V. Borrel

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.

β-Delayed neutron emission of the isotopes 20C, 40,41,42P, 43,44S☆

Abstract Very neutron-rich isotopes have been produced by the fragmentation of 55 MeV/u 48 Ca projectiles from GANIL onto a 181 Ta target. The nuclei were separated by means of the doubly achromatic spectrometer LISE and implanted into a semiconductor telescope for identification in Z and A . Their β-delayed neutron radioactivity is observed by means of a 4π β -n coincidence detector. The results include first measurements of the half-life T 1 2 and the neutron emission probability P n for the isotopes 20 C, 40,41,42 p and 43,44 S. They are compared to current theoretical predictions.

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.

Beta-delayed proton decay of the Tz = −52 isotope 31Ar☆

Abstract Beta-delayed proton radioactivity has been observed for the T z = − 5 2 isotope 31 Ar which has been produced by projectile fragmentation of 36 Ar at 85 MeV/u. The 31 Ar measured half-life is T 1 2 = 15 ± 3 ms , in good agreement with shell-model calculations; spectra of beta-delayed protons have been obtained. No evidence was found for direct two-proton-decay of 31 Ar.

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 studies of far from stability nuclei near N = 28

Abstract Beta-decay half-lives and β-delayed neutron-emission probabilities of the very neutron-rich nuclei 43 P, 42,44,45 S and 44–46 Cl, 47 Ar, which lie at or close to the N=28 magic shell, have been recently measured through β or β-n time correlation measurement. The results are compared to recent model predictions and indicate a rapid weakening of the N=28 shell effect below 48 Ca. The nuclear structure effects reflected in the decay properties of the exotic S and Cl isotopes may be the clue for the astrophysical understanding of the unusual 48 Ca 46 Ca abundance ratio measured in the solar system.

First beta-decay studies of the neutron-rich isotopes 53–55Sc and 56–59V

The neutron-rich isotopes Sc53-55 and V56-59 have been produced at GANIL in interactions of a 64.5 MeV/u Cu-65 beam with a Be-9 target. They were separated by the doubly achromatic spectrometer LISE3. Beta-decay half-lives and subsequent low-energy gamma-rays were observed for the first time. The present results are compared to QRPA model predictions. The quick drop of the half-life observed at N = 33 for Ca-53(20)33 is water V-56(23)33 and absent for Sc-54(21)33, indicating a vanishing of the N = 32 subshell north to Ca-52(32). In an astrophysical context, these neutron-rich isotopes represent r-process progenitors which, after beta-decay, would produce the correlated isotopic over-abundances of Cr-54, Fe-58, Ni-64 in certain refractory inclusions of meteorites

Elastic scattering of a secondary 11Li beam on 28Si at 29 MeV/nucleon

Abstract The elastic scattering of a secondary 11 Li beam (29 MeV/nucleon) on a 28 Si target has been measured for the first time. To compensate for the low intensity of the secondary beam, an efficient detecting system permitting one to obtain reliable experimental data has been used. An attempt has been made to reproduce the data in a phenomenological analysis and with coupled-channel calculations with a double-folding optical potential, with energy- and density-dependent effective interaction and realistic densities. In coupled-channel calculations with the folding optical potential a better description is achieved if a neutron halo of 11 Li is taken into account.

Identification of 100Sn and others proton drip-line nuclei in the reaction 112Sn(63 MeV/nucl.)+natNrmNi

Abstract Six new neutron-deficient nuclei 103Sb, 104Sb, 98In, 91Pd, 89Rh and 87Ru have been identified among the products of quasi-fragmentation of a 112Sn beam (63 MeV/nucl.), in addition to the previously reported 100Sn and 102Sn. Decays of over forty short-lived ( T 1 2 from ≈50 ns to 70μ s ) isomeric states including new isomer 66mAs were observed in the same reaction.

8B studied as a secondary beam at GANIL

Abstract The LISE spectrometer at GANIL has been used to form a 8 B secondary beam. Using a multiple silicon telescope, data have been obtained for the reaction and break-up cross sections, as well as for the parallel momentum distribution of 7 Be from break-up. For the first time, separate contributions to the break-up of diffraction and/or Coulomb dissociation and absorption mechanisms have been determined. The ensemble of data supports the existence in 8 B of a “pigmy” halo.