Algebraic description of multiple quadrupole excitations of collective states in nuclei: (I). Representation matrices for U(6)

An algorithm for the calculation of representation matrices for the totally symmetric representations [N] of the group U(6) is described. Applications to multiple quadrupole excitation processes in nuclei are given.     

Collective excitations in the 123, 125I nuclei

The odd-proton nuclei ¹²³I and ¹²⁵I have been studied in the reactions ¹²¹Sb(α, 2n)¹²³I and ¹²³Sb(α, 2n)¹²⁵I, respectively. The level schemes, spin and parity assignments are based on results obtained from singles γ-ray spectra (E_α = 27 MeV) and excitation functions, from measurements of delayed γ-rays, γ-γ coincidences, internal conversion electrons and γ-ray angular distributions. High-spin positive- and negative-parity bands with energies up to 2948 keV and spins up to $\text{23}\text{2}$ in ¹²³I and with energies up to 3775 keV and spins up to $\text{27}\text{2}\text{in}{}^{125}\text{I}$ have been established. In the decay schemes of both nuclei two separated structures of levels have been observed. One group of levels shows a strong ΔJ = 2 quasirotational pattern predicted by the particle-vibration coupling model. The ΔJ = 1 sequence on a $\text{9}\text{2}$⁺ state is assigned as a rotational structure built on the axially symmetric deformed state $\text{9}\text{2}{}^{\mathrm{+}}\text{[404]}$. In ¹²³I a 28 ns isomer at 2660.0 keV has been found.     

Momentum distributions in nuclei measured with relativistic heavy ions

In a peripheral reaction between relativistic heavy ions, where one nucleon is knocked out of the projectile, the momentum distribution of the remaining fragment reflects the momentum distribution of the knocked out nucleon. This has been proven in a previous paper. Here we study how the final-state interaction between the knocked out nucleon and the observed fragment influences the result: The real part of the optical potential which describes the final-state interaction shifts the experimental momentum distribution by a value 〈k_∥〉 of a few tens of MeV/c and the imaginary part reduces the cross sections by a factor 2 roughly. We also derive the cross section for a proton as target.     

Solution of Bohr's collective Hamiltonian for transitional odd-mass nuclei

A numerically feasible method, based on the use of deformed phonons, is developed for the diagonalization of the collective quadrupole Hamiltonian for a system with an odd particle coupled to an anharmonic even core. Examples: the transition from prolate to oblate via γ-unstable shapes and furthermore the $\text{h}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ spectra of the nuclei ¹⁸⁷Ir and ¹⁹⁷Tl.     

In-beam study of 80Kr; Quasiparticle excitations in nuclei around mass 80

The excited states in ⁸⁰Kr have been studied in the reactions ⁷⁷Se(α, n), ⁷⁸Se(α, 2n), ⁸⁰Se(α, 4n) and ⁶⁵Cu(¹⁸O, p2n) by using in-beam γ-ray spectroscopy. In addition to γγ-coincidences, excitation functions and angular distributions, linear polarization of γ-rays and conversion electrons were also measured. All together, 32 levels have been identified up to spin 14 at an excitation energy of 6.7 MeV in ⁸⁰Kr. For 21 of these levels the mean lifetime could be determined by Doppler shift methods and by the pulsed-beam γ-timing method. The B(E2) values of 30–60 W.u., derived for many transitions, indicate strong collectivity and the existence of several band structures is suggested. Above 2.5 MeV 2-quasiparticle (qp) excitations become important. The excitation energies of ⁸⁰Kr and its neighbours ^{77, 78, 79}Kr, ⁷⁷Br and ⁸¹Rb have been analysed in terms of the cranked shell model. In ^78,80Kr two-proton excitations have been found to be responsible for the observed band crossing. Quasiparticle excitations strongly influence the pairing and stabilize the deformation. The anomalies in the negative-parity bands of ⁸¹Rb and ⁷⁷Br are interpreted as a crossing of a 3qp and a 1qp band and the relatively low frequency of the crossing point is ascribed to the blocking effect.     

Elastic and inelastic scattering of 14C from medium heavy nuclei

The elastic and inelastic scattering of ¹⁴C at ⁵¹ MeV from targets of ⁴⁰Ca, ⁵⁶Fe, ⁶⁰Ni, ⁶⁶Zn and ⁸⁸Sr has been measured using a Q3D spectrometer. The ¹⁴C-nucleus potentials have been derived by optical-model analysis of the observed elastic scattering; the inelastic scattering differential cross sections were interpreted in the distorted-wave Born approximation and also in the coupled-channels approach. The analysis yields ¹⁴C-nucleus potentials that closely resemble ^{12, 13}C and ¹⁶O potentials.     

Total photonuclear absorption cross section for Pb and for heavy nuclei in the Δ-resonance region

The partial sum σ⁽²⁾(E_γ) = Σ_i?2 σ(γ, in…), representing the inclusive cross section for all reaction channels in which at least two neutrons are emitted, has been measured with a quasimonochromatic photon beam obtained by the in-flight annihilation of monoenergetic positrons, and neutron multiplicity counting. These experimental results, taken with photon energies E_γ from 145 up to 440 MeV for Pb and with photon energies E_γ= 235 MeV and 330 MeV for Al, Cu, Zr, Sn, Ho, Ta and U, are subsequently used to determine the total photonuclear absorption cross section σ(tot: E_γ) and to study the dependence upon the mass number A of the normalized cross section σ(tot: E_γ)/A. These results are then compared with other information on the total photonuclear absorption cross section.     

Rotational excitations in residual nuclei produced by stopped negative pions on targets of Ho, Lu, Lu, Hf, and Ta

Reactions produced by stopped negative pions on targets of ¹⁶⁵Ho, ¹⁷⁵Lu, ¹⁷⁶Lu, ¹⁷⁹Hf, and ¹⁸¹Ta are investigated by measuring photons coincident with the stopped pions. These photons were detected with an 80 cm³ Ge(Li) detector and identified as pionic X-rays or rotational γ-rays from the residual nuclei. Pion capture is found to result in the excitation of high-spin states. Yields for the residual nuclei are inferred from the observed γ-transitions and compared to calculations based on the exciton model for pre-equilibrium emission followed by particle evaporation. Preliminary results of a pion stop, γn coincidence experiment are presented and compared to the calculations.     

The use of exotic heavy ion transfer reactions to study light neutron rich nuclei

The ¹⁸O + ^{207, 208}Pb reaction at E = 93 MeV has been used to measure the mass excesses of ^{21, 22}O ¹⁹N and ¹⁷C. values of 8.095 ± 0.075, 9.29 ± 0.18, 15.96 ± 0.15 and 21.10 ± 0.22 MeV, respectively, were obtained. These results are in good agreement with previous measurements. A comparison with current theoretical predictions is also presented.     

Energy surfaces,collective potentials and collective inertia parameters

The Hill-Wheeler equation is used to derive a collective Hamiltonian from an energy surface. Two simple ansatz are proposed for the collective Hamiltonian, which is then calculated explicitly in a hierarchy of approximations. The consistency of the method is verified in an illustrative example.     

Dynamics of high energy heavy ion collisions — nuclear matter at high density and temperature

Nuclear collisions at E_LAB > 50 MeV/n are studied using the nuclear fluid dynamical and the cascade model. Evidence for the predicted compression effects has been found in recent experimental data: The preferential sidewards emission of light fragments as well as the azimuthal p-p correlations indicate the presence of collective flow phenomena. The analysis of future 4π exclusive experiments in terms of the kinetic flow tensor can yield information about the equation of state of the compressed nuclear matter. We also discuss the role of pions and strange particles as probes for the properties of nuclear matter at high excitation energy.     

Low lying octupole excitations in 15N

The inelastic electron scattering form factors for the $\text{5.27}\text{MeV}\text{(}\text{5}\text{2}{}^{\mathrm{+}}\text{), 7.15}\text{MeV}\text{(}\text{5}\text{2}{}^{\mathrm{+}}\text{), 7.30}\text{MeV}\text{(}\text{3}\text{2}{}^{\mathrm{+}}\text{)}\text{and}\text{7.56}\text{MeV}\text{(}\text{7}\text{2}{}^{\mathrm{+}}\text{)}$ levels have been measured in the momentum transfer range between 0.6 and 1.1 fm^?1. The resulting B(E3) values are discussed in terms of weak coupling models and the spherical shell model of Zuker, Buck and McGrory.     

Intrinsic and rotational excitations in 175Lu

The γ-ray cascades following the ¹⁷⁶Yb(p, 2n)¹⁷⁵Lu reaction are studied with Ge(Li) detectors and interpreted within the level scheme of ¹⁷⁵Lu. According to this interpretation the $\text{1}\text{2}$⁺[411], $\text{1}\text{2}$^?[541], $\text{5}\text{2}$⁺[402], $\text{7}\text{2}$⁺[404] and $\text{9}\text{2}$^?[514] rotat $\text{15}\text{2}$⁺, $\text{21}\text{2}$^?, $\text{13}\text{2}$⁺, $\text{19}\text{2}$⁺ and $\text{19}\text{2}$^? members, respectively. Fu band head is determined to be 626.6 keV. The data for the rotational bands are discussed within the particle-rotor model and compared with the available information on the corresponding bands in the adjacent Lu isotopes.     

Effects of non-orthogonality and virtual excitations in direct reactions (III)

Formulae are presented concerning the effects of virtual Coulomb excitation on the transfer of nucleons between heavy ions at energies below the Coulomb barrier. These formulae are applied towards calculation for the ²⁰⁸Pb(¹⁶O, ¹⁷O)²⁰⁷Pb reaction at 69.1 MeV. Generally, the effect is found to be of the order of 10% but it is not discernable owing to the small reaction cross sections. Suggestions are presented for extending the analysis to higher energies where the situation is more favourable.     

Feeding and lifetimes of yrast levels in Hf nuclei

Feeding intensities and times, as well as lifetimes of yrast levels in doubly even ^{166, 168, 170}Hf nuclei have been measured. The reactions ^{122, 124}Sn(^{48, 50}Ti, 4n)^166–170₇₂Hf have been investigated using the recoil-distance Doppler-shift method. The lifetimes, and hence the B(E2) values measured, show the same trends of rather small deviations from the rigid rotor as in the known ₆₈Er and ₇₀Yb cases. In addition to the fast feeding component, showing behaviour rather similar to that of the Yb nuclei, another type of slow feeding, related to the low spin yrast levels, is observed.     

Collectivity and the role of two-proton and two-neutron excitations in 82Kr

Excited states in ⁸²Kr have been studied in the reaction ⁸⁰Se(α, 2n) by using in-beam γ-ray spectroscopy. Measurements of γγ coincidences, excitation functions, angular distributions and the linear polarization of the γ-rays have been carried out. All together, 38 levels have been identified up to $\text{I = 12}\text{h}\text{?}$ and excitation energies up to 6 MeV. For 23 of these levels the mean lifetime could be determined by Doppler shift and the pulsed-beam γ-timing methods. Most of the levels could be grouped into collective bands according to measured B(E2) values. Two sequences, I^π = 8⁺, 10⁺, 12⁺, are interpreted as rotation-aligned 2q.p. bands built up on $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ protons and neutrons, respectively. Interaction matrix elements between these bands were deduced as V ? 10 keV. Additional states, I^π = (6⁺), 7⁺, 8⁺, indicate coexistence of rotation-aligned and deformation-aligned 2q.p. excitations. An odd-parity ΔI = 1 band is ascribed to proton excitations of the type $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{?}\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$. Several fast M1 transitions with B(M1) = 0.1–1 W.u. have been observe states of equal spin. They are interpreted as being due to configuration mixing.     

Target fragments in collisions of 1.8 GeV/nucleon Fe nuclei with photoemulsion nuclei, and the cascade-evaporation model

Nuclear photographic emulsion is used to study the dependence of the characteristics of target-nucleus fragments on the masses and impact parameters of interacting nuclei. The data obtained are compared in all details with the calculation results made in terms of the Dubna version of the cascade-evaporation model (DCM).     

Nucleon channel coupling in electrodisintegration of nuclei

The process of nuclear excitation above threshold for nucleon decay in high energy electron scattering is considered. On the basis of the particle-hole shell model a formalism is proposed which allows one to describe electroexcitation of nuclei in a unified manner both in the resonance and quasielastic scattering regions. Numerical calculations were made for the ¹²C nucleus taking into account the mixing of proton and neutron configurations (channels) of the particle-hole type in the continuum.     

Damping of nuclear collective motion in the extended mean-field theory

The dissipation mechanism in slow nuclear collective motion is studied in the frame of the extended mean-field theory. The collective motion is treated explicitly by employing a travelling single-particle representation in the semi-classical approximation. The rate of change of the collective kinetic energy is determined by: (i) one-body dissipation, which reflects uncorrelated particle-hole excitations as a result of the collisions of particles with the mean field, (ii) two-body dissipation, which consists of simultaneous 2 particle-2 hole excitations via direct coupling of the residual two-body interactions, and (iii) potential dissipation due to the redistribution of the single-particle energies as a result of the random two-body collisions. In contrast to the first two processes the potential dissipation exhibits memory effects due to the large values of the local equilibration times.