Fission fragment energy correlation measurements for the sub-barrier (nth, f) of 241Am and 243Am

Fission-fragment mass and kinetic-energy distributions and their correlations have been studied for the sub-barrier fission of the odd-odd ²⁴²Am and ²⁴⁴Am nuclei resulting from thermal neutron capture in ²⁴¹Am and ²⁴³Am. Unwanted events were eliminated by a coherence test based on the time of flight. The ²⁴³Am mass distribution is more asymmetric and shows structures at μ_H ≈ 139 and 144, compared with that for ²⁴¹Am which is smooth and structureless. The structure at μ_H ≈ 144 is caused by deformed neutron shells at N ≈ 88 in the heavy fragment and $\text{N ≧ 60}$ in the light fragment. While the 〈E_K〉 at symmetry for ²⁴¹Am is ≈ 19 MeV higher than that for ²³⁵U, it is shifted upwards only ≈ 6.5–10 MeV in the other mass regions. However, 〈E_K〉 at symmetry for ²⁴³Am is ≈ 6 MeV lower than for ²⁴¹Am and this decrease tapers down to ≈ 1 MeV for μ_H > 135. These data show a decrease in the total fragment deformation for ²⁴¹Am at symmetry as predicted by calculations. However, the ²⁴³Am data show a sudden change back to higher deformation.     

Giant resonance in the total photoabsorption cross section of Z ≈ 90 nuclei

Total photoabsorption cross sections of ²³²Th, ²³⁵U, ²³⁸U and ²³⁹Pu have been measured in the giant resonance region by the absorption method. Measured cross sections were approximated by two Lorentz lines. Lorentz line parameters, integrated cross sections, deformation parameters and quadrupole moments are given. The analysis of the nuclear optical anisotropy evolution with the increase of Z shows that Z ≈ 90 nuclei seem to be transitional, similar to N ≈ 90 nuclei. A comparison of experimental cross sections with dynamic collective model calculations has been performed. The evidence that quadrupole photoabsorption occurs in the 20–25 MeV energy region is also presented.     

Dependence of Doubly Charged Ion Formation by the Two-Electron Mechanism on the intensity of Laser Radiation

Experimental dependences of the doubly charged ion yield N ²⁺ on the laser radiation intensity F are analyzed for the case of ion formation by the two-electron mechanism. It is shown that the region where the dependence N ²⁺(F) is characterized by the degree of nonlinearity K≈2.5 corresponds to the saturation of the process of doubly charged ion formation. This region is described well by theoretical calculations performed for Gaussian laser beams.     

Landau fragmentation and deformation effects in dipoSe response of sodium clusters

Recent experimental data on the dipole plasmon in axial sodium clusters Na _N ⁺ with 11 ≤ N ≤ 57 are analyzed within a self-consistent separable random-phase approximation (SRPA) based on the deformed Konh-Sham functional. Good agreement with the data is achieved. The calculations show that, while in light clusters plasmon properties (gross structure and width) are determined mainly by deformation splitting, in medium clusters with N τ 50 the Landau fragmentation becomes decisive. Moreover, in medium clusters shape isomers come to play with contributions to the plasmon comparable with the ground state one. As a result, commonly used methods of the experimental analysis of cluster deformation become useless and correct treatment of cluster shape requires microscopic calculations.     

Two mutually-repelling self-avoiding walks: A Monte Carlo study

We consider two self-avoiding walks (SAWs) on a square lattice, beginning simultaneously from two arbitrary nearest neighbours. The walks cross neither themselves nor each other, and grow simultaneously. Using Monte Carlo technique we study the variation, with the length N of the walks, of the average end-to-end distance 〈R_N〉 of each walk and of their average separation 〈S_N〉. We find 〈R_N²〉∞N^2ν, 〈S_N²〉∞N^2λ, where ν ≈ λ ≈ 0.75 for two ordinary SAWs and ν ≈ λ ≈ 0.67 for two growing SAWs in two dimensions. For two directed SAWs, we find 〈R_N∥/⊥²〉 ∞ N^2ν∥/⊥ and 〈S_N∥/⊥² 〉 ∞ N^2λ_∥/⊥, where ν_∥ ≈ 1.00, ν_⊥ ≈ 0.56, λ_∥ ≈ 0.50 and λ_⊥ ≈ 0.59 in two dimensions. We thus find an indirect excluded volume effect on one directed SAW, due to the other.     

Transition from strong to weak coupling in SU(N) lattice gauge theories

A possible explanation is proposed for the crossover from strong to weak coupling region in SU(N) lattice gauge theories. We predict the pointswhere the crossover takes place for all SU(N)M: For example, g² ≈ 2.0 for SU(2), g² ≈ 1.0 for SU(3) and lim_N→∞Ng²(SU(N) ≈ 2.0.     

Collective rotations of fission isomers in actinide nuclei

Using the total-Routhian-surface(TRS)method,the rotational behaviors of fission isomers in the second well of actinide nuclei234 242U,236 244Pu and238 246Cm were investigated.The pairing-deformation-frequency self-consistent TRS calculations reproduced reasonably the experimental moments of inertia extracted from spectroscopic data.It is calculated that,in these largelyelongated(β2≈0.65 andβ4≈0.03)fission isomers,theν12[981]neutron andπ12+[651]proton align simultaneously at rotational frequencyω≈0.4 0.6 MeV(corresponding to spin I≈80),which leads to clear upbending in moments of inertia(MoI’s).Our calculations have indicated that the hexadecapole deformationβ4influenced significantly the frequency of the rotational alignment of the proton12+[651]orbit.     

Experimental observation of “configuration” modes of bistable centers in CdF2:In crystals

New “configuration” modes, which were predicted by us for CdF₂:In crystals, have been revealed at the frequencies ν₁ ≈ 32.4 cm^?1 and ν₂ ≈ 96.3 cm^?1 for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m ₁ m ₂/(m ₁ + 2m ₂) of the In ion (m ₁) and two F ions (2m ₂) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF₂ (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N ₁) and shallow (N ₂) states. The obtained ratio N ₂/N ₁ ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (ε′) and imaginary (ε″) parts of the dielectric constant has been found (at a frequency of 25 cm^?1, Δε′ ≈ 0.2 and Δε″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF₂:In, related to the impurity lattice modes, has also been revealed.     

New estimate of the nucleon σ-terms

From low energy πN and KN data it is found that σ(πN) ≈ 70 MeV and (u₀/u₈)_N ≈ 1.     

Neutronh 11/2 alignments in the triaxial nucleus133La

High spin states have been studied in¹³³La via the¹²²Sn (¹⁵N, 4ny) fusion evaporation reaction. Bands build on low lying h_11/2,g_7/2 and d_5/2 proton states have been identified. At higher spin a h_11/2 neutron alignment is observed. The softness with respect to the triaxial deformation makes the nuclear shape sensitive to the quasiparticle configurations and coexistence between states withy ≈ + 30°,y ≈ ? 30° andy ≦ ? 60° was found. The results have been interpreted using total routhian surface (TRS) model calculations.     

High-spin spectroscopy of162Hf

Excited states in¹⁶²Hf were investigated up to spinI ≈ 38 using the anti-Comptonspectrometer array HERA with 21 detectors. In addition, some information was obtained on¹⁶¹Hf. The analysis of triple coincidences was crucial for the construction of the level schemes. The results are interpreted within the framework of the cranked shell model and are compared to neighbouring isotopes and isotones, showing Fermi level and deformation effects. The systematic behaviour of the band crossings in the Hf isotopes and theN=90 isotones is discussed.     

Mass and energy characteristics of the 241Pu(nth, f) fragments

Fission-fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of ²⁴¹Pu. The global mass distribution is smooth, apart from a small shoulder at μ_H ≈ 144 probably due to the deformed shells N ≈ 88 and N ? 60 in the heavy and the light fragment respectively. When low excitation events are selected, these structures become more pronounced. Furthermore, there is a sudden increase of 1–2 MeV in $\text{E}\text{?}\text{(μ)}$ for masses around 85 and above 155 which is probably associated with a spherical shell N = 50 in the light fragment. Finally, the prompt neutron emission curve $\text{v(m}{}^1\text{)}$ is calculated.     

Equilibrium properties of fast-rotating headed nuclei

We report on calculations of the equilibrium deformation in excited heated rotating nuclei. At A ～ 150–200 and temperature t > t_c ≈ 1.2 MeV the shell effects turn out to be small to compete with the variations of the liquid drop component of the energy. The transition from the shape of a “cool” nucleus to that of a “hot” nucleus takes place at t_c and in deformed nuclei resembles a phase transition. The stiffness parameter with respect to shape variations at t_c is anomalously low.     

Rotational properties of the odd-Z transfermium nucleus 255Lr by a particle-number-conserving method in the cranked shell model

Experimentally observed ground state band based on the 1/2-[521] Nilsson state and the first exited band based on the 7/2-[514]Nilsson state of the odd-Z nucleus ~(255)Lr are studied by the cranked shell model(CSM) with the paring correlations treated by the particle-number-conserving(PNC) method. This is the first time the detailed theoretical investigations are performed on these rotational bands. Both experimental kinematic and dynamic moments of inertia(J~(1)and J~(2)) versus rotational frequency are reproduced quite well by the PNC-CSM calculations. By comparing the theoretical kinematic moment of inertia J~(1) with the experimental ones extracted from different spin assignments, the spin 17/2~-→13/2~- is assigned to the lowest-lying 196.6(5) ke V transition of the 1/2~-[521] band, and 15/2~-→11/2~- to the 189(1) ke V transition of the 7/2~-[514] band, respectively. The proton N = 7 major shell is included in the calculations. The intruder of the high- j low-? 1 j_((15)/2)(1/2~-[770]) orbital at the high spin leads to band-crossings at ω≈0.20( ω≈0.25) Me V for the 7/2~-[514] α =-1/2(α = +1/2) band, and at ω≈0.175 Me V for the1/2~-[521] α =-1/2 band, respectively. Further investigations show that the band-crossing frequencies are quadrupole deformation dependent.     

Measurement of excited state number densities of oxygen and nitrogen in a non-equilibrium plasma

Number densities of several excited states of atomic oxygen and nitrogen have been measured in the decaying non-thermal plasma of a θ-pinch afterglow. The spatial variation of the electron density and temperature as functions of time after initiation of main bank discharge have also been measured to facilitate a comparison of the excited state number densities with model calculations. Measurements of the atomic oxygen excited states indicate that quintet to triplet spin exchange collisions and doubly excited states must be included in the model. The measured populations of the excited atomic nitrogen states agree well with those calculated at high density (N_e≈ 10¹⁴ cm^?3), but disagree badly at lower densities (N_e ≈ 10¹² cm^?3). The discrepancies seem to be real since they are larger than expected measurement uncertainty.     

Theoretical analysis of photoelectron angular distributions in hydrogen chloride

Multiple-scattering calculations for the ≈X² II and ≈A²Σ⁺ ionizations of HCl are analysed by a partitioning procedure to explain the angular distributions around the Cooper minima. A formal analogy to resonances is pointed out. The Cooper minima in the valence ionizations of Cl₂ are discussed qualitatively.     

Rotational consequences of stable octupole deformation in nuclei

The characteristic features of high-spin spectra of octupole-deformed nuclei are demonstrated by means of Woods-Saxon-Bogolyubov cranking calculations. The rotational spectra of Ra and Th nuclei are studied. The experimental data suggests shape changes with increasing neutron number from N ? 130 (nearly spherical shapes) through N ? 134 (octupole-deformed shapes) to N ? 140 (well-deformed reflection-symmetric shapes). The octupole mixing between the high-j intruder states and normal-parity orbitals leads to specific patterns of quasiparticle spectra characterised by a quantum number referred to as simplex. The influence of octupole deformation on high-spin properties of nuclear spectra like spin alignment, band interaction, etc. are discussed.     

Backbending in theN=96 isotones

The backbending in the even-even,N=96 isotones can be quantitatively accounted for by the rotation-alignment of the spins of neutrons in i 13/2 orbits, as shown by comparing the aligned angular momentum and relative Routhian for thes-bands in these isotones and for the i 13/2 bands in the corresponding isotopes withN=91. The influence of protons on this backbending situation is shown to be indirect, acting through a change of the nuclear deformation, which yields a change of the moment of inertia of the g.s. band and of the non-rigid character of the rotation. The experimental data on theN=96 and 97 isotones are in reasonable agreement with cranking model calculations. Possible reasons for the inhibition of backbending in the h 9/2 proton bands in the odd-Z, N=96 isotones, all related to a change of deformation, are presented.     

In-beam γ-ray studies of complex band structures and of isomeric states in 152, 153Dy nuclei

The high-spin level structures of ¹⁵²Dy and ¹⁵³Dy were studied experimentally with ^{154, 155}Gd(α xnγ) in-beam reactions, and for ¹⁵²Dy also with ${}^{\mathrm{144,\; 146}}\text{Nd}\text{(}{}^{12}\text{C, x}\text{n}\text{γ)}$ reactions. The experiments included measurements of singles γ-ray and conversion-electron spectra, γ-ray angular distributions and E_γ-t and E_γ-E_γ-t coincidences. A multiplicity filter set-up was used to study the feeding and decay of isomeric states in ¹⁵²Dy. In ¹⁵²Dy about twenty so far unknown levels were found, including two high-spin isomeric states with $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 60}\text{and}\text{≈ 13}\text{ns}$ at excitation energies E_x ≈ 5.04 and 6.08 MeV, respectively. These states are compared with recent calculations on yrast traps. The level scheme of ¹⁵³Dy contains 28 levels up to E_x = 4.1 MeV and $\text{J}{}^{\mathrm{\pi }}\text{= (}\text{37}\text{2}{}^{\mathrm{+}}\text{)}$. Band structures in both nuclei are discussed in comparison with other N = 86 and N = 87 isotones.     

Comparison of recent massive muon pair data with the asymptotically free parton model

We present calculations of vW₂ and of massive muon pair production cross sections in the kinematic ranges of recent experiments. These calculations test the asymptotically free parton model and excellent agreement with the data is found. Estimates of the transverse momentum 〈q_⊥〉_μμ expected in massive dimuon experiments are made. For s = 750 GeV² and M_μμ ≈ 10 GeV, we predict 〈q_⊥〉_μμ ≈ 1.4 GeV/c.