K. Rykaczewski

Identification of the doubly-magic nucleus 100Sn in the reaction 112Sn+natNi at 63 MeV/nucleon

Abstract We report on the production of the doubly-magic nucleus 100 Sn and other proton-rich nuclei in the A ∼ 100 region in the reaction 112 Sn + nat Ni at 63 MeV/nucleon. The experiment was carried out using the high acceptance device SISSI and the Alpha and LISE3 spectrometers at GANIL. The identification of the reaction products ( A , Z and Q ) was made using the measurements of time-of-flight, energy-loss and kinetic energy.

Decay studies of neutron-rich isotopes of manganese, iron, cobalt, nickel, copper and zinc

Abstract A 46 mg/cm2 thick natW target was irradiated with an 11.5 MeV/u 82Se beam. On-line massseparated samples of projectile-like neutron-rich products from multi-nucleon transfer reactions were investigated by βγ spectroscopy in the mass region A = 52–79. Decay properties of the nuclei 61Mn (0.71 s), 64Fe (2.0 s), 65Co (1.25 s), 69Ni (10 s) and 73mZn (5.8 s) were measured for the first time. For 67Ni a decay scheme could be constructed using information from new γ-transitions. The known decay schemes of 60Mn, 63Fe, 63Co and 69Cu were extended. New half-life values of neutron-rich isotopes are compared with theoretical predictions.

Decay studies of neutron-rich products from 76Ge induced multinucleon transfer reactions including the new isotopes 62Mn, 63Fe and 71, 72, 73Cu

Abstract By irradiating a nat W target with 9 MeV/u 76 Ge ions, neutron-rich isotopes in the chromiumgermanium region were produced. On-line mass-separated samples were investigated. The new isotopes 62 Mn, 71 Cu, 72 Cu and 73 Cu were identified and their half-lives measured to be 0.88(15), 19.5(16), 6.6(1) and 3.9(3) s, respectively. The half-life of 63 Fe, a nucleus which has previously been detected using particle-identification techniques, was measured to be 4.9(5) s. Decay schemes were derived in most cases. Additional spectroscopic information was obtained on the decays of 73 Zn and 74 Zn. We give evidence that the earlier reported γ-ray activity attributed to 67 Ni belongs to the decay of the 70 Cu isomers. At A = 67 we observed a new β-ray activity of 21(1) s half-life which is here assigned to the decay of 67 Ni.

Beta-decay half-lives of new neutron-rich chromium-to-nickel isotopes and their consequences for the astrophysical r-process

Abstract Beta-decay studies of the new neutron-rich isotopes 58,59Cr, 63Mn, 66,67Co and 69Ni, yielding distinctly shorter half-lives than the corresponding theoretical predictions, are presented. The influence of the short half-lives on the r-process calculations of nuclear abundances is discussed. It is concluded that a significantly higher neutron density than the one obtained from explosive helium burning in supernovae is need to reproduce the observe abundances around A=80.

Identification of μs-isomers produced in the fragmentation of a 112Sn beam

Abstract Decays of over forty short-lived ( T 1 2 from ≈ 50 ns to 70 μs) isomeric states including a new isomer 66mAs produced in the fragmentation of a 112Sn beam (58 A·MeV, 63 A·MeV) on a natNi target were observed at the final focus of the LISE3 spectrometer at GANIL. Their detection, based on the slow (≈ 10 μs) time correlation of identified ions with the characteristic γ-radiation, represents a novel method to search for new isomers and can be used for unambiguous isotope identification for projectile fragment separator experiments. Isomeric yields and isomer-to-total production ratios were determined.

New neutron-rich 179Yb and 181,182Lu isotopes produced in reactions of 9 MeV/u 136Xe ions on tantalum and tungsten targets

Abstract The new neutron-rich isotopes 179 Yb and 181,182 Lu were produced in multinucleon transfer reactions by irradiating nat W/Ta targets with 9 MeV/u 136 Xe ions, and identified by mass separation and decay spectroscopy. The measured half-lives of 179 Yb, 181 Lu and 182 Lu are 8.1±0.8, 3.5±0.3 and 2.0±0.2 min, respectively. The properties of the excited states of 181,182 Hf are discussed. The possibility of studying neutron-rich nuclei outside the classical fission-product regions is demonstrated.

Decay studies of neutron-rich radium and actinium isotopes, including the new nuclides 232Ra and 232,234Ac☆

Abstract Isotopes of the elements francium to neptunium were produced in multinucleon-transfer reactions of 11.4 MeV/u 238 U ions with nat W/Ta targets. On-line mass-separated samples were investigated by γ, X and β singles and coincidence measurements. New spectroscopic information was gained on the 231 Ac decay. For the first time, β-delayed γ-rays from 232 Ra and 232,233,234 Ac were observed. Decay schemes for these isotopes are presented, and half-lives of the latter four nuclides were determined to be 250(50) s, 119(5) s, 145(10) s and 44(7) s, respectively. The comparison with theoretical half-life predictions gave remarkable agreement with gross-theory calculations.

Beta-decay of 20Mg

Abstract The β-decay of 20Mg was investigated. A secondary beam of 20Mg ions, produced in reactions between a 95 A·MeV24Mg-beam and a natNi-target, was isotopically separated by means of the LISE3 spectrometer at GANIL. This secondary beam was implanted into a silicon detector array surrounded by germanium γ-detectors. The β-delayed proton and γ-ray data, measured for this short-lived nucleus ( T 1 2 = 95 ± 3 ms ), were incorporated into an improved 20Mg→20Na decay scheme. The 2645 keV level in 20Na is of importance for the breakout from the astrophysical hot CNO-cycle and the onset of the rapid proton capture process via the reaction 19Ne(p,γ)20Na. An upper limit of 0.1% for the β-decay feeding of the 2645 keV level and a lower limit for the corresponding log ft value of 6.24 were determined. The implications of this result for the spin and parity assignment of the 2645 keV state are discussed. By comparing the 20Mg β-decay into the proton-unbound 3001 keV state in 20Na and the isospin-mirrored decay into the particle-bound 3488 keV level in 20F, an asymmetry ft + ft − − 1 = 1.69 −0.65 +0.86 was observed. The comparison of the experimentally determined B(GT) values for the Gamow-Teller β-decay of 20Mg with a calculation performed in a full sd-shell model space yields an overall quenching of the Gamow-Teller strength of 0.84, corresponding to an observed fraction of 71% of the calculated strength. The measured Gamow-Teller strength above excitation energies of 3 MeV is more fragmented than predicted by the shell-model calculation.

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.

Decay properties of the new isotopes94Ag and95Ag

By using a40Ca beam of the UNILAC accelerator the new isotopes94Ag and95Ag were produced through58Ni(40Ca,p3n) and58Ni(40Ca,p2n) reactions, respectively, and identified by detectingβ-delayed protons from mass-separated sources.94Ag represents the heaviest N=Z nucleus with known decay properties. Theβ-decay half-lives of94Ag and95Ag were determined to be 0.42±0.05 s and 2.0±0.1 s, respectively. The measuredβ-decay properties are compared with model predictions.