Nucleus Excitation Energy Research Articles

Studies of superdeformation in the [formula omitted] region

Recent advances in the physics of superdeformed nuclei in the A⋍ 150 mass region are reviewed. Topics covered include the intensity and the feeding pattern of yrast bands as a function of i) compound nucleus excitation energy and input angular momentum, ii) the mass of Gd isotopes and iii) beam-target combinations. Recent data on excited bands in 149Gd and on the deexcitation of the 152Dy band are presented. Plans for new γ-ray spectrometers are discussed and future directions of the field are briefly outlined.

Particle-hole symmetry, F-spin, and r-process parameters.

We exploit approximate symmetry under particle-hole conjugation and the systematics associated with a classification scheme (inspired by the neutron-proton interacting boson model) to obtain estimates of binding energies and low-lying excitation energies for even-even r-process nuclei drawn from the major proton and neutron shells, 50Z82 and 82N126. We anticipate that our simple formalism for binding energies, depending on only six parameters, has an accuracy of \ensuremath{\sim}0.8 MeV. We argue that the particular dual particle-hole conjugation symmetry required to relate excitation energies in neutron-rich nuclei to the known excitation energies of the corresponding conjugated nuclei holds to within 10% or so.

The limit of stability of proton-rich thallium isotopes: a search for the decay of177Tl

The fusion reaction74Se+106Cd→180Pb* at a compound nucleus excitation energy Ex ≃ 40 MeV, has been used in a search for the decay of the unknown nucleus177Tl. Evaporation residues were velocity and mass analysed prior to implantation into a position sensitive silicon surface barrier detector. No evidence was discovered for the proton or alpha-decay of177Tl for a cross-section > 10−5+10 nb. This indicates either that177Tl decays too rapidly (t1/2 < 1 μs) by proton emission for the decay to be detected, or that177Tl is produced with a cross-section less than the limit established in the present experiment. The following nuclear decay half-lives were measured with improved accuracy;177Hg (t1/2 = 130±5 ms),178Hg (t1/2=250±25 ms) and177Au (t1/2=1180±70 ms). The experiment also provided the first direct confirmation of the correct mass assignment of the alpha-decay, Eα=6.26 MeV, to the decay of176Au. Two additional halflife measurements of proton-rich Osmium isotopes are also presented from a previous similar experiment;165Os (t1/2=73±8 ms) and166Os (t1/2 = 194±17 ms).

Pre-scission 1H and 4He emissions in 16O+197Au reaction.

Pre-scission $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in $^{16}\mathrm{O}$${+}^{197}$Au reactions are measured in coincidence with fission fragments in the compound nucleus excitation energy of 48 to 99 MeV. Observed coincident energy spectra of $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ are shifted towards lower energies compared to statistical model calculations. The data are analyzed in terms of the spin-dependent level density and emission barriers for the charged particles. The analysis shows that the effect of the spin-dependent level density on the energy spectrum is small and the effective emission barriers for particle emissions are reduced by 1.0\ifmmode\pm\else\textpm\fi{}0.4 MeV for $^{1}\mathrm{H}$ and 1.8\ifmmode\pm\else\textpm\fi{}0.4 MeV for $^{4}\mathrm{He}$ relative to the corresponding fusion barriers calculated by using optical potentials.

Critical excitation energy in fusion-evaporation reactions.

The widths of the residue-mass distributions are used as a measure of the excitation energy of hot nuclei. They increase with the recoil velocity but saturate much below the calculated velocity ${v}_{\mathrm{CN}}$ for complete fusion. Above these critical excitations energies (4.5--6.4 MeV/nucleon) derived from the recoil velocity, the normal evaporation of nucleons and small clusters disappears, if a hot and equilibrated compoundlike nucleus is formed at all.

State densities and spectrum fluctuations: information propagation in complex nuclei

At excitation energies in nuclei where the state density is unambiguously defined there is a sharp separation between the smoothed spectrum (which defines the density) and fluctuations about it which have recently been studied with a view to understanding some aspects of quantum chaos. We briefly review these two complementary subjects, paying special attention to: the role of the effective interaction in determining the density; the calculation of interacting-particle state and level densities, and of expectation values of interesting operators; the information about the effective nucleon-nucleon interaction which is carried both by the density and the fluctuations.

The new nuclei162Os and156Ta and theN=84 alpha emitting isomers

The reactions58Ni+102Pd→160W and58Ni+106Cd→164Os were investigated to search for new decay data of neutron deficient nuclei. Excitation energies of the compound nuclei covered a range from 47 to 89 MeV. Velocity separation of the evaporation residues and position time correlations with the a decays of the implanted nuclei were used. The following new decay data were measured:162Os (Eα=(6611 ±30) keV, T1/2=(1.9±0.7) ms);158W (T1/2=(0.9±0.3) ms);158mW (E=1.88 MeV, Eα=(8280±30) keV, T1/2=(0.01-1) ms);155mLu (Eα=(5575±10) keV); β decay of156Ta (T1/2 > 10 ms) to the 8+ yrast isomer in156Hf. A cross section of 5μb was measured for the new isotope156Ta produced in a p3n evaporation channel from160W at 64 MeV excitation energy.

Temperatures and excitation energies of hot nuclei in the reactions of 32S+Ag and 16O+Ag at 30 MeV/nucleon.

For the reactions of 30 MeV/nucleon /sup 16/O and /sup 32/S with Ag, singles and coincidence measurements have been made for heavy residues, fragments (3less than or equal tozless than or equal to14), and light charged particles (zless than or equal to2). Mass-velocity correlations for the residues and fragment-residue coincidences indicate that increasing residue velocities do correspond to increasing excitation energy. Excitation energies as high as 90% of those which would result from complete fusion are reached. The spectra of light particles detected in coincidence with residue groups having different average velocities are analyzed with a moving source fit. When recoil effects are properly taken into account, excellent fits to the data are obtained. From the energy spectra and multiplicities of particles emitted from a fusion-like source the initial temperatures of the primary composite nuclei are determined. The results suggest that a plateau temperature near 6.5 MeV is reached above excitation energies of 3 MeV/nucleon. The temperatures are compared to those resulting from various model calculations.

Production and characterization of hot nuclei in the reactions of 19 and 35 MeV/u 14N with 145Sm

Hot nuclei produced in the reactions of 261 MeV and 490 MeV 14N with 154Sm have been studied. The initial properties of these nuclei; excitation energies, angular momenta and temperatures, have been characterized through measurements of residue velocities, gamma ray multiplicities and α-particle energy spectra. Nuclei with excitation energies as high as 400 MeV and temperatures as high as 6 MeV are produced. Determinations of the variation of temperature with excitation energy for nuclei of A ⋍ 160 indicate that the apparent level density parameter a, defined as E ∗|T 2 changes from A/8 at low energies to ≈ A/13 at 400 MeV excitation energy. Effective particle emission barriers suggest large shape fluctuations during the de-excitation cascade. At 35 MeV/u the variation of angular momentum transfer with linear momentum transfer in incomplete fusion reactions is in reasonable agreement with values calculated using a geometric overlap model.

Excitation of a target nucleus in the firestreak model

The nuclear firestreak model is used to calculate the excitation energy of residual nuclei in nucleus-nucleus collisions. The incorporation of diffuse surfaces of interacting nuclei has introduced the dependence of the excitation of the nucleus on the energy of the projectile. The shear viscosity of nuclear matter is also taken into account. A “nuclear thermometer” is proposed to measure the temperature of the excited target nucleus.

Landau-parameters and isoscalar nuclear excitations in a relativistic mean-field theory

The parameters of Landau's Fermi-liquid theory are calculated for nuclear matter in the framework of a relativistic mean-field theory. Self-interactions of the scalar meson field are taken into account. Isoscalar excitation energies of finite nuclei are calculated with these parameters in a fluid-dynamical approach.

On the limiting excitation energy of nuclei

Abstract The limiting temperature of compound nuclei is estimated to be around T ≅ 5 MeV. Above this temperature the formation of nuclear droplets and light gaseous-like particles becomes a competing process. This temperature is lower than that following from thermal Hartree-Fock calculations. An experiment is proposed to verify the onset of multiframentation.

Structure and Decay Properties of Low-Spin States at High Excitation Energy

The structure and decay properties of levels between 1 and 40 MeV of excitation energy in rare earth nuclei have been studied. In particular, coupling phenomena and signatures of non-statistical cooling are discussed.

Incomplete fusion in the 40Ar+68Zn reaction: Trends and limits.

The heavy fragments emitted in the $^{40}$Ar${+\mathrm{}}^{68}$Zn reaction have been studied at $^{40}\mathrm{Ar}$ energies of 14.6, 19.6, 27.6, and 35 MeV/nucleon. Fusion-evaporation and fusion-fission components resulting from complete and incomplete fusion were observed, and cross sections have been measured for both components at the four energies. The velocity distributions of the evaporation residues have been analyzed and the linear momentum transferred to the compound system deduced. Compared to other results, the momentum transfer appears to depend on the asymmetry of the system and on the projectile mass. The mass, excitation energy, and temperature of the compound nuclei formed have been determined. Fusion disappears slightly above 35 MeV/nucleon, which corresponds to a compound nucleus excitation energy of 500 MeV (5.9 MeV/nucleon) and to a temperature of 6.8 MeV. The results are discussed in terms of various theoretical models.

Kinematic coincidence technique to identify γ-decaying highly excited states in light nuclei

A simple kinematic coincidence technique is described for separating γ-decaying states from particle decaying states populated in two-body reactions. The method is especially suited to the discovery and further study of γ-decaying states at high excitation energies in light nuclei, but has not previously been widely exploited. Experimental applications using the reactions 12C( 16O, α) 24Mg and 12C( 6Li, d) 16O are presented and these illustrate the range of results obtainable. In particular, the method provides an excellent means of measuring the probability for γ-decay, Γ γ/Γ, of individual excited states. The general properties of reactions suitable for use with the coincidence technique are discussed.

Nuclear Fluid Dynamics at a Finite Temperarute

An irrotational nuclear fluid dynamical scheme, on the. basis of the quantal action principle, is presented, where the effect of temperature is taken into account. A low temperature expansion, retaining terms up to T', is considered. The temperature dependence of the energies of isoscalar giant resonances is calculated. The nuclear radius, chemical potential, excitation energy and incompressibility of hot nuclei are investigated as functions of the temperature.

The ΔTz = + 1 Gamow-Teller excitation of N = 28 isotones

Results of shell-model calculations of Gamow-Teller strength distributions are presented for 50Ti → 50Sc, 52Cr → 52V and 54Fe → 54Mn. The results support the previous shell-model estimates that a large fraction of Gamow-Teller strength is concentrated at low excitation energy in the daughter nuclei. The strength distributions are in fairly good agreement with those of forward-angle cross sections of T 0 + 1 isospin states of the intermediate-energy (p, p') reaction on N = 28 isotones. An estimate of quenching in the T 0 → T 0 + 1 τσ mode transition is attempted. Calculations are performed also for the first 2 + and 4 + parent states in the context of electron capture at the late stages of stellar evolution.

Influence of the giant dipole resonance in the statistical decay of the compound nucleus160Er

Evaporation residue excitation functions and statisticalγ-ray spectra have been measured in the heavy ion fusion reaction16O+144Nd →160Er betweenE Lab=76 and 105 MeV. In agreement with statistical model calculations, the evolution of theγ-ray spectra as a function of the compound nucleus excitation energy is consistent with the emission of high-energyγ rays (Eγ>10 MeV) in the early stage of the decay process. The description of the evaporation residue distributions by the statistical model is improved if aγ strength function including a giant dipole resonance built on each excited state and describing the shape of the measured statisticalγ-ray spectra is used in the calculations. The present analysis shows, however, that there is a strong correlation between the parameters of the resonance deduced from the fit to theγ-ray spectra and other parameters of the statistical model, in particular the level density parameter.

Gamow-Teller electron capture strength distributions in stars: Unblocked iron and nickel isotopes

Abstract Results of shell-model calculations of the Gamow-Teller strength distributions for the (n, p) or electron capture direction are presented for 54 Fe → 54 Mn, 56 Fe → 56 Mn, 56 Ni → 56 Co, 60 Fe → 60 Mn, and 64 Fe → 64 Mn . In the cases of 56Fe, 60Fe, and 64Fe calculations were performed on both the ground and first excited states. These and similar transitions are characterized by large amounts of Gamow-Teller strength at low excitation energy in the daughter nuclei. The question of quenching in the (n, p) direction remains open. It is expected that the late stages of presupernova stellar evolution (before neutron shell blocking) will depend on the character and distribution of Gamow-Teller strengths for electron capture transitions such as these.

F-spin multiplets in rare-earth nuclei

Excitation energies in rare-earth nuclei belonging to F-spin multiplets are found to be nearly constant for the ground and gamma bands, whereas they vary for the quasi beta band. Implications for the IMB-2 Hamiltonian are discussed.