Intruder Band Research Articles

Intruder band mixing in an ab initio description of 12Be

The spectrum of ▪ exhibits exotic features, e.g., an intruder ground state and shape coexistence, normally associated with the breakdown of a shell closure. While previous phenomenological treatments indicated the ground state has substantial contributions from intruder configurations, it is only with advances in computational abilities and improved interactions that this intruder mixing is observed in ab initio no-core shell model (NCSM) predictions. In this work, we extract electromagnetic observables and symmetry decompositions from the NCSM wave functions to demonstrate that the low-lying positive parity spectrum can be explained in terms of mixing of rotational bands with very different intrinsic structure coexisting within the low-lying spectrum. These observed bands exhibit an approximate SU(3) symmetry and are qualitatively consistent with Elliott model predictions.

Collective-model description of shape coexistence and intruder states in cadmium isotopes based on a relativistic energy density functional

Low-energy structure of even-even $^{108-116}$Cd isotopes is analyzed using a collective model that is based on the nuclear density functional theory. Spectroscopic properties are computed by solving the triaxial quadrupole collective Hamiltonian, with parameters determined by the constrained self-consistent mean-field calculations within the relativistic Hartree-Bogoliubov method employing a universal energy density functional and a pairing force. The collective Hamiltonian reproduces the observed quadrupole phonon states of vibrational character, which are based on the moderately deformed equilibrium minimum in the mean-field potential energy surface. In addition, the calculation yields a low-lying excited $0^+$ band and a $\gamma$-vibrational band that are associated with a deformed local minimum close in energy to the ground state, consistently with the empirical interpretation of these bands as intruder bands. Observed energy spectra, $B(E2)$, and $\rho^2(E0)$ values are, in general, reproduced reasonably well.

An Application of Interacting Boson Model-2 (IBM-2) Configuration Mixing in Tin Nuclei

Using IBM-2 configuration mixing calculations, the normal and intruder 2p-2h bands in even-even tin isotopes are examined. The states of the normal and intruder bands were computed separately and then mixed using a basic band-mixing Hamiltonian. The experimental data for energy levels and electronic transition probability from current and past investigations are compared.

Shape coexistence in Sr isotopes

Sr isotopes are located in the mass region $A\approx 100$, where a very quick onset of nuclear deformation exists, being other notable examples of this area Yb, Zr, and Nb nuclei. The presence of the proton subshell closure $Z=40$ allows the existence of particle-hole excitations that produces low-lying intruder bands. Purpose: The goal of this work is the study of the nuclear structure of the even-even $^{92-102}$Sr isotopes through the accurate description of excitation energies, $B(E2)$ transition rates, nuclear radii and two-neutron separation energies. Method: The interacting boson model with configuration mixing will be the framework to calculate all the observables of the Sr isotopes. Only two types of configurations will be considered, namely, 0particle-0hole and 2particle-2hole excitations. The parameters of the model are determined using a least-squares procedure for the excitation energies and the $B(E2)$ transition rates. Results: For the whole chain of isotopes, the value of excitation energies, $B(E2)$'s, two-neutron separation energies, nuclear radii, and isotope shifts have been obtained, with a good agreement between theory and experiment. Also, a detailed analysis of the wave functions have been performed and, finally, the mean-field energy surfaces and the value of the nuclear deformation, $\beta$, have been obtained. Conclusions: The presence of low-lying intruder states in even-even Sr isotopes have been confirmed and its connection with the onset of deformation has been clarified. Lightest Sr isotopes present a spherical structure while the heaviest ones are clearly deformed. The rapid onset of deformation at neutron number $60$ is due to the crossing of the regular and intruder configurations and, moreover, both families of states present an increase of deformation with the neutron number.

105Tc nuclear structure and systematic evolution of states of 1/2+[431] intruder band in odd-A 95,97,99,101,103,105,107Tc isotopes

Previously observed negative and positive parity states of 105Tc were studied in the framework of the particle-rotor model. Transition properties and experimental energies were compared to the predictions of model calculations. A systematic study of the evolution of the intruder π1/2+[413] band in the nuclear structure of odd-A technetium isotopes 95,97,99,101,103,105,107Tc is presented as well. The existence of this intruder band has been argued previously in 95,97,99,101,103Tc isotopes (partially populated) and fully observed and confirmed in 105Tc. It will be shown that changes in deformation and subsequently the position of the Fermi level, vis a vis the 1/2+[431] intruder orbital originating from the π(d5/2, g7/2) subshells, predominantly affect these systematic changes. All four interpreted experimental rotational bands are naturally predicted by the rotational model as bands build on states of good Ω originating from the 5/2+[422], 5/2–[303], 3/2–[301], and 1/2+[431] orbitals near the Fermi level in deformed 105Tc (strong coupling). Further experimental investigation about missing data are needed for those observed in low lying states in both 105Tc and 103Tc to confirm the presence of the 1/2–[301] rotational band that is well defined in lighter 95,97,99,101Tc isotopes.

High-j Proton h11/2 and g7/2 Intruder Bands in 113In

Excited states of 113In have been populated through the heavy-ion fusion evaporation reaction 110Pd(7Li, 4n)113In. A new band with the configuration of a proton d5/2 orbital is identified. Two ΔI = 2 intruder bands, built on the πh11/2 and the πg7/2 orbitals, have been extended to spins (63/2−ħ and (55/2+)ħ, respectively. The negative-parity πh11/2 intruder band shows a smooth increase in aligned spin, which is attributed to a strong proton-neutron interaction. The properties of the positive-parity πg7/2 band are discussed based on tilted axis cranking model calculations, and the features of the antimagnetic rotation for this band are shown after backbend. Furthermore, the contributions of the two-shears-like mechanism, the neutron (gd)ν shell and the core rotation are investigated for the positive-parity πg7/2 band.

Shape coexistence and multiparticle-multihole structures in Cd110,112

From detailed spectroscopy of Cd-110 and Cd-112 following the beta(+)/EC decay of In-110,In-112 and the beta(-) decay of Ag-112, the presence of very weak decay branches from nonyrast states is revealed. In Cd-112, 2(5)(+) -> 0(4)(+) and 4(6)(+) -> 2(5)(+) transitions are observed that yield B(E2; 2(5)(+) -> 0(4)(+)) = 34 +/- 15 W.u. and B(E2; 4(6)(+) -> 2(5)(+)) = 77 +/- 30 W.u., respectively, clearly indicating a collective structure. In 110Cd, a weak decay branch from the 4(6)(+) level to the 2(5)(+) level is observed, and from a lifetime measurement following the (n, n gamma' reaction, B(E2; 4(6)(+) -> 2(5)(+)) = 55 +/- 14 W.u. is determined. A new branch is also observed for the decay of the 6(4)(+) level to the 4(6)(+) state, indicating that the sequence 2(5)(+), 4(6)(+), and 6(4)(+) forms part of a collective structure. The presence of 3(3)(+) and 5(2)(+) levels spaced between the previous sequence is highly suggestive of a gamma band built on the 0(2)(+) shape-coexisting intruder state. The 0(4)(+) levels in Cd-110,Cd-112,Cd-114 have preferred decays to the lowest 2(+) members of the intruder bands, and for 114Cd a previous measurement had established an enhanced B(E2; 0(4)(+) -> 2(3)(+)). The energy systematics of the 0(2)(+), 0(3)(+), and 0(4)(+) levels all display the characteristic parabolic-shaped pattern, suggesting that they are built on multiparticle-multihole proton excitations. The results are compared with beyond-mean-field calculations that reproduce qualitatively the observed levels and their decays and suggest that the 0(1)(+), 0(2)(+), 0(3)(+), and 0(4)(+) levels and the excited states built on them possess different deformations.

Relevance of the Nuclear Structure of the Stable Ge Isotopes to the Neutrino-less Double-Beta Decay of 76Ge

Gamma-ray detection following the inelastic neutron scattering reaction on isotopically enriched material was used to study the nuclear structure of 74Ge. From these measurements, low-lying, low-spin excited states were characterized, new states and their decays were identified, level lifetimes were measured with the Doppler-shift attenuation method (DSAM), multipole mixing ratios were established, and transition probabilities were determined. New structural features in 74Ge were identified, and the reanalysis of older 76Ge data led to the placement of the 2+ member of the intruder band. In addition, a number of previously placed states in 74Ge were shown not to exist. A procedure for future work, which will lead to meaningful data for constraining calculations of the neutrinoless double-beta decay matrix element, is suggested.

Shape coexistence and collective low-spin states in Sn112,114 studied with the (p,p′γ) Doppler-shift attenuation coincidence technique

Proton-scattering experiments followed by the coincident spectroscopy of $\gamma$ rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in $^{112}$Sn and $^{114}$Sn, to determine their lifetimes and extract reduced transitions strengths $B(\Pi L)$. The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted $\gamma$ rays of excited states in coincidence. The novel $(p,p'\gamma)$ DSA coincidence technique was employed to measure sub-ps nuclear level lifetimes. 74 level lifetimes $\tau$ of states with $J = 0 - 6$ were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, $sd$ IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in $^{114}$Sn for the first time. The $1^-$ candidate in $^{114}$Sn fits perfectly into the systematics observed for the other stable Sn isotopes. The $E2$ transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in $^{112,114}$Sn. The collectivity in this configuration is comparable to the one observed in the Pd nuclei, i.e. the 0p-4h nuclei. Strong mixing between the $0^+$ states of the normal and intruder configuration might be observed in $^{114}$Sn. The general existence of QOC states in $^{112,114}$Sn is supported by the observation of QOC candidates with $J \neq 1$.

Quasiparticle structure of low spin states of neutron rich odd odd 152, 154, 156Eu isotopes

Projected shell model has been employed to study systematically the well deformed neutron rich odd odd isotopes with neutron number from 89 to 93. We perform calculations for rotational bands and analyze the band structure of low lying states. Our calculations predict two 2-quasiparticle (qp) intruder bands based on I=6− and I=7− in 152Eu and one 2-qp band based on I=7− in 154Eu. A systematic study of yrast spectra and rotational bands in 152−156Eu isotopes is presented and compared with the experimental data. The projected shell model is found to be successful in the investigation of the yrast bands and other 2-qp rotational bands in these neutron rich doubly odd isotopes.

Band structure inSn113

The structure of collective bands in $^{113}\mathrm{Sn}$, populated in the reaction $^{100}\mathrm{Mo}(^{19}\mathrm{F},\phantom{\rule{0.16em}{0ex}}p5n)$ at a beam energy of 105 MeV, has been studied. A new positive-parity sequence of eight states extending up to 7764.9 keV and spin $(39/{2}^{+})$ has been observed. The band is explained as arising from the coupling of the odd valence neutron in the ${g}_{7/2}$ or the ${d}_{5/2}$ orbital to the deformed 2p-2h proton configuration of the neighboring even-$A$ Sn isotope. Lifetimes of six states up to an excitation energy of 9934.9 keV and spin $47/{2}^{\ensuremath{-}}\mathrm{belonging}$ to a $\mathrm{\ensuremath{\Delta}}I=2$ intruder band have been measured for the first time, including an upper limit for the last state, from Doppler-shift-attenuation data. A moderate average quadrupole deformation ${\ensuremath{\beta}}_{2}=0.22\ifmmode\pm\else\textpm\fi{}0.02$ is deduced from these results for the five states up to spin $43/{2}^{\ensuremath{-}}$. The transition quadrupole moments decrease with increase in rotational frequency, indicating a reduction of collectivity with spin, a feature common for terminating bands. The behavior of the kinematic and dynamic moments of inertia as a function of rotational frequency has been studied and total Routhian surface calculations have been performed in an attempt to obtain an insight into the nature of the states near termination.

Conversion electron study of 110Cd: Evidence of new E0 branches

Excited states of 110Cd were studied with conversion electron spectroscopy following the $ \beta^{+}/EC$ decay of 110In. Internal conversion coefficients from K-shell electrons are extracted from $\gamma$ - $ e^{-}$ coincidences and compared with expected values for E1, M1, and E2 transitions, allowing the assignment of the transition multipolarities. The $ \alpha_{K}$ values for transitions connecting the $ 4^{+}$ and $ 6^{+}$ members of the deformed intruder band and the ground-state band show evidence for E0 components. The extracted $ \rho^{2}(E0)\cdot 10^{3}$ value for the $ 4^{+}_{3}\rightarrow 4^{+}_{1}$ 708 keV transition is determined to be $ 106^{+98}_{-91}$ .

Reflection symmetry of the near-yrast excitations in145Ba

Excited states in ${}^{145}$Ba, populated in spontaneous fission of ${}^{248}$Cm, have been studied by means of $\ensuremath{\gamma}$ spectroscopy, using high-fold $\ensuremath{\gamma}$ coincidences measured with the EUROGAM2 array of Ge detectors. The 507.7-keV level, which has been assigned in this work spin and parity 9/2${}^{+}$, belongs to the ${i}_{13/2}$ intruder band. We have identified in ${}^{145}$Ba a new 9/2${}_{2}^{\ensuremath{-}}$ level at 346.3 keV and a band on top of it. The negative parity indicates that it is not due to an octupole excitation. The position of the 5/2${}^{\ensuremath{-}}$ ground state, relative to the positive-parity band, could be reproduced by calculations with a reflection-symmetric potential and we do not observe a parity doublet to the ground state. Therefore we conclude that octupole excitations in ${}^{145}$Ba are most likely due to octupole vibrations coupled either to the reflection-symmetric ground state or to the ${i}_{13/2}$, decoupled neutron configuration.

High Spin States of 113In

Level structures of 113In are studied through the fusion-evaporation reaction 110Pd(7Li,4n)113In at a beam energy of 50 MeV. Two new bands are established for the first time. Here the ΔI = 2 band based on proton excitation is assigned to the πg−29/2⊗νh211/2d5/2 configuration. In addition, the backbending associated with the additional aligned h11/2 neutron pair is found in the negative parity yrast band. The experimental results are compared with self-consistent tilted axis cranking relativistic mean field calculations. Several decay paths of the πh11/2 intruder band are established, by which the level energies and spins of the πh11/2 intruder band are determined. The bandcrossing delay of the πh11/2 intruder bands around the Z = 50 shell and the dynamic moment of inertia of the πg−19/2⊗νh211/2 bands in 113,115,117,119 In are studied systematically.

Level structures of 113In

Level structures of 113In have been studied through the fusion-evaporation reaction 110Pd (7Li ,4n113In at a beam energy of 50MeV. More than 40 new \( \gamma\) transitions have been observed in 113In . Two new dipole bands are established and one of them is proposed as a magnetic rotation band with the configuration of \(\ensuremath \pi g_{9/2}^{-1}\otimes\nu h_{11/2}^{2} g_{7/2}^{2}\) . Self-consistent tilted axis cranking relativistic mean field calculations have been performed to interpret the rotational structure. Several decay paths of the \(\ensuremath \pi h_{11/2}\) intruder band have been established, by which the level energies and spins of the \(\ensuremath \pi h_{11/2}\) intruder band are determined.

Structural evolution of the intruder band in 118Sn

Excited states of the positive-parity intruder band in 118Sn have been studied via the 116Cd(7Li, 1p4n) reaction at 7Li energy of 48 MeV using techniques of in-beam γ-ray spectroscopy. This intruder band has been observed up to 7187 keV with spin (16+). The structural evolution of this intruder band with increasing angular momentum has been discussed in terms of the aligned angular momentum and the ratio of the E-Gamma Over Spin (E-GOS) curve.

Experimental study of the 2p-2h band inSn111

The {delta}I=2 intruder band in {sup 111}Sn, built upon the 4074.3 keV state, was studied. The states were populated in the {sup 100}Mo({sup 20}Ne, {alpha}5n) reaction at a beam energy of 136 MeV. Mean lifetimes of five states up to 8737.2 keV (spin 43/2{sup -}) have been measured for the first time using the Doppler shift attenuation method. In addition, an upper limit of mean lifetime has been estimated for the 9860.0 keV (spin 47/2{sup -}) state. The B(E2) values, derived from the present lifetime results, indicate a quadrupole deformation of {beta}{sub 2}=0.28{+-}0.02 for the 31/2{sup -} state and decrease progressively with spin, suggesting a reduction in collectivity. The dynamic moment of inertia for the band also decreases continuously up to the highest observed frequencies. These results, along with the predictions of a total Routhian surface calculation, suggest that the {delta}I=2 band in {sup 111}Sn undergoes a change of shape from collective prolate to triaxial with increase in spin and possibly terminates in a noncollective oblate state at a high spin.

Excitation energy and deformation of the1/2+[431] intruder band inTc107

The already detailed study of $^{107}\mathrm{Tc}$ nucleus was complemented by a search for microsecond isomers at very low energy. For this purpose, this neutron-rich nucleus was produced by thermal-neutron-induced fission of $^{241}\mathrm{Pu}$. We have found a new 30.1 keV microsecond isomeric state which deexcites to the ground state by a strongly-hindered $E1$ transition. This isomer was identified as the $3/{2}^{+}$ level of the $1/{2}^{+}$[431] intruder band in $^{107}\mathrm{Tc}$ and is also the lowest-lying member of the band. The very low energy of the band head suggests a large quadrupole deformation. From a comparison with $^{105}\mathrm{Tc}$, where more information is known about the intruder band, it is deduced that the $1/{2}^{+}$[431] band has a quadrupole deformation, ${\ensuremath{\epsilon}}_{2}\ensuremath{\geqslant}0.35$ and a possible triaxial shape, $\ensuremath{\gamma}\ensuremath{\approx}{20}^{\ifmmode^\circ\else\textdegree\fi{}}$.

Study of intruder band in 112Sn

Excited states of the positive-parity intruder band in 112Sn, populated in the 100Mo( 20Ne, α 4 n ) reaction at a beam energy of 136 MeV, have been studied. The band has been observed up to 11570.0 keV with spin (24 +). Mean lifetimes have been measured for six states up to the 22 +, 10335.1 keV level and an upper limit of the lifetime has been estimated for the 11570.0 keV (24 +) state. The B(E2) values, derived from the present lifetime results, correspond to a moderate quadrupole deformation of β 2 ∼ 0.18 for states with spin J π ⩾ 12 + , and the decrease in B(E2) for the 14 + → 12 + transition is consistent with a ν ( h 11 / 2 ) 2 alignment at ℏ ω ∼ 0.35 MeV , predicted by a cranked shell-model calculation. Total Routhian surface calculations predict a triaxial shape following the alignment.

Magnetic and intruder rotational bands inIn113

Excited states in {sup 113}In were populated via the reactions {sup 100}Mo({sup 18}O,p4n){sup 113}In and {sup 110}Pd({sup 7}Li,4n){sup 113}In. The two known {delta}J=2 intruder bands, based on the {pi}g{sub 7/2}xd{sub 5/2} and {pi}h{sub 11/2} orbitals, have been extended by 8({Dirac_h}/2{pi}) to spins (49/2{sup +})({Dirac_h}/2{pi}) and (55/2{sup -})({Dirac_h}/2{pi}), respectively. The previous finding of three sequences of {delta}J=1 {gamma}-ray transitions has been confirmed. A self-consistent cranked shell-model calculation gives a good description of the contrasting alignment patterns of the two {delta}J=2 intruder bands. The intruder bands, the known sequences of M1 transitions, and spherical levels together represent a coexistence of three different excitation modes in this nucleus.