G. Reusen

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.

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.

Beta decay of neutron-deficient even-mass indium isotopes: Evidence for population of highly-excited states in the cadmium-daughter nuclei

Abstract By using mass-separated sources, positrons as well as β-delayed protons and γ-rays were measured from the β-decay of 102 In ( T 1 2 = 22 ± 1 s ) and 100 In ( T 1 2 = 6.1 ± 0.9 s ). In particular, the average β + (β + + EC ) ratio for the decay of 102 In was measured to be 0.76(4) and the level scheme of 102 Cd was extended. Whereas no direct β + EC feeding of the identified 102 Cd levels was observed, there is evidence for population of 102 Cd states at high excitation energy followed by statistical γ-decay. This feature, which has been deduced on the basis of the measured positron spectrum and the measured β + (β + + EC ) ratio of the 102 In decay, is compared to the decay properties of the heavier odd-odd indium isotopes 104 In, 106 In and 108 In, and is also used to interpret the observed β-delayed proton data of 102 In and 100 In. For the first time, large-space shell-model calculations have been performed for the Gamow-Teller decay of heavy odd-odd nuclei. The decay results from these calculations are in good overall agreement with the gross decay properties of 102 In and 100 In deduced from the experiment.

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.

Production and acceleration of radioactive ion beams at Louvain-La-Neuve

Abstract The motivation, experimental setup and status report of the program for producing and accelerating radioactive ion beams at Louvain-la-Neuve are described.

The Gamow-Teller decay of 105Sn to three-particle states in 105In

Abstract The EC/ β + decay of 105 Sn was reinvestigated by using 58 Ni(5 MeV/u) + 50 Cr reactions, chemically selective on-line mass separation and γ-ray spectroscopy. The half-life of 105 Sn has been determined as 34 ± 1 s. Out of 104 γ transitions ascribed to the 105 Sn decay, 89 have been placed in the decay scheme including 52 excited states of 105 In. From the EC/ β + feeding of individual states, the distribution of the Gamow-Teller (GT) strength has been derived. It is shown that the main part of the GT strength is associated with the feeding of 105 In levels having excitation energies above 3 MeV. This observation can be interpreted as a sign of dominant feeding of three-quasiparticle states in 105 In, which correspond to the π( g 9 2 ) −1 νg 7 2 νd 5 2 shell-model configuration spread over many levels. The sum of the GT strength deduced from the present gamma-ray data of B Σ (GT) = 1.46 provides a lower limit to the total GT strength. Many weak transitions, mainly from high-energy levels, may not have been detected in this study and therefore part of the strength may be missed. An indirect support for this conclusion has been obtained from the analysis of the indium KX-rays intensity. This analysis indicates about 50% contribution of the electron-capture to the beta decay of 105 Sn, which is interpreted as a sign of a predominant feeding of the high-energy 105 In states with B Σ (GT) ⩾ 3. The observed GT distribution and strength will be compared to results obtained from a finite-Fermi-system theory and a large-basis shell-model calculation. The core-polarization and higher-order hindrance factors will be discussed.

Extraction efficiency of n-13 (t1/2 = 9.96 min) atoms from a graphite target - comparison between off-line and online obtained results

An off-line release study N-13 (T1/2 = 9.96 min) produced by a proton induced reaction on a graphite target (POCO-graphite EDM3, density = 1.84 g/cm3, grain size approximately 3-mu-m) has been performed. The activation energy for the diffusion process is determined to be 6.15(16)x10(5) J/Mol. With this acitivation energy, extraction efficiences for N-13 are obtained at different temperatures and are compared to on-line measured extraction efficiencies.

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.

OCTUPOLE DEFORMATION IN AC-223

Abstract The level structure of 223Ac has been obtained from a study of the α-decay of mass separated 227Pa. Two levels connected by a fast E1 transition and with spin and parity 5+ 2 and 5− 2 were found: they form a parity doublet suggestive of octupole deformation in this nucleus. The Coriolis matrix element obtained for the mixing between the low-lying states is found to be attenuated, in agreement with theoretical expectations for stable octupole deformation.