R. Anne

The achromatic spectrometer LISE at GANIL

Abstract The achromatic spectrometer LISE installed at GANIL is used for the production and identification of either very neutron- or proton-rich nuclei obtained in the fragmentation of intermediate-energy heavy-ion beams at 0°. The characteristics and advantages of the system are described.

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.

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.

Experimental evidence for the particle stability of 34Ne and 37Na

The neutron drip line in a neon–magnesium region has been explored by the projectile fragmentation of a 59.8 A MeV 48Ca beam using the new fragment separator LISE 2000 at GANIL. New neutron-rich isotopes, 34Ne and 37Na, have been observed together with some evidence for the particle instability of 33Ne and 36Na.

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.

Zero degree measurements of isotopic distributions in 44 MeV/u 86Kr-induced reactions for the production of nuclei far from stability

Abstract The velocity and isotopic distributions of nuclei produced in 44 MeV/u 86Kr-induced reactions on 27Al, 103Rh and 197Au targets are investigated by means of the magnetic spectrometer LISE. These distributions are compared with two different models, and indicate that no real projectile fragmentation occurs, but rather highly dissipative processes such as incomplete fusion or deep-inelastic transfer. Consequently, the yields of nuclei far from stability is much lower relative to those observed with lighter projectiles such as 40Ar at the same energy.