M. Gorska

Superallowed Gamow-Teller decay of the doubly magic nucleus 100Sn

The shell structure of atomic nuclei is associated with ‘magic numbers’ and originates in the nearly independent motion of neutrons and protons in a mean potential generated by all nucleons. During β+-decay, a proton transforms into a neutron in a previously not fully occupied orbital, emitting a positron–neutrino pair with either parallel or antiparallel spins, in a Gamow–Teller or Fermi transition, respectively. The transition probability, or strength, of a Gamow–Teller transition depends sensitively on the underlying shell structure and is usually distributed among many states in the neighbouring nucleus. Here we report measurements of the half-life and decay energy for the decay of 100Sn, the heaviest doubly magic nucleus with equal numbers of protons and neutrons. In the β-decay of 100Sn, a large fraction of the strength is observable because of the large decay energy. We determine the largest Gamow–Teller strength so far measured in allowed nuclear β-decay, establishing the ‘superallowed’ nature of this Gamow–Teller transition. The large strength and the low-energy states in the daughter nucleus, 100In, are well reproduced by modern, large-scale shell model calculations.

On-line yields obtained with the ISOLDE RILIS

The ISOLDE resonance ionization laser ion source (RILIS) allows to ionize efficiently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed 23–34 Mg, 26–34 Al, 98–132 Cd, 149 Tb, 155–177 Yb, 179–200 Tl, 183–215 Pb and 188–218 Bi. The obtained yields are presented together with measured release parameters which allow to extrapolate the release efficiency towards more exotic (short-lived) nuclides of the same elements. 2002 Elsevier Science B.V. All rights reserved.

Experimental approach towards shell structure at 100Sn and 78Ni

The status of experimental approach to Sn-100 and Ni-78 is reviewed. Revised single particle energies for neutrons are deduced for the N=Z=50 shell closure and evidence for low lying I-pi=2(+) and 3(-) states is presented. Moderate E2 polarization charges of 0.1 e and 0.6 e are found to reproduce the experimental data when core excitation of Sn-100 is properly accounted for in the shell model. For the neutron rich Ni region no conclusive evidence for an N=40 subshell is found, whereas firm evidence for the persistence of the N=50 shell at Ni-78 is inferred from the existence of seniority isomers. The disappearance of this isomerism in the mid nug(9/2) shell is discussed.

0(gs)+ -->2(1)+ transition strengths in 106Sn and 108Sn.

The reduced transition probabilities, B(E2; 0(gs)+ -->2(1)+), have been measured in the radioactive isotopes (108,106)Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation gamma rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0(gs)+ -->2(1)+)=0.222(19)e2b2 for 108Sn and B(E2; 0(gs)+-->2(1)+)=0.195(39)e2b2 for 106Sn were determined relative to a stable 58Ni target. The resulting B(E2) values are approximately 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.

Cd-98(48)50: The two-proton-hole spectrum in Sn-100(50)50

Excited states in {sup 98}Cd, two proton holes from {sup 100}Sn, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the ({pi}g{sub 9/2}){sup {minus}2} two-proton-hole spectrum below a T{sub 1/2}=0.48(16) {mu}s isomer is deduced and compared to shell-model predictions. A tentative I{sup {pi}}=(8{sup +}) assignment, as suggested by systematics, yields a strongly reduced B(E2,8{sup +}{r_arrow}6{sup +})=0.44({sup +20}{sub {minus}10}) W.u., corresponding to an effective proton charge of e{sub {pi}}=0.85({sup +20}{sub {minus}10}) e , which is at variance with existing theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}Excited states in Cd-98, two proton holes from Sn-100, were identified and studied for the first time, using in-beam spectroscopy with highly selective ancillary detectors. The structure of the (pi g(9/2))(-2) two-proton-hole spectrum below a T-1/2 = 0.48(16) mu s isomer is deduced and compared to shell-model predictions. A tentative I-pi = (8(+)) assignment, as suggested by systematics, yields a strongly reduced B(E2,8(+) --> 6(+)) = 0.44((+20)(-10)) W.u., corresponding to an effective proton charge of e(pi) = 0.85((+20)(-10))e, which is at variance with existing theoretical predictions.

Direct observation of long-lived isomers in 212Bi.

Long-lived isomers in (212)Bi have been studied following (238)U projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of (212)Bi was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of >1910 keV. It was also found to have an extended Lorentz-corrected in-ring half-life >30 min, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies.

Gamow–Teller strength distribution near 100Sn. The beta decay of 102In

One of the main motivations of today’s β-decay studies is to investigate whether the presence of many bound nucleons affects the value of the Gamow—Teller (GT) axialvector coupling constant. The quest for 100Sn is driven by the hope that its particulary simple decay scheme may help to perform improved tests of the models used to calculate the summed GT strength (B GT). Since 100Sn can not yet be produced in sufficient quantity, the experiments are restricted to nearby nuclei. In this paper we report on a study of the β decay of 102In, performed at the GSI Online Mass Separator. This odd—odd nucleus has a very complicated decay scheme and thus represents indeed a great challenge. The loss of numerous weak transitions in high resolution mesurements is suspected to be one of the main reasons for incomplete B GT measurements. Therefore, the experiment with a cube-like array of 6 Euroball Cluster high-resolution germanium detectors (Euroball Cube), with 7 crystals per Cluster, was complemented by the measurement with a total-absorption gamma spectrometer (TAS) [1].

g-factor measurements of μs isomeric states in neutron-rich nuclei around 68Ni produced in projectile-fragmentation reactions

We report the first g factor measurement on microsecond isomers of neutronrich nuclei produced in projectile-fragmentation reactions at intermediate energies. The nuclides in the vicinity of 68 Ni were produced and spin oriented following the fragmentation of a 76 Ge, 61.4 MeV u −1 beam at GANIL. The LISE spectrometer was used to select the nuclei of interest. The time-dependent perturbed angular distribution (TDPAD) method was applied in combination with th eh eavy-ion–gamma correlation technique to measure the g factors of 69m Cu (J π = 13/2 + , T1/2 = 350 ns) and 67m Ni (J π = 9/2 + , T1/2 = 13.3 µs). Specific details of the experimental technique and the comparison of the results (|g( 69m Cu) |= 0.225(25) and |g( 67m Ni) |= 0.125(6)) with theoretical calculations are discussed. These results provide another indication of the importance of proton excitations across the Z = 28 shell gap.

Production of very neutron-deficient isotopes near 100Sn via reactions involving light-particle and cluster emission

Abstract The production of very neutron-deficient isotopes near 100Sn has been investigated by using on-line mass separation of evaporation residues produced by heavy-ion induced complete-fusion reactions. We measured the cross sections for 99Cd, 100In, 101Sn and 102In via 58Ni+58Ni fusion reactions followed by cluster emission, and via 58Ni+50Cr fusion reactions accompanied by evaporation of protons, neutrons or α particles. Both types of reactions yield similar cross sections for the production of exotic nuclei near 100Sn. The data are discussed in comparison with results obtained from statistical-model calculations.

Structure of 55Ti from relativistic one-neutron knockout

Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV -hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1/2- state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N=34 gap in 54Ca.