角动量对断前粒子发射壳效应的影响

用Smoluchowski方程研究了角动量对一个轻的闭壳核¹³²Sn裂变前粒子蒸发壳效应的影响,发现壳对断前粒子发射的影响敏感地依赖于这个裂变系统的角动量.对可能的原因进行了讨论     

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.     

Probing Shell Correction at High Spin by Neutron Emission of Doubly Magic Nuclei 208pb and 132Sn

Shell effects in particle emission for two doubly magic nuclei 132 Sn and 208 Pb were studied in the framework of Smoluchowski equation taking into account temperature and spin-dependent shell correction. It is shown that the shell effects in the enission of pre-scission neutrons are sensitive to the spin dependence of the shell correction at a moderate excitation energy. Therefore, we propose to use neutron multiplicity as an observable to probe the shell correction at high spins.     

Probing Shell Correction at High Spin by Neutron Emission of Doubly Magic Nuclei 208Pb and 132Sn

Shell effects in particle emission for two doubly magic nuclei ^132Sn and ^208pb were studied in the framework of Smoluchowski equation taking into account temperature and spin-dependent shell correction. It is shown that the shelle ffects in the emission of pre-scission neutrons are sensitive to the spin dependence of the shell correction at a moderate excitation energy. Therefore, we propose to use neutron multiplicity as an observable to probe the shell correction at high spins.     

Influence of Isotope on Shell Effects of Pre-scission Particle Evaporation

The shell effects on the particle evaporation prior to fission for three Pb isotopes, 204pb, 20sPb, and 212pb,as well as three Sn isotopes, 128Sn, 132Sn, and 136Sn, are explored by a diffusion model. Calculations show that the magnitude of shell effects in the emission of particles changes with the neutron-to-proton ratio N/Z of these fissioning nuclei, and this change is affected significantly by the spin and excitation energy of the system. It is shown that high angular momentum enhances the dependence of shell effects on the N/Z while high excitation energy weakens such a dependence.     

Influence of Isotope on Shell Effects of Pre-scission Particle Evaporation

The shell effects on the particle evaporation prior to fission for three Pb isotopes, ^204Pb, ^208Pb, and ^212pb,as well as three Sn isotopes, ^128Sn, ^132Sn, and ^136Sn, are explored by a diffusion model. Calculations show that the magnitude of shell effects in the emission of particles changes with the neutron-to-proton ratio N/Z of these fissioning nuclei, and this change is affected significantly by the spin and excitation energy of the system. It is shown that high angular momentum enhances the dependence of shell effects on the N/Z while high excitation energy weakens such a dependence.     

Self-Similar Strcture Shell Model

In Order to extend the conventional shell model(SM) calculation with harmonic oscillator bases to halo nuclei, a self-similar, structure shell model(SSM) is proposed. The SSM is achieved by a rescaling of both the Kinetic and potential energy term of the harmonic oscillator and a mean field imitation, so that the single particle orbit in SSM has state (orbit) -dependent frequency. The large r. m. s. radius and the thick neutron skin for halo nuclei as well as the bound state properties of Borromean nuclei such as ⁶He, ¹¹Li and ¹⁴Be can be reproduced.     

Angular Momentum Projected Deformed HF Spectra of 46Ti and 48Cr With MSDI

Taking the modified surface delta interaction (MSDI) and using a spherical shell model basis set, we carry out the deformed Hartree-Fock (HF) calculation for the even-even nuclei ⁴⁶Ti and ⁴⁸Cr in the fp shell, and construct the deformed HF intrinsic states which are the Slater determinants from the HF single-particle states.The angular momentum projection program is then carried out and the reasonable results are obtained.     

Influence of Shell on Pre-scission Particle Emission of a Doubly Magic Nucleus 208Pb

Using Smoluchowski equation, we study the shell effects on the emission of light particles in the fission process of a doubly magic nucleus ²⁰⁸Pb. Calculated results show that shell has a considerable effect on the neutron emission and that shell effect gradually becomes weak with increasing excitation energy. In addition, a dependence of shell effects in the neutron emission on the angular momentum has been found.     

Inner Shell Excitations of Lithium Studied by Fast Electron Impact

Electron energy loss spectra for the inner shell excitations of atomic lithium are measured at an incident electron energy of 2500eV and scattering angles of 0^o, 2^o, 4^o and 6^o. Two optically forbidden transitions of （ls2s^2）^2S and （ ls2s ^3 S）3s^2 S are observed. The generalized oscillator strength ratios for ls（2s2p^3 P）2 P^0 to 1s（ 2s2p^1 P）2 P^0 were determined, and they are independent of the momentum transfer.     

Stady on the Energy Spectra of fp Odd A Nuclei 47Ti,47V,49Cr With PDHF Method

The approximate angular momentum projected Hartree-Fock(PDHF)method is used to some odd-A nuclei in the fp shell:⁴⁷Ti,⁴⁷V,⁴⁷Cr,⁴⁹Cr.Their energy spectra are calculated and compared with the experimental data.The results are fairly satisfactory.In addition,the characteristics of those spectra are analyzed with the particle-rotor model.It is found that those spectra possess the properties of the no alignment limit Spectra.     

Angular momentum and energy dependence of fission andn,p,4He emission from194Hg compound nucleus

Spin and temperature dependence of the fission and particle emission is studied for¹⁹⁴Hg. The compound nucleus is described using the Strutinsky shell correction approach extended for finite angular momenta and temperature. The shell corrections to the potential energy, free energy and the angular momentum are calculated using the Woods-Saxon average field. Results are compared with the experimental data and show a good qualitative agreement. It is found that the inclusion of the shell effects is necessary to understand the decay properties of¹⁹⁴Hg even for temperatures as high as 1.5–2.0 MeV.     

Application of the Modified BUU Model to Reaction Induced by 16O on 197An at 20MeV≤E/u≤180 MeV:Dependence of the Angalar and Linear Momentum Transfer on Impact Parameter and Energy

The angular and linar momentum transfered to target residue on 20 to 180MeV/u ¹⁶O+¹⁹⁷Au system are calculated with BUU(Boltzmann-Uehling-Uhlenbeck) model in which the angular momentum conservation is added.The dependence of angular and linar momentum on impact parameter and incident energy is discussed.The deviations between calculated values of(1mt) and those from viola system are compared.The results of the simulations suggest that the saturation of angular momentum transfer with incident energy increase mainly comes from the loss of capture ability of the target on projectile.     

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.     

Particle-Number Conserving Analysis for High K Multi-Quasiparticle Bands in Odd-A Deformed Nuclei 173,175Hf

Using the Particle-number Conserving (PNC) method for treating the cranked shell model, the high K multi-quasiparticle bands in odd-A deformed nuclei ^173,175Hf are analyzed, including the variation with rotational frequency of the moment of inertia, angular momentum alignment and occupation probability of each cranked Nilsson orbital. No free parameters are involved in the PNC calculation and the experimental results are reproduced well. The microscopic mechanism of the difference between the multi-quasiparticle high K bands and the yrast bands in neighboring even-even nuclei is investigated, where the blocking effects of high j intruder orbitals near the Fermi surface play a crucial role.     

Investigation of the Type of Quadrupole Pairing Force in Superdeformed Nuclei

The variations of moments of inertia of superdeformed (SD) bands ¹⁹⁴Hg(1)and ¹⁹²Hg(1) with angular momentum (rotational frequency) are investigated using the particlenumber conserving (PNC) method for treating the cranked shell model. Calculations show that if both the monopole and Y₂₀ quadrupole pairing forces are taken into account, the observed co variation of the moments of inertia of ¹⁹⁴Hg(1) and ¹⁹²Hg(1) can be reproduced very well both in the low-and high- ranges, in particular the J⁽²⁾ downturn of ¹⁹⁴Hg(1) and the flattening in J⁽²⁾ of ¹⁹²Hg(1) at>0.40MeV. On the contrast, the calculated J⁽²⁾'s with the Y_2±1 or Y_2±2 quadrupole pairing forces are in disagreement with the experiments. The microscopic mechanism of the co variation of J⁽²⁾ is clearly exhibited in the PNC calculation (the contributions to J⁽²⁾ from various major Shells and individual cranked Nilsson orbitals, the occupancy of each cranked Nilsson Orbital etc). The variation of moment of inertia is the result of the competition among the shell effect (single particle motion in a deformed potential), pairing correlation, Pauli blocking effect and Coriolis (anti-pairing) interaction.     

MEASUREMENTS OF ANGULAR MOMENTUM EFFECT OF FISSION BARRIER

The fission cross sections and the evaporation residue cross sections for ¹²C+²⁰⁹Bi and ¹⁴N+Pb reactions were measured with the gold surface barrier silicon detectors and the mica nuclear track detectors.The critical angular momentum l_er was deduced from the measured evaporation residue cross section σ_er on the basis of the sharp-off model.The fission barrier with the particular angular momentum i derived from the condition Γ_f/Γ_n=1 at l=l_e The angular momentum effect of the fission barrier was studied experimentally.     

Particle-Number Conserving Calculation for Low-Lying Excited High-K Multi-Quasiparticle Bands in 172,174Hf

Using the particle-number conserving method for treating the cranked shell model,the microscopic mechanism of the variation of the kinematic moments of inertia with rotational frequency ω for the rare-earth deformed nuclei ^172,174Hf is investigated. The observed J⁽¹⁾ and angular momentum alignment are reproduced satisfactorily in the PNC calculation,in which no free parameters are involved. The PNC analysis shows that the difference between the variations of J⁽¹⁾ with ω for the high K multi-quasiparticle bands and for ground state band is mainly due to the Pauli blocking effect of high j intruder orbitals near the Fermi surface.     

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.     

APPROXIMATE ANGULAR MOMENTUM PROJECTION OF THE DEFORMED HARTREE-FOCK STATE FOR K=1

Using our approximate angular momentum projection method with one parameter, the projection spectra of the axially symmetric Hartree-Fock intrinsic states are studied. The approximate and the exact projection matrix elements are obtained and compared each other. The projection integrals for angular momentum J=1 to 11 are given. The energy spectra of the nuclei ⁴⁴Ti and ⁵⁰Ti are calculated, and the results coincide with the observed energy levels fairly well.