Stagnancy of the pygmy dipole resonance

The pygmy dipole resonance(PDR) of nickel isotopes is studied using the deformed random phase approximation method. The isoscalar character of the pygmy resonance is confirmed, and the correlation between the pygmy resonance and neutron skin thickness is discussed. Our investigation shows a linear correlation between PDR integral cross section and neutron skin thickness when the excess neutrons lie in pf orbits, with a correlation rate of about 0.27 fm~(-1). However, in more neutron-rich nickel isotopes, the growth of the pygmy dipole resonance is stagnant. Although the neutron skin thickness increases, the whole skin is not active. There is an inertial part in the nuclei~(70-78)Ni which does not participate in the pygmy resonance actively and as a result, contributes little to the photo-absorption cross section.     

Proton electric pygmy dipole resonance

The evolution of the low-lying E1 strength in proton-rich nuclei is analyzed in the framework of the self-consistent relativistic Hartree-Bogoliubov model and the relativistic quasiparticle random-phase approximation (RQRPA). Model calculations are performed for a series of N=20 isotones and Z=18 isotopes. For nuclei close to the proton drip line, the occurrence of pronounced dipole peaks is predicted in the low-energy region below 10 MeV excitation energy. From the analysis of the proton and neutron transition densities and the structure of the RQRPA amplitudes, it is shown that these states correspond to the proton pygmy dipole resonance.     

Isospin character of the pygmy dipole resonance in 124Sn

The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (α, α'γ) coincidence method at Eα=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structure: one group of states which is excited in (α, α'γ) as well as in (γ, γ') reactions and a group of states at higher energies which is only excited in (γ, γ') reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.     

The pygmy dipole resonance in Ni and the neutron skin

A search of the pygmy resonance in ⁶⁸Ni was made using the virtual photon technique. The experiment was carried out using the radioactive beam ⁶⁸Ni at 600 A MeV, produced with fragmentation of ⁸⁶Kr at 900 A MeV on a ⁹Be target. The ⁶⁸Ni beam was separated by a fragment separator, and the γ-rays produced at the interaction with the Au target were detected with the RISING and FRS set-up at the GSI laboratory in Germany, also including the HECTOR array. The measured γ-ray spectra show a peak centered at approximately 11 MeV, whose intensity can be explained in term of an enhanced strength of the dipole response function (pygmy resonance). A pygmy structure of this type was also predicted by different models for this unstable neutron-rich nucleus. Correlations between the behavior of the nuclear symmetry energy, the neutron skins, and the percentage of energy-weighted sum rule (EWSR) exhausted by the pygmy dipole resonance (PDR) are investigated by using different random phase approximation (RPA) models.     

Fine structure of the giant dipole resonance in O and nuclear coexistence

It is shown that nuclear coexistence can account for most of the fine structure observed for the giant dipole resonance in ¹⁶O.     

Nature of the pygmy dipole resonance in 140Ce studied in (alpha, alpha' gamma) experiments

A concentration of electric-dipole excitations below the particle threshold, which is frequently denoted as the pygmy dipole resonance, has been studied in the semimagic nucleus 140Ce in (alpha, alpha' gamma) experiments at E alpha = 136 MeV. The technique of alpha-gamma coincidence experiments allows the separation of E1 excitations from states of other multipolarities in the same energy region and provides an excellent energy resolution to allow a detailed analysis for each state. The experimental results show that the PDR splits into two parts with different nuclear structure: one part which is excited in (alpha, alpha' gamma) as well as (gamma, gamma') experiments and one part which is excited only in (gamma, gamma').     

Microscopic nature of the pygmy dipole resonance: the stable Ca isotopes

The electric dipole strength distribution in 44Ca has been measured up to 10 MeV in high resolution photon scattering experiments for the first time. The data obtained have been compared to earlier measurements on (40,48)Ca in order to view the evolution of the electric pygmy dipole resonance (PDR). Calculations that were performed within the framework of the microscopic extended theory of finite Fermi systems, which adds contributions of the quasiparticle-phonon coupling to random phase approximation calculations, give a qualitative agreement with the experimental data for all three isotopes. We have shown that it is necessary to include this coupling to describe the PDR.     

Excitation of pygmy dipole resonance in neutron-rich nuclei via Coulomb and nuclear fields

We study the nature of the low-lying dipole strength in neutron-rich nuclei, often associated with the pygmy dipole resonance. The states are described within the Hartree-Fock plus RPA formalism, using different parametrizations of the Skyrme interaction. We show how the information from combined reaction processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states.     

Electromagnetic cross sections of double giant dipole resonances in 136Xe and 208Pb within the phonon damping model

The electromagnetic cross sections of the double giant dipole resonances (DGDR) in 136Xe and 208Pb are calculated using the strength functions obtained within the phonon damping model. The parameters of the model have been selected to describe reasonably well the single giant dipole resonance in these nuclei. The results are found in an overall agreement with the recent experimental data for the DGDR cross sections in exclusive measurements at near-relativistic energies.     

A possible proton pygmy resonance in ~(17)Ne

The low-lying electric dipole strengths in proton-rich nuclei 17F and 17Ne, which can be produced at HIRFL-CSR in Lanzhou, are investigated. In the framework of the covariant density functional theory the self-consistent relativistic Hartree Bogoliubov model and the relativistic quasiparticle random phase approximation with the NL3 parameter set and Gogny pairing interaction are adopted in the calculations. A pronounced dipole peak appears below 10 MeV in17Ne, but does not occur in 17F. The prop erties of t...     

X-ray study of the deep inelastic reactions 1,160-MeV136Xe+Pb and136Xe+232Th

The spectra ofK x-rays emitted by the target-like reaction products in the deep inelastic collisions 1,160-MeV¹³⁶Xe+Pb and¹³⁶Xe+²³²Th were investigated. Target-like x-ray emission probabilities per projectile-like and per target-like product were determined and compared with theoretical predictions of atomic vacancy production in the higher-Z particle (1s }-vacancy production). It is shown that a large fraction of the x-rays is produced by internal conversion ofγ-rays in the higher-Z reaction product. Measurement of target-like x-rays in coincidence with projectile-like and with target-like reaction products allowed the determination of the average survival probability against fission and of theZ-distribution of the highly excited target-like nuclei as a function of theQ value (total kinetic energy loss). The observed average survival probabilities agree qualitatively with theoretical predictions of Wilcke et al. Nuclearreaction-time effects on the 1s σ-vacancy production in these collisions could not be demonstrated with any certainty.     

Fine structure effects and phase transition of Xe nanocrystals in Si

We report on an X-ray diffraction study performed on Xe agglomerates obtained by ion implantation in a Si matrix. At low temperature, Xe nano-crystals were formed in Si with different average sizes according to the preparation procedure. High resolution diffraction spectra were detected as a function of the temperature, in the range 15–300 K, showing evidence of fine structure effects in the growth mode of the Xe nanocrystals. We report the first experimental observation of fcc crystalline agglomerates with a lattice parameter expanded by the epitaxial condensation on the Si cavities, whereas for small agglomerates randomly oriented evidence of a contracted lattice was found. For these nanocrystals, a solid-to-liquid transition temperature, size dependent, was detected; above the transition temperature, a fluid phase was observed. Neither overpressurized clusters were detected at any temperature, nor preferential binary size distribution as reported for a metal matrix.     

A dipole band in 124Xe

High-spin states in ¹²⁴Xe were populated by means of the ¹¹⁰Pd(¹⁸O,4n) reaction. In-beam γ rays were measured using the GASP spectrometer. A dipole band, similar to those previously found in other nuclei of this mass region, was identified in ¹²⁴Xe.     

The decay of the136I isomers to the energy levels of136Xe

The gamma ray spectra associated with the decay of the¹³⁶I isomers have been measured by means of a Ge(Li) detector. The iodine samples were produced by using the technique of on-line isotope separation of the fission products of a²³⁵U target at the R2-O reactor in Studsvik, Sweden. On the basis of the measured singles and Ge-Ge coincidence spectra a new level scheme was constructed for¹³⁶Xe. A comparison with theoretical calculations is included. The half-life analysis of the gamma peaks as well as the beta andK X-rays associated with mass 136 iodine showed that there exist three separate half-lives: 48 ± 2 s, 83 ± 3 s and 100 ± 3 s, the last one being not previously reported. The possibility of the existence of three isomeric states in¹³⁶I is discussed.     

Investigation of the (d,p)-reaction on136, 132Xe in inverse kinematics

The one-neutron transfer reactions d(^132,136 Xe, p)^133,137 Xe have been investigated in inverse kinematics with xenon beams incident on deuterium loaded titanium targets. The angular distributions of the protons, measured with a detector array of 100 PIN-photodiodes, have been analyzed using standard DWBA. Generally, good agreement is obtained with results previously obtained in reactions induced by light-ion beams.     

A search for deeply bound pionic states of xenon produced in the 136Xe(d, 3He)135Xeπ-bound reaction

A search for deeply bound pionic states of xenon produced in the ¹³⁶Xe(d, ³He)¹³⁵Xe_π-bound reaction at E _d = 500 MeV is reported. The population of the 1s pionic-atom state of ¹³⁵Xe is observed on the predicted level of the cross section ～ 40 μb/sr. The binding energy of the 1s state, B = 2.9 ± 0.5 MeV, however, is lower than theoretically predicted.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Mass and double-beta-decay Q value of 136Xe

The atomic mass of 136Xe has been measured by comparing cyclotron frequencies of single ions in a Penning trap. The result, with 1 standard deviation uncertainty, is M(136Xe)=135.907 214 484 (11) u. Combined with previous results for the mass of 136Ba [Audi, Wapstra, and Thibault, Nucl. Phys. A 729, 337 (2003)10.1016/j.nuclphysa.2003.11.003], this gives a Q value (M[136Xe]-M[136Ba])c(2)=2457.83(37) keV, sufficiently precise for ongoing searches for the neutrinoless double-beta decay of 136Xe.     

Nuclear spin resonance of (129)Xe doped with O(2)

Spin-lattice relaxation of (129)Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O(2) impurity concentrations. Direct measurements of the ground state magnetic properties of the O(2) are difficult because the ESR (electron spin resonance) lines of O(2) are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O(2) molecules and to the O(2) magnetic relaxation. From these measurements, the value of the D[Sz(2)-(1/3)S(2)] spin-Hamiltonian term of the triplet spin ground state of O(2) can be determined. The temperature and magnetic field dependence of the measured paramagnetic O(2)-induced excess line width of the (129)Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D=0.19 meV (2.3 K), and with the reasonable assumption that the E[S(x)(2)-S(y)(2)] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2K of the (129)Xe spin-lattice relaxation rate, T(1n)(-1)(T), is also accounted for by our model. Using an independent determination of the true O(2) concentration in the Xe-O(2) solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O(2) at 2.3 K and 0.96 T is determined to be approximately 1.4 x 10(-8)s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized (129)Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.