E. Casarejos

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.

Measurement of the complete nuclide production and kinetic energies of the system {sup 136}Xe+hydrogen at 1 GeV per nucleon

We present an extensive overview of production cross sections and kinetic energies for the complete set of nuclides formed in the spallation of {sup 136}Xe by protons at the incident energy of 1 GeV per nucleon. The measurement was performed in inverse kinematics at the GSI fragment separator. Slightly below the Businaro-Gallone point, {sup 136}Xe is the stable nuclide with the largest neutron excess. The kinematic data and cross sections collected in this work for the full nuclide production are a general benchmark for modeling the spallation process in a neutron-rich nuclear system, where fission is characterized by predominantly mass-asymmetric splits.

Spallation residues in the reaction Fe-56 + p at 0.3-A-GeV, 0.5, 0.75, 1.0-A GeV and 1.5 A GeV

The spallation residues produced in the bombardment of ^(56)Fe at 1.5A, 1.0A, 0.75A, 0.5A, and 0.3A GeV on a liquid-hydrogen target have been measured using the reverse kinematics technique and the fragment separator at GSI (Darmstadt). This technique has permitted the full identification in charge and mass of all isotopes produced with cross sections larger than 10^(-2) mb down to Z=8. Their individual production cross sections and recoil velocities at the five energies are presented. Production cross sections are compared with previously existing data and with empirical parametric formulas, often used in cosmic-ray astrophysics. The experimental data are also extensively compared with the results of different combinations of intranuclear cascade and deexcitation models. It is shown that the yields of the lightest isotopes cannot be accounted for by standard evaporation models. The GEMINI model, which includes an asymmetric fission decay mode, gives an overall good agreement with the data. These experimental data can be directly used for the estimation of composition modifications and damages in materials containing iron in spallation sources. They are also useful for improving high-precision cosmic-ray measurements.

Isotopic Yield Distributions of Transfer- and Fusion-Induced Fission from 238U+12C Reactions in Inverse Kinematics

A novel method to access the complete identification in atomic number Z and mass A of fragments produced in low-energy fission of actinides is presented. This method, based on the use of multi- nucleon transfer and fusion reactions in inverse kinematics, is applied in this work to reactions between a 238U beam and a 12C target to produce and induce fission of moderately excited actinides. The fission fragments are detected and fully identified with the VAMOS spectrometer of GANIL, allowing the measurement of fragment yields of several hundreds of isotopes in a range between A ~ 80 and ~ 160, and from Z ~ 30 to ~ 64. For the first time, complete isotopic yield distributions of fragments from well-defined fissioning systems are available. Together with the precise measurement of the fragment emission angles and velocities, this technique gives further insight into the nuclear-fission process.

Production of medium-mass neutron-rich nuclei in reactions induced by Xe 136 projectiles at 1 A GeV on a beryllium target

Production cross sections of medium-mass neutron-rich nuclei obtained in the fragmentation of {sup 136}Xe projectiles at 1 A GeV have been measured with the FRagment Separator (FRS) at GSI. The measured cross sections are compared to {sup 238}U fission yields and model calculations to determine the optimum reaction mechanism to extend the limits of the chart of the nuclides around the r-process waiting point at N=82.

Transfer reactions in inverse kinematics: An experimental approach for fission investigations

Inelastic and multinucleon transfer reactions between a U-238 beam, accelerated at 6.14 MeV/u, and a C-12 target were used for the production of neutron-rich, fissioning systems from U to Cm. A Si telescope, devoted to the detection of the targetlike nuclei, provided a characterization of the fissioning systems in atomic and mass numbers, as well as in excitation energy. Cross sections and angular and excitation-energy distributions were measured for the inelastic and transfer channels. Possible excitations of the targetlike nuclei were experimentally investigated for the first time, by means of gamma-ray measurements. The decays from the first excited states of 12C, B-11, and Be-10 were observed with probabilities of 0.12-0.14, while no evidence for the population of higher-lying states was found. Moreover, the fission probabilities of U-238, Np-239 and Pu-240,Pu-241,Pu-242 and Cm-244 were determined as a function of the excitation energy.

Experimental investigation of the residues produced in the 136Xe + Pb and 124Xe + Pb fragmentation reactions at 1A GeV

The nuclide cross sections and longitudinal velocity distributions of residues produced in the reactions of {sup 136}Xe and {sup 124}Xe at 1A GeV in a lead target were measured at the high-resolution magnetic spectrometer, the fragment separator (FRS) of GSI. The data cover a broad range of isotopes of the elements between Z=3 and Z=56 for {sup 136}Xe and between Z=5 and Z=55 for {sup 124}Xe, reaching down to cross sections of a few microbarns. The velocity distributions exhibit a Gaussian shape for masses above A=20, while more complex behavior is observed for lighter masses. The isotopic distributions for both reactions preserve a memory on the projectile N/Z ratio over the whole residue mass range.

Proton-proton correlations observed in two-proton decay of 19Mg and 16Ne

Proton-proton correlations were observed for the two-proton decays of the ground states of 19 Mg and 16 Ne. The trajectories of the respective decay products, 17 Ne + p + p and 14 O + p + p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.

Transient and quasi-stationary dissipative effects in the fission flux across the barrier in 1/A-GeV U-238 on deuterium reactions

Isotopic cross sections of all projectile residues with Z above 23 produced in collisions induced by 238U at 1 A GeV on deuterium have been measured. The isotopic distributions reflect the role of evaporation and fission in the formation process of these nuclei. The comparison of the measured cross sections with Monte-Carlo de-excitation codes including an analytical description of the dynamics of fission shows the sensitivity of the data to nuclear dissipation. Moreover, the large excitation-energy range covered in this experiment together with the high accuracy of the measured cross sections allowed to clearly separate and quantify the role of transient and quasi-stationary dissipative effects in the fission-decay width.