More Detailed Study of Deformation Effects in Prescission Particle Emission by a Diffusion Model

Deformation effects on particle emission in a fission process of 251Es nucleus as functions of excitationenergy, angular momentum, and viscosity coefficient have been investigated in detail within the framework of Smolu-chowski equation. Our calculations show that high excitation energy, low angular momentum, and large viscosity willenhance the influence of deformation on multiplicity of prescission particles, and that the roles of these three parameterswill become weak with decreasing deformation.     

Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

Independent isomeric yield ratios of¹³²I were radiochemically determined in alpha particle induced fission of²³⁸U in the energy range 25–44 MeV. Fission fragment angular momenta were deduced from the measured isomeric yield ratios using spin dependent statistical model analysis. It was seen that angular momentum of¹³²I increases with increase of excitation energy and angular momentum of the fissioning nucleus. Comparison of the present data on¹³²I in²³⁸U(α,f) with the literature data for the same product in²³⁸U(p, f) and²³⁸U(γ, f) at various excitation energies show that fragment angular momentum strongly depends on the input angular momentum in the range of excitation energy considered. Experimental fragment angular momentum at all excitation energies were seen to be in agreement with the theoretical values calculated based on thermal equilibration of the various collective rotational degrees after considering the occurence of multichance fission. Thus, strong effect of input angular momentum as well as the statistical equilibration among the various collective rotational degrees of freedom in medium energy fission is corroborated.     

On the impact of large amplitude pairing fluctuations on nuclear spectra

The influence of large amplitude pairing fluctuations is investigated in the framework of beyond mean field symmetry conserving configuration mixing calculations. In the numerical application the finite range density dependent Gogny force is used. We investigate the nucleus ⁵⁴Cr with particle number and angular momentum projected wave functions considering the axial quadrupole deformation and the pairing gap degree of freedom as generator coordinates. We find that the effects of the pairing fluctuations increase with the excitation energy and the angular momentum. The self-consistency in the determination of the basis states plays an important role.     

Microscopic calculations of fission barriers and critical angular momenta for excited heavy nuclear systems

An extended version of Strutinsky's macro-microscopic method is used to calculate effective potential energies for rotating, excited heavy compound nuclei undergoing fission. Nuclear deformation is parameterized in terms of Lawrence's family of shapes. A two-center single-particle potential corresponding to these shapes is employed, with BCS pairing added. Statistical excitation is introduced by temperature-dependent occupation of (quasi-) particle energy levels. We calculate shell corrections to the energy, the free energy and the entropy as functions of deformation and temperature. The associated average quantities are derived from a temperature-dependent liquid drop model. The resulting static deformation energy is augmented by the rotational energy to yield the isothermal effective potential energy as a function of deformation, temperature and angular momentum. Moments of inertia are obtained from the adiabatic cranking model with temperature-dependent pairing included.We have also calculated the effective potential for constant entropy rather than constant temperature. Although this isentropic process physically is more appropriate than the isothermal process, it has not been treated before. For the same amount of excitation energy in the spherical state of the compound nucleus, the isentropic barriers turn out higher than the isothermal ones. For both processes we have extracted the critical angular momentum (defined as the one for which the barrier approximately vanishes) as a function of excitation. Our model is applied to the super-heavy nuclei ²⁷⁰110, ²⁷⁸110, ²⁹⁸114, ²⁹²118 and ³²²128, which have been tried to form in krypton and argon induced heavy ion reactions.     

Nuclear Level Density with Non-zero Angular Momentum

The statistical properties of interacting fermions have been studied for various angular momentum with the inclusion of pairing interaction. The dependence of the critical temperature on angular momentum for several nuclei,have been studied. The yrast energy as a function of angular momentum for 28 Si and 24Mg nuclei have been calculated up to 60.0 MeV of excitation energy. The computed limiting angular momenta are compared with the experimental results for 26Al produced by 12C 14N reaction. The relevant nuclear level densities for non-zero angular momentum have been computed for 44Ti and l36Ba nuclei. The results are compared with their corresponding values obtained from the approximateformulas.     

超重复合核的存活概率

考虑角动量相关性的情况下计算了超重核259Db的裂变位垒Bf.计算了258Rf,259Db,266Hs和267Mt 4个超重核单中子蒸发的存活概率Wsur及其随角动量分波的变化关系.给出了中子蒸发宽度、裂变宽度和超重核蒸发一个中子的概率对激发能与角动量分波的依赖关系. Considering the dependence on angular momentum， the fission barriers of super heavy nuclei 259Db have been calculated. The survival probability for 258Rf， 259Db，266Hs，267Mt compound and its relation with angular momentum have been estimated. The neutron emission width， the fission width and the realization probability for one neutron emission and their relation with excitation energy and angular momentum are also presented.     

Nuclear Level Density with Non-zero Angular Momentum

The statistical properties of interacting fermions have been studied for various angular momentum with the inclusion of pairing interaction. The dependence of the critical temperature on angular momentum for several nuclei, have been studied. The yrast energy as a function of angular momentum for 28Si and 24 Mg nuclei have been calculated up to 60.0 MeV of excitation energy. The computed limiting angular momenta are compared with the experimental results for ^26Al produced by ^12C＋ 14N reaction. The relevant nuclear level densities for non-zero angular momentum have been computed for ^44Ti and ^136 Ba nuclei. The results are compared with their corresponding values obtained from the approximate formulas.     

重离子碰撞中超重核的形成机制(英文)

在双核模型框架下,用数值解主方程方法计算了超重核的熔合几率。 明确描述了包含能量、角动量和碎片形变弛豫的相对运动,并与核子扩散过程相耦合。因此,用微观方法推导出的核子跃迁几率是与时间相关的。所计算的以Pb为靶的冷熔合超重核形成截面和以48Ca为弹核的热熔合超重核形成激发函数与已知的实验值在合理的范围内符合。In the dinuclear system conception, the master equation is solved numerically to calculate the fusion probabilities of super heavy nuclei. The relative motion concerning the energy, the angular momentum and the fragment deformation relaxations is explicitly treated to couple with the diffusion process. The nucleon transition probabilities, which are derived microscopically, are related with the energy dissipation of the relative motion, thus they are time dependent. The formation cross sections of the super heavy nuclei from Pb based cold fusion and excitation functions from 48Ca induced hot fusion are reasonably consistent with known experimental data.     

Optical and sound helical structures in a Mandelstam-Brillouin mirror

It is shown theoretically that the phase conjugation of a speckle optical field in a Mandelstam-Brillouin mirror is accompanied by the excitation of helical hypersonic waves with a step equal to one-half of the optical wavelength. The excitation of these waves violates the initial isotropy of the dielectric medium. The predicted effect admits clear physical interpretation based on the angular momentum conservation. The angular momentum transfer from the light to the medium occurs in the vicinity of an optical singularity (optical vortex line) due to reversal of the light orbital angular momentum by the phase-conjugation mirror. The excitation of hypersonic waves transferring the angular momentum is the necessary condition for the reversal of the angular momentum of the reflected light.     

Correlations of fission fragment angular momentum with collective and intrinsic degrees of freedom

In the present work angular momenta of the fragments corresponding to¹³²I^m,g have been deduced from the radiochemically determined independent isomeric yield ratios and statistical model based analysis in neutron induced fission of²³⁵U,²³⁹Pu and²⁴⁵Cm and spontaneous fission of²⁵²Cf. These data along with similar data on¹³⁴I, reported earlier from this laboratory, bring out the effects of deformed 66n and spherical 82n shells on fragment angular momentum showing also an inverse correlation of the latter with elemental yields. Quantitative estimates of fragment scission point deformation and the coefficient of change of fragment angular momentum with kinetic/excitation energy have been deduced and are seen to be in good agreement with the expected theoretical estimates.     

Momentum transfer in light-ion-induced fission reactions

Angular correlations and angular distributions of the fission fragments produced in the bombardment of a ²³²Th target with protons, deuterons and α-particles in the energy range between 35 and 1000 MeV/nucleon have been measured. From these measurements, the distributions of linear momentum imparted to fissioning nuclei have been deduced in the various energy regimes; dominating reaction mechanisms are classified according to the fraction of the available incident momentum transferred to the target. The experimental results are compared to the predictions of intra-nuclear cascade calculations. An optimum excitation energy supported by the fissioning nuclei could be the dominant limitation to momentum transfer at high incident energies. The angular distributions of the fission fragments were used to extract fission cross sections and upper limits of the angular momentum imparted to the fissioning nuclei.     

Q-Deformed Bosons at Quasi-classical Limit of SO（3）

When a boson interacts with another to form a composite system with SO（3） dynamic symmetry, it is shown that there exists the q-deformed 5osonic excitation satisfying the q-deformed Heisenberg commutation relation in the quasi-classical limit that the angular momentum j for SO（3） is large, but not infinite. In second quantization this quasi-excitation is associated with the boson realization of SO（3） Lie algebra. Physically, the phenomena of q-deformed excitation can happen in many models of quantum dynamics, such as super emission from a system of many identical two-level atoms, the spin wave in Heisenberg chain, the high spin precession and the coherent output of Bose-Einstein atoms in a trap. Especially, in these models, the deformation parameter q is well defined intrinsically by a conservative quantity, such as the total atomic number and the angular momentum.     

Excitation of nuclei by photon beams carrying orbital angular momentum

A drop in the efficiency of nuclear excitation through transitions of high multipolarity is related to the increase in the angular momentum difference between the nuclear states involved in the excitation transition. Such transitions need photons with a high angular momentum. It is well known that photon beams carrying a well-defined and arbitrarily high value of angular momentum about the beam axis can be produced. We discuss some features in the excitation of nuclei with the beams.     

Isomeric yield ratios of fission products in the system of 24 MeV proton-induced fission of238U

Isomeric yield ratios of 30 fission products in 24 MeV proton-induced fission of²³⁸U were measured by the use of the ion-guide isotope separator on-line. The obtained isomeric yield ratios were converted to the angular momenta of primary fission fragments based on the statistical model. The deduced angular momenta were examined from various aspects. It is found that in general the angular momentum continuously increases with the fragment mass number including the region of symmetric mass division. However, there are some exceptions. For Sn isotopes the deduced angular momenta are quite small due to the spherical shape of the nuclear shell configuration. It is also concluded from the consideration of the charge distribution that the angular momentum of fission product scatters considerably within the narrow range of mass division. The dependence of the angular momentum on the available energy of fragments at scission point indicates that the individual fragment possesses a characteristic deformation at scission and/or the deduced angular momentum is seriously affected by the particle excitation after scission.     

High spin properties of 124Ba

The ¹²⁴Ba nucleus is investigated on the basis of the method of statistical mechanics by assuming the nucleons to move in triaxially deformed Nilsson potential. The variation in the Fermi energies of protons and neutrons is studied as a function of spin and temperature. The Fermi energies determined as a function of angular momentum is used to study the dependence of shell correction on angular momentum using the Strutinsky smoothing procedure. The most important observation is that the shell correction is almost the same for all spins for ¹²⁴Ba. The spin cutoff parameter and the single particle level density parameter are studied as a function of spin and temperature. Constant entropy lines drawn by plotting the excitation energy against angular momentum are found to be roughly at constant energy above the yrast line and are almost equally spaced. It is observed that no yrast traps are present for ¹²⁴Ba.     

Controlling the angular momentum composition of a Rydberg electron wave packet

Sequences of phase-locked laser pulses have been employed to control the orbital angular momentum character of an electron wave packet, which is initially created from a superposition of s and d Rydberg series. By an intelligent choice of phase, which depends on the excitation energy and the quantum defects, we are able to selectively pump down either all or a fraction of one or other angular momentum component, and by employing multichannel quantum-defect theory we are able to analyze the quantum-state distribution in detail.     

Dynamical effects in the population of compound nuclei

Evidence for the influence of reaction dynamics on the population of compound nuclei will be presented. Large dissipation in certain heavy-ion reactions leads to long compound nucleus formation times which can change the initial angular momentum and excitation energy distribution of the compound nucleus. This effect can be observed by measuring high energy γ-rays from the giant dipole resonance. The overall neutron multiplicity, however, is not a sensitive observable. The long formation times might even influence the final spin distribution of evaporation residues, which could possibly explain the entrance channel dependent population of superdeformed bands.     

Energy and angular momentum transfers from an electromagnetic wave to a copper ring in the UHF band

Electromagnetic waves could carry orbital angular momentum. Such momentum can be transferred to macroscopic objects and can make them rotate under a constant torque. Based on experimental observations, we investigate the origin of orbital angular momentum and energy transfer. Due to angular momentum and energy conservation, we show that angular momentum transfer is due to the change in the sign of angular momentum upon reflection. This leads to a rotational Doppler shift of the electromagnetic wave frequency, ensuring energy conservation.     

Properties of hot residual nuclei produced in pA reactions

The predictions for the production of nuclides and particles in proton-induced reactions are important e.g. for the detailed design of spallation neutron sources or accelerator-driven-systems. Computational tools are required that are able to describe quantitatively the two-stage process i.e. intra-nuclear cascade followed by evaporation-fission. The first stage is a highly non-equilibrated process in which incoming proton deposits in hot residual nucleus both excitation energy and angular momentum. The CBUU transport model calculation for few targets, for the proton energy range 0.4-2.0 GeV are presented, with the idea to find global parametrizations for the distributions of charge, mass, excitation energy, angular momentum of hot residual nuclei.Received: 30 September 2002, Published online: 22 October 2003PACS: 24.10.-i Nuclear reaction models and methods - 24.10.Lx Monte Carlo simulations (including hadron and parton cascades and string breaking models)     

同位旋对断前粒子发射的壳结构依赖性

用扩散模型研究了壳对幻数附近的核,即²⁰⁴Pb,²⁰⁸Pb,²¹²Pb和¹²⁸Sn,¹³²Sn,¹³⁶Sn,在裂变过程中蒸发轻粒子多重性的影响.发现壳能够影响粒子发射的同位旋依赖性,并且该影响的大小与复合系统的自旋和激发能有关.计算表明角动量在壳影响同位旋相关的粒子发射中起到了显著的作用,而高激发能弱化了壳的影响.