电子和原子核散射研究原子核结构(英文)

本文综述了近年来我们组利用电子散射结合相对论平均场模型对奇特核结构的研究。我们发展了相对论平均场框架下的磁电子散射方法,并用其研究一些中子晕及质子晕核的基态组态,例如,23O,17,19C和23Al。研究发现,原子核不同组态的弹性磁形状因子彼此差别很大。其次,我们发展了相对论平均场框架下的弹性库伦电子散射方法,并用该方法研究了奇特核的电荷分布。研究发现,丰质子核中扩展的电荷密度分布可以通过库伦电子散射来测量。这种方法还被进一步推广用于计算弹性宇称不守恒电子散射,研究了一些典型原子核的中子密度分布,例如,Ca同位素链,N=50同中子素链以及N=Z的双幻核。结果表明,宇称不守恒非对称度的振幅主要由质子和中子形状因子极小值之间的距离决定。这些结果为下一代电子-核对撞机上的电子散射实验提供了有用的参考。     

原子核结构的相对论平均场描述

简要评述近几年相对论平均场理论的发展及其在核结构研究中取得的部分进展. 主要内容包括中子晕, 质子晕, 巨晕, 激发态晕等奇特核现象, 超重核结构, 超核结构以及原子核的赝自旋对称性和反核子谱的自旋对称性等.     

电子和原子核散射研究原子核结构

本文综述了近年来我们组利用电子散射结合相对论平均场模型对奇特核结构的研究.我们发展了相对论平均场框架下的磁电子散射方法,并用其研究一些中子晕及质子晕核的基态组态,例如,23O,17,29C和23Al.研究发现,原子核不同组态的弹性磁形状因子彼此差别很大.其次,我们发展了相对论平均场框架下的弹性库伦电子散射方法,并用该方法研究了奇特核的电荷分布.研究发现,丰质子核中扩展的电荷密度分布可以通过库伦电子散射来测量.这种方法还被进一步推广用于计算弹性宇称不守恒电子散射,研究了一些典型原子核的中子密度分布,例如,Ca同位素链,N=50同中子素链以及N=Z的双幻核.结果表明,宇称不守恒非对称度的振幅主要由质子和中子形状因子极小值之间的距离决定.这些结果为下一代电子-核对撞机上的电子散射实验提供了有用的参考.     

利用直接核反应研究轻丰中子核的奇特结构

直接核反应(包括弹性散射、非弹性散射、转移反应等)是研究丰中子核奇特结构的常用实验手段之一。以6,8He、11Li、11,12Be、14,15B、15,16C等N=8附近核的结构研究为例，综述了如何利用这几种反应从实验上研究丰中子核的奇特结构。利用6,8He，11Be等晕核在质子和氘靶上的弹性散射数据，拟合得到了这些反应体系的有效相互作用(光学势)。利用16C在质子和氘靶上的非弹性散射数据，抽取了16C 的形变参数，结果表明16C的形变不可忽略。利用单核子转移反应，定量研究了8He, 11,12Be等丰中子核基态及低占据态中的p-，s-，以及d-波成分的强度。结果表明:8He基态中的4个价中子，除了可以构成正常的 (0p_3/2)4 组态外，还有一定的几率构成($ 0 p_{3/2} $)2(0p_1/2)2组态；12Be的基态以d波闯入为主，而其临近的11Be的基态以s波闯入为主。     

中能重离子碰撞中晕核反应动力学

基于同位旋相关量子分子动力学研究了中子晕核和质子晕核的核反应动力学，着重研究了松散的晕结构对于重离子碰撞中的碎裂和动量耗散中的特殊作用. 为了突出晕核松散结构在反应动力学中的特殊作用. 同时计算了在完全相同入射道条件下相等质量稳定弹核引起核反应. 通过对晕核和稳定核核反应动力学结果的对比分析，发现晕核的松散结构对核反应动力学的作用和影响是重要的. 如这种松散结构明显的增加了碎裂多重性；相反, 减小了原子核阻止(动量耗散). 关键词： 中子晕核 质子晕核 重离子碰撞 核反应动力学     

放射性核束引起反应中轻带电粒子发射的同位旋效应

研究了69MeV/u³⁶Ar轰击Be靶的碎裂产物与Si靶反应引起的前角区轻带电粒子的发射产额与原子核同位旋、原子核序数的关系.发现前角区轻带电粒子发射产额对于丰质子核的结构非常敏感,与同位旋存在一种指数关系.因此可以把前角区轻带电粒子发射产额作为探测丰质子核结构的一种手段,也是一种新的验证一个核是否具有质子晕结构的实验证据.     

利用中高能破碎和敲出反应研究非稳定核结构

中高能区的破碎和敲出反应,由于机制相对简单、对核的表面敏感、理论处理比较成熟等原因,在非稳定核的晕结构、壳移动等奇特性质研究中发挥了独到作用。针对次级束流强度较弱、能散较大等特点,实验上发展和应用了逆运动学有效立体角完全测量、轻重靶结合、高精度消色差磁谱仪、前向中子谱仪、在束γ测量等技术手段,大大提升了实验的探测能力和选择性。从物理问题探索和实验路线演变的角度,回顾了非稳定核破碎和敲出反应的发展过程与启示,并探讨未来的走向。     

相对论和非相对论理论模型研究奇异核的性质

采用相对论和非相对论理论模型可描述奇异核的性质 .相对论平均场理论预言了2 6,2 7,2 8P存在一个质子晕 ,而27,28,29S存在两个质子晕 .最近 ,MSU的最新的实验发现了2 6,2 7,2 8P核存在一个质子晕.采用相对论 Hartree- Fock理论研究了 Fock项和矢量介子对奇异核性质的贡献,研究表明交换项和矢量介子对非常丰中子核性质的影响非常不同于对稳定线附近核性质的影响.同时,采用形变的Hartree- Fock- Bogoliubov理论研究了某些轻核同位素链的性质和形变.     

简单的原子极端的核——对奇异氦的激光捕捉与探测

氦-8 (8He) 是丰中子核.它由2个质子与6个中子组成,平均寿命为0.2s,其中子丰度是在已知的所有原子核中最高的.氦-8的结构中有一个紧密束缚的α粒子,环绕于α粒子有4个松散的中子,其分布范围较大,形成中子晕.因为氦-8有这特殊的结构,它为研究基础核作用力提供了一个有意义的系统.文章作者近期使用原子阱及激光冷却技术成功地捕捉到这个奇异的氦同位素,并对阱内单原子做了精密的激光光谱测量,由此首次得到氦-8的核电荷半径.这个结果可与不同的核结构理论计算值相比较,有助于了解在极端丰中子环境中的核作用力以及核结构.     

远离β稳定线奇异核性质的理论研究

回顾了对远离β稳定线奇特核性质的理论研究现状，并用相对论平均场理论研究了远离β稳定线奇特核的性质．包括轻核的中子晕和质子晕、远离β稳定线核的壳效应和超重核的性质等.The present situation of studies on exotic nuclei far from the β stable line is simply reviewed and then the relativistic mean field study on these nuclei has been carried out. This includes studies on neutron halos and proton halos in light nuclei, on nuclear shell effects of nuclei far from the stability, and on the properties of superheavy nuclei.     

Total interaction and proton-removal cross-section measurements for the proton-rich isotopes 7Be, 8B,and 9C

In an experiment performed at the FRS of GSI, we measured total interaction cross sections for ⁷Be, ⁸B, and ⁹C, one-proton-removal cross sections for ⁸B and ⁹C as well as two-proton-removal cross sections for ⁹C on targets ranging from carbon to lead at an energy of 285 MeV/nucleon. In addition, we performed measurements at 142 MeV/nucleon for ⁸B. The experimental results are compared to different calculations. Glauber-type calculations with different model·density distributions show that, down to incident energies of about 50 MeV/nucleon, total interaction cross-section measurements with light targets are not sensitive to an extended proton distribution in ⁸B. However, at lower incident energies, a tail in the proton density distribution is needed to explain the total interaction cross sections. Total interaction cross-section measurements with high-Z targets in the present experiment show a significant increase of the cross sections due to low-lying electromagnetic strength.     

Fusion cross sections for 10,11B+12C at sub-Coulomb energies

Total fusion cross sections for the ¹⁰B + ¹²C and ¹¹B + ¹²C reactions have been determined over a 5 MeV (c.m.) energy range extending to ≈ 3 MeV below the Coulomb barrier. Absolute γ-ray yields for specific transitions in the de-excitation of the heavy products following compound nucleus decay were measured using a Ge(Li) detector. Statistical model calculations of the decay modes of the compound nucleus have been used to deduce, from the γ-ray data, cross sections for single proton, neutron and α-particle emission, and to determine total cross sections for compound nucleus formation. No evidence has been found for sub-Coulomb resonances in either reaction. The total reaction cross sections are compared with optical model calculations using different parameter sets and the observed trend in the very low energy cross sections is discussed relative to other reactions in the same mass region.     

Isotopic effects in high-energy nuclear reactions and isospin correlations of fragmentation cross sections

The experimental study of cross sections and kinematic characteristics of H, He and Li isotopes produced in the interaction of 660 MeV protons with separated isotopes of the target nuclei B, Ni, Sn and Sm, has been performed. The two-dimensional isospin correlation of cross sections for secondary particles in the nucleon-nucleus inelastic interaction is discussed. The systematization formula for fragmentation cross sections in broad regions of fragment and target nucleus masses over a large interval of incident proton energies is given.     

中能Abrasion-Ablation模型的核内核子密度分布修正

将具体的核内核子(包括中子、质子)密度分布引入到中能Abrasion-Ablation模型中,用修正后的模型计算了30MeV/u ⁴⁰Ar+^natAg反应中的弹核碎裂过程.结果表明,在非常周边的碰撞中,修正后的模型计算的碎片截面与修正前的计算相比有明显的差别,反映了核密度边缘弥散的影响.与实验结果的比较验证了中能弹核碎裂碎片能量相对于束流能量的降低是由摩擦作用引起的.     

The (n, α) reaction on 11B induced by 14.4 MeV neutrons

The ¹¹B(n, α)⁸Li reaction has been studied at 14.4 MeV in a geometry that allows simultaneous detection of the emitted and recoil particles. The angular distributions have been measured for the transitions to the ground and first excited states of the ⁸i nucleus and compared with PWBA for knock-on and heavy-particle pick-up processes. The total cross sections for the respective transitions have been deduced.     

色散耦合道光学模型对28Si的核子散射数据分析

在光学势中引入色散关系, 相比于通常的耦合道光学模型, 极大地减少了光学势 参数的个数. 采用基于软旋转子模型的耦合道方法对较轻形变核²⁸Si核的中子和质子散射实验数据进行了计算分析, 拟合得到一组色散光学势参数. 计算的中子总截面、以及中子、质子的弹性与非弹性散射微分截面与实验数据有较好的符合.     

The scale dependent nuclear effects in parton distributions for practical applications

The scale dependence of the ratios of parton distributions in a proton of a nucleus A and in the free proton, , is studied within the framework of the lowest order leading-twist DGLAP evolution. By evolving the initial nuclear distributions obtained with the GRV-LO and CTEQ4L sets at a scale , we show that the ratios are only moderately sensitive to the choice of a specific modern set of free parton distributions. We propose that to a good first approximation, this parton distribution set-dependence of the nuclear ratios can be neglected in practical applications. With this result, we offer a numerical parametrization of for all parton flavours i in any , and at any and any for computing cross sections of hard processes in nuclear collisions. Received: 10 August 1998 / Published online: 27 April 1999     

Charmonium production in deep inelastic scattering at HERA

The electroproduction of and mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers and photon-proton centre of mass energies GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic production is measured as a function of and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic and meson production is measured as a function of . The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions. Received: 10 March 1999 / Published online: 8 September 1999     

Differential cross sections of proton compton scattering at photon laboratory energies between 700 and 1000 MeV

Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. 78 data points are presented as angular distributions at photon lab energies of 700, 750, 800, 850, 900, and 950MeV. The c.m. scattering angle ranges from 40°–130°, corresponding to a variation of the four momentum transfer squared betweent=?0.10 tot=?0.96 GeV² at 700 and 950 MeV, respectively. Two additional differential cross sections have been measured at 1000MeV, 35.6° and 47.4°. The angular distributions show forward peaks whose extrapolations to 0° are consistent with calculated forward cross sections taken from literature. The small angle data (|t| ?0.2 GeV²) together with the calculated cross sections at 0° are also consistent with the assumption of a slope parameterB of 5 GeV^?2. For the first time a rerise of the angular distributions towards backward angles has been observed. It becomes less steep with increasing energy. The most interesting feature of the angular distributions is a sharp structure which appears betweent=?0.55 GeV² at 700MeV andt=?0.72 GeV² at 950 MeV. Such a rapid varation of the differential cross section witht has never been ovserved in elastic hadron-hadron scattering or photoproduction processes. It indicates the existence of a dynamical mechanism which could be a peculiarity of Compton scattering.     

丰质子核反应中轻带电粒子发射的同位旋效应

采用同位旋相关的Boltzmann—Langevin方程计算了核素^12-15N和^17-20Ne反应中轻带电粒子发射的同位旋效应。^12-15N与^28Si靶的反应结果显示轻带电粒子的产生截面有明显的同位旋效应,^12N的轻带电粒子产生截面突然增大,与实验得出的结论相同,由此检验了所采用的计算方法的可行性。同时还计算了^17-20Ne与^9Be靶的反应,发现^17Ne的轻带电粒子产生截面也是突然增大,并且其质子分布有较大的弥散,据此认为^17Ne可能具有晕结构。 Within the Boltzmann-Langevin equation, the isospin effects on the production cross sections of light charged particles （LCP） in the reaction of proton rich nuclei were studied. We have calculated the LCP productio n cross sections of the reaction ^12-15N ＋ ^28Si at 55 MeV/u and ^17-20Ne ＋ ^9Be at 60 MeV/u. The LCP production cross sections of ^12N increase abnormally in comparison with those of ^13-15N. The result accorded with experimental one. It also happened to ^17-20Ne. It suggests that ^17Ne is probably a proton halo nucleus.