Search Results (45 total)

The reduced transition probabilities, B(E2;0_gs⁺→2₁⁺), have been measured in the radioactive isotopes ^108,106Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation γ rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0_gs⁺→2₁⁺)=0.222(19)e²b² for ¹⁰⁸Sn and B(E2;0_gs⁺→2₁⁺)=0.195(39)e²b² for ¹⁰⁶Sn were determined relative to a stable ⁵⁸Ni target. The resulting B(E2) values are ∼30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N=Z=50 shell closure.

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 first excited 2⁺ state of the unstable isotope ¹¹⁰Sn has been studied in safe Coulomb excitation at 2.82  MeV/u using the MINIBALL array at the REX-ISOLDE post accelerator at CERN. This is the first measurement of the reduced transition probability of this state using this method for a neutron deficient Sn isotope. The strength of the approach lies in the excellent peak-to-background ratio that is achieved. The extracted reduced transition probability, B(E2:0⁺→2⁺)=0.220±0.022e²b², strengthens the observation of the evolution of the B(E2) values of neutron deficient Sn isotopes that was observed recently in intermediate-energy Coulomb excitation of ¹⁰⁸Sn. It implies that the trend of these reduced transition probabilities in the even-even Sn isotopes is not symmetric with respect to the midshell mass number A=116 as ¹⁰⁰Sn is approached.

The β-decay properties of ₂₈⁷²Ni₄₄ and ₂₉⁷²Cu₄₃ have been studied at the LISOL facility of Louvain-La-Neuve and at the CERN-ISOLDE facility, respectively. These neutron-rich nuclei have been produced in the proton-induced fission of ²³⁸U. Their decay schemes are presented and the lifetime T_1/2=6.63(3) s of ⁷²Cu was measured. No β-decaying isomeric state was found in ⁷²Cu, in line with a suggested spin (2) for its ground state. Spin and parity assignments of the observed excited states in the odd-odd nucleus ⁷²Cu are proposed and discussed in terms of coupling between the valence proton and neutrons. Comparison is made with a schematic shell-model picture of ⁷²Cu and with large-scale shell-model calculations performed in the (2p_3/21f_5/22p_1/21g_9/2) shell space outside the doubly magic ₂₈⁵⁶Ni₂₈ core.

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 temperature dependence of the magnetic specific heat has been studied experimentally and theoretically in the semimagnetic semiconductor Pb_1−xEu_xTe for x=0.027 and 0.073, over the temperature range from 0.5 to 10 K, in magnetic fields up to 2 T. There was a maximum in the magnetic specific heat between 1 and 3 K even in zero and low magnetic fields; this maximum shifted toward higher temperatures with increasing magnetic field. The experimental data have been analyzed in the framework of a model in which we assume that the ground states of europium ions are split even without an external magnetic field. We present arguments which support this assumption and we show that it is possible to find a physical mechanism leading to the splitting which can explain the experimental results.

Candidates for the two-phonon quadrupole-octupole 1^- state and the two-phonon mixed-symmetry 1_ms⁺ state have been identified in the N=52 isotope ⁹⁶Ru using the nuclear resonance fluorescence technique at the bremsstrahlung facility of the Stuttgart Dynamitron accelerator. Detailed information on energies, spins, branching ratios, and transition strengths of four new dipole excitations in ⁹⁶Ru have been obtained. The observed dipole excitations are nearly at the same energies as in ⁹⁴Mo, and the transition probabilities are comparable to those for the decay of the (2₁⁺⊗3₁^-)1^- and the (2₁⁺⊗2_ms⁺)1_ms⁺ states in ⁹⁴Mo.

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.

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.

A β-decay study of the even mass ^74,76,78Cu isotopes toward levels in ^74,76,78Zn was performed at the ISOLDE mass separator. The copper isotopes were produced in proton- or neutron-induced fission reactions on ²³⁸U, laser ionized, mass separated, and sent to a β-γ detection system. Half-lives, decay schemes, and possible spin configurations were obtained for the copper isotopes. The results are compared with calculations using schematic forces as well as large-scale shell-model calculations with realistic forces.

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.

Using resonant laser ionization, β-decay studies, and for the first time mass measurements, three β-decaying states have been unambiguously identified in ⁷⁰Cu. A mass excess of -62 976.1(1.6)   keV and a half-life of 44.5(2) s for the (6^-) ground state have been determined. The level energies of the (3^-) isomer at 101.1(3) keV with T_1/2=33(2)   s and the 1⁺ isomer at 242.4(3) keV with T_1/2=6.6(2)   s are confirmed by high-precision mass measurements. The low-lying levels of ⁷⁰Cu populated in the decay of ⁷⁰Ni and in transfer reactions compare well with large-scale shell-model calculations, and the wave functions appear to be dominated by one proton–one neutron configurations outside the closed Z=28 shell and N=40 subshell. This does not apply to the 1⁺ state at 1980 keV which exhibits a particular feeding and deexcitation pattern not reproduced by the shell-model calculations.

The ⁷⁰Ni nucleus has been produced at the LISOL facility and its β decay to ⁷⁰Cu has been observed. In parallel, the ⁷⁰Cu nucleus has been produced at the ISOLDE facility. A new third β-decaying isomeric state in ⁷⁰Cu has been identified, partly with the aid of the in-source laser spectroscopy method. Its measured half-life is T_1∕2=33(2)  s. In addition, two isomeric transitions in ⁷⁰Cu have been observed at energies of 101.1(3) and 141.3(3)  keV, and it allowed the relative placement of the three ⁷⁰Cu isomeric states and their tentative spin and parity assignments. The new ⁷⁰Cu isomer was found to be weakly populated in the β decay of ⁷⁰Ni. It allowed the construction of the ⁷⁰Ni decay scheme. Furthermore, the β decay of the three ⁷⁰Cu isomers to ⁷⁰Zn has been measured and their decay schemes are presented. The ⁷⁰Ni β decay to the isomeric states in ⁷⁰Cu and their subsequent isomeric decay and β decay to ⁷⁰Zn are discussed within the extreme shell-model picture of two valence nucleons outside the semidouble magic ₂₈⁶⁸Ni₄₀ core and it yields satisfactory results. Large-basis shell-model calculations using ₂₈⁵⁶Ni₂₈ as the core and a realistic effective interaction support rather well the suggested interpretation.

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.

An excitation function for resonance elastic scattering of α particles on ¹⁸O and ¹⁸Ne was measured using the method of inverse geometry with a very thick target. Spectroscopic information was obtained for 23 levels in the excitation energy region from 11.9 to 13.7  MeV in ²²Ne. Twelve of them are new. General features of α-cluster bands in ²²Ne are analyzed in the framework of the potential model with a deep potential well. Predictions for the 11⁻ level in ²²Ne, as well as for the isotopic shift of the cluster levels in ²²Mg, are given. Evidence is presented that new perspectives on the study of nuclear structure and nuclear spectroscopy can be obtained in complimentary measurements of α-cluster states in mirror N≠Z nuclei.

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.

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.

Temperature dependence of magnetic specific heat and magnetic susceptibility has been studied experimentally and theoretically in the semimagnetic semiconductor Pb_1-xMn_xTe for x=0.024 and x=0.056, over the temperature range from 0.5 to 15 K, in magnetic fields up to 4 T. There was usually a maximum in the magnetic specific heat around 1 K in zero and low magnetic fields; the maximum shifted toward higher temperatures with increasing magnetic field. The experimental data have been analyzed in the framework of magnetic cluster models. An analysis of the influence of local lattice distortions and sp-d exchange coupling has been performed.

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.