J. Benlliure

Relativistic radioactive beams: A new access to nuclear-fission studies

The secondary-beam facility of GSI Darmstadt was used to study the fission properties of 70 short-lived radioactive nuclei. Most of them have not been accessible so far in conventional fission experiments. Relativistic secondary projectiles were produced by fragmentation of a 1 A GeV 238U primary beam and identified in nuclear charge and mass number. Using these reaction products as secondary beams, the giant resonances, mostly the giant dipole resonance, were excited by electromagnetic interactions in a secondary lead target, and fission from excitation energies around 11 MeV was induced. The fission fragments were identified in nuclear charge, and their velocity vectors were determined. Elemental yields and total kinetic energies have been obtained for a number of neutron-deficient actinides and preactinides. The characteristics of multimodal fission of nuclei around 227Th were systematically investigated. The proton even–odd effect was determined for all systems.

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.

Production of neutron-rich isotopes by cold fragmentation in the reaction 197Au + Be at 950 A MeV

Abstract The production cross sections and longitudinal-momentum distributions of very neutron-rich isotopes have been investigated in the fragmentation of a 950 A MeV 179 Au beam in a beryllium target. Seven new isotopes ( 193 Re, 194 Re, 191 W, 192 W, 189 Tl, 187 Hf and 188 Hf) and the five-proton-removal channel were observed for the first time. The reaction mechanism leading to the formation of these very neutron-rich isotopes is explained in terms of the cold-fragmentation process. An analytical model describing this reaction mechanism is presented.

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.

Discovery and cross-section measurement of neutron-rich isotopes in the element range from neodymium to platinum with the FRS

Abstract:Using the high-resolution performance of the fragment separator FRS at GSI we have discovered 60 new neutron-rich isotopes in the atomic number range of 60⩽Z⩽78. The new isotopes were unambiguously identified in reactions with a

Evaporation residues produced in the spallation reaction 238U+p at 1 A GeV

The production of heavy nuclides from the spallation-evaporation reaction of 238U induced by 1 GeV protons was studied in inverse kinematics. The evaporation residues from tungsten to uranium were identified in-flight in mass and atomic number. Their production cross-sections and their momentum distributions were determined. The data are compared with empirical systematics. A comparison with previous results from the spallation of 208Pb and 197Au reveals the strong influence of fission in the spallation of 238U.

Measurement of nuclide cross-sections of spallation residues in 1-A-GeV U-238 + proton collisions

The production of heavy nuclides from the spallation-evaporation reaction of 238U induced by 1 GeV protons was studied in inverse kinematics. The evaporation residues from tungsten to uranium were identified in-flight in mass and atomic number. Their production cross-sections and their momentum distributions were determined. The data are compared with empirical systematics. A comparison with previous results from the spallation of 208Pb and 197Au reveals the strong influence of fission in the spallation of 238U.

Odd-even effects observed in the fission of nuclei with unpaired protons

Nuclear-charge yields of fragments produced by fission of 214,…,223Ac, 220,…,229Th, 224,…,232Pa, and 231,…,234U have been measured. These radioactive nuclei were produced as secondary beams, and fission was induced in flight at 420 A MeV by electromagnetic excitation in a lead target. The excitation-energy distribution at fission is estimated to be centered at 11 MeV. The nuclear-charge distributions of the fission fragments of odd-Z actinium and protactinium isotopes show a pronounced odd-even structure: in asymmetric charge splits, the unpaired proton predominantly sticks to the heavy fragment. A model is proposed which attributes this effect to the single-particle level densities of the nascent fission fragments. This model also explains the strongly enhanced odd-even effect observed in the most asymmetric parts of the nuclear-charge distributions after fission of the even-Z nuclei 220,…,229Th and 231,…,234U. It is concluded that proton pairs are broken in an early stage of the fission process.

Fission of nuclei far from stability

Abstract The secondary-beam facility of GSI provided the technical equipment for a new kind of fission experiment. Fission properties of short-lived neutron-deficient nuclei have been investigated in inverse kinematics. The measured element distributions reveal new kinds of systematics on shell structure and even–odd effects and lead to an improved understanding of structure effects in nuclear fission. The relevance of these studies for some presently considered applications is described. Prospects for future experiments are discussed.

Production of new neutron-rich isotopes of heavy elements in fragmentation reactions of 238U projectiles at 1A GeV

The production of heavy neutron-rich nuclei has been investigated using cold-fragmentation reactions of {sup 238}U projectiles at relativistic energies. The experiment performed at the high-resolving-power magnetic spectrometer Fragment Separator at GSI made it possible to identify 40 new heavy neutron-rich nuclei: {sup 205}Pt, {sup 207-210}Au, {sup 211-216}Hg, {sup 214-217}Tl, {sup 215-220}Pb, {sup 219-224}Bi, {sup 223-227}Po, {sup 225-229}At, {sup 230,231}Rn, and {sup 233}Fr. The production cross sections of these nuclei were also determined and used to benchmark reaction codes that predict the production of nuclei far from stability.