Neutron Particle-hole Configurations Research Articles

Low-energy isovector and isoscalar dipole response in neutron-rich nuclei

The self-consistent random phase approximation (RPA), based on the framework of relativistic energy density functionals, is employed in the study of isovector and isoscalar dipole response in $^{68}$Ni, $^{132}$Sn, and $^{208}$Pb. The evolution of pygmy dipole states (PDS) in the region of low excitation energies is analyzed as a function of the density-dependence of the symmetry energy for a set of relativistic effective interactions. The occurrence of PDS is predicted in the response to both the isovector and isoscalar dipole operators, and its strength is enhanced with the increase of the symmetry energy at saturation and the slope of the symmetry energy. In both channels the PDS exhausts a relatively small fraction of the energy-weighted sum rule but a much larger percentage of the inverse energy-weighted sum rule. For the isovector dipole operator the reduced transition probability $B(E1)$ of the PDS is generally small because of pronounced cancellation of neutron and proton partial contributions. The isoscalar reduced transition amplitude is predominantly determined by neutron particle-hole configurations, most of which add coherently, and this results in a collective response of the PDS to the isoscalar dipole operator.

High resolution particle spectroscopy in 208Pb

By using the Q3D magnetic spectrograph of the Maier-Leibnitz-Laboratorium at München, particle-hole states in 208Pb are investigated. With the reaction 208Pb(p, p') at energies 14 < Ep < 18MeV an instrumental resolution of δEp′Ep′ = 3 · 10−4 is achieved for protons without energy loss in the target. For lower energies Ep′ an exponential tail with a width proportional to the effective target thickness appears. Below Ex = 8.2 MeV more than 250 states in 208Pb are observed. Excitation energies are derived with uncertainties of 0.1 keV for strongly excited states by calibration with known data from Nuclear Data Sheets. 208Pb(p, p') via isobaric analog resonances (IAR) in 209Bi allows to determine the neutron particle-hole components of each state in 208Pb. The selective excitation in an IAR yields the parity for each state. Spin and dominant neutron particle-hole configurations of a state are determined from the mean cross section and the shape of the angular distribution for 208Pb(p, p'). From about 120 particle-hole states predicted by the shell model in 208Pb below Ex = 6.1 MeV, about 50 states with negative parity and 30 states with positive parity are identified.

Erratum to: Neutron particle-hole structure of states in 208Pb determined from the proton decay of the d5/2 isobaric analog resonance in 209Bi

Proton scattering on 208Pb via isobaric analog resonances (IARs) in 209Bi generates neutron particle-hole configurations in excited states of 208Pb . The spin and parity of the neutron particle correspond to the spin and parity of the resonance. We performed experiments on the 208Pb (p, p’) reaction via all known isobaric analog resonances in 209Bi with the Q3D magnetic spectrograph in Munchen. We observed more than 130 levels in the range of excitation energies E x < 6.4 MeV. In the proton decay of the d 5/2 resonance, 29 levels have cross-sections larger than 5% of the maximum cross-section of any level. Six levels in the range 4.6 < E x < 5.2 MeV correspond to states with known spins and major $\ensuremath \nu(d_{5/2}^{+1} p_{1/2}^{-1})$ strength. Six levels in the range 5.7 < E x < 6.4 MeV contain most of the $\ensuremath \nu(d_{5/2}^{+1} p_{3/2}^{-1})$ strength. At least one level originates from an unresolved doublet. Among the states with a major $\ensuremath \nu(d_{5/2}^{+1} p_{3/2}^{-1})$ component, only states with spins 1- and 3- are firmly identified. The comparison to previous data taken with semiconductor detectors reveals details of the structure of at least 18 states in the range 4.6 < E x < 6.4 MeV.

Relativistic RPA plus phonon-coupling analysis of pygmy dipole resonances

Abstract The relativistic random-phase approximation (RRPA) plus phonon-coupling (PC) model is applied in the analysis of E1 strength distributions in 208 Pb and 132 Sn, for which data on pygmy dipole resonances (PDR) have recently been reported. The covariant response theory is fully consistent: the effective nuclear interaction NL3 is used both to calculate the spectrum of single-nucleon Dirac states, and as the residual interaction which determines the collective phonon states in the relativistic RPA. It is shown that the picture of the PDR as a resonant oscillation of the neutron skin against the isospin saturated proton–neutron core, and with the corresponding RRPA state characterized by a coherent superposition of many neutron particle–hole configurations, remains essentially unchanged when particle–vibration coupling is included. The effect of two-phonon admixtures is a weak fragmentation and a small shift of PDR states to lower excitation energy. Even though the PDR calculated in the extended model space of p h ⊗ phonon configurations contains sizeable two-phonon admixtures, it basically retains a one-phonon character and its dynamics is not modified by the coupling to low-lying surface vibrations.

Lifetime of a new high-spin isomer in 150 Dy

A new high-spin isomer in 150Dy has been observed at an excitation energy of 10.3 MeV by combining the inverse-kinematic reaction induced by a pulsed beam of 132Xe and the $\gamma$ -ray recoil-shadow technique. The half-life of this isomeric state has been determined to be $T_{1/2} = 1.6 \pm 0.6$ ns using the conventional centroid-shift method with the 141Pr(16O, p6n)150Dy reaction at 165 MeV. The mechanism producing high-spin isomers in N = 83,84 isotones is qualitatively discussed in terms of the difference of the neutron particle-hole configuration between the high-spin isomer and the lower-lying state.

Two-nucleon excitations in 66 148 Dy82 from Gamow-Teller decay of the 67 148 Ho81 6? isomer

Two-particle excitations in the two-proton nucleus148Dy populated inβ-decay of 10-s 148Ho(6−) were investigated throughγ-ray measurements following on-line mass separation. Theπh 11/2s1/2 two-proton doublet and the (vh 9/2 s 1 2/−1 )5− neutron particle hole configuration are firmly identified, and theπh 11/2 d 5 2/−1 and πh11/2D3/2 multiplets are also observed. The results suggest dominantπh 11/2vs 1 2/−1 character for the148Ho 6−-isomer.

Generalized neutron particle-hole states in an extended unified model

Abstract Negative-parity levels in the doubly even N = 82, Z nuclei, with 3.0 MeV ⪅ E x ⪅ 6.0 MeV are described in an extended unified-model approach, where neutron hole states in the Z = 50, N = 82 closed shell core, (i.e. 2 d 3 2 −1 ,3 s S 1 2 −1 ,2 d 5 2 −1 ,1 g 7 2 −1 ) are coupled to the low-lying levels ( E x ⪅ 2.0 MeV) of the odd-neutron N = 83, Z nuclei. This particular configuration space of generalized neutron particle-hole states (GNPH) is particularly suited for describing negative-parity levels obtained in proton inelastic scattering through isobaric analogue resonances (IAR), corresponding to the N = 83, Z low-lying nuclear levels. Level schemes as well as partial decay widths and angular distributions are calculated and compared extensively with the available experimental data. Also spectroscopic factors, as well as wave functions, deduced from the experimental results are studied in detail. Thus in the cases of 136 Xe, 138 Ba, 140 Ce, 142 Nd and 144 Sm, some of the important neutron particle-hole configurations can uniquely be determined in the energy region 3.0 MeV ⪅ E x ⪅ 6.0 MeV.

Spins of generalized neutron particle-hole states in 140Ce with excitation energies above Ex=4 MeV

Spins are assigned to states in 140Ce observed in the proton decay of resonances which are the isobaric analogues of the first (J π= 3 2 −) , second (J π= 1 2 −) and fifth (J π= 5 2 −) excited states in 141Ce. Spins 2 −, 1 −, 2 −, 2 − and 1 − are assigned to the states at E x = 4.158, 4.182, 4.580, 4.748 and 4.77 MeV in 140Ce, respectively. The states at E x = 5.026, 5.196, 5.424 and 5.466 MeV are shown to have spins 2 − or 3 −. The assignments are based on the assumption that only the generalized neutron particle-hole configurations | d 3 2 −1⊗ 141 Ce(Jβ)〉 and | s 1 2 −1 ⊗ 141 Ce (Jβ)〉 contribute to the proton widths measured on the resonance Jβ.

Gross structure in excitation functions for207Pb(p, p′)207Pb in the region of overlapping isobaric analogue resonances

Prominent resonances in the reaction207Pb(p, p′)207Pb leading to the low-lying states in207Pb(1/2− g.s., 5/2− 0.57 MeV, 3/2− 0.89 MeV, 7/2− 2.33 MeV) are observed betweenE p =16 and 18 MeV. The structures in the excitation functions are interpreted as being due to analogues of states in208Pb which are mixtures of neutron particle-hole configurations.