J.P. Dufour

The GSI projectile fragment separator (FRS): A Versatile magnetic system for relativistic heavy ions

The projectile fragment separator FRS designed for research and applied studies with relativistic heavy ions was installed at GSI as a part of the new high-energy SIS/ESR accelerator facility. This high-resolution forward spectrometer has been successfully used in first atomic and nuclear physics experiments using neon, argon, krypton, xenon, and gold beams in the energy range from 500 to 2000 MeV/u. For the first time relativistic xenon and gold fragments have been isotopically separated. In this contribution we describe first experiments characterizing the performance of this spectrometer.

Projectile fragments isotopic separation: Application to the lise spectrometer at GANIL

The isotopic separation of intermediate-energy (30 to 100 MeV per nucleon) heavy-ion projectile fragments is theoretically and experimentally investigated. Physical separation of nuclides is achieved through magnetic analysis (AZ sensitivity) and differential stopping-power effects (Z−2 sensitivity). Depending on the specific case, a complete mass and charge separation may be achieved or a few nuclides may be selected simultaneously. The projectile fragments isotopic separation method described allows us to obtain an experimental mass separation AΔA ≈ 100. Simple formulae are given to determine the optimum factors for the best separation and transmission of the device. First applications to the spectroscopy of exotic nuclei lead to the new observation of the beta decay of 37P at GANIL and 15B at CERN.

Measurement of isotopic cross-sections of spallation residues in 800-A/MeV Au-197 + p collisions

Abstract The spallation of 197 Au by 800 MeV protons was investigated in inverse kinematics at GSI, Darmstadt, by use of a 197 Au beam bombarding a liquid-hydrogen target. The fragment separator (FRS) was used to select and identify the reaction products prior to β decay. The individual production cross sections and the kinematic properties of 380 isotopes for all elements between mercury ( Z=80 ) and neodymium ( Z=60 ) have been measured. A comparison with a Monte-Carlo calculation based on the two-step model of the spallation reaction is given. The isotopic cross-section distribution of iridium isotopes is compared to that resulting from the aluminium-induced fragmentation of 197 Au. The mean kinetic energies of the fragments are deduced from the experimental data. The importance of the new data to improve our understanding of the spallation mechanism is discussed.

Isotopic production cross sections of fission residues in 197Au-on-proton collisions at 800 A MeV

Abstract Interactions of 197Au projectiles at 800 A MeV with protons leading to fission are investigated. We measured the production cross sections and velocities of all fission residues which are fully identified in atomic and mass number by using the in-flight separator FRS at GSI. The new data are compared with earlier measurements of the characteristics of fission in similar reactions. Both the production cross sections and the recoil energies are relevant for a better understanding of spallation reactions.

31Ar and 27S: Beta-delayed two-proton emission and mass excess

Abstract Beta-delayed two-proton decay is reported for the T z = − 5 2 isotopes 31 Ar and 27 S wnich were produced by fragmentation of 36 Ar at 85 MeV/u. The two-proton nature of this decay is established on the basis of the number of events which trigger two detectors or more in a silicon-detector telescope. The proton energy spectra give experimental values for the daughter energy levels and for the branching ratios. The deduced values of the 31 Ar and 27 S masses are discussed and compared with mass predictions. For both nuclei, other channels like β3p and βpα are to be searched for since the observed β2p and βp branches do not fully account for the total decay width of the isobaric analog states in the daughter nuclei. Partial decay schemes are proposed for 31 Ar and 27 S.

Reaction cross section measurements of the neutron-rich isotopes8,9,11Li at 80 MeV/nucleon

Total interaction cross sections have been measured for8Li on C and Pb targets, for9Li on C, Al, Cu, Sn and Pb targets as well as for11Li on C, Sn and Pb targets. For each beam, we also used a plastic scintillator as target. The measurements with the scintillator targets are used to extract reduced nuclear radii of the lithium isotopes. These radii are then used for the calculation of the nuclear part of the total cross section for the other targets. The total electromagnetic-dissociation (EMD) cross sections have been deduced and are compared to different models. A strong target-charge-dependent EMD cross section is measured for11Li reaching 2.96−0.82+0.84 b for the Pb target. In the9Li case, a large EMD cross section for high-Z targets has been observed which amounts to 0.75 ± 0.45 b for the Pb target. The EMD cross sections of both,9Li and11Li, may be understood by the giantdipole-resonance model.

Longitudinal momenta and production cross-sections of isotopes formed by fragmentation of a 500 A×MeV86Kr beam

Abstract At the projectile-fragment separator FRS at GSI, longitudinal momentum distributions and production cross-sections were investigated for reactions between a 500 A× MeV 86 Kr beam and beryllium, copper and tantalum targets. Fragments in a wide A Z range were studied. For fragments close to the projectile the measured longitudinal momentum distributions agree qualitatively with a semi-empirical parametrization. Deviations from this simple picture are found, however, for the momentum widths of light fragments and for the momentum transfer to those fragments that differ in their A Z ratio from the most probable value. The experimental data on production cross-sections are used for testing the predictions obtained from a semi-empirical parametrization, a geometrical abrasion model and an intranuclear-cascade model. The cross-section for the formation of the doubly magic nucleus 78Ni in 86Kr fragmentation is estimated to be 17 pb.

New neutron-rich isotopes in the scandium-to-nickel region, produced by fragmentation of a 500 MeV/u86Kr beam

We have measured production cross-sections of the new neutron-rich isotopes58Ti,61V,63Cr,66Mn,69Fe,71Co and neighbouring isotopes that have been identified as projectile fragments from reactions between a 500 MeV/u86Kr beam and a beryllium target. The isotope identification was performed with the zero-degree magnetic spectrometer FRS at GSI, using in addition time-of-flight and energy-loss measurements. The experimental production cross-sections for the new nuclides and neighbouring isotopes are compared with an empirical parametrization. The resulting prospects for reaching even more neutron-rich isotopes, such as the doubly-magic nuclide78Ni, are discussed.

The decay modes of proton drip-line nuclei with A between 42 and 47

Neutron-deficient isotopes with Z=21 to 26 have been produced as projectile-like fragments of an intense 58Ni GANIL beam of 69 MeV/nucleon. The nuclei selected by the upgraded LISE3 spectrometer were identified and implanted in a silicon detector telescope. The 43Cr, 47Fe and 46Fe isotopes were identified for the first time whereas 45Fe, 45Mn, 44Mn and 42V were not observed, indicating probable instability of these nuclei against particle emission. Measurements of the half-lives of 43Cr and 46Mn have been performed and the analysis of their measured beta-delayed proton spectra has given, through the isobaric multiplet mass equation, an empirical estimation of their masses. Half-lives of 44Cr, 43V, 47Fe and 46Fe have also been measured. A discussion of various mass predictions for nuclei at the proton drip-line is given.

Beta delayed multi-neutron radioactivity of 17B, 14Be, 19C☆

Abstract The three nuclei 17B, 14Be, 19C have been produced and separated by the LISE spectrometer at GANIL. Half-lives and beta-delayed neutron multiplicities have been determined for these nuclei. The study of 17B has allowed us to observe for the first time the beta-delayed four-neutron decay mode with a branching ratio p4n=(4±3)×10−3.