-E coincidence matrix and twoasymmetrized ADO matrices. A total of36new transitions have been identified andplaced in this scheme. positive and negative parity level which have been substantiallyextended up to21+(Ex=9784.9keV) and21-(10738keV).
As a conclusion，the level scheme of92Nb has been substantially extended on theprevious level scheme, The Shell Model calculations have been performed performedin the model spaceπ(0f5/2,1p3/2,1p1/2,0g9/2) ν(1p1/2,0g9/2,1d5/2,0g7/2) and the results werein line with the experimental results. The systemical analyzes for the isotones of N=51showed that there were some rules for the high spins level structure of nuclei in thisregion.
The Recoil Distance Doppler Shift (RDDS) method has been well developed. Aseries of methods such as installation of target and the stopper foil and the delicatemeasurement of the distance between the target and the stopper foil and the steps of inbeam experiment were mastered in the process of building the Plunger. And theapparatus will have a good application for ground. The lifetimes of the excited nuclearstates can be measured and more information of the nuclear structure can be gotten byPlunger.
引文
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[32] PIIPARINEN M. et al. High-spin spectroscopy of the142Eu,143Eu and144Eu nuclei[J]. Nucl. Phys. A1996,605,191.
[33] SCHUCK C et al. High-Spin States in154Er and Parallel Proton-andNeutron-Core Breaking[J]. Nucl Phys,1989, A496:385.
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[41] LIU M L et al. High-spin level scheme and decay of the67-μs isomerin142Pm[J]. Phys.Rev.C,2004,70,014304.
[42] CONFORT J R and SCHIFFER J P. Particle-Hole Multiplets in88Y and theMechanism of the (3He,t) Reaction [J]. Phys.Rev.C.1971,4,803.
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[47] GLASMCHER T et al. Evidence for Single Particle Structure ofHigh Spin States in144Pm and145Pm[J]. Phys. Rev. C,1993,47,2586.
[48] RZACA-URBAN T et al. Study of the High-Spin Structure of146Pm[J].Nucl. Phys A,1995,588,767.
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[53] FUNKE L. et a1. Neutron-core excitations in the N=50nucleus89YNuclear Physics A1992,541,241.
[54] HE L et al. Experimental study of high-spin states in stable nucleus91Zr (in Chinese). Sci Sin-Phys Mech Astron,2012,42:249–258.
[55] JAKOB G et a1. Spin dependence of d5/2neutron configurations in thewave functions of92,94Zr from g-factor measurements[J]. PhysicsLetters B,1999,468,13–19.
[56]HAUSMANN M et al. Lifetime study of particle-hole excitations in thesemimagic nucleus93Tc[J]. Phys. Rev. C,2003,68,024309.
[57] GHUGRE S S. et a1High-spin states in94Tc and the shell-modelinterpretation[J]. Phys. Rev. C1995,51,2809.
[58] REIF J., WINTER G, SCHWENGNER R, PRADE H, and KAUBLER L. Break-upof the N=50core in89Y [J]. Nucl.Phys. A,1995,587,449.
[59] RAINOVSKI G. et a1. High-spin structure of the spherical nucleus90Y[J]. Phys. Rev. C,2002,65,044327.
[60] http://www.nndc.bnl.gov/masses/mass.mas03
[61] AUDI G，WAPSTRA A H and THIBAULT C. The AME2003atomic massevaluation.(Ⅱ). Tables, graphs and references[J]. Nucl. Phys.A,2003,729,337.
[62] BROWN B A and RAE W D M2007nushell@MSUMSU-NSCL report.
[63] KHARRAJA B et a1. Level structures of96,97,98Ru at high angularmomentum[J]. Phys.Rev.C,1998,57,83.
[64] KHARRAJA B et a1Nuclear structure of94,95Mo at high spins[J].Phys.Rev.C,1998,57,2903.
[65] GHUGRE S S. et a1High-spin states in97,98Rh[J]. Phys.Rev.C,1998,58,3243.
[66] FUKUCHI T et a1. High-spin isomer in93Mo[J]. Eur. Phys. J. A,2005,24,249.
[67] GLICKSTEIN S S et a1. Neutron inelastic scattering from91Zr[J]. PhysRev C,1974,10,173–179.
[68] RUDOLPH D. et a1. Electromagnetic decay properties of high spinstates in91Tc[J]. Phys Rev C,1994,49,66.
[69] AMUSA A and LAWSON R D. High spin states in91Tc[J]. Z. Phys. A,1983,314,205.
[70] PRAGYA SINGH et a1. Spectroscopy of high spin states in89Mo and90Mo[J]. Phys Rev C,1992,45,2161.
[71] GHUGRE S S and PATEL S B. High-spin states in the odd-odd nucleus92Tc[J]. Phys Rev C,1995,51,1136.
[72] STEFANOVA EA et a1. Structure of high-spin states in89Sr and90Sr[J].Phys Rev C,2001,63,064315.
[73] PATTABIRAMAN N S, CHINTALAPUDI S N, GHUGRE S S et al. Level structureof92Mo at high angular momentum: Evidence for Z=38,N=50core excitation [J]. Phys. Rev. C,200265,044324.
[74] HAUSMANN M et a1. Lifetime study of particle-hole excitations in thesemimagic nucleus93Tc[J]. Phys Rev C,2003,68,024309.
[75] GALINDO E et a1. Transitionstrengths between particle holeexcitationsin95Ru[J]. Phys Rev C,2004,69,024304.
[76] ROTH H A et a1.Gaps in the yrast level structure of the N=50isotones93Tc,94Ru, and95Rh at high angular momentum[J]. Phys Rev C,199450,1330.
[77] GHUGRE S S and DATTA S K. Shell model study of the high spin statesin the N=50isotones92Mo,93Tc,94Ru, and95Rh[J]. Phys.Rev.C1995,52,1881.
[78] ARNELL S E et a1. Yrast level structure of the neutron deficient N=49isotones Tc, Ru, Rh, and Pd up to high angular momentum[J].Phys.Rev.C,1994,49,51.
[79] PRAGYA SINGH, PILLAY R G and SHEIKH J A. Dynamic response functionand large-amplitude dissipative collective motion[J]. Phys. Rev. C,1993,48,1690.
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[81] FUKUCHI T et al. High-spin isomer in93Mo[J]. Eur. Phys. J. A2005,24,249.
[82] GHUGRE S S et a1. Spectroscopy of95Ru at high spins[J]. Phys Rev C,1994,50,1346.
[83] JANSSEN R V F. et al Level structure of94,95,96Tc at high spins andshell‐modelcalculations Phys. Rev. C,1999,61,024302.