Two-fluid model description of isovector giant resonances in fast rotating nuclei

Isovector giant resonances of arbitrary multipolarity in fast rotating nuclei are studied by solving the inviscid two-fluid equation of relative motion in a rotating frame of reference. Both Coriolis and centrifugal forces are taken into account. The resulting expressions display in a quite simple way general features of giant multipole resonances of fast rotating nuclei, in addition to a good agreement with other calculations for the giant dipole resonance. Typical values for the resonance energies and their fragmentation due to nuclear deformation and rotation are given. In particular, enormously large resonance splitting should occur in the superdeformed states.     

Particle-hole calculation of the isobaric analog and isovector monopole resonances

The correlated proton-particle—neutron-hole spectrum is calculated for N > Z nuclei using a Skyrme type interaction and the response function method. The basis of the calculation is a complete one-particle—one-hole space with the continuum included. As a result the distribution of the isovector monopole strength in the analog nucleus is obtained. This distribution has a narrow peak which corresponds to the isobaric analog resonance and at higher energies a broad peak which is the isovector monopole resonance. The coupling between these two states is inherent in the calculation.     

The structure of 209Bi: (I). Microscopic structure from analog resonances

Excitation functions for the ²⁰⁹Bi(p, p′) reaction have been measured for incident proton energies from 14.0 to 15.5 MeV to study the population of states in ²⁰⁹Bi via decay of the isobaric analog resonances in the compound nucleus ²¹⁰Po. The analogs correspond to parent states in the lowest one MeV of excitation in ²¹⁰Bi. Strong resonances are observed in the excitation functions for twenty proton groups corresponding to levels between 2.75 and 3.65 MeV in ²⁰⁹Bi. From these results, levels which possess microscopic 2p-1h structure involving the orbitals are identified and their ²¹⁰Bi parentage determined. The resonant scattering results together with other direct inelastic scattering data allow the spins and parities for many of the levels to be deduced. The results of a microscopic calculation, in which empirical two-body matrix elements are employed to describe the interactions between the orbitals, are also reported and compared with the experimental results for levels in ²⁰⁹Bi. The excellent agreement between theory and experiment demonstrates that the general character of these levels can be understood microscopically in terms of their 2p-1h structure.     

The width of the 5.11 MeV state in 10B and isovector parity mixing

We review estimates for effects of parity mixing on the α-capture of vector polarized ⁶Li to the 5.16 MeV, 2⁺, T = 1 state in ¹⁰B. The cross section depends on the ⁶Li polarization direction because of isovector parity mixing with the 5.11 MeV, 2^?, T = 0 state. The effect is enhanced due to isospin conservation. The α-width of the 5.11 MeV state, an important parameter for calculating the enhancement, has been measured to be 0.98 ± 0.07 keV. The consequences of parity mixing are reevaluated using best available values for the relevant parameters.     

A microscopic theory of giant electric isovector resonances

The electric multipole isovector (τ = 1) giant resonances for Δτ_z = 0, ± 1 are studied using the self-consistent HF-RPA theory.The distributions of strength, energies, the isospin compositions and other properties of the J = 0⁺, 1^?, 2⁺ resonances in the ⁴⁸Ca, ⁹⁰Zr, ¹²⁰Sn and ²⁰⁸Pb regions are calculated. Sum rules for the charge-exchange Δτ_z = ±1 excitations are derived.     

Shell-model study of giant dipole resonances in open-shell nuclei using the Lancgos method

A microscopic investigation of giant-resonance states in open-shell nuclei is proposed using for the representation of the wave functions the shell-model basis in a large model space. Instead of an exact diagonalization of the hamiltonian, which is essentially impossible for the large model spaces considered, an iterative procedure is used, which is based on the Lanczos algorithm for matrix diagonalization. The choice for the initial state in this iteration ensures that the complete transition strength to the resonance of interest is taken into account, and the iteration allows an increasingly accurate estimate of the spreading of this transition strength to more complicate configurations. An application of this method to the giant dipole resonance in ²⁰Ne yields stable results after a few iteration steps and demonstrates the efficiency of this method.     

Experimental displacement energies of isobaric analog states in the 1f72 shell

Q-values for the (p, t), (p, ³He) and (p, d) reactions for twenty-five isobaric analog states in the $\text{1}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ shell have been accurately measured. The corresponding experimental displacement energies are compared with recent model predictions.     

A microscopic description of the negative parity resonances in the mass-4 system by the coupled-channels method (II)

The coupled-channels method for the treatment of the continuum is used to study the negative parity excited states of the ⁴He nucleus in the Tamm-Damcoff approximation. It is shown that the splittings within the SU(4) supermultiplet are well reproduced in this calculation when the spin-dependent forces responsible for the removal of the degeneracy are taken appropriately into account. The location of the T = 0, 1^? resonance and the admixture of spurious c.m. excitation in the intrinsic excitation are discussed. In addition to interpreting the observed spectrum, the available experimental data for the particle channels are well described by the theory.     

Study of the fission decay of the isoscalar monopole resonance via the 238U(α, α′) reaction at 0°

The fission decay of ²³⁸U has been investigated using inelastic scattering of 120 MeV ga-particles to excite the ²³⁸U nucleus. Angular correlations of the fission fragments have been measured for excitation energies between 5.7 and 15.7 MeV in coincidence with inelastically scattered α-particles between 0 and 3°. The difference in yield for fission in coincidence with inelastically scattered α-particles between 0–1.35° and 1.35–3° was used to deduce the fission decay of the giant monopole resonance. It was found that in the fission decay channel (22 ± 5)% of the E0 EWSR strength is located between 8 and 15 MeV excitation energy. The distribution of the deduced monopole strength is in agreement with recent theoretical calculations predicting splitting of the giant monopole resonance in deformed nuclei.     

On the observability of “molecular resonances” in 24Mg through the 10B + 14N entrance channel

Excitation functions of the (¹⁴N, α) reaction on ¹⁰B have been measured simultaneously at about 20 angles in the energy range 12.0 ≦ E_c.m. ≦ 15.5 MeV. The experimental data do not give evidence for the existence of resonances in ²⁴Mg through the ¹⁴N on ¹⁰B collision.     

Radiative width for the 10.35−6.92 MeV transition in 16O

From a measurement of the cross section for ¹²C(α, γ)¹⁶O at the E_α = 4.253±0.014 MeV(E = 10.352±0.011 MeV) resonance, the radiative width for the 10.35 MeV (J^π = 4⁺) to 6.92 MeV (J^π = 2⁺) transition was determined to be Γ_γ = 58±7 meV. This is somewhat higher than the value predicted by large j-j shell-model calculations, but agrees well with predictions based on the rotational and the SU(3) models for a 4p-4h band, and the deformed- basis calculation of Green and Brown.     

The mean life of the second 0+ state in 42Ca

The mean life of the second excited state in ⁴²Ca (1.84 MeV, O⁺) was measured by a direct electronic timing technique to be 558 ± 8 ps. This result compares with a previous measurement of 480 ± 30 ps which yields a high B(E2) value. The present result gives a B(E2; 0₂⁺ 1.84 MeV → 2₁⁺ 1.52 MeV) value of 484 e² · fm⁴ or 55 single-particle units; the theoretical implications of this are discussed.The accurate mean life was extracted using a convoluted function to describe both the inherent time resolution of the experimental system and the exponential decay of the state of interest.     

Gamma-ray decay from the analogue state of the Ca g.s.

The γ-decay from three fine-structure components of the ⁴⁹Ca ground state analogue has been investigated. Angular distributions have been measured for the decay from the strongest component. Absolute γ-widths are found from the γ-spectra, and the proton and neutron widths from fits to the ⁴⁸Ca(p, p) and (p, n) excitation functions. A group of eight states with E_x around 7 MeV is found to carry a significant part of the M1 strength. A comparison with the β-decay from ⁴⁹Ca has been made.     

The reaction 36S(p,γ)37Cl: (I). Excitation energies and γ-ray branchings of bound states deduced from resonances in the range Ep = 500–2000 keV

Yield curves of the reaction ³⁶S(p, γ)³⁷Cl have been measured over the range E_p = 500–2000 keV with a highly enriched (81%) ³⁶S target. Proton energies, with a precision of typically 0.3 keV, and strengths are presented for the nearly 200 observed resonances. Several previously reported resonances, among which the well-known $\text{J}{}^{\mathrm{\pi }}\text{=}\text{7}\text{2}{}^{\mathrm{?}}\text{, E}{}_{\mathrm{p}}\text{= 1887}\text{keV}$ analogue resonance, are proven to be multiplets.At 75 selected resonances in the ranges E_p = 500–1200 and 1800–2000 keV the decay schemes have been studied. These measurements also provide rather detailed information on the γ-ray branching ratios of more than 50 bound states of which the majority has not been observed previously. Precision excitation energies have been determined; for the levels with E_x < 5 MeV the median uncertainty amounts to 30 ppm. The reaction Q-value is Q = 8386.34 ± 0.23 keV.These precision data invalidate several previous spin and parity assignments to low-lying bound states of ³⁷Cl. They also provide a basis for the lifetime measurements and spin and parity assignments to be discussed in the following paper.     

Damping width of double resonances

Damping width of the double giant dipole resonance of¹³⁶Xe excited in relativistic heavy ion collisions is calculated by diagonalizing a microscopic Hamiltonian in a basis containing one-, two- and three-phonon states. The coupling between these states is determined making use of the fermion structure of the phonons. The resulting width of the double giant dipole resonance is close to √2 times the width of the single giant dipole resonance.     

Damping width of double resonances

Damping width of the double giant dipole resonance of ¹³⁶Xe excited in relativistic heavy ion collisions is calculated by diagonalizing a microscopic Hamiltonian in a basis containing one-, two- and three-phonon states. The coupling between these states is determined making use of the fermion structure of the phonons. The resulting width of the double giant dipole resonance is close to \(\left( {3_1^ + } \right)\) times the width of the single giant dipole resonance.     

Study of isovector charge-exchange excitations and their decay via the 16O(3He,tp) reaction

The nucleus ¹⁶F was studied via the ¹⁶O(³He, t) reaction at 81 MeV. Differential cross sections for many states were obtained and interpreted with DWBA calculations, using microscopic wave functions and an effective projectile-nucleon interaction. Proton decay to several states in ¹⁵O was observed and angular correlations for protons in coincidence with tritons detected at θ = 0° were measured. Several spin-parity assignments have been made. The distribution of isovector ΔL = 1 strength could be deduced. The analog of the giant dipole resonance ($\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{? 9.5}\text{MeV}\text{)}$ is strongly excited. The magnetic quadrupole strength has two strong components, one low, at E_x = 0.424 MeV, and one high, at $\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{? 7.5}\text{MeV}$. Evidence is given for a proportionality between cross section and M2 strength for transitions to J^π = 2^? states, which possibly make the (³He, t) reaction a suitable tool for determining quantitatively isovector M2 (or B_ij) strengths.     

Coulomb displacement energies and decay widths of isobaric analog states from Sm(3He,t) at θ = 0°

The ^{144,147,148,149,150,154}Sm(³He, t)^{144,147,148,149,150,152,154}Eu reactions leading to ground-state isobaric analog states (IAS) have been studied at θ = 0° and E(3He) = 45.9 MeV. The Coulomb displacement energies decrease more rapidly than $\text{A}{}^{\mathrm{<mtext>?1</mtext><mtext>3</mtext>}}$. Approximately 110 keV of the total decrease of about 470 keV from A = 144 (spherical) to A = 154 (deformed) can be ascribed to deformation. No discontinuity is apparent at the transition from spherical to deformed shapes at N = 88–90. This is attributed to two effects: (i) rms radii increase with static deformations and with dynamical vibrations; (ii) Coulomb displacement energies depend on rms charge radii and on the rms radii of the neutron excess. The data suggest neutron deformations greater than proton deformations for A = 148 and 150 but smaller for A = 152 and 154. The IAS widths increase from ～30 keV to ～90 keV and can be attributed to mixing with the (T₀?1) component of a proposed isovector giant monopole resonance.