Search Results (65 total)

New sub-μs isomers have been observed in the neutron-rich Sn isotopes. ^125,127,129Sn nuclei have been produced in a relativistic fission reaction of ²³⁸U on a ⁹Be target at 750 A·MeV and by the fragmentation of ¹³⁶Xe at 600 A·MeV populating high-spin yrast states. In addition to the already known μs isomers, three new ones with sub-μs half-lives have been observed. These yrast isomers are the high-spin members of the ν(d_3/2^-1h_11/2^-2) and νh_11/2^-n, seniority v=3 multiplets leading to isomeric (23/2⁺) and (27/2^-) states, respectively. Added to the already known 19/2⁺μs isomers in this region the current work completes the systematic information of neutron-hole excitations toward the filling of the last h_11/2 orbital at N=82. The results are discussed in the framework of state-of-the-art shell-model calculations using realistic interactions.

The γ decay of excited states in the waiting-point nucleus ¹³⁰Cd₈₂ has been observed for the first time. An 8⁺ two-quasiparticle isomer has been populated both in the fragmentation of a ¹³⁶Xe beam as well as in projectile fission of ²³⁸U, making ¹³⁰Cd the most neutron-rich N=82 isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an N=82 shell quenching at Z=48. They allow us to follow nuclear isomerism throughout a full major neutron shell from ⁹⁸Cd₅₀ to ¹³⁰Cd₈₂ and reveal, in comparison with ⁷⁶Ni₄₈ one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions.

The electromagnetic decay properties of high-spin states in ⁵²Mn have been studied through various experiments with the GASP and EUROBALL arrays plus the ISIS light charged-particle detector and the Neutron Wall. From γ-γ particles coincidence measurements, spins, and parities of these states and branching ratios of their decay γ rays have been determined. Using the Doppler-shift attenuation method the mean life of some states have been established. These results are compared with large-scale shell-model calculations in the full fp shell.

Gamma decays from excited states up to J^π=6⁺ in the N=Z-2 nucleus ⁵⁴Ni have been identified for the first time. Level energies are compared with those of the isobars ⁵⁴Co and ⁵⁴Fe and of the cross-conjugate nuclei of mass A=42. The good but puzzling f_7/2 cross-conjugate symmetry in mirror and triplet energy differences is analyzed. Shell model calculations reproduce the new data but the necessary nuclear charge-dependent phenomenology is not fully explained by modern nucleon-nucleon potentials.

The reduced transition probabilities B(E2;0⁺→2₁⁺) of the neutron-rich ⁷⁴Zn and ⁷⁰Ni nuclei have been measured by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d’Ions Lourds via interactions of a 60A  MeV ⁷⁶Ge beam with a Be target. The B(E2) value for ⁷⁰Ni₄₂ is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to ⁷⁴Zn₄₄. The enhanced proton core polarization in ⁷⁰Ni is attributed to the monopole interaction between the neutron in the g_9/2 and protons in the f_7/2 and f_5/2 spin-orbit partner orbitals. This interaction could result in a weakening of magicity in ⁷⁸Ni₅₀.

The unstable neutron-deficient ¹⁰⁸Sn isotope has been studied in inverse kinematics by intermediate-energy Coulomb excitation using the RISING/FRS experimental setup at GSI. This is the highest Z nucleus studied so far with this method. Its reduced transition probability B (E2;0_g.s.⁺→2₁⁺) has been measured for the first time. The extracted B(E2) value of 0.230(57)e²  b² has been determined relative to the known value in the stable ¹¹²Sn isotope. The result is discussed in the framework of recent large-scale shell model calculations performed with realistic effective interactions. The roles of particle-hole excitations of the ¹⁰⁰Sn core and of the Z=50 shell gap for the E2 polarization are investigated.

The monopole effect of the tensor force is presented, exhibiting how spherical single-particle energies are shifted as protons or neutrons occupy certain orbits. An analytic relation for such shifts is shown, and their general features are explained intuitively. Single-particle levels are shown to change in a systematic and robust way, by using the π+ρ meson exchange tensor potential, consistently with the chiral perturbation idea. Several examples are compared with experiments.

Proton radioactivity from ¹⁰⁵Sb has been reinvestigated at the GSI on-line mass separator. The nucleus was produced in the reaction ⁵⁰Cr(⁵⁸Ni,1p2n), and collected with a tape transport system. A double-sided Si strip detector was used for proton spectroscopy. The direct proton decay of ¹⁰⁵Sb reported in an earlier experiment at Berkeley was not observed. The present data imply an upper limit ~10^-3 for the ground-state proton decay branching ratio for proton energies higher than 430 keV.

We studied the β-decay properties of the N=Z+1 nucleus ⁹⁵Ag by measuring β-delayed γ rays and β-γ-γ coincidences with a plastic scintillator as β detector and a Ge-detector array. The ⁹⁵Ag nuclei were produced by means of the ⁵⁸Ni(⁴⁰Ca,p2n) reaction and separated with the GSI online mass separator. The previously reported level scheme of the ⁹⁵Pd daughter nucleus was extended considerably. The deduced level scheme is compared with different shell-model calculations with or without breaking the ¹⁰⁰Sn core.

We have observed direct one-proton decay of the (21⁺) isomer in the N=Z nuclide ⁹⁴Ag into high-spin states in ⁹³Pd by detecting protons in coincidence with γ-γ correlations and applying γ gates based on known ⁹³Pd levels. Two decay branches have been identified, with proton energies of 0.79(3) and 1.01(3) MeV and branching ratios of 1.9(5)% and 2.2(4)%, respectively. The corresponding partial half-life values are 21(6) and 18(4) s. The Q value of the direct proton decay of the (21⁺) isomer was found to be 5.78(3) MeV. The very small reduced widths of the observed proton decays might reflect dominating collective configurations in the (21⁺) isomer, and the fine structure of the proton spectrum might indicate a strong deformation of this state.

New shell model Hamiltonians are derived for the T=1 part of the residual interaction in the f_5∕2  p_3∕2  p_1∕2  g_9∕2 model space based on the analysis and fit of the available experimental data for ₂₈⁵⁷Ni₂₉–₂₈⁷⁸Ni₅₀ isotopes and ₂₉⁷⁷Cu₅₀–₅₀¹⁰⁰Sn₅₀ isotones. The fit procedure, properties of the determined effective interaction as well as new results for valence-mirror symmetry and seniority isomers for nuclei near ⁷⁸Ni and ¹⁰⁰Sn are discussed.

The decay of the (7⁺) and (21⁺) isomers of the N=Z isotope ⁹⁴Ag was studied at the GSI on-line mass separator by measuring β-delayed protons, γ rays, proton-γ and proton-γ-γ coincidences as well as the β-strength distribution. We have observed high-spin (up to 39∕2) states in ⁹³Rh populated by proton emission following the β decay of the ⁹⁴Ag isomers. The major part of the population is related to the β decay of the known (7⁺) isomer whose half-life is 0.61(2)  s. The assignment of the high-spin (21⁺) isomer in ⁹⁴Ag with a half-life of 0.39(4)  s has been confirmed. The excitation energy and β-decay energy of the (21⁺) isomer were measured to be at least 5.4 and 17.7  MeV, respectively. At this excitation energy, the (21⁺) isomer is expected to be unbound to direct one-proton, two-proton, or α decays. The remarkably long half-life of the (21⁺) isomer with the highest spin and excitation energy ever observed for β-decaying nuclei makes a new textbook example of a nuclear high-spin trap. The branching ratios for β-delayed proton emission are about 20% and 27% for the decays of the (7⁺) and (21⁺) isomers, respectively. The properties of the experimentally identified ⁹³Rh levels are discussed in comparison to shell-model predictions.

A core-excited I^π=(12⁺) spin-gap isomer was identified in ⁹⁸Cd in an experiment at EUROBALL IV. It was found to feed the known I^π=(8⁺) seniority isomer by an E4 transition. Half-lives of T_1∕2=0.23(+4 / −3)  μs and 0.17(+6 / −4)  μs were measured for the two states at E_x=6635  keV and 2428  keV, respectively. From the excitation energy of the core-excited isomer a ¹⁰⁰Sn shell gap of 6.46(15)  MeV is inferred. The measured E4 and E2 strengths, ¹⁰⁰Sn core excitations and the origin of empirical polarization charges are discussed in the framework of large-scale shell model calculations. An E2 polarization charge for protons of δe_π<0.4  e is found, which corresponds to the empirical value δe_π=0.45(+20 / −25)  e in the pure proton hole valence space.

The neutron-rich isotope ²¹⁸Bi has been produced in proton-induced spallation of a uranium carbide target at the ISOLDE facility at CERN, extracted from the ion source by the pulsed-release technique and resonant laser ionization, and its β decay is studied for the first time. A half-life of 33(1)s was measured and is discussed in the self-consistent continuum-quasi particle-random-phase approximation framework that includes Gamow-Teller and first-forbidden transitions. A level scheme was constructed for ²¹⁸Po, and a deexcitation pattern of stretched E2 transitions 8⁺→6⁺→4⁺→2⁺→0⁺ to the ground state is suggested. Shell-model calculations based on the Kuo-Herling interaction reproduce the experimental results satisfactorily.

A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of ²³⁸U at 750  MeV∕nucleon has been undertaken at the SIS∕FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around A∼190 were identified and the corresponding isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning γ cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.

Nuclei in the A∼180 region have been populated and investigated in a series of multinucleon transfer and deep-inelastic reactions involving an 11.4  MeV per nucleon ¹³⁶Xe beam produced by the GSI UNILAC accelerator, impinging on a selection of tantalum and tungsten targets. The reaction products were released from a thermal ion source and subsequently mass selected using the GSI on-line mass separator. The unexpectedly high yield of γ rays associated with the decay of the well established K^π=37∕2⁻,t_1∕2=51.4  min isomer in  ₇₂¹⁷⁷Hf and anomalous half-life characteristics associated with this decay lead to these data being interpreted as the β⁻ decay of a high-K isomer in the mother nucleus, ¹⁷⁷Lu. By comparing the experimental findings with the predictions obtained from multi-quasiparticle blocked-BCS-Nilsson calculations, the proposed decay is suggested to be from a K^π=39∕2⁻ five-quasiparticle state in  ₇₁¹⁷⁷Lu. A half-life of 7±2  min is determined for this β-decay path which is estimated to have an excitation energy of ≈3.9  MeV above the ¹⁷⁷Lu ground state.

The yrast J^π=8⁺ states in neutron-rich ^70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8⁺ isomeric states in ^72,74Ni nuclei via fragmentation of ⁷⁶Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of ^70,72Co nuclei was performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of ^70,72Co to ^70,72Ni were observed using the EXOGAM germanium detectors. The half life of ⁷²Co was measured to be 62(3) ms and the level sequence of the lowest excited states in ⁷²Ni was suggested, with the 2⁺ state at 1096  keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8⁺ isomer in ⁷²Ni are discussed.

Three γ-decaying isomers in the N=Z+1 nucleus ⁹⁵Ag have been identified for the first time. The ⁹⁵Ag nuclei were produced via the ⁵⁸Ni(⁴⁰Ca,p2n) reaction and separated by using the GSI on-line mass separator. The emitted γ rays were measured with a multi-Ge detector setup in anticoincidence with positrons. Three individual γ-decay cascades have been identified and assigned to ⁹⁵Ag, representing the depopulation of isomers at 344-, 2531-, and 4859-keV excitation energy with tentative spin-parity assignments of (1/2^-), (23/2⁺), and (37/2⁺), respectively. The spin and parity assignments are based on a comparison with shell-model predictions employing empirical interactions.

The γ rays following the 72s ¹⁵⁰Ho 2^- Gamow-Teller β decay have been investigated with the CLUSTER CUBE setup, an array of six EUROBALL CLUSTER Ge detectors in close cubic geometry, providing a γ ray detection sensitivity of 2×10^-5 per β-parent decay for γ-ray energies up to 5 MeV. The fine structure of the Gamow-Teller resonance at 4.4-MeV excitation in ¹⁵⁰Dy has been studied. The resolved levels are compared with Shell Model predictions.

Large-scale shell model calculations were performed in the full Z=82–126 proton model space π(0h_9/2, 1f_7/2, 0i_13/2, 2p_3/2, 1f_5/2, 2p_1/2) employing the code NATHAN. The modified Kuo-Herling interaction was used, no truncation was applied up to protactinium (Z=91) and seniority truncation beyond. The results are compared to experimental data including binding energies, level schemes, and electromagnetic transition rates. An overall excellent agreement is obtained for states that can be described in this model space. Limitations of the approach with respect to excitations across the Z=82 and N=126 shells and deficiencies of the interaction are discussed.

The β decay of ¹⁰⁰In, the one proton hole and one neutron particle neighbor to ¹⁰⁰Sn, was investigated at the GSI on-line mass separator by using germanium detectors and a NaI total-absorption spectrometer. On the basis of βγγ coincidences, the ¹⁰⁰In decay scheme was established for the first time. The ground-state spin and parity for ¹⁰⁰In are discussed by investigating β feeding of levels in ¹⁰⁰Cd and β-delayed proton emission to ⁹⁹Ag. The half-life was remeasured and found to be 5.9(2) s. The Q_EC value was determined from the measured EC/β⁺ ratio for the β-delayed protons to be 10.08(23) MeV. The main fraction of the β feeding was established to populate the region of 6 MeV excitation energy, which corresponds to a total Gamow-Teller (GT) strength of 3.9(9) and a centroid at 6.4 MeV. Large-scale shell-model calculations employing a realistic interaction are used to assign configurations to states in ¹⁰⁰In and ¹⁰⁰Cd. The GT β-decay strength distribution measured in the total absorption experiment is compared to shell-model predictions. The deduced overall hindrance of the GT strength agrees with the values predicted for the ¹⁰⁰Sn GT decay.

A systematic study of the population probabilities of microsecond isomers produced following the fragmentation of ²⁰⁸Pb projectiles at 1 GeV/nucleon has been undertaken at the SIS/FRS facility at GSI Darmstadt. Gamma decays from approximately 20 isomeric states, mainly in the rare-earth and transitional nuclei with A∼180, were identified and the corresponding isomeric ratios deduced. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). The model is found to represent an upper limit for the population of isomeric states in relativistic projectile fragmentation.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

The neutron-rich isotopes ^68–74Ni have been produced at the LISOL facility in the fission of ²³⁸U induced by a 30-MeV proton beam. They have been extracted selectively through resonant laser ionization and electromagnetic mass separation. Production rates have been deduced and deviate from theoretical predictions. The β decay of the isotopes has been investigated by means of β-γ and γ-γ spectroscopy. Half-lives have been determined for ^68–74Ni, and the level scheme has been extended for ⁶⁹Cu and newly established for ^71,73Cu. The constructed schemes show an unexpected and sharp lowering of the πf_5/2 orbital with respect to the πp_3/2 ground state. This feature is ascribed to the monopole term of the residual proton-neutron interaction and is well reproduced by realistic shell-model calculations.