C.O. Bacri

INDRA, a 4π charged product detection array at GANIL

Abstract INDRA, a new and innovative highly segmented detector for light charged particles and fragments is described. It covers geometrically 90% of the 4π solid angle and has very low detection thresholds. The detector, operated under vacuum, is axially symmetric and segmented in 336 independent cells allowing efficient detection of high multiplicity events. Nucleus identification down to very low energy threshold (≈ 1 A MeV) is achieved by using ionization chambers operated with low pressure C 3 F 8 gas. Residual energies are measured by a combination of silicon (300 μm thick) and cesium iodide (5 to 14 cm in length) detectors. Very forward angles are covered by fast counting phoswich scintillators (NE102/NE115). Charge resolution up to Z = 50 is achieved on a large energy dynamic range (5000 to 1 for silicon detectors). Isotopic separation is obtained up to Z = 3. The treatment of the signals is performed through specifically designed and highly integrated modules, most of which are in the new VXIbus standard. Full remote control of parameter settings, including visualization of signals, is thus allowed. The detector is continuously monitored with a laser source and electronic pulsers and is found stable over several days. Energy calibration procedures, making use of specific detectors and the ability of the GANIL accelerator to deliver secondary beams, have been developed. First experiments were performed in the spring of 1993.

Determination of the 13N(p, γ)14O reaction rate through the Coulomb break-up of a 14O radioactive beam

Abstract In stellar-evolution models, the 13N(p, γ)14O reaction plays an important role in the hot CNO cycle. Its reaction rate depends directly on the magnitude of the radiative width of the 5.17 MeV level in 14O. That width has been measured using the Coulomb break-up technique. A 70 MeV u 14O beam was excited in the Coulomb field of a 208Pb target, the 13N and proton fragments being recorded using a magnetic spectrometer and CsI scintillators, respectively. The experimental value Γγ = 2.4 ± 0.9 eV is in overall agreement with other recent 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.

Isotopic dependence of the nuclear caloric curve

The A/Z dependence of projectile fragmentation at relativistic energies has been studied with the ALADIN forward spectrometer at SIS. A stable beam of (124)Sn and radioactive beams of (124)La and (107)Sn at 600 MeV per nucleon have been used in order to explore a wide range of isotopic compositions. Chemical freeze-out temperatures are found to be nearly invariant with respect to the A/Z of the produced spectator sources, consistent with predictions for expanded systems. Small Coulomb effects (DeltaT approximately 0.6 MeV) appear for residue production near the onset of multifragmentation.

Characterization of the scission point from fission-fragment velocities

The isotopic-yield distributions and kinematic properties of fragments produced in transfer-induced fission of 240Pu and fusion-induced fission of 250Cf, with 9 MeV and 45 MeV of excitation energy respectively, were measured in inverse kinematics with the spectrometer VAMOS. The kinematic properties of identified fission fragments allow to derive properties of the scission configuration such as the distance between fragments, the total kinetic energy, the neutron multiplicity, the total excitation energy, and, for the first time, the proton- and neutron-number sharing during the emergence of the fragments. These properties of the scission point are studied as functions of the fragment atomic number. The correlation between these observables, gathered in one single experiment and for two different fissioning systems at different excitation energies, give valuable information for the understanding and modeling of the fission process.

Multifragmentation in Xe(50 AMeV) + Sn: Confrontation of theory and data

Abstract We compare in detail central collisions Xe(50 AMeV) + Sn, recently measured by the INDRA collaboration, with the Quantum Molecular Dynamics (QMD) model in order to identify the reaction mechanism which leads to multifragmentation. We find that QMD describes most of the data quite well, in the projectile/target region as well as in the midrapidity zone where also statistical models can be and have been employed. The agreement between QMD and data allows us to use this dynamical model to investigate the reaction in detail. We arrive at the following observation: (a) the in-medium nucleon-nucleon cross section is not significantly different from the free cross section, (b) even the central collisions have a binary character, (c) most of the fragments are produced in the central collisions, (d) the simulations as well as the data show a strong attractive in-plane flow resembling deep inelastic collisions, and (e) at midrapidity the results from QMD and those from statistical model calculations agree for almost all of the observables with the exception of d 2 σ dZ dE . This renders it difficult to extract the reaction mechanism from midrapidity fragments only. According to the simulations the reaction shows a very early formation of fragments, even in central collisions, which pass through the reaction zone without being destroyed. The final transverse momentum of the fragments is very close to the initial one and due to the Fermi motion. A heating up of the systems is not observed and hence a thermal origin of the spectra cannot be confirmed. The disagreement between the simulations and the data for some observables is presumably due to a force range which is too large as compared to reality but necessary to keep the nuclei stable in these semiclassical approach.

Peripheral collisions with 200 MeV/nucleon krypton ions

Abstract Using a magnetic spectrometer, a precision measurement of N/Z distributions and fragment velocity distributions was performed at 0.6° and 1.5° in peripheral collisions of 200 MeV / nucleon 84Kr on a gold target. The data are compared with both the participant-spectator model and the intranuclear cascade model. It is shown that the primary fragments underwent substantial excitation, which cannot be accomodated with the cold fragmentation.

N/Z DEPENDENCE OF PROJECTILE FRAGMENTATION

The N/Z dependence of projectile fragmentation at relativistic energies has been studied in a recent experiment at the GSI laboratory with the ALADiN forward spectrometer coupled to the LAND neutron detector. Besides a primary beam of 124Sn, also secondary beams of 124La and 107Sn delivered by the FRS fragment separator have been used in order to extend the range of isotopic compositions of the produced spectator sources. With the achieved mass resolution of ΔA/A ≈ 1.5%, lighter isotopes with atomic numbers Z ≤ 10 are individually resolved. The presently ongoing analyses of the measured isotope yields focus on isoscaling and its relation to the properties of hot fragments at freeze-out and on the derivation of chemical freeze-out temperatures which are found to be independent of the isotopic composition of the studied systems. The latter result is at variance with the predictions for limiting temperatures as obtained with finite-temperature Hartree-Fock calculations.

Mass and Isospin Effects in Multifragmentation

A systematic study of isospin effects in the breakup of projectile spectators at relativistic energies has been performed with the ALADiN spectrometer at the GSI laboratory (Darmstadt). Four different projectiles 197Au, 124La, 124Sn and 107Sn, all with an incident energy of 600 AMeV, have been used, thus allowing a study of various combinations of masses and N/Z ratios in the entrance channel. The measurement of the momentum vector and of the charge of all projectile fragments with Z>1 entering the acceptance of the ALADiN magnet has been performed with the high efficiency and resolution achieved with the TP-MUSIC IV detector. The Rise and Fall behavior of the mean multiplicity of IMFs as a function of Zbound and its dependence on the isotopic composition has been determined for the studied systems. Other observables investigated so far include mean N/Z values of the emitted light fragments and neutron multiplicities. Qualitative agreement has been obtained between the observed gross properties and the predictions of the Statistical Multifragmentation Model.

Can we really measure the internal energy of hot nuclei with a 4π detection array

The second generation of high-quality detection arrays gave hope to nuclear physicists to finally obtain an experimental equation of state of nuclear matter. In spite of this progress, the measurement of the internal energy of a hot nucleus remains a very difficult task. This paper illustrates this difficulty by a methodological study of a classical technique of excitation energy measurement used in the Fermi energy range. The aim of this study is to verify the validity, the accuracy and the experimental limits of these measurements. It is shown that it is difficult to have a real experimental mastery of the source reconstruction and calorimetry at least for limited bombarding energies and violent collisions.