Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in²³⁵U(n _th,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.     

Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997     

Dissociation and fission of small sodium and strontium clusters

Fission of charged small sodium and strontium clusters has been studied by the ab initio density functional theory methods. Dissociation energies and fission barriers have been calculated for all possible fission channels for the Na₁₀²⁺ and Sr₇²⁺ clusters. The dissociation energies and fission barriers have been analyzed as functions of the daughter fragment size.     

Investigation of polar emission in 252Cf and 235U + nth fission

The energy distributions and relative intensities of protons, deuterons, tritons and α-particles emitted along the fission axis during spontaneous fission of ²⁵²Cf were measured simultaneously with both fission fragment energies. The absolute intensity of particles, the mass distribution of fragments, the total kinetic energy and total excitation energy of both fragments were subsequently deduced from the experimental data. Statistical model calculations based on a hypothesis that the polar particles are evaporated from fission fragments have been performed for ²⁵²Cf and ²³⁶U fission. Although some experimental results agree remarkably well with the evaporation hypothesis, the considered model cannot describe many features of the polar emission phenomenon.     

Branching ratios of successive emission (up to three) of C2n + (n = 1–5) fragments in asymmetrical fission processes of C60 r+ ions (r = 4–6)

Highly charged C₆₀ molecules are produced in collisions between neutral C₆₀ and multiply charged ions within a large range of temperatures. Successive emission of one, two or three light monocharged fragments referred as one-, two- and three-step processes have been observed. The experimental mass branching ratios for the emission of one C_2n⁺ fragment from C₆₀⁶⁺, C₆₀⁵⁺ and C₆₀⁴⁺ ions are compared with the theoretical values using a statistical model. From hotter C₆₀⁶⁺ ions, branching ratios for three-step processes have been measured and the data are in good agreement with an estimation using the branching ratios in one-step process.     

Studies of mass and energy correlations in thermal neutron fission of U-235 accompanied by long range alpha particles

Several characteristics of fission accompanied by long range alpha particles (LRA) have been studied in the thermal neutron induced fission of²³⁵U. The kinetic energies of fission fragments and the LRA were measured with a back-to-back ionization chamber and semiconductor detectors respectively. The kinetic energies of the two fragments and the LRA in LRA fission, along with the energies of pair fragments in the normal binary fissions, were recorded event by event on a magnetic tape by means of a four-parameter data acquisition system. The data were analysed to study the dependence of different quantities in LRA fission on the fragment mass ratio, LRA energy and the total kinetic energy of the fission fragments. It is seen that the most probable energy of LRA increases significantly for near symmetric mass divisions. The total kinetic energy for all mass ratios in LRA fission is found to be (2.6±0.7) MeV larger than that in binary fission. The difference in the total kinetic energies in LRA and binary fissions is seen to be dependent on mass ratio. This result may suggest that the scission configuration in LRA fission is different for different mass ratios. Correlations between the fission fragment and LRA energies have been studied for several mass ratios. It is seen that the most probable fragment kinetic energyĒ _k varies nearly linearly with the LRA energyE _a for various mass divisions but the variation of the most probable LRA energyĒ _a with fragment kinetic energyE _k is found to deviate from linearity for several mass ratios. From a least square fit to the variation ofĒ _k withE _a it is found that the slope (dĒ _k/dE_a) increases with the increase in mass ratio. The present results are discussed to arrive at a better understanding of the scission configuration in the fission accompanied by LRA emission.     

Mass distribution studies in the <Superscript>19</Superscript>F + <Superscript>197</Superscript>Au reaction

Mass distribution and evaporation residue measurements have been carried out in the reaction ¹⁹F + ¹⁹⁷Au using the recoil catcher technique followed by off-line γ-ray spectrometry. The random neck rupture model (RNRM) has been used to compute the variance of the mass distribution ( σ²_A) and the average kinetic energy ( ˉ) of the fission fragments for the present system. The results of model calculations have been found to be in good agreement with the experimental observations. Measured evaporation residue cross-sections have been compared with the statistical model calculations.     

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

A theoretical study of the Y$\mathsf{_{3}}$O clusters

Hybrid density functional calculations are performed to study the structural and electronic properties of neutral, anionic and cationic Y₃O clusters. The most stable structures of these clusters are found to be triply bridging oxygen atom structures with C_S symmetry. The ground states of Y₃O, Y₃O^- and Y₃O ⁺ are doublet (²A), triplet (³A) and singlet (¹A), respectively. The calculated electron affinities and ionization potentials are in good agreement with the available experimental data. Time-dependent density functional theory is used to calculate the low-lying excited states. A theoretical assignment for the features in the experimental photoelectron spectra is given.Received: 5 November 2003, Published online: 20 January 2004PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 31.15.Ew Density-functional theory - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.)     

Fission-mode calculations for 239U,a revision of the multi-modal random neck-rupture model

In the course of an experimental study of the fragment characteristics after neutron-induced fission of ²³⁸U with incident neutron energies between 1.2 and 5.8 MeV fission-mode calculations in the frame of the multi-modal random neck-rupture model have been performed. During these calculations some technical parts of the model have been revised. The identification of fission modes is now based on unequivocal and reproducible criteria. The Rayleigh criterion has consequently been applied to determine each possible scission configuration. The most remarkable new results are that all physically relevant fission modes branch off in the second potential minimum exhibiting different outer barriers, and that the total kinetic energy distribution of the fission fragments is a direct consequence of the Rayleigh criterion. The fragment characteristics as the mean mass, the mean total kinetic energy and the corresponding width obtained from the fission-mode calculations compare reasonably well with the experimental findings.For the first time the weighted fission cross sections through each particular fission mode have been analyzed simultaneously using a Hill-Wheeler type expression for the transmission through a double-humped fission barrier. The results support the picture of individual outer barriers with slightly different penetrabilities and a slightly lower inner barrier.     

Kinetic energy distributions of the fragment ions from multiply ionized C2H6 as functions of the charge state of the intermediate states

Here, we report the kinetic energy distributions (KEDs) of the fragment ions produced from multiply ionized ethane (C₂H₆) molecules in single electron capture collisions with 1.2 MeV C²⁺. To systematically investigate the fragmentation dynamics, the KEDs were obtained as functions of the charge state of the intermediate C₂H₆^r+* ions r transiently generated prior to fragmentation. r was determined from coincidence measurement of the fragment ions and the number of emitted electrons. The KEDs are drastically different depending on the number of broken C–H bonds. The underlying causes are explained by the variation of the relative contributions of the multiply ionized states and preferential fragmentation pathways. For instance, CH_n⁺ fragment ions with smaller n exhibit lower KEs because they are likely to be correlated with H⁺ emission, which carries away a large portion of the KE release. In addition, we report the KEDs of H₃⁺ produced from doubly and triply charged states.     

纳秒激光诱导C60分子碎裂中轻碎片离子C+n(n＜30)的产生机理研究

采用飞行时间质谱计测量了纳秒激光诱导C₆₀分子碎裂中轻碎片离子C⁺_n(n≤11)的初始平均动能,结果显示轻碎片离子具有相同的初始平均动能(约为0.34eV),并且该动能在一定范围内不随激光通量的变化而明显改变.结合前人的实验结果,对纳秒激光诱导C₆₀分子碎裂中轻碎片离子C⁺_n(n＜30)的主要产生模式作了新的阐述,即C< 关键词： 飞行时间质谱计 轻碎片离子 笼形结构塌陷 初始动能     

Beta-delayed fission and neutron emission: Consequences for the astrophysicalr-process and the age of the galaxy

Beta-delayed neutron emission and beta-delayed fission probabilities (P _n andP _{β d j} ) were calculated for neutron-rich nuclei between the beta stability line and the neutron-drip line in the range 10≦Z≦100 and 75≦Z≦100, respectively. These results were obtained by applying recentβ-strength function calculations, fission barrier height predictions, and a neutron optical potential from infinite nuclear matter calculations. An area of ～100% fission probability is predicted around Z=94,N=168 extending beyond the well-known island of spontaneous fission in that region. Astrophysicalr-process calculations were performed including the calculatedP _n and Pβ d f values. This puts the method to determine the age of the Galaxy by the actinide chronometers on a reliable basis. An excellent overall agreement with the observedr-abundance distribution is obtained. The predictedr-process production ratios for the Chronometrie pairs²³²Th/²³⁸U,²³⁵U/²³⁸U and²⁴⁴Pu/²³⁸U result in an age of the Galaxy oft _G =(20.8± ₄ ² )×10⁹ a, which is by almost a factor two larger than earlier predictions by this method, but in accordance with recent astronomical observations from globular clusters. The predicted island of 100%β-delayed fission acts as a sink to ther-process with the consequence that no superheavy elements are produced in nature.     

ConCm±(n=1-5; m=1,2)团簇的密度泛函理论研究

本文采用基于自旋极化的密度泛函理论系统研究了Co_nC_m^± (n=1-5; m=1,2)团簇的几何结构和电子结构特性. 将Co_nC^± (n=2-5)团簇中的一个Co替换为C原子, 个体的基态几何结构发生明显变化; 在Co_nC₂^± (n=1-5)团簇的生长序列中, 发现从n=3开始团簇中的两个C原子有彼此分离分布的趋势, 我们分析, 这是Co金属能够维持单壁碳纳米管(SCNTs)保持开口生长, 成为非常有效的一种催化剂的重要原因. 同时, 将Co_nC^± (n=2-5)团簇中添加一个Co原子后系统的总磁矩出现大幅下降的趋势, 但仍保持奇偶交替的规律. 通过比较中性及带电的Co_nC以及Co_nC₂ (n=1-5)团簇的碎裂能, 本工作发现: 由实验获取的SCNTs应均为带正电的体系, 这一结论与已有的实验模型拟合得很好.     

关于Al_nC和Al_nC~+(n=1—8)团簇结构和稳定性的研究

通过采用7种密度泛函理论DFT方法对AlnC进行计算,所得结果与实验数据比较,选择了B3lyp方法和6-311G(d)基组对AlnC及AlnC+(n=1—8)团簇进行结构优化和频率分析,得到了AlnC及AlnC+基态以及亚稳态结构.当n从小到大变化时,这些团簇的结构从平面向立体过渡,平面构型以三角形为主,立体构型主要是三棱柱笼状结构;在这些团簇中的高对称性结构中,中性团簇和阳离子只能有其一是稳定构型;在所研究的团簇中,Al2C和Al5C团簇较为稳定.     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002     

Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.     

Light particle accompanied quasifission in superheavy composite systems

The dynamics of the excited superheavy system with Z = 118 in the reaction ⁸⁶Kr + ²⁰⁸Pb at E _Kr = 460, 500, and 600 MeV has been investigated. The mass and kinetic energy of binary fragments were measured by the time-of-flight method. Double-differential distributions of protons and α particles were measured in coincidence with fragments. The proton spectra can be described considering only evaporation from fragments. Evidence of the neck fragmentation was obtained from analysis of double-differential α spectra. Properties of the α-particle neck fragmentation component are close to those known from the ternary fission of actinide nuclei, but the multiplicity is much larger than can be expected from extrapolation of the ternary fission data. The text was submitted by the authors in English.     

Compact variational wave functions for bound states in three-electron atomic systems

The variational procedure to construct compact and accurate wave functions for three-electron atoms and ions is developed. The procedure is based on the use of six-dimensional Gaussoids written in the relative four-body coordinates r ₁₂, r ₁₃, r ₂₃, r ₁₄, r ₂₄, and r ₃₄. The nonlinear parameters in each basis function have been carefully optimized. Using these variational wave functions, we have determined the energies and other bound state properties for the ground 1² S-states in a number of three-electron atoms and ions. The three-electron atomic systems considered in this work include the neutral Li atom and nine positively charged lithiumlike ions: Be⁺, B²⁺, C³⁺, ..., Na⁸⁺, and Mg⁹⁺. Our variational wave functions are used to determine the hyperfine structure splitting and field shifts for some lithium-like ions. The explicit formulas of the Q ⁻¹ expansion are derived for the total energies of these three-electron systems. The article is published in the original.