超重核合成时的驱动势与热熔合反应截面

在双核模型框架下,双核系统生成超重复合核的机理是由双核中的弹核的核子全部转移到靶核产生的,而核子转移是由双核系统驱动势确定的.对有的反应道,核子转移与中质比变化路径之间有比较复杂的关系.原则上动力学方程与驱动势都应该是中子和质子的二维显函数.为处理方便,采用与中质比相关的核子转移路径的选择来取驱动势,得到了接近实验值的超重核合成蒸发剩余截面. 关键词： 超重核 熔合反应 驱动势 激发函数     

Q值效应对多核子转移反应~(136)Xe+~(208)Pb的影响（英文）

基于改进的量子分子动力学(ImQMD)模型,模拟了~(136)Xe+~(208)Pb在质心系入射能量为E_(c.m.)=617MeV的多核子转移反应。为了研究这个特殊的反应体系,将不同核子数转移前后形成的类弹与类靶对应的Q值的指数概率分布引入模型中考虑Q值效应,并将计算结果与实验探测的总动能损失分布和质量分布进行了比较。我们发现考虑Q值效应后的计算结果更好地再现了实验数据。在少于10个核子的转移过程中,核子的转移明显受到了Q值效应的驱动。在更多的核子转移过程中,Q值效应明显压低了反应产物的生成截面。这是因为大量的核子转移主要发生在小碰撞参数,对应的复合体系的寿命由于Q值效应明显减小。在ImQMD与ImQMD+Q的计算结果比较中,我们还发现,Q值效应明显增加了比208Pb更丰中子的中子数为126的初级碎块生成截面。     

铪的新同位素:186Hf

利用兰州重离子加速器提供的60MeV/u的¹⁸O离子束轰击天然钨靶,通过奇异的多核子转移反应生成¹⁸⁶Hf.采用快速放射化学分离技术从钨及反应产物的混合物中分离出铪,由高纯锗(HPGe)探测器测量样品的γ活性,观测到了¹⁸⁶Hf的子体¹⁸⁶Taγ活性的生长、衰变行为.由此表明:本实验首次合成并鉴别了重丰中子新核素¹⁸⁶Hf,并测定它的半衰期为(2.6±1.2)min.     

Q值效应对多核子转移反应136Xe+208Pb的影响

基于改进的量子分子动力学(ImQMD)模型，模拟了136Xe+208Pb在质心系入射能量为$E_{\rm{c.m.}}$=617 MeV的多核子转移反应。为了研究这个特殊的反应体系，将不同核子数转移前后形成的类弹与类靶对应的$Q$值的指数概率分布引入模型中考虑$Q$值效应，并将计算结果与实验探测的总动能损失分布和质量分布进行了比较。我们发现考虑$Q$值效应后的计算结果更好地再现了实验数据。在少于10个核子的转移过程中，核子的转移明显受到了$Q$值效应的驱动。在更多的核子转移过程中，$Q$值效应明显压低了反应产物的生成截面。这是因为大量的核子转移主要发生在小碰撞参数，对应的复合体系的寿命由于$Q$值效应明显减小。在ImQMD与ImQMD+Q的计算结果比较中，我们还发现，$Q$值效应明显增加了比208Pb更丰中子的中子数为126的初级碎块生成截面。     

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。     

丰中子核反应的HBT效应研究

用同位旋相关的量子分子动力学模型模拟了丰中子弹核引起(Be同位素)的核反应的中子-质子动量关联函数(即HBT), 我们发现在小相对动量的中子-质子的HBT敏感于弹核的结合能, 同时质子--晕中子的HBT依赖于核的单中子分离能. 我们还研究了核态方程和介质中核子--核子截面的对HBT的影响.     

新核素238Th的鉴别

利用60MeV/u的¹⁸O离子束轰击天然铀靶,通过多核子转移反应生成重丰中子新核素²³⁸Th.观测到了²³⁸Th的子体²³⁸Paγ活性的生长、衰变行为,鉴别了²³⁸Th的存在,并测得它的半衰期为(9.4±2.0)min.同时,发现了一条能量为(89.0±0.3)keV、半衰期为(8.9±1.5)min的新γ射线,指定为²³⁸Th的衰变γ射线.     

核子发射数的数学模拟:扩展BUU模型

本文在原来BUU方法的基础上,辨认了靶核与炮弹中的中、质子,同时在模拟核子的初始分布中考虑了小液滴模型修正,引入了表面弥散度、中子皮和形变参数等,使核子的分布更真实,这对丰中子、丰质子核反应以及非对称核反应尤为重要.考虑到中能核反应时,自由核子-核截面中,不同核子间的散射截面大约是同类核子间散射截面的三倍的事实,在BUU碰撞项中体现了这一点,这样可合理地取出不同核子-核子间碰撞流的问题.我们在利用这个扩展的BUU模型研究44Mev/A Kr+Au反应之前,有效地检验了模型的稳定性问题,并在文中合理地定义了核子发射的条件,得到了反应过程中弹核擦碎的中子、质子数与碰撞参数的关系以及类弹靶的关联.     

原子核的大质量转移反应和丰中子超重核的产生(英文)

迄今为止,人们合成的超重核都是缺中子的,无论是熔合-裂变反应还是碎化过程都无法使产物达到周期表的“东北区域”。而重核之间(如U核之间) 的近垒大质量转移反应则可能是目前生成丰中子超重核和达到未知丰中子重核区域的唯一途径。在改进的量子分子动力学(ImQMD) 模型结合统计模型框架内,研究了U+U等反应体系的大质量转移反应,计算了反应产生的初生态碎片的质量和电荷分布,并成功再现了产物的终态质量和电荷分布。通过比较3 个反应136Xe+248Cm,48Ca+248Cm和238U+248Cm 产生的106号元素的截面大小,揭示了U+U等重核大质量转移反应对产生丰中子超重核是非常有利的。For elements with Z > 100 only neutron deficient isotopes have been synthesized so far. The “northeast area” of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes.The large mass transfer reactions in near barrier collisions of heavy (U-like) ions seem to be the only reaction mechanism allowing us to produce neutron rich heavy nuclei including those located at the superheavy(SH) island of stability and unexplored area of heavy neutron-rich nuclides. This study is extremely important for nuclear astrophysical investigations and, in particular, for the understanding of the r process. In this paper within the Improved Quantum Molecular Dynamics (ImQMD) model combining with the statistical-evaporation model, the large mass transfer reactions, like 238U+238U have been studied. The charge and mass distributions of transiently formed primary fragments are investigated within the ImQMD model and de-excitation processes of those primary fragments are described by the statistical decay model. The mass distribution of the final products in 238U+238U collisions is obtained and compared with the recent experimental data. Through compared the formation cross sections of transfermium element 106 by three reactions of 136Xe+248Cm, 48Ca+248Cm and 238U+248Cm, it is explored that the large mass transfer reactions, like U+U are very benefit for the production of SH nuclei.     

40,48Ca+90,96Zr融合反应的动力学模型研究

应用改进的量子分子动力学模型,在严格挑选初始核考虑弹靶结构效应的基础上,研究了近垒和垒上融合反应^40,48Ca+^90,96Zr. 研究表明: 4个反应的理论计算截面与实验值很好符合; 丰中子反应⁴⁰Ca+⁹⁶Zr的垒下融合截面比其他3个反应有明显增强的现象.为了理解丰中子反应⁴⁰Ca+⁹⁶Zr与⁴⁰Ca+⁹⁰Zr相比垒下融合截面增强,而Ca+⁹⁶Zr垒下融合截面没有明显增强的原因, 进一步分析了484个反应的融合位垒,中子转移与融合位垒的关系、中子转移与Q值的关系,结果表明: 正反应Q值会引起核子(特别是中子)转移的增强,从而导致动力学融合位垒的下降和垒下融合截面增强.     

Some regularities in the production of isotopes with 4 ≦ Z ≦ 9 in the interaction of 22Ne with 232Th

In the bombardment of ²³²Th with ²²Ne ions with an energy of 172 MeV the energy spectra and production cross sectionslfor isotopes of elements ranging ifrom Be to F have been measured at an emission angle of 12°. It is shown that all of the isotopes detected have been produced by deep inelastic collisions of the initial nuclei, i.e. the kinetic energies of the reaction products are close to the exit Coulomb barriers. It is found that the energy spectra widths (FWHM), relative yields at 12° and 40° and the Q_gg dependences of isotopic production cross sections differ considerably for stable, neutron-deficient and neutron-rich isotopes. This difference can be interpreted as being due to a contribution from secondary processes such as α-particle and nucléon evaporation from the excited ²²Ne and light transfer reaction products. The data obtained can be employed to choose optimal conditions for the detection of the extremely neutron-rich isotopes of light elements produced in multinucleon transfer reactions.     

Heavy ion induced transfer reactions on 148Nd

Transfer reactions induced by ¹⁶O and ¹⁸O beams on ¹⁴⁸Nd were measured with a time-of-flight setup at 72 MeV incident energy. The angular distributions are bell shapes having their maxima at angles somewhat below the grazing angle. The excitation in the final nuclei takes place (if possible) near the optimum Q-value and is spread over 5 MeV for the one-particle transfer reactions and up to 10 MeV for the multiparticle transfers. The cross sections for the individual channels are explained mostly by Q-window considerations. In spite of the differences in the individual channels the total transfer cross section integrated over excitation energy, angle, and all channels turns out to be the same same for both ¹⁶O and ¹⁸O beams. This cross section amounts to 20 % of the total reaction cross section and nicely fills the gap between the measured fusion cross section and the total reaction cross section obtained from optical model calculations based on elastic scattering data.     

New neutron-rich 179Yb and 181,182Lu isotopes produced in reactions of 9 MeV/u 136Xe ions on tantalum and tungsten targets

The new neutron-rich isotopes ¹⁷⁹Yb and ^181,182Lu were produced in multinucleon transfer reactions by irradiating ^natW/Ta targets with 9 MeV/u ¹³⁶Xe ions, and identified by mass separation and decay spectroscopy. The measured half-lives of ¹⁷⁹Yb, ¹⁸¹Lu and ¹⁸²Lu are 8.1±0.8, 3.5±0.3 and 2.0±0.2 min, respectively. The properties of the excited states of ^181,182Hf are discussed. The possibility of studying neutron-rich nuclei outside the classical fission-product regions is demonstrated.     

The 51V(n, γ)52V reaction studied with polarized neutrons and polarized vanadium nuclei

Circular polarization of γ- radiation following capture of polarized thermal neutrons by unoriented nuclei and the angular distribution of such radiation, using polarized neutrons and polarized nuclei, made it possible to assign spins to 13 levels of ⁵²V. In addition the admixtures of spin-3 and spin-4 reaction channels could be derived for 20 primary γ-ray transitions. With this information interference phenomena in the ⁵¹V(n, γ) reaction were studied. The low-lying levels of ⁵²V are compared with shell-model calculations. The capture mechanism is discussed in terms of a doorway-state model.     

Particle transfer reactions with the time-dependent Hartree-Fock theory using a particle number projection technique

A particle-number projection technique is used to calculate transfer probabilities in the 16O+208Pb reaction below the fusion barrier. The time evolution of the many-body wave function is obtained with the time-dependent Hartree-Fock (TDHF) mean-field theory. The agreement with experimental data for the sum of the proton-transfer channels is good, considering that TDHF has no parameter adjusted on the reaction mechanism. Some perspectives for extensions beyond TDHF to include cluster transfers are discussed.     

Coincidence spectroscopy of pion-nucleaus interactions in 60Ni

The preevaporation phase of the interaction of 70 MeV pions with ⁶⁰Ni has been studied by measuring multinucleon removal spectra via prompt γ-ray spectra in coincidence with energetic charged particles. The data provide a direct estimate of the reaction cross section due to inelastic scattering and of the multiplicity of energetic protons emitter by the excited nucleus.     

The interaction of 538 MeV 56Fe with 238U

Binary coincident fragments from the ⁵⁶Fe+²³⁸U reaction at 538 MeV have been studied. Fragment energies were measured and fragment masses determined by a kinematic method. Three types of event are defined by suitable adjacent limits in the mass versus energy event space. The angular distributions of cross section, energy and mass have determined for each event region and particular attention paid to that part containing possible fissions following complete fusion. The total reaction cross section consists of about 50 % of binary fragmentation channels. The remaining channels correspond to a ternary process of heavy recoil fission.     

Nuclear reactions of 24 MeV 7Li on 10B

The charged-particle reaction channels accessible in the bombardment of ¹⁰B targets by ⁷Li nuclei of 24 MeV incident energy are investigated by measuring all ejectiles up to Be. Except for tritons, the reactions with light ejectiles proceed mainly via the formation of a compound nucleus, whereas some of the reactions with ejectiles heavier than the α-particle can be described by a direct mechanism.     

Giant resonances in the fused system induced by 40Ca + 40Ca reactios

The giant quadrupole resonance of the fused system obtained in a ⁴⁰Ca + ⁴⁰Ca reaction is investigated in the framework of the TDHF approximation. A large splitting of the resonance due to the deformation of the composite system is observed. These results are confirmed by a direct fluid-dynamical calculation of the resonance. An estimation of the energy stored in the resonance is also presented.     

The (d, 6Li) reaction on 24Mg, 26Mg and 28Si at 35 MeV

Data for the (d, ⁶Li) reaction on targets of ²⁴Mg, ²⁶Mg and ²⁸Si have been obtained at 35 MeV bombarding energy. Angular distributions were measured for low-lying states in the residual nuclei. Zero-range distorted-wave Born approximation (DWBA) calculations have been used to analyze the data. The DWBA calculations account for the shapes of the experimental distributions reasonably well. The observation of significant population of unnatural parity states implies, however, that other transfer mechanisms may be important. The experimental spectroscopic factors are in qualitative agreement with those obtained from SU(3) theory.