Results Of Shell-model Calculations Research Articles

High-spin states of 218Th

High-spin states in the N = 128 nucleus 218Th have been investigated following fusion–evaporation reactions, using the recoil-decay tagging technique. Due to the short-lived nature of the ground state of 218Th prompt γ rays have been correlated with the α decay of the daughter nucleus 214Ra. The level scheme representing the decay of excited states has been extended to (16+) with the observation of six previously unreported transitions. The observations are compared with the results of shell model calculations and within the context of the systematics of neighbouring nuclei.

Investigation of strengths distributions of Gamow-Teller in sd and fp shell nuclei

The strength distribution of Gamow-Teller (GT) in the 26Si→26Al, 28Si→28P, 42Sc→42Ca and 44Sc→44Ca transitions are calculated by using shell model with full basis which assume the valence nucleons in the sd and fp shells. The calculation of the shell model were conducted in the sd and fp shells without any restriction by using USDB and USDA interactions for sd shell and GXFP1A, KB3G and FPD6 effective interactions for fp shell. Our theoretical results of shell model calculations compared with the data obtained from (3He, t) and (p, n) charge-exchange reactions. Calculated B(GT) strength distributions agree well with the recently available experimental data and with the summed transition strengths B(GT).

Nuclear Weak Rates and Detailed Balance in Stellar Conditions

Abstract Detailed balance is often invoked in discussions of nuclear weak transitions in astrophysical environments. Satisfaction of detailed balance is rightly touted as a virtue of some methods of computing nuclear transition strengths, but I argue that it need not necessarily be strictly obeyed in astrophysical environments, especially when the environment is far from weak equilibrium. I present the results of shell model calculations of nuclear weak strengths in both charged-current and neutral-current channels at astrophysical temperatures, finding some violation of detailed balance. I show that a slight modification of the technique to strictly obey detailed balance has little effect on the reaction rates associated with these strengths under most conditions, though at high temperature the modified technique in fact misses some important strength. I comment on the relationship between detailed balance and weak equilibrium in astrophysical conditions.

Identification of levels above 6- isomeric state in 66Cu

Excited states in odd-odd 66Cu were investigated in a reaction between a 136MeV 30Si beam and a 65Cu target with the Indian National Gamma Array. Six new transitions have been identified including four transitions feeding the 600ns \(6^{-}\) isomeric state from an investigation of prompt-prompt and prompt-delayed coincidence events. The results of the present work have extended the level structure of this nucleus up to \(I^{\pi}=(9^{-})\). In addition, new information on the set of \( \pi p_{3/2} \nu g_{9/2}\) multiplets in this nucleus have been added. Shell model calculations were performed within the \( fpg_{9/2}\) and \( f_{5/2}pg_{9/2}\) model spaces. The results of shell model calculations using the \( fpg_{9/2}\) model space have been observed to be in better agreement with experimental excitation energies up to the highest spin observed. The results of the present work highlight the necessity of \( f_{7/2}\) proton holes to describe the positive as well as negative parity states in 66Cu.

Decay of 72Co and microsecond isomers in even-mass neutron-rich nickel isotopes

A new excited state at 2164 keV has been identified in 72Ni with tentative spin and parity of 4+ based on comparison to shell model calculations. This level is suggested to be populated via the β-decay of an isomeric state in 72Co, and is postulated to be one of the key states that play a role in the disappearance of isomers in Ni. The known low-energy levels in 72Ni are compared with results of shell model calculations using the NR78 interaction to follow the seniority of the observed states. In addition, the half-life of the 8+ isomer in 76Ni has been measured as 630(90) ns and the full cascade of transitions from this isomer has been verified experimentally in this work.

High-spin structures of3688Kr52and3789Rb52: Evolution from collective to single-particle behavior

The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were built from triple gamma-ray coincidence data and gamma-gamma angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two levels schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.

Core-coupled states and split proton-neutron quasiparticle multiplets in122−126Ag

Neutron-rich silver isotopes were populated in the fragmentation of a 136Xe beam and the relativistic fission of 238U. The fragments were mass analyzed with the GSI Fragment separator and subsequently implanted into a passive stopper. Isomeric transitions were detected by 105 HPGe detectors. Eight isomeric states were observed in 122-126Ag nuclei. The level schemes of 122,123,125Ag were revised and extended with isomeric transitions being observed for the first time. The excited states in the odd-mass silver isotopes are interpreted as core-coupled states. The isomeric states in the even-mass silver isotopes are discussed in the framework of the proton-neutron split multiplets. The results of shell-model calculations, performed for the most neutron-rich silver nuclei are compared to the experimental data.

Spin-dipole excitations in16O and tensor correlations

The multipole dependent effects induced by tensor correlations on the charge-exchange spin-dipole (SD) modes of the light nucleus ${}^{16}$O are studied within a Hartree-Fock (HF) plus self-consistent random-phase approximation (RPA), using Skyrme-type interactions. The role played by the triplet-even and triplet-odd tensor terms are examined separately to explain either the softening or hardening of the excitation energies of SD modes. The unperturbed and RPA summed strengths, calculated with and without the tensor force, are discussed and compared with the results of shell-model calculations.

The first 2+ excited state of 24O

The unbound excited states of the most neutron-rich dripline oxygen isotope, O (Z=8, N=16), have been investigated using the 24O(p,p′)O* reaction at the beam energy of 63 MeV/nucleon in inverse kinematics at RIKEN. The 24O beam was produced in the projectile-like fragmentation process of a 95 MeV/nucleon 40Ar primary beam on the Be production target. The decay energy spectrum of 24O* in 23O+n channel was reconstructed in the invariant mass of the neutron and the charged fragment detected in coincidence. The first unbound excited state of 24O has been observed at Ex=4.68+0.21−0.14 14 MeV (preliminary) along with the evidence for another higher lying state at around 7.3 MeV. The energy of the former state is compared with existing data and the results of shell-model calculations.

E1strength inPb208within the shell model

The dipole response of the doubly magic nuclide $^{208}\mathrm{Pb}$ was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 9.0 and 15.0 MeV. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ data combined with $(\ensuremath{\gamma},n)$ data from the literature are compared with results of shell-model calculations and calculations using a quasiparticle random-phase approximation. The shell-model calculations including (2p-2h) excitations describe the experimental $E1$ strength well and reproduce the spreading of the giant dipole resonance by applying a small smearing width only.

First g-factor measurements on semi-magic 36S and their implications for the rigidity of the [formula omitted] shell closure

The g-factor of the very short-lived (τ=0.12(1) ps) 21+ state at 3.290 MeV in 36S, g(21+)=+1.3(5), has been determined for the first time by utilizing the Coulomb excitation of 36S beams in inverse kinematics combined with the transient field technique. In addition, for the 31− state (with τ=0.9(1) ps) at 4.192 MeV, a value of g(31−)=+0.8(5) was obtained. In the same experiment, the g-factor of the 21+ state in 40Ar has been remeasured using the α-transfer reaction on the same carbon target. Lifetimes of several excited states in both 36S and 40Ar were also redetermined using the Doppler-shift-attenuation method. The new experimental g-factors and deduced B(E2) values were compared with results of shell-model calculations with the WBT effective interaction and the sdfp shell model space. Comparisons with previous results for 32,34,38,40S and 36,38,40Ar shed light on how the properties of the Ar and S isotopes change as the N=20 shell closure is approached, crossed, and left behind.

Measurement of thegfactor of the41+state inGe70

The g factors of the 2{sub 1}{sup +},3{sub 1}{sup -}, and 4{sub 1}{sup +} states of {sup 70}Ge have been measured by the transient field technique. The measured value, g(4{sub 1}{sup +})=+0.5(2) is in good agreement with the results of shell-model calculations within the full fp configuration space and with the nuclear g-factor systematics in this region. The measured g(4{sub 1}{sup +}) is consistent with the recently reported positive g(4{sub 1}{sup +}) value for {sup 68}Zn, the N=38 isotone of {sup 70}Ge. The determination of the logarithmic slope of the particle-{gamma} angular correlations is discussed for two typical arrangements of transient field experiments.

Structure of the isomeric states inSb123,125

Excited states in $^{123,125}\mathrm{Sb}$ have been studied following the $^{122,124}\mathrm{Sn}$($^{7}\mathrm{Li}$,\ensuremath{\alpha}2n)$^{123,125}\mathrm{Sb}$ reactions at beam energies of 35 and 37 MeV, respectively. Conversion coefficients for transitions depopulating isomeric states in $^{125}\mathrm{Sb}$ have been measured using an electron spectrometer. This has allowed firm ${J}^{\ensuremath{\pi}}$ assignments to be made and enabled the structure of the isomeric states to be compared with those in ${}^{121,127\ensuremath{-}131}$Sb and in the neighboring even-$A$ tin isotopes. The half-lives of the ${J}^{\ensuremath{\pi}}={\frac{23}{2}}^{+}$ isomeric states in $^{123}\mathrm{Sb}$ and $^{125}\mathrm{Sb}$ have been measured to be 66(4) \ensuremath{\mu}s and 272(16) ns, respectively. The half-lives of the ${J}^{\ensuremath{\pi}}={\frac{19}{2}}^{+},{\frac{19}{2}}^{\ensuremath{-}}$, and ${\frac{15}{2}}^{\ensuremath{-}}$ states in $^{125}\mathrm{Sb}$ have been measured as 31(2) ns, 28.0(7) \ensuremath{\mu}s, and 4.1(2) \ensuremath{\mu}s, respectively. The transition probabilities for the transitions depopulating these states are compared with those in ${}^{121,127\ensuremath{-}131}$Sb and with the results of shell-model calculations using the Oxbash code and the SN100PN interaction in a restricted model space.

Beta-delayed γ-ray studies of πf7/2 - νpf shell nuclei

We have utilized the selective process of β decay to populate low-energy excited states in the neutron-rich 22Ti, 23V, 24Cr, and 25Mn nuclei. The goal was to systematically track the monopole shift of the νf5/2 single-particle level with increased occupancy of the πf7/2 orbital. The β-decay properties of the parent nuclides, along with the low-energy structure of the daughters, are presented and compared with the results of shell model calculations employing the GXPF1 interaction.

On the unusual properties of the 282 keV state in 135Sb

Recently the first excited state in 135Sb has been observed at the unexpectedly low excitation energy of only 282keV and interpreted as mainly d 5/2 proton coupled to the 134Sn core. Based on theoretical considerations it was suggested that its low excitation energy is related to a relative shift of the proton d 5/2 and g 7/2 orbits induced by the neutron excess. We have measured the lifetime of the 282keV state by the advanced time-delayed βγγ(t) method. The measured half-life, T 1/2 = 6.1(4)ns, yields exceptionally low limits of B(M1;5/21 +→7/21 +)≤3.0×10-4 μ 2 N and B(E2;5/21 +→7/21 +)≤54e 2 fm 4. These strongly hindered M1 and slow E2 transition rates are similar to those for the transition de-populating the first excited state at 405keV in 211Bi. Results of shell model calculations with realistic interactions are presented. The M1 decay rate was found to be extremely sensistive both to the wave function and to the M1 effective operator.

Isospin structure ofJπ=1+states inNi58andCu58studied byNi58(p,p') andNi58(He3,t)Cu58measurements

Isospin is a good quantum number under the assumption that the nuclear interaction is charge independent. An analogous structure of excited states is expected for nuclei with the same mass number A but with different z components Tz of the isospin T, where Tz=(N−Z)/2. The analogous structure has been studied for the isobaric nuclei Ni58 and Cu58 by comparing the transitions from the Ni58 ground state (initial isospin Ti=1 and Jπ=0+) to the M1 and the Gamow-Teller (GT) states (Jπ=1+) in Ni58 and Cu58, respectively. For this purpose, proton inelastic scattering (p,p') at Ep=160 MeV and the charge-exchange (He3,t) reaction at 140 MeV/nucleon were both measured at 0°, exciting final states with isospin Tf=1 and 2 and Tf=0, 1, and 2, respectively. High energy and scattering-angle resolutions were achieved by applying complete beam matching techniques. On the basis of the correspondence between excitation energies and transition strengths, isospin values Tf=1 and 2 of analog GT and M1 states were identified. The distribution of Tf=2 states was also compared with results of Ni58(d,He2), Ni58(t,He3), and Ni58(n,p) experiments, in which only Tf=2 states are excited. The obtained GT strength distribution is compared with the results of shell-model calculations.7 MoreReceived 31 March 2006DOI:https://doi.org/10.1103/PhysRevC.75.034310©2007 American Physical Society

Low-lying levels in Cr59: Inadequacy of the fp model space and onset of deformation

The low-lying levels in $^{59}\mathrm{Cr}$ have been studied with the $^{13}\mathrm{C}(^{48}\mathrm{Ca},2p)$ reaction at a beam energy of $130\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Data on $^{59}\mathrm{Mn}$, produced via the $pn$-evaporation channel, have set limits on the spin of the ground state in $^{59}\mathrm{Cr}$ and enabled some spin-parity assignments of excited states to be made. The structure obtained in this study is clearly inconsistent with results of shell-model calculations within the full $fp$ shell and requires inclusion of the $\ensuremath{\nu}{g}_{9∕2}$ orbital. The sequence of states is understood within Nilsson model calculations assuming a moderate oblate ground-state deformation.

High-spin states in the 97Tc nucleus

High-spin states in the 97Tc nucleus have been studied by in-beam γ-ray spectroscopy with the reaction 82Se(19F,4nγ) at 68 MeV incident energy. Excited states have been observed up to about 8 MeV excitation and spin 43/2. The observed level scheme is compared with results of shell model calculations.

The yrast spectroscopy of neutron-rich nuclei produced in deep-inelastic processes

The highly sensitive GASP array at the INFN Legnaro Laboratory was used to study the \(\gamma\)-ray de-excitation of neutron-rich nuclei produced in the deep-inelastic processes which occur when 230 MeV 36S ions interact with a target of 176Yb. Yrast decay schemes were identified in over forty target-like fragments and in over twenty projectile-like fragments. Analysis of the data has resulted in extensions to the yrast decay sequences of the target-like species, 176Hf, 166Er, 172Yb, and 152Sm. New transitions have also been observed in a number of projectile-like species including 34P and 41Cl. Experimental results are compared with the results of shell model calculations.

Yrast excitations inN=81nuclei132Sband133Tefrom248Cmfission

Gamma rays in the ${}^{132}\mathrm{Sb}$ and ${}^{133}\mathrm{Te}N=81$ isotones near doubly magic ${}^{132}\mathrm{Sn}$ have been studied at Gammasphere using a ${}^{248}\mathrm{Cm}$ fission source. Previously unknown yrast cascades in the two nuclei were identified in cross coincidence with known \ensuremath{\gamma} rays from complementary Rh and Ru fission fragments. The ${}^{132}\mathrm{Sb}$ levels are explained as proton-neutron hole states as well as core excited states of $2p\ensuremath{-}2h$ character, while the interpretation of the ${}^{133}\mathrm{Te}$ level scheme is mainly based on results of shell model calculations using empirical proton-proton interaction energies from ${}^{134}\mathrm{Te}$ together with estimated proton-neutron hole interactions.