Residual Neutron-proton Interaction Research Articles

Non-perturbative symmetry breaking of parton distributions

Six rotational bands in the odd-odd nucleus 174Ta have been populated with the 160Gd(19F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π12− [541] ⊗ ν12− [521] band and semi-decoupled π12− [541] ⊗ ν72− [633] band are observed, in addition to the high-K couplings of the π92− [514] ⊗ ν72+ [633], π92− [514] ⊗ ν52− [512], π72+ [404] ⊗ ν72+ [633], and π52+ [402] ⊗ ν52− [512] configurations. The signature splitting of the π12− [541] ⊗ ν72+ [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i132 neutrons in the π12− [541] bands of odd-Z nuclei.

EXCEPTIONAL STRUCTURES IN THE ODD-ODD NUCLEUS 250Md

The low-energy two-quasiparticle bandhead energies for the odd-odd Z=101 nucleus 250 Md are evaluated using a zero range residual neutron-proton interaction. The 250Md ground state is seen to have the spin-parity Iπ=0- corresponding to the singlet band from the configuration {p : 7/2[514]⊗n:7/2[624]} in violation of the Gallagher-Moszkowski (GM) coupling rule. The situation here is shown to be almost identical to that for the rare-earth nucleus 166 Ho , which is the only well-established exception to the GM rule known so far. Analysis of the expected low energy spectrum, including the rotational levels, for 250 Md reveals the occurrence of an as-yet-unobserved long-lived high-spin Iπ = 7- isomeric state around (80±30) keV with dominant ε and α decay modes.

Rotational alignment nearN=Zand proton-neutron correlations

The effects of the residual proton-neutron interactions on bandcrossing features are studied by means of shell model calculations for nucleons in a high-$j$ intruder orbital. The presence of an odd-nucleon shifts the frequency of the alignment of two nucleons of the other kind along the axis of rotation. It is shown that the anomalous delayed crossing observed in nuclei with aligning neutrons and protons occupying the same intruder subshell can be partly attributed to these residual interactions.

Charge-changing transitions in an extended Lipkin-type model

Charge-changing transitions are considered in an extended Lipkin-Meshkov-Glick (LMG) model taking into account explicitly the proton and neutron degrees of freedom. The proton and neutron Hamiltonians are taken to be of the LMG form and, in addition, a residual proton-neutron interaction is included. Model charge-changing operators and their action on eigenfunctions of the model Hamiltonian are defined. Transition amplitudes of these operators are calculated using exact eigenfunctions and then the RPA approximation. The best agreement between the two kinds of calculation is obtained when the correlated RPA ground state, instead of the uncorrelated HF ground state, is employed and when the proton-neutron residual interaction, besides the proton-proton and neutron-neutron residual interactions, is taken into account in the model Hamiltonian.

High-spin states in 171Lu

Rotational bands in the odd-proton nucleus 171Lu have been populated with the 160Gd( 19F, α4n) reaction. High-spin states were observed to a maximum spin of 73 2 − using the NORD-BALL array. Several new three-quasiparticle bands were observed, including one based on the π 1 2 − [541]⊗ν 7 2 + [633]⊗ν 1 2 − [521] configuration. The signature splitting of this band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. The contribution of this interaction to the increased crossing frequency observed for the alignment of i 13 2 neutrons in the π 1 2 − [541] band is considered. Strong E1 transitions from high-spin levels in the π 1 2 + [411] band to the π 1 2 − [541] band are observed.

Like particles vs proton-neutron pairs: Phase transitions in realistic model spaces

The quasiparticle random phase approximation (QRPA) and its renormalized version (RQRPA) are used to investigate the particle number fluctuations and the number of particle pairs with $J=0,$ $T=1$ and $J=1,$ $T=0$ in the ground state of ${}^{76}\mathrm{Ge}$ as a function of the residual proton-neutron interaction. The study is performed in realistic model spaces including many single particle orbitals. While the RQRPA remains stable after the QRPA collapse the sudden increase in particle number fluctuations exhibits its limitations. The analysis of the number of pairs shows that the isoscalar-isovector phase transition found in exact calculations is causing the QRPA collapse and is missed in the RQRPA formalism.

Structure of excited bands in 162Tm and residual interactions

The nucleus 162Tm has been populated by the 130Te( 37Cl,5n) reaction, and the γ-cascades detected with the NORDBALL array. Eight coupled rotational bands involving the proton orbitals, g 7 2 [404] 7 2 + , d 3 2 [411] 1 2 + , d 5 2 [402] 5 2 + , h 11 2 [523] 7 2 − and h 9 2 [541] 1 2 − and neutron orbitals i 13 2 [642] 5 2 + , h 9 2 [523] 5 2 − and f 7 2 [521] 3 2 − have been established in 162Tm, to spins varying from 18 to 47ℏ. Various phenomena which can be related to residual neutron-proton interactions are discussed. In particular, signature inversion in the π [541]1 2 − ⊗ ν [642]5 2 + bands based on the h 9 2 quasiproton and i 13 2 quasineutron, rewpectively, may be explained by a fairly large p-n interaction. In addition, interactions at the crossings of bands with constituent quasiparticles differing both in parity and signature have been extracted with strength of the order of a few (4–12) keV. Such interactions also require terms beyond the standard mean field description.

Semiclassical description of the shears mechanism and the role of effective interactions

By deriving the angle between the proton and neutron spin vectors ${j}_{\ensuremath{\pi}}$ and ${j}_{\ensuremath{\nu}}$ in the shears bands in ${}^{198,199}\mathrm{Pb},$ we present a semiclassical analysis of the $B(M1)$ and $B(E2)$ transition probabilities as a function of the shears angle. This provides a semiempirical confirmation of the shears mechanism proposed by Frauendorf using the tilted-axis-cranking model. In addition, we propose that the rotational-like behavior observed for these bands may arise from a residual proton-neutron interaction.

New isomer in80Y

The {beta}{sup +}/EC decay of mass-separated A=80 nuclei was studied by means of {beta}- and {gamma}-ray spectroscopy after the bombardment of a {sup 24}Mg target with {sup 58}Ni ions at 190 MeV. A new 1{sup {minus}} isomer at 228.5(1) keV has been identified in {sup 80}Y with a half-life of 4.7(3) s. This new isomer decays to the 4{sup {minus}} ground state with an 81(2){percent} branch and by {beta}{sup +}/EC transitions to levels in {sup 80}Sr with a 19(2){percent} branch. The half-life of the {sup 80}Y ground-state decay has been remeasured to be 30.1(5) s. A level scheme for the low-lying states in {sup 80}Y is presented. Hartree-Fock-Bogolyubov calculations show a large prolate deformation for these states, and two-quasiparticle + rotor model calculation results suggest that the low-lying states can be reproduced by inclusion of an effective proton-neutron residual interaction. The dominating Nilsson configurations for the 4{sup {minus}} ground state and the 1{sup {minus}} isomeric state have been found to originate from the parallel and antiparallel coupling of the proton [422]5/2{sup +} and the neutron [301]3/2{sup {minus}} orbitals. {copyright} {ital 1998} {ital The American Physical Society}

Band structures and proton-neutron interactions in 174Ta

Six rotational bands in the odd-odd nucleus 174Ta have been populated with the 160Gd( 19F,5n) reaction. High-spin states were identified using the NORDBALL array. Both signatures of the doubly decoupled π 1 2 − [541] ⊗ ν 1 2 − [521] band and semi-decoupled π 1 2 − [541] ⊗ ν 7 2 − [633] band are observed, in addition to the high-K couplings of the π 9 2 − [514] ⊗ ν 7 2 + [633] , π 9 2 − [514] ⊗ ν 5 2 − [512] , π 7 2 + [404] ⊗ ν 7 2 + [633] , and π 5 2 + [402] ⊗ ν 5 2 − [512] configurations. The signature splitting of the π 1 2 − [541] ⊗ ν 7 2 + [633] band is inverted from the expected splitting, and this is interpreted as being due to a residual proton-neutron interaction. It is shown empirically that this interaction, together with deformation changes, can account for the increased crossing frequency associated with the alignment of i 13 2 neutrons in the π 1 2 − [541] bands of odd-Z nuclei.

Tensor force in doubly odd deformed nuclei

We study the nucleus ${}^{176}\mathrm{Lu}$ within the framework of the particle-rotor model. The aim of this study is to obtain information on the residual neutron-proton interaction in doubly odd deformed nuclei. To this end, both zero-range and finite-range interactions are considered with particular attention focused on the role of the tensor force, which is still a controversial issue. A detailed comparison of the calculated results with experimental data provides clear evidence of the importance of the tensor-force effects.

Signature inversion in semi-decoupled bands: residual interaction between h protons and i neutrons

Abstract Semi-decoupled bands based on the πh 9 2 ⊗ vi 13 2 configuration are observed in 162 Tm, 164 Tm and 174 Ta. Spins assigned to these bands imply an inversion of the expected signature splitting, which is interpreted as being the result of a residual proton-neutron interaction.

An extended Lipkin-type model with residual proton-neutron interaction

The Lipkin-Meshkov-Glick (LMG) model is extended to explicitly take into account the proton and neutron degrees of freedom. The proton and neutron Hamiltonians are taken to be of the LMG form, and, in addition, a residual proton-neutron interaction is include. Exact solutions in an SU(2) ⊗ SU(2) basis as well as the RPA solutions for the energy spectrum of the model Hamiltonian are obtained. The spectrum of the exact solutions is degenerate in the limit of no proton-neutron residual interaction, but this degeneracy is totally removed when this type of residual interaction is turned on. The spectrum obtained with RPA is compressed or expanded, as compared to the LMG model with the same total number of nucleons ( N), depending of the relative magnitudes of the like- and unlike-particle residual interactions. Furthermore, the behavior of the first collective excited state is studied as function of the interaction parameters of the model using the exact and RPA methods. At a given N, it was found that the RPA result is closer to the exact one when the like- and unlike-particle residual interactions are both taken into account in the model Hamiltonian, as compared to the case when residual interaction of only one type (e.g. either like- or unlike-particle type) is involved.

Interactions between a multitude of rotational bands in well deformed odd nuclei: A new method for spin and parity assignment

Using the ( 18O,5n) 163Er reaction a multitude of rotational bands have been established with firm spin and parity assignments in 163Er. In 17 out of 24 band crossings E2 cross-band transitions have been observed. The interaction strength varies between ∼ 1 and ∼ 50 keV with many of the values in the range 10–15 keV. These interactions sample a variety of the lowest (multi) -quasiparticle configurations. For some bands, in particular those with high K-values, the configurations can be established rather well. Quite complicated changes in the wavefunctions must occur at these crossings, and to explain the observed interaction strengths coupling to various vibrational degrees of freedom, in addition to possible residual neutron-proton interactions, may be needed.

Double beta decay and the proton-neutron residual interaction

The validity of the pn-QRPA and -RQRPA descriptions of double beta decay transition amplitudes is analyzed by using an exactly solvable model. It is shown that the collapse of the QRPA is physically meaningful and that it is associated with the appearance of a state with zero energy in the spectrum. It is shown that in the RQRPA this particular feature is not present and that this approach leads to finite but otherwise spurious results for the double beta decay transition amplitudes near the point of collapse.

High-spin states in odd-odd 164Lu

High-spin states in odd-odd 164Lu have been studied using 19F and 23Na induced fusion-evaporation reactions. Three strongly coupled rotational bands have been established and three decoupled bands were tentatively assigned to 164Lu. The band crossing frequencies, energy sig-nature splittings, and relative transition probabilities were deduced from experimental data and compared with neighboring even-even and odd- A nuclei. An anomalously large band crossing frequency was observed in the excited band, and a pronounced signature inversion was observed in the yrast band of 164Lu. These anomalies may be associated with residual neutron-proton interactions.

Empirical proton-neutron interactions near closed shells: A simple shell-model approach.

We point out that the proton-neutron residual interaction is able to account for the recently observed properties in the empirical interaction energies between the last odd proton and neutron in nuclei adjacent to closed shells. Using zero-range forces, the specific difference between the average particle-particle and particle-hole interaction matrix elements is the major factor. We extend the arguments to cover also nuclei with two valence proton and two valence neutron configurations outside of a closed shell (we treat both two-particle two-particle, two-hole two-hole, and two-particle two-hole situations). Here we derive the particular binding energy contribution due to the proton-neutron interaction and give applications to heavy nuclei.

Near yrast states in doubly odd 214Fr.

High spin states of doubly odd $^{214}\mathrm{Fr}_{127}$ have been investigated using in-beam \ensuremath{\gamma}-ray and conversion electron spectroscopy techniques through the $^{206}\mathrm{Pb}$${(}^{11}$B, 3n) and $^{208}\mathrm{Pb}$${(}^{11}$B, 5n) fusion-evaporation reactions. Completely new spectrocopic information has been obtained. The yrast level structure is established up to spin (${19}^{+}$) and some information on \ensuremath{\gamma} transitions from higher-lying levels is also obtained. Two new isomers ${\mathit{T}}_{1/2}$=174(20) ns and ${\mathit{T}}_{1/2}$=11(2) ns were found. Configuration assignments for the low-lying levels are discussed. Information on residual proton-neutron interactions is extracted.

Microscopic approach to the U(6/4 x 4)

Relations between the ${\mathrm{Spin}}^{\mathrm{BFF}}$(6) symmetry Hamiltonian for odd-odd systems and the interacting boson-fermion-fermion model (IBFFM) Hamiltonian are investigated. An exact equivalence is established by slightly revising the exchange term of the IBFFM Hamiltonian. The ${\mathrm{Spin}}^{\mathrm{BFF}}$(6) symmetry is realized approximately in IBFFM by extending the quasiparticle configuration space for odd fermions. The proton-neutron residual interaction associated with the appearance of dynamical symmetry is compared to effective proton-neutron forces.

Proton-neutron interaction in rotating odd-odd nuclei and its effects on signature splitting

Effects of the residual proton-neutron interaction in the rotating frame are studied by analyzing the signature inversion observed in odd-odd nuclei. In particular, the importance of configuration mixing induced by the interaction is stressed. Spectra given by the cranking model are improved appreciably but the effects of theresidual interaction seem insufficient to reproduce experimental data.