Multi-quasiparticle Configurations Research Articles

Microscopic study of band structures of neutron-rich 153,155,157Sm isotopes

A systematic study of band structures of some odd mass Samarium isotopes with neutron numbers 91 ≤ N ≤ 95 is carried out by employing projected shell model. The theoretical results have been obtained for band head energies, energy spectra, transition energies and moment of inertia. The calculations on comparison with the experimental data show a reasonably good agreement. The band spectra and transition energies for higher spin states are also predicted in these isotopes. The calculations successfully provide good understanding of the change in structure of the yrast bands and excited bands in terms of single and multi-quasiparticle configurations. The future experimental work will test the predictions made in this paper.

Phenomenological description of non-axial shapes of some doubly even neutron deficient barium isotopes

The evolution of non-axial shapes and rotational structure in even–even 122–132Ba isotopes has been investigated microscopically with the application of a triaxial projected shell model (TPSM). Based on calculations, the shape instability and softness towards γ deformation are predicted for these isotopes. In the present work, the application of three-dimensional angular-momentum projection on multi-quasiparticle configurations, constructed from the triaxially deformed mean field, provides a reliable description of the yrast and near yrast structure in these nuclei. The theoretically obtained backbends demonstrate the importance of decoupling of multi-quasiparticle states for these nuclei. The uniform behaviour of transition probabilities B(E), shown by the present calculations, authenticate the global validity of Grodzin’s product rule. Besides this, the deformation systematics and odd–even staggering in γ bands have also been discussed in the present work.

A novel method for stellar electron-capture rates of excited nuclear states

We propose a novel shell-model method to calculate stellar electron-capture (EC) rates of highly-excited nuclear states. This method is based on the Projected Shell Model that can incorporate high-order multi-quasiparticle configurations in a large model space. By taking the EC calculation from 59Co to 59Fe as the first example, we study the effects of nuclear excitations (including both single-particle and collective excitations) on the stellar EC rates. It is found that the predicted EC rates exhibit large variations as functions of excitations in both parent and daughter nuclei, which sensitively depend on the density and temperature in the stellar environment. We emphasize the importance of inclusion of multi-quasiparticle configurations constructed in a large model space for the calculation of EC rates used in astrophysical studies.

Microscopic study of electromagnetic properties and band spectra of neutron deficient 133,135,137Sm

A microscopic high spin study of neutron deficient and normally deformed 133,135,137Sm has been carried out in projected shell model framework. The theoretical results have been obtained for the spins, parities and energy values of yrast and excited bands. Besides this, the band spectra, band head energies, moment of inertia and electromagnetic transition strengths are also predicted in these isotopes. The calculations successfully give a deeper understanding of the mechanism of the formation of yrast and excited bands from the single and multi-quasi particle configurations. The results on moment of inertia predict an alignment of a pair of protons in the proton (1h11/2)2 orbitals in the yrast ground state bands of 133-137Sm due to the crossing of one quasiparticle bands by multi-quasiparticle bands at higher spins. The discussion in the present work is based on the deformed single particle scheme. Any future experimental confirmation or refutation of our predictions will be a valuable information which can help to understand the deformed single particle structure in these odd mass neutron deficient 133-137Sm.

Microscopic insight into the nuclear structure properties of odd-mass 101−109Cd isotopes

The purpose of the present work is to study the negative parity yrast bands in odd-mass 101−109Cd nuclei in the framework of Projected Shell Model (PSM). The results are obtained for the energy spectrum, reduced transition probabilities [B(E2)], rotational alignment etc. The yrast and excited band spectra of these isotopes are found to arise from the quasiparticle states around the Fermi level. The yrast and excited band spectra are found to depend sensitively on the different single particle and multi-quasiparticle configurations at low- and high-spins respectively. This work also substantiates the observed presence of anti-magnetic rotation in 105,107,109Cd nuclei.

Shell-model method for Gamow-Teller transitions in heavy deformed odd-mass nuclei

A shell-model method for calculating Gamow-Teller (GT) transition rates in heavy deformed odd-mass nuclei is presented. The method is developed within the framework of the projected shell model. To implement the computation requirement when many multi-quasiparticle configurations are included in the basis, a numerical advancement based on the Pfaffian formula is introduced. With this new many-body technique, it becomes feasible to perform state-by-state calculations for the GT nuclear matrix elements of $\ensuremath{\beta}$-decay and electron-capture processes, including those at high excitation energies in heavy nuclei which are usually deformed. The first results, ${\ensuremath{\beta}}^{\ensuremath{-}}$ decays of the well-deformed $A=153$ neutron-rich nuclei, are shown as the example. The known $log(ft)$ data corresponding to the $B({\mathrm{GT}}^{\ensuremath{-}}$) decay rates of the ground state of $^{153}\mathrm{Nd}$ to the low-lying states of $^{153}\mathrm{Pm}$ are well described. It is further shown that the $B$(GT) distributions can have a strong dependence on the detailed microscopic structure of relevant states of both the parent and daughter nuclei.

Multiple band structures in Ge70

High-spin states in $^{70}\mathrm{Ge}$ were studied using the $^{55}\mathrm{Mn}$($^{18}\mathrm{O},p2n$) fusion-evaporation reaction at a beam energy of 50 MeV. Prompt $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were measured using the Florida State University Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. An investigation of these coincidences resulted in the addition of 31 new transitions and the rearrangement of four others in the $^{70}\mathrm{Ge}$ level scheme, providing a more complete picture of the high-spin decay pattern involving both positive- and negative-parity states with multiple band structures. Spins were assigned based on directional correlation of oriented nuclei ratios, which many times also led to unambiguous parity determinations based on the firm assignments for low-lying states made in previous work. Total Routhian surface calculations, along with the observed trends in the experimental kinematic moment of inertia with rotational frequency, support the multiquasiparticle configurations of the various crossing bands proposed in recent studies. The high-spin excitation spectra predicted by previous shell-model calculations compare favorably with the experimental one determined from this study.

A systematic study of band structure and electromagnetic properties of neutron rich odd mass Eu isotopes in the projected shell model framework

The positive and negative parity rotational band structure of the neutron rich odd mass Eu isotopes with neutron numbers ranging from 90 to 96 are investigated up to the high angular momentum. In the theoretical analysis of energy spectra, transition energies and electromagnetic transition probabilities we employ the projected shell model. The calculations successfully describe the formation of the ground and excited band structures from the single particle and multi quasiparticle configurations. Calculated excitation energy spectra, transition energies, exact quantum mechanically calculated B(E2) and B(M1) transition probabilities are compared with experimental data wherever available and a reasonably good agreement is obtained with the observed data. The change in deformation in the ground state band with the increase in angular momentum and the increase in neutron number has also been established.

Decay patterns of multi-quasiparticle bands—a model independent test of chiral symmetry

Nuclear chiral systems exhibit chiral symmetry bands, built on left-handed and right-handed angular momentum nucleon configurations. The experimental search for such chiral systems revealed a number of suitable candidates, however an unambiguous identification of nuclear chiral symmetry is still outstanding. In this work it is shown that the decay patterns of chiral bands built on multi-quasiparticle configurations are different from those involving different single-particle configurations. It is suggested to use the observed decay patterns of chiral candidates as a new model-independent test of chiral symmetry.

Systematic study of multi-quasiparticleK-isomeric bands in tungsten isotopes by the extended projected shell model

Background: The interplay between collective and single-particle degrees of freedom is an important structure aspect to study. The nuclei in theA ≈ 180 mass region are often denoted as good examples to study such problems because these nuclei are known to exhibit many rotational bands based on multi-quasiparticle K isomers. Purpose: A large set of high-quality experimental data on high-K isomeric states in the A ≈ 180 mass region has accumulated. A systematic description of them is a theoretical challenge as it requires a method going beyond the usual mean field with multi-quasiparticle configurations built in the shell-model basis. The K-isomer data provide an ideal testing ground for theoretical models. Method: The recently extended projected shell model (PSM) by the Pfaffian method is employed with a sufficiently large configuration space including up to 10 quasiparticles. The restoration of rotational symmetry which is broken in the deformed mean field is obtained by means of angular-momentum projection. With axial symmetry in the basis deformation, each multi-quasiparticle state, classified by a K quantum number, represents the major component of a rotational K band. Shell-model diagonalization in such a projected basis defines the K mixing, which is the key ingredient of the present method. Results: Quasiparticle structure and rotational properties of high-K isomers in even-even neutron-rich 174−186W isotopes are described. The rotational evolution of the yrast and near-yrast bands is discussed with successive band crossings. Multi-quasiparticle K isomers and associated rotational bands in each W isotope are studied with detailed quasiparticle configurations given. Electromagnetic transition properties are also studied and the calculated B(E2), B(M1), and g-factors are compared with experiment if data exist. Conclusions: Many nuclei of the A ≈ 180 mass region exhibit properties of an axially symmetric shape and K is approximately a good quantum number. For such nuclei, the extended PSM assuming an axially symmetric basis but including K mixing through diagonalization of the two-body Hamiltonian is an appropriate method to study multi-quasiparticle K isomers and K violations in these states. For special examples where one finds highly K-forbidden transitions the present model needs to be further improved.

In-beam spectroscopy of medium- and high-spin states inCe133

Medium and high-spin states in $^{133}$Ce were investigated using the $^{116}$Cd($^{22}$Ne, $5n$) reaction and the Gammasphere array. The level scheme was extended up to an excitation energy of $\sim22.8$ MeV and spin 93/2 . Eleven bands of quadrupole transitions and two new dipole bands are identified. The connections to low-lying states of the previously known, high-spin triaxial bands were firmly established, thus fixing the excitation energy and, in many cases, the spin parity of the levels. Based on comparisons with cranked Nilsson-Strutinsky calculations and tilted axis cranking covariant density functional theory, it is shown that all observed bands are characterized by pronounced triaxiality. Competing multiquasiparticle configurations are found to contribute to a rich variety of collective phenomena in this nucleus.

Configurations and hindered decays of [formula omitted] isomers in deformed nuclei with [formula omitted

Spectroscopic information on the decay properties of high-K isomers in deformed and transitional nuclei has been evaluated and collated. Assigned multi-quasiparticle configurations are included. Factors that control the transitions strengths, such as various contributions to K mixing, are outlined. The systematics of K-forbidden transitions for different multipolarities are discussed for selected cases in terms of the hindrances, FW, and of the reduced hindrance factor per degree of K forbiddenness, fν, where ν=|ΔK−λ|, ΔK is the K-value difference between the initial and final state and λ is the transition multipole order. With the improved statistics for E1, M1 and E2 transitions, a factorization into the product of the underlying multipolarity-dependent transition strength and a ν-dependence, due to K forbiddenness (f0), is possible. This suggests a weaker dependence on K forbiddenness than is commonly assumed.

Crossing of large multiquasiparticle magnetic-rotation bands inBi198

High-spin states in the doubly-odd $^{198}$Bi nucleus have been studied by using the $^{185,187}$Re($^{16}$O, xn) reactions at the beam energy of 112.5 MeV. $\gamma-\gamma$ coincidence were measured by using the INGA array with 15 Compton suppressed clover HPGe detectors. The observed levels have been assigned definite spin-parity. The high spin structure is grouped into three bands (B1, B2 and B3), of which two (B1 and B2) exhibit the properties of magnetic rotation (MR). Tilted axis cranking calculations were carried out to explain the MR bands having large multi-quasiparticle configurations. The calculated results explain the bands B1 and B2 very nicely, confirming the shears mechanism and suggest a crossing of two MR bands in both the cases. The crossing is from 6-qp to 8-qp in band B1 and from 4-qp to 6-qp in band B2, a very rare finding. A semiclassical model has also been used to obtain the particle-hole interaction strengths for the bands B1 and B2, below the band crossing.

The study of energy bands in neutron-deficient 123,125Ce nuclei

Theoretical investigation of energy bands of 123,125 Ce is performed by applying the projected shell model approach. The present calculation is viewed to reproduce the known experimental data of these nuclei, qualitatively. Not only this, the configurations of diverse bands are also obtained by an analysis of the band diagrams. The low-lying states of the bands are seen to arise from one quasiparticle configuration whereas the high-spin states arise from multi-quasiparticle configurations. The signature partner band of negative parity band is predicted in 123 Ce. The ground state bands and relative energies of some bands with respect to them are predicted in 123,125 Ce that can be helpful for future experiments.

Mixing effects on K-forbidden transition rates from the 6+ isomers in the N = 104 isotones

The problem of the very different E2 decay rates from the two-quasineutron Kπ = 6+ isomers in the N = 104 isotones 172Er, 174Yb, 176Hf, and 178W is investigated using the triaxial projected shell model with inclusion of multi-quasiparticle configurations. It is demonstrated that the highly K-forbidden transition from the 6+ isomer to the ground-state band is sensitive to mixing with the 6+ state of the γ-vibrational band. Thus the inter-band transitions, and lifetimes, depend on the relative position of the γ-band and the isomeric state in each isotone.

Triaxial projected shell model study of the rapid changes inB(E2)for180−190Pt isotopes

The mass region with a proton number just below the magic number $Z=82$ is known to exhibit a rich variety of shape phenomena. Inspired by the recent extensive experimental measurements of the transition probabilities for the yrast bands in some Pt isotopes in this mass region, we have performed a detailed investigation of ${}^{180\ensuremath{-}190}$Pt using the triaxial projected shell model approach (TPSM). It is demonstrated that by performing the exact three-dimensional angular-momentum projection on multi-quasiparticle configurations, constructed from the traxially deformed mean field, the TPSM provides a consistent description of the yrast band structures, the $\ensuremath{\gamma}$-vibrational band, and the second ${0}^{+}$ band in these nuclei. Further, the observed rapid variations in the quadrupole transition probability along the yrast line of these isotopes are well reproduced in the present study.

A New High-Spin Level Scheme of 128I

High-spin states in the odd-odd nucleus 128I are investigated via the 124Sn(7Li,3n)128I reaction at 28 and 32 MeV beam energies. A new level scheme of 128I is established up to high-spin states at Iπ = 16, including 48 levels and 72 γ transitions. The present level scheme is largely different from the one in a recent publication due to identification of several doublet and triplet γ transitions and their proper placements in the level scheme. The high-spin level structure exhibits no obvious collective properties and is possibly associated with two and multi-quasiparticle configurations.

Particle-hole optical model and strength functions for high-energy giant resonances

A formulation of the particle-hole optical model is proposed for describing the contribution of the fragmentation effect to the formation of strength functions for high-energy giant resonances. The model is based on the Bethe-Goldstone equation for the energy-averaged particle-hole Green’s function. In this equation, the particle-hole interaction that is induced by a virtual excitation of multiquasiparticle configurations and in which, upon averaging over energy, an imaginary part is contained is taken into account. An analogy with the single-quasiparticle optical model is discussed.

Decay studies of K isomers in 254No

A detailed $ \gamma$ spectroscopic decay study of two K isomers in 254No was performed. In addition to the previously reported $ \gamma$ lines two new transitions of E = 778 , 856keV could be attributed to the decay pattern of 254m1No ( T 1/2 = 275±7 ms). The population of an excited band built up on this isomer ( $\ensuremath K^{\pi} =8^{-}$ by the decay of 254m2No ( T 1/2 = 198±13 μs) could be proven by measuring delayed $ \gamma$ - $ \gamma$ coincidences between transitions stemming from the decay of both isomeric states. The energies of the band members could be established up to $\ensuremath I^{\pi} = 15^{-}$ . A spontaneous fission branch of (2.0±1.2)×10-4 was measured for 254m1No , an upper limit of $ \le$ 1.2×10-4 was estimated for 254m2No . These values demonstrate the high stability of multi-quasiparticle configurations against spontaneous fission. Evidence for an $ \alpha$ decay branch of 254m1No in the order of 1×10-4 was found.

Band structures in near spherical 138Ce

The high spin states of N = 80 138Ce have been populated in the fusion evaporation reaction 130Te( 12C, 4n) 138Ce at E beam = 65 MeV . The γ transitions belonging to various band structures were detected and characterized using an array of five Clover Germanium detectors. The level scheme has been established up to a maximum spin and excitation energy of 23 ℏ and 9511.3 keV, respectively, by including 53 new transitions. The negative parity Δ I = 1 band, developed on the 6536.3 keV 15 − level, has been conjectured to be a magnetic rotation band following a semiclassical analysis and comparing the systematics of similar bands in the neighboring nuclei. The said band is proposed to have a four quasiparticle configuration of [ π g 7 2 h 11 2 ] ⊗ [ ν h 11 2 ] − 2 . Other band structures are interpreted in terms of multi-quasiparticle configurations, based on Total Routhian Surface (TRS) calculations. For the low and medium spin states, a shell model calculation using a realistic two body interaction has been performed using the code OXBASH.