Low-lying Positive Parity States Research Articles

Intruder structures observed inTe122through inelastic neutron scattering

The excited levels of $^{122}\mathrm{Te}$ to $3.3\ensuremath{-}\phantom{\rule{0.3em}{0ex}}\text{MeV}$ excitation have been studied using $\ensuremath{\gamma}$-ray spectroscopy following inelastic neutron scattering. The decay characteristics of these levels have been determined from $\ensuremath{\gamma}$-ray excitation functions, angular distributions at ${E}_{n}\ensuremath{-}1.72,2.80$, and $3.35\phantom{\rule{0.3em}{0ex}}\text{MeV}$, Doppler shifts, and $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences. Electromagnetic transition rates were deduced for many levels, as were multipole-mixing and branching ratios. Level energies and electromagnetic transition rates were compared to interacting boson model (IBM) calculations, both with and without intruder-state mixing, and to particle-core coupling model calculations. The energies of low-lying levels of $^{122}\mathrm{Te}$ are well described by the IBM with intruder-state mixing calculations, and observed transition rates support emerging intruder bands built on ${0}^{+}$ levels. The other models considered do not produce enough low-lying positive parity states; however, U(5) energies to the four quadrupole-phonon level agree very well with observations when states with large intruder configurations are ignored. Mixed-symmetry and quadrupole-octupole excitations have been investigated, but mixing with other configurations and fragmentation of strength prohibit a clear identification of these states.

Microscopic structure of low-lying positive parity states in nuclei near shell closure

We adopt the quasiparticle-phonon model to investigate the microscopic structure of some low-lying states recently discovered in nuclei around shell closure. The study determines quantitatively the phonon content of these states and shows that their main properties are determined by a subtle competition between particle-particle and particle-hole quadrupole interactions and by the interplay between orbital and spin-flip motion. The results are in overall agreement with experiments and consistent with the interacting boson model and shell model calculations.

Decay of mass-separated183Ptm (43 s)and183Ptg (6.5 min)to183Ir

The decays of ${}^{183}{\mathrm{Pt}}^{m}$ (43 s, ${J}^{\ensuremath{\pi}}={\frac{7}{2}}^{\ensuremath{-}})$ and ${}^{183}{\mathrm{Pt}}^{g}$ (6.5 min, ${J}^{\ensuremath{\pi}}={\frac{1}{2}}^{\ensuremath{-}})$ have been studied with mass-separated sources from the UNISOR facility. Multiscaled spectra of $\ensuremath{\gamma}$ rays, x rays, and conversion electrons, as well as $\ensuremath{\gamma}\ensuremath{\gamma}t,$ $\ensuremath{\gamma}Xt,$ $e\ensuremath{\gamma}t,$ and $\mathrm{eXt}$ coincidences, were obtained. A revision of the low-lying positive-parity states is necessary in order to accommodate a strong 58 keV transition which is previously unreported. Other additions are made to the low-energy part of the ${}^{183}\mathrm{Ir}$ scheme. The systematic features of the neutron-deficient odd-mass Ir isotopes are discussed.

Radioactive decay of80Yand low-lying states in80Sr

The ${\ensuremath{\beta}}^{+}/\mathrm{EC}$ decay of a mass-separated ${}^{80}\mathrm{Y}$ source has been studied by $\ensuremath{\beta}$- and $\ensuremath{\gamma}$-ray spectroscopy following the bombardment of a thin ${}^{24}\mathrm{Mg}$ target with ${}^{58}\mathrm{Ni}$ ions at 190 MeV. An extended decay scheme with 14 new states in ${}^{80}\mathrm{Sr}$ has been identified including the first observation of negative-parity states. The low-lying positive-parity states have been found to be clustered indicating a pattern of one-, two-, and possibly three-phonon multiplets characteristic of an anharmonic vibrator. Some states in ${}^{80}\mathrm{Sr}$ can also be grouped into a $\ensuremath{\gamma}$-vibrational band. In general the low-lying states in light strontium isotopes show vibrationlike collectivity which evolves to rotational behavior with increasing spin and decreasing neutron number. The observed ${}^{80}\mathrm{Y}\ensuremath{\beta}$-decay fragmentation and $\mathrm{log}\mathrm{ft}$ values have also been used to assign spin and parity of ${4}^{\ensuremath{-}}$ and ${1}^{\ensuremath{-}}$ to the parent ground state and the 228.5 keV isomeric state, respectively.

Analytical expressions for electromagnetic transition rates in the SU(3) limit of thespdfinteracting boson model

Collective negative parity states are believed to be related to octupole degrees of freedom in the nucleus in the geometrical model @1#. In the algebraic model, their description is the interacting boson model ~IBM! including s, p, d, and f bosons @2‐7#. One advantage of the algebraic model is the availability of analytical expressions for many physical quantities. The SU~3! limit describes a rotational spectrum @3‐5#. Analytical expressions for the energy can be written explicitly in terms of the Casimir operators of the corresponding group chain. Besides the energy spectrum, electromagnetic transition properties are important in determining the structure of a nucleus. In the sd-IBM, electromagnetic transition rates are obtained analytically @6,8#. Numerical studies have shown that the low-lying positive parity states are well-described by Ns dbosons, and the low-lying negative parity states are well described by coupling one pf boson with N21 sd bosons. In this limit, the sd bosons and pf bosons interact only via the quadrupole interaction. This is the weak correlation case. In this study, we adopt this assumption. In the strong correlation, sd bosons and pf bosons interact strongly, and the pf bosons and sd bosons are mixed strongly. Lac and Morrison @9# have derived analytical expressions in the strong correlation case using the 1/ N expansion technique. It is the purpose of this paper to give analytical expressions of E1, E2, E3, M1, and M2 transition rates involving low-lying negative parity states in the SU(3) sd 3SU(3) pf limit. The paper is organized as follows. In Sec. II, we give a brief description of the method of the calculation. In Sec. III we give the results. In Sec. IV we apply the results to some deformed nuclei. In Sec. V we give a brief summary.

Decay of 129Ce to low-lying positive-parity states in 129La

Low-lying levels of the 129La isotope have been investigated through the β+/EC decay of 129Ce. The radioactive nuclei were produced in the 94Mo+40Ca reaction at a bombarding energy of 255 MeV and transported with a He-jet system. On-line mass separation was used to select the 129Ce beta-decay γ-γ-t, X- γ- t, e−-γ -t coincidence measurements as well as time multi-analysis were performed. Internal conversion electrons were recorded with a magnetic selector and multipolarities deduced. The level scheme of 129La is discussed and compared with the calculations made in the frame of the neutron-proton Interacting Boson-Fermion Model (IBFM-2).

Comment on "Large-space shell-model calculations for light nuclei"

In a recent publication Zheng, Vary, and Barrett reproduced the negative quadrupole moment of Li-6 and the low-lying positive-parity states of He-5 by using a no-core shell model. In this Comment we question the meaning of these results by pointing out that the model used is inadequate for the reproduction of these properties.

Exotic cluster states in actinide nuclei.

We calculate exotic cluster states for heavy nuclei in the actinide region, using potentials based on an $\ensuremath{\alpha}\ensuremath{-}\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}$ potential which itself reproduces the properties of the low-lying positive parity states of ${}^{212}$Po. We find that by suitably scaling the parameters of the $\ensuremath{\alpha}\ensuremath{-}\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{e}$ potential so as to reproduce the exotic decay lifetimes, we obtain a good description of the spectra and electromagnetic decay properties of the ground state rotational bands of the actinide nuclei.

Positive parity states in 11Be.

The low-lying positive parity states of $^{11}\mathrm{Be}$ are calculated in a coupled channels treatment of a valence neutron interacting with a deformed core. The loosely bound nature of the valence neutron is taken into account by using a Woods-Saxon potential. Comparisons are made to shell-model predictions and to data. The model reproduces the measured spectrum quite well for realistic parameters of the neutron-core interaction.

Pion inelastic scattering to low-lying positive-parity states in 20Ne.

We have measured pion inelastic scattering from $^{20}\mathrm{Ne}$ with 180 MeV ${\mathrm{\ensuremath{\pi}}}^{+}$ and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$ at 12\ifmmode^\circ\else\textdegree\fi{} and from 15\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{} in 5\ifmmode^\circ\else\textdegree\fi{} steps and with 120 MeV ${\mathrm{\ensuremath{\pi}}}^{+}$ from 15\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{} in 5\ifmmode^\circ\else\textdegree\fi{} steps. We report here on states in the first four ${\mathit{K}}^{\mathrm{\ensuremath{\pi}}}$=${0}^{+}$ bands, including the 6.73MeV ${0}^{+}$ and the 8.78-MeV $^{6+}$ levels. Coupled-channels processes are shown to be important for many states.

Quasielastic proton knockout from 16O.

The spectral function of the $^{16}\mathrm{O}$(e,e'p${)}^{15}$N reaction has been measured in quasielastic parallel kinematics. Momentum distributions are extracted for transitions to several discrete states, with emphasis on the low-lying positive parity states of $^{15}\mathrm{N}$. Spectroscopic factors and bound state wave functions are deduced from a DWIA analysis employing five different optical potentials. Coupled channels effects are investigated for the first four states of $^{15}\mathrm{N}$ and are found to be minimal. The spectroscopic results of the 1/${2}^{\mathrm{\ensuremath{-}}}$ ground state and the first 3/${2}^{\mathrm{\ensuremath{-}}}$ excited state indicate a 28%\ifmmode\pm\else\textpm\fi{}5.4% reduction of the valence 1p shell strength with respect to the IPSM sum rule limit for $^{16}\mathrm{O}$. The spectroscopic strength for knockout of protons from the 2s1d shell, integrated up to a missing energy of 40 MeV, is found to be 0.269\ifmmode\pm\else\textpm\fi{}0.033. The distribution of proton spectroscopic strength is determined from a multipole decomposition of the $^{16}\mathrm{O}$ spectral function.

6p-2h core excitations in 20O.

The effects of intruder states arising from general positive deformations on the excitation energies, the two-neutron transfer spectroscopic amplitudes for the reaction $^{18}\mathrm{O}$(t,p${)}^{20}$O, and some electric quadrupole transition amplitudes between some low-lying positive parity states in $^{20}\mathrm{O}$ are presented. The intruder states Hamiltonian matrices for infinite and zero deformations and the corresponding predictions obtained from them by mixing core-excited states and shell model states are also presented together with a general comparison of the results with experiments and with pure (sd${)}^{4}$ shell model results.

Excited charm mesons in semileptonic B-bar decay and their contributions to a Bjorken sum rule.

It has recently been shown that hadrons containing a single heavy quark exhibit a new flavor-spin symmetry of QCD. We exploit this symmetry to obtain model-independent predictions for the 14 form factors in weak decays from the ground-state pseudoscalar meson {ital P}{sub {ital Q}{ital i}} of a heavy quark {ital Q}{sub {ital i}} to the low-lying positive-parity excited states of a heavy quark {ital Q}{sub {ital j}} in terms of two universal functions of momentum transfer. These predictions are of interest in the study of {ital {bar B}}{r arrow}{ital D}{sub 2}{sup *}(2460), {ital D}{sub 1}(2420), {ital D}{sub 1}({similar to}2360), and {ital D}{sub 0}{sup *}({similar to}2360) semileptonic decays. We also discuss the connection between these results and the slope of the function {xi} (which determines the {ital {bar B}}{r arrow}{ital D} and {ital {bar B}}{r arrow}{ital D}{sup *} transition form factors) given by a heavy-quark sum rule suggested by Bjorken.

A 12C+ 12C cluster model of 24Mg

A recently proposed cluster model of 24Mg based on two 12C component nuclei is re-examined with a view to justifying some of its assumptions and parameter values. The 12C clusters are allowed to occupy their 0 1 + (0.00 MeV), 2 1 + (4.44 MeV), O 2 + (7.65 MeV), 3 1 − (9.64 MeV) or 1 1 − (10.84 MeV) states, and hence give rise to a variety of bands of states of both positive and negative parity in 24Mg. Coupled-channels calculations, invoking the bound-state diagonalisation approximation, are performed to evaluate the energies and wave functions of these states up to excitation energies of about 16 MeV and are found to describe almost the entire experimentally known T = 0 spectrum in this region. Reduced E2 electromagnetic strengths for transitions between the low-lying positiveparity states are also calculated in good agreement with measured values, without the need to introduce effective charges. Finally, form factors for elastic and inelastic electron scattering (the latter via the 2 1 + and unresolved 4 1 + + 2 2 + states) from 24Mg are examined, and found to imply a charge radius somewhat larger than that deduced from other analyses, and a breakdown of the applicability of the plane-wave Born approximation at the higher values of the momentum transfer.

Spectroscopy of the ODD-ODD nucleus 76As and its supersymmetric description

Gamma rays and conversion electrons following thermal-neutron capture in 75As have been measured using the Ge pair spectrometer PN4, the bent crystal spectrometers GAMS and the β-spectrometer BILL at the ILL. Coincident γ-rays were recorded at the CEN/SCK Mol, and average resonance capture at En = 2 and 24 keV has been studied at the HFBR of BNL. The neutron binding energy in 76As was determined to be 7328.498 ± 0.075 keV. The resulting level scheme for 76As greatly improves the knowledge on this odd-odd nucleus. The low-lying positive-parity states are discussed in the U(5) limit of the Uv(6/12)⊗Uπ(6/12) supersymmetry using the same hamiltonian to describe the quartet 76Se-77Se-75As-76As having the same total number of bosons plus fermions.

Low-lying positive-parity dipole states from a quadrupole-octupole boson Hamiltonian

The e.m. properties of the lowestJπ=1+ eigenstates of a phenomenological quadrupole-octupole boson Hamiltonian are extensively studied. The first two states are shown to be of collectiveM1 nature. The specific signature as well as their similarities to theM1 state describing the scissors mode are discussed.

Nuclear structure of116In

The low-lying positive parity states of116In have been described within the framework of the interacting boson-fermion-fermion/odd-odd truncated quadrupole phonon model (IBFFM/OTQM). The calculations reproduce the energy levels and known electromagnetic moments. γ-branching ratios have also been calculated and compared with the existing experimental data.

Levels in124Te populated in the decay of124Sb

The levels in124Te have been investigated by studying the gamma rays emitted following the decay of 60d124Sb. Gamma-gamma coincidence spectra have been taken with gates set at 603, 646, 709, 714 and 723 keV gamma rays by using 105 cm3 Ge(Li)-110 cm3 HPGe coincidence system. 69 gamma rays have been observed and nine of them are reported for the first time. Four new levels in124Te at 2512, 2550.2, 2808 and 2814.5 keV are proposed. Levels reported in literatures at 2412 and 2733 keV are excluded from the level scheme of124Te based on the present study. The experimentally determined energy spectrum of low-lying positive parity states, B(E2) ratios and electric quadrupole moment of the first excited state are compared with the IBM-1 predictions.

Low-lying positive parity states in 195Ir following double neutron capture

Multiple neutron capture reactions have been studied on 193Ir targets and gamma and electron spectroscopy performed. Double neutron capture events were separated from all others and a level scheme of 195Ir deduced. Gamma-ray energies and multipolarities are tabulated. A neutron capture cross-section for the reaction 194 Ir( n, γ) of 1500±290 b has been determined and the neutron binding energy of 195Ir measured to be 7231.92 (6) keV. The results are discussed in terms of perturbed spin(6) symmetry and U( 6 4 ) supersymmetry. A σ < σ max family of levels has clearly been identified.

Shell-model calculation of 99Tc beta decay in astrophysical environments.

Using the shell-model Lanczos method, we calculate the Gamow-Teller matrix elements for beta transitions of astrophysical interest from excited states of $^{99}\mathrm{Tc}$ ${((7/2)}^{+}$,141 keV; ${(5/2)}^{+}$,181 keV) to the ground and excited states of $^{99}\mathrm{Ru}$. The level schemes of low-lying positive-parity states in these nuclei and in the analogous isotonic nuclei $^{97}\mathrm{\ensuremath{-}}^{97}$Mo are reproduced fairly well within a model space consisting of low-seniority excitations in the 1g2d shell, which are mixed via the Kallio-Kolltveit effective interaction. The calculations lead to a $^{99}\mathrm{Tc}$ half-life of \ensuremath{\sim}20 yr at a stellar temperature of 3\ifmmode\times\else\texttimes\fi{}${10}^{8}$ K typical for the s process with a $^{22}(\mathit{\ensuremath{\alpha}}$,n${)}^{25}$Mg neutron source. Referring to recent studies of thermal-pulse s-process models, we stress that a substantial amount of $^{99}\mathrm{Tc}$ can survive the s process in spite of its enormously enhanced decay rate. The factual observation of $^{99}\mathrm{Tc}$ at the surface of certain stars, therefore, does not necessarily contradict an s-process scenario with $^{22}\mathrm{Ne}$ as a neutron source.