A. T. Reed

Search for 28O and study of neutron-rich nuclei near the N = 20 shell closure

Abstract A search for 28 O with a 78 AMeV beam of the neutron-rich isotope 36 S has been performed for the first time. Evidence for the unbound character of 28 O was obtained. In the same experiment the half-lives of the very neutron-rich isotopes 27,29 F and 30 Ne were measured and those for 28,29 Ne and 30,31 Na reexamined. The results are compared to shell-model predictions and conclusions drawn regarding the extent of the region of deformation around N = 20.

K-forbidden transitions from multi-quasiparticle states

Abstract Four-quasiparticle, K π = 12 + states in 172 Hf and 178 W, with half-lives less than 2 ns, are found to decay to their respective K π = 0 + ground-state bands, in competition with transitions to intermediate- K , two-quasiparticle structures. All decay transitions are weakly hindered. When taken together with other K -isomer decay rates, an overall trend of decreasing hindrance with increasing excitation energy is evident. Estimates based on density-of-states considerations approximately reproduce the trend of the data, but also indicate the need to include other K -mixing effects.

DRIPLINE NUCLEI PRODUCED BY QUASI-FRAGMENTATION OF THE 32,34,36S PRIMARY BEAMS

Some regularities in the production of isotopes with are investigated in reactions induced by beams at on targets , and . A search for using a beam of the neutron-rich has been performed. Evidence for particle instability of was obtained. In the same experiment, the half lives of the very neutron-rich isotopes and were measured for the first time.

Characteristics of neutron-rich nuclei around shell closures N=20 and 28

Abstract The paper presents a review of experimental results obtained at GANIL and FLNR Dubna on the study of the particle stability of neutron-rich nuclei near the neutron drip line. Very recent data on mass measurements of neutron-rich nuclei in the range A =29–47 at GANIL and some characteristics of binding energies in this region are presented. Nuclear binding energies are very sensitive to the existence of shells and provide clear signatures of shell closures. Experiments have been mostly performed in cooperation GANIL–Orsay–Dubna–Řež–Bucharest–Liverpool.

Superdeformation in

Abstract The superdeformed nucleus 193 Pb has been studied following a heavy-ion fusion-evaporation reaction with the Gammasphere spectrometer array. Three new superdeformed bands, two of them probably signature partners, have been observed, bringing the total number of superdeformed bands known in this nucleus to nine. An estimate of the g -factor associated with one of the previously known pairs of strongly coupled excited bands has been made, confirming their configuration. The properties of the bands are compared with the results of cranked shell model calculations; good agreement is found and configurations are assigned to each band. The two new signature partner bands are suggested to be based on the two signatures of the [512]5/2 neutron orbital. The third new band is probably based on the 7 3 intruder orbital. In addition, the high-energy region of the spectrum in coincidence with the yrast superdeformed bands has been carefully studied. The current data provide no evidence for the previously reported transitions linking the superdeformed and normal-deformed states with energies around 2.2 – 2.4 MeV.

^193

Abstract This paper describes the results of two complementary experiments which studied the properties of the well-deformed nucleus 171 Hf. The first experiment, with a thin self-supporting target, extended the rotational bands built upon the [633]7 / 2, [512]5 / 2 and [521]1 / 2 configurations up to spins of 73 / 2–85 /2ℏ . The configurations of these bands and observed band crossings are discussed within the framework of the cranked-shell model. The second experiment employed a backed target in order to measure the lifetimes, by the Doppler Shift Attenuation method, and thereby establish deformations for some of the states in the collective rotational bands. The extracted deformations are found to be consistent with those predicted from theoretical Total Routhian Surface calculations. These deformations provide strong evidence that the high-spin states in 171 Hf, and perhaps more importantly, in the region where the high- K ( K π =19/2 + and K π =23/2 − ) isomeric states decay, retain their well-deformed axial symmetry.

Pb: Observation of three additional excited bands

A K{sup {pi}}=8{sup {minus}} strongly coupled rotational band, which is built upon a 23 ns isomeric state, has been established in {sup 170}Hf. The single-particle configuration of the band has been determined from the in-band to out-of-band branching ratios and associated {vert_bar}(g{sub K}{minus}g{sub R}){vert_bar}/Q{sub 0} values. The {gamma}-ray decay of the K{sup {pi}}=8{sup {minus}} isomeric state proceeds through a K{sup {pi}}=6{sup +} intrinsic state, in accordance with the {ital K}-selection rule. A complex array of delayed {gamma}-ray transitions have been observed to link this K{sup {pi}}=6{sup +}, intrinsic bandhead state into the yrast states. In particular, a new {Delta}K=8, 1141 keV, E1 transition links the K{sup {pi}}=8{sup {minus}} isomeric state directly to the ground-state band. The {gamma}-ray reduced hindrance factor for this E1 decay in {sup 170}Hf is the smallest {Delta}K=8 transition observed in the Hf nuclei. However, its value is consistent with that expected on the basis of the increased Coriolis mixing which occurs in the lighter Hf isotopes. {copyright} {ital 1999} {ital The American Physical Society}

High-Spin States and Lifetime Measurements in

This contribution focussed on studying nuclear pairing via its effects on the nuclear moment of inertia using K-isomeric states as a probe. The reason for studying the effects on K-isomeric states is that their underlying single-particle configuration can be uniquely defined through a measurement of the in-band to out-of-band branching ratios. The angular momentum limits to populating K-isomers were also addressed.

^171

The heavy odd-odd N=Z nuclei {sup 78}Y,{sup 82}Nb,{sup 86}Tc have been produced at GANIL by fragmentation of a 60 MeV/u {sup 92}Mo beam on a natural nickel target. Their lifetime have been measured for the first time, using the identification-implantation and subsequent time correlated {beta} counting technique at the LISE spectrometer. The obtained half-lives for {sup 78}Y,{sup 82}Nb,{sup 86}Tc are consistent with the microscopic predictions. These nuclei are good candidates to extend the mass range for the V{sub ud} matrix element evaluation and our experimental results highlight the interest of further investigation of their excited states and {beta} decay schemes as a function of isospin. {copyright} {ital 1998 American Institute of Physics.}

Hf

The heavy odd-odd nuclei , , , have been produced at GANIL by fragmentation of a beam on a natural nickel target. The species of interest were selected and identified by their energy loss in a silicon detector and their time of flight through the LISE spectrometer. The ions were implanted in a silicon strip detector in which their -decay was observed. The measured half-lives for , , correspond to superallowed transitions and may sign Fermi transitions on the basis of values. Our experimental results pave the way for further and more precise measurements of these nuclei.