Search Results (56 total)

By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei ^129-132Sn and ^133,134Sb, their dipole-strength distributions have been measured. A sizable fraction of “pygmy” dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei (²⁰⁸Pb and N=82 isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (a₄=32.0±1.8 MeV and p_o=2.3±0.8 MeV/fm³) are deduced as well as neutron-skin thicknesses R_n-R_p of 0.24±0.04 fm for ¹³²Sn and of 0.18±0.035 fm for ²⁰⁸Pb, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.

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 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.

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.

The dipole strength distribution above the one-neutron separation energy was measured in the unstable ¹³⁰Sn and the double-magic ¹³²Sn isotopes. The results were deduced from Coulomb dissociation of secondary Sn beams with energies around 500  MeV/nucleon, produced by in-flight fission of a primary ²³⁸U beam. In addition to the giant dipole resonance, a resonancelike structure (“pygmy resonance”) is observed at a lower excitation energy around 10 MeV exhausting a few percent of the isovector E1 energy-weighted sum rule. The results are discussed in the context of a predicted new dipole mode of excess neutrons oscillating out of phase with the core nucleons.

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 report the first measurement of a ratio λ_βb/λ_βc of bound-state (λ_βb) and continuum-state (λ_βc) β^--decay rates for the case of bare ²⁰⁷Tl⁸¹⁺ ions. These ions were produced at the GSI fragment separator FRS by projectile fragmentation of a ²⁰⁸Pb beam. After in-flight separation with the Bρ-ΔE-Bρ method, they were injected into the experimental storage-ring ESR at an energy of 400.5A  MeV, stored, and electron cooled. The number of both the ²⁰⁷Tl⁸¹⁺ ions and their bound-state β^--decay daughters, hydrogenlike ²⁰⁷Pb⁸¹⁺ ions, were measured as a function of storage time by recording their Schottky-noise intensities. The experimental result, λ_βb/λ_βc=0.188(18), is in very good agreement with the value of 0.171(1) obtained from theory employing spectra of allowed transitions.

The γ-cascades following the Gamow Teller beta decay of ¹⁴⁸Dy 0⁺ have been investigated with the total absorption gamma spectrometer at GSI. Our measurements confirm that the Gamow-Teller resonant state is populated with a log  ft=3.9 and that there is very little mixing of this state with the background J=1 states. In contrast to previous measurements, the experimental method presented here is sensitive to very small admixtures at high excitation energy. Previously unobserved, weak beta-branches to levels in ¹⁴⁸Tb are also reported.

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.

The one-proton knockout channel from ⁶He (240  MeV/u) impinging on a carbon target has been investigated. The triton fragments originating from this channel were detected in coincidence with the two neutrons. A broad structure, peaked at 3 MeV above the t+2n threshold, is observed in the t+n+n-relative energy spectrum. It is shown that this structure is mainly due to a I^π=1/2⁺ resonance as expected for the ⁵H ground state, and from the observed angular and energy correlations, being used for the first time in ⁵H studies, that the neutrons to a large extent occupy the p shell.

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.

Electromagnetic and nuclear inelastic scattering of the halo nucleus ¹¹Be have been investigated by a measurement of the one-neutron removal channel, utilizing a secondary ¹¹Be beam with an energy of 520 MeV/nucleon impinging on lead and carbon targets. All decay products, i.e., ¹⁰Be fragments, neutrons, and γ rays have been detected in coincidence. Partial cross sections for the population of ground and excited states in ¹⁰Be were determined for nuclear diffractive breakup as well as for electromagnetically induced breakup. The partial cross sections for ground-state transitions have been differentiated further with respect to excitation energy, and the dipole-strength function associated solely with transitions of the halo 2s_1/2 neutron to the continuum has been derived. The extracted dipole strength integrated from the neutron threshold up to 6.1 MeV excitation energy amounts to 0.90(6)e² fm². A spectroscopic factor for the ν2s_1/2⊗¹⁰Be(0⁺) single-particle configuration of 0.61(5) and a root-mean-square radius of the 2s_1/2 neutron wave function of 5.7(4) fm have been deduced.

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.

Fragmentation of secondary beams of neutron-rich, unstable ^19,20,21O isotopes at beam energies near 600 MeV/nucleon was studied by measuring the production cross sections for carbon, nitrogen, and oxygen fragments. Data for stable ^17,18O beams were obtained as well. The measurements serve to illuminate the isospin dependence of the fragmentation process. The experimental results are compared to those from empirical parametrization and those from abrasion-ablation models.

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.

We have measured production cross sections of projectilelike fragments in the reaction of neutron-deficient ¹¹²Sn with a ⁹Be target at an energy of E_lab=1A GeV. Results have been obtained for neutron-deficient products along the N=Z line down to the subnanobarn range. We find very good agreement with the recently revised empirical parametrization EPAX. In addition, a few one- and two-proton pickup cross sections have been measured. The former ones are well reproduced by intranuclear-cascade calculations.

Low-lying resonance states in ⁷He(⁶He+n), formed after fragmentation reactions of a 227 MeV/nucleon ⁸He beam on a carbon target, have been studied. Coincidences between ⁶He nuclei and neutrons, corresponding to the one-neutron knockout channel in ⁸He, were selected. The relative energy spectrum in the ⁶He+n system shows a structure, which is interpreted as the ⁷He ( I^π  =  3/2^-) ground state, unbound with 0.43(2) MeV relative to the ⁶He+n system and a width of Γ  =  0.15(8) MeV overlapping with an excited ( I^π  =  1/2^-) state observed at 1.0(1) MeV with a width of Γ  =  0.75(8) MeV.

In two complementary measurements, a cubelike array of six Euroball-Cluster germanium detectors and a total-absorption γ spectrometer were used to investigate the β decay of ⁹⁸Ag, a three-proton-hole and one-neutron-particle nucleus with respect to the ¹⁰⁰Sn core. The half-life and Q_EC value of the decay were determined to be 47.7(0.3) s and 8200(70) MeV, respectively. A total of 438 γ rays (414 new) was observed, and 173 levels (163 new) in ⁹⁸Pd have been identified. The Gamow-Teller (GT) β-decay strength distribution from the total-absorption γ spectroscopy reveals a large GT resonance around 6 MeV with a width of about 2 MeV. The hindrance factor for the total GT strength, summed from the ground state up to 7.8 MeV excitation energy in ⁹⁸Pd, amounts to 4.6(6) with reference to a shell-model calculation which yields good agreement with the shape of the experimental GT resonance.

We have measured the Coulomb dissociation of ⁸B into ⁷Be and a proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S₁₇-factor for the ⁷Be(p,γ)⁸B reaction at E_c.m.  =  0.25–2.78 MeV is deduced yielding S₁₇(0)  =  20.6±1.2 (expt) ±1.0(theor)  eV b. This result agrees with the presently adopted zero-energy S₁₇-factor obtained in direct-reaction measurements and with the results of other Coulomb-dissociation studies performed at 46.5 and 51.2 MeV/nucleon.

In two complementary measurements, a cubelike array of 6 Euroball-Cluster germanium detectors and a total-absorption γ-spectrometer were used to investigate the β decay of ⁹⁷Ag, a three proton-hole nucleus with respect to the ¹⁰⁰Sn core. The half-life and Q_EC value of the decay of the 9/2⁺ ground state of ⁹⁷Ag were determined to be 25.9(4) s and 6.98(11) MeV, respectively. A total of 603 γ rays (578 new) was observed, and 151 levels (132 new) in ⁹⁷Pd have been identified. An interesting β-delayed γ cascade was observed, which comprises 6 γ-transitions with a deexcitation pattern involving an initial increase of the level spin. The Gamow-Teller (GT) β-decay strength distributions from the two measurements reveal a large GT resonance around 4 MeV with a width of about 1.8 MeV. The hindrance factor for the total GT strength, summed from the ground state up to 6 MeV excitation energy in ⁹⁷Pd, amounts to 4.3(6) with reference to a shell-model prediction. This factor is discussed in comparison with a core polarization and a Monte Carlo shell-model calculation.

Peripheral fragmentation of a 287 MeV/nucleon beam of the halo nucleus ¹¹Li incident on a carbon target has been studied in a fragment-neutron coincidence experiment. The momentum distribution of the ¹⁰Li fragments gives access to the momentum distribution of the removed neutron in ¹¹Li. From the shape of the distribution, the (1s_1/2)² contribution to the mixture of (1s_1/2)² and (0p_1/2)² components in the ground-state wave function was determined to be (45±10)%. The angular correlation between the knocked out neutron and the one from the decay of ¹⁰Li shows a skew distribution signaling interference of these two different parity states.

The three-body breakup ⁶He→⁴He+n+n is studied experimentally, using a secondary ⁶He ion beam of 240 MeV/nucleon incident on carbon and lead targets. Integrated cross sections for one- and two-neutron knockout and differential cross sections dσ/dE^* and dσ/dϑ for inelastic nuclear or electromagnetic excitations into the ⁶He continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10±2)% of the energy-weighted Thomas-Reiche-Kuhn sum rule or (40±8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact from which the root-mean-square distance between the α core and the two valence neutrons of r_α-2n=(3.36±0.39) fm is derived. The known I^π=2⁺ (1.80 MeV) resonance in ⁶He is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter δ₂=(1.7±0.3) fm or B(E2,0⁺→2⁺)=(3.2±0.6)e² fm⁴ are derived. No clear signature could be obtained for predicted higher-lying 2⁺ resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying ⁴He+n+n system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process ⁴He(2n,γ)⁶He is briefly discussed.

Measurements of deexcitation γ rays in coincidence with the momentum distribution of the projectile residues produced in reactions of the type ⁹Be(²⁸P,²⁷Si+γ)X at energies around 65 MeV/u are used to study single-nucleon stripping to individual states. The cross sections are compared with calculations based on an eikonal model description of the reaction and the shell model. The measurements indicate that the halo character of the ground state and other detailed spectroscopic information can be derived using knockout reactions in inverse kinematics.