P. Dendooven

COLLECTIVE STRUCTURE OF THE NEUTRON-RICH NUCLEI, RU-110 AND RU-112

Abstract The collective structure of 110 Ru and 112 Ru has been studied at the IGISOL facility through the beta decay of 110 Tc and 112 Tc. Neutron-rich technetium isotopes were produced by the 238 U(p, f) reaction at 20 MeV bombarding energy. The new isotope 112 Tc was found to decay with a half-life of 280(30) ms. The observed energies of the 2 + and 4 + ground state band levels in 108,110,112 Ru are almost the same. However, a steady energy decrease of the proposed gamma band head is observed with increasing neutron number, suggesting importance of triaxiality in these nuclei. In fact, only in 192 Os the second 2 + state has been observed at lower excitation than in 112 Ru. The static deformation of 108,110,110 Ru isotopes has been investigated in microscopic lattice Hartree-Fock calculations. The obtained potential energy surfaces predict that these isotopes are deformed (β ∼ 0.3), with a quasidegeneracy associated with γ-deformation, and suggest that the γ-asymmetry softness increases with the neutron number for neutron-rich ruthenium isotopes.

Life time measurements of 0+ intruder states in 190,192,194Pb

Abstract The fine structure in the α-decay of mass-separated 194,196,198 Po is studied. Feeding to the 0 + ground state and the first excited 0 + intruder state in 190,192,194 Pb is observed. From α-e-t coincidences between the feeding α-ray and the E0 transition, the half life of the 0 + intruder state in 190,192,194 Pb is measured. The mixing between intruder state and ground state is deduced.

STATUS-REPORT OF THE LEUVEN ISOTOPE SEPARATOR ONLINE (LISOL)

Abstract The mass separator LISOL, on-line to the CYCLONE cyclotron, is described as it is presently configured. Improvements in the cyclotron beams are mentioned, and separator developments to accommodate the various experimental stations are presented. Modifications to the separator to permit ion guide operation are given. Yields of some short-lived isotopes are presented, for heavy-ion induced reactions.

INTRUDER STATES IN ODD ODD TL NUCLEI POPULATED IN THE ALPHA-DECAY OF ODD ODD BI ISOTOPES

The alpha-decay of mass separated Bi-190,192,194,196 isotopes has been studied with the LISOL on-line separator. Time sequential alpha-singles spectra were taken and for Bi-190,192,194, alpha-Xt and alpha-gamma-t coincidences were collected. Transition probabilities of levels in Tl-186,188,190 were measured using a fast timing alpha-gamma-t and alpha-e-t coincidence set up. The alpha-decay of 196Bi and the beta-decay of 190Bi are observed for the first time. In all the Bi nuclei presently studied, unhindered alpha-decay of Bi starting from the high- (10-) and low-spin (3+) isomer populating intruder-based states in Tl was observed. The excitation energy of these intruder states is comparable with the intruder states in the neighboring odd-even and even-even nuclei.

A LASER ION-SOURCE FOR ONLINE MASS SEPARATION

A laser ion source based on resonance photo ionization in a gas cell is proposed. The gas cell, filled with helium, consists of a target chamber in which the recoil products are stopped and neutralized, and an ionization chamber where the atoms of interest are selectively ionized by the laser light. The extraction of the ions from the ionization chamber through the exit hole and skimmer is similar to the ion-guide system. The conditions to obtain an optimal system are given. The results of a two-step one-laser resonance photo ionization of nickel, and the first results of laser ionization in a helium buffer gas cell are presented.

PRODUCTION AND MASS SEPARATION OF SHORT-LIVING NEUTRON-DEFICIENT ACTINIDES

Abstract Light-ion reactions on radioactive as well as stable targets, in combination with the ion-guide technique represent a new and promising approach to the problem of the production and mass separation of short-living neutron-deficient actinides.

ALPHA-DECAY STUDY OF PB-188 AND HG-180,HG-182

Theα decay of mass-separated188Pb and180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asα-X-t andα-e-t coincidence events were collected. Fine structure in theα decay of188Pb was measured for the first time, feeding the 21+ and 02+ states in184Hg. Theα decay of180,182Hg feeds low-lying states in176−178Pt. One of these states could be identified as a 0+ state. The deexcitation pattern could be established. As a by-product an alpha-decay scheme of180Au towards levels in176Ir is given.Theα decay of mass-separated188Pb and180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asα-X-t andα-e-t coincidence events were collected. Fine structure in theα decay of188Pb was measured for the first time, feeding the 21+ and 02+ states in184Hg. Theα decay of180,182Hg feeds low-lying states in176−178Pt. One of these states could be identified as a 0+ state. The deexcitation pattern could be established. As a by-product an alpha-decay scheme of180Au towards levels in176Ir is given.

The alpha-branching ratios of the pb-188, pb-190, pb-192 isotopes

Theα-branching ratios (bα) of192,190,188Pb are measured using mass-separated sources. Different experimental set-ups are used — one detector as well as two detector set-ups — thereby detecting theα particles from the parent and/or viaα decay formed daughter nuclei, theβ-delayed gamma radiation from the parent and/or viaβ decay formed daughter nuclei and the Tl KX rays from electron capture decay. Values forbα of 6.2(6) 10−5 and 4.0(4) 10-3 were found for192,190Pb respectively. For188Pb, limits on thebα values were obtained: 0.03<bα<0.10. A careful analysis of the previously reportedbα values showed that the discrepancies in thebα values were not due to inadequate correction procedures, as was suggested, but to experimental uncertainties in the efficiency determination of the different detection set-ups and to an unreliableβ-decay scheme for188Pb. Thebα obtained in this work show that the leadα decay is not faster than the Hgα decay and that there is no need to assume a disappearance of theZ=82 shell closure halfway betweenN=82 andN=126.

SPIN-FLIP BETA- DECAY OF EVEN-EVEN DEFORMED-NUCLEI RU-110 AND RU-112

Neutron-rich nuclides110Ru and112Ru produced in symmetric fission of238U by 20 MeV protons have been studied at the IGISOL facility by means ofβ-ray,γ-ray and conversion electron spectroscopy. A total of 12 and 6γ-transitions were observed in the decays of110Ru and112Ru, respectively. Multipolarities were determined for a few transitions. The beta decay half-life was determined to be 11.6±0.6 s for110Ru and 1.75±0.07 s for112Ru. As a side product, a new value of 2.1±0.3 s for theβ half-life of the112Rh 1+ state was obtained. The decay energy measured with the plastic scintillator was 2.81 ±0.05 MeV for110Ru and 4.52 ±0.08 MeV for112Ru. The beta decay schemes of110Ru and112Ru isotopes indicate that the main fraction of their beta strength resides in two 0+→1+ spin-flip transitions with their logft values between 4.4 and 4.7. The decay energies and the energies of the 1+ GT states are compared with the macroscopic-microscopic model calculations. The observed GT-strengths are discussed in the framework of the deformed single-particle model.

THE INFLUENCE OF SHAPE CHANGES ON THE ALPHA-DECAY IN THE BI-191(M,G)-]TL-187-]AU-183 CHAIN

The191Bim,g →187T1 →183Au a-decay chain has been studied using mass — separated sources. The a — branching ratios have been measured. Out of the reduced a — decay widths, it is clear that the πs1/2 — πS1/2 and πh9/2 → πh9/2 transitions are not hindered although they involve strong shape changes.