Reaction mechanism analyses performed with a 4pi detector for the systems 208Pb + Ge, 238U + Ni and 238U + Ge, combined with analyses of the associated reaction time distributions, provide us with ...evidence for nuclei with Z=120 and 124 living longer than 10(-18) s and arising from highly excited compound nuclei. By contrast, the neutron deficient nuclei with Z=114 possibly formed in 208Pb + Ge reactions have shorter lifetimes, close to or below the sensitivity limit of the experiment.
A model is proposed to describe emission of light charged clusters during the cascade stage in nucleon-induced spallation reactions. It consists in implementing a surface percolation procedure into ...the Liège intranuclear cascade (INCL4) model: when a nucleon is ready to leave the nuclear surface, it is allowed to drag along a cluster of nucleons, under some conditions of closeness in phase space. This possibility relies on the instantaneous dynamical phase space occupancy in the nuclear surface. The following clusters are considered:
d,t,
3
He,
4
He
. Good agreement is obtained with experimental data relative to heavy and medium-heavy targets at two different energies. It is shown that the implementation of light cluster emission in the cascade stage also improves our previous results for residue mass spectra.
The fusion-fission cross sections of the 4He+238U and 6He+238U systems have been measured, at Louvain-la-Neuve, for energies around and below the Coulomb barrier, using an array of Si detectors ...surrounding a UF4 target. The data taken with 4He are in good agreement with previous data and with the coupled channel fusion calculation performed with ECIS. The 6He data show a regular trend with a large enhancement below the barrier which is attributed to the halo structure of the 6He nucleus.
The intranuclear cascade (INC) plus evaporation model for nucleon-nucleus interactions in the 200 MeV–1.2 GeV range is revisited. The standard Liège INC model supplemented by the Dresner ...evaporation-fission model is used and shown to give a good overall agreement with experimental data, basically neutron double differential cross-sections and residue mass spectra. Small systematic discrepancies are identified and shown to correspond to the single scattering contribution. Improvements of the INC model, linked with in-medium cross-sections, refraction of particles at the nuclear surface, treatment of the Pauli blocking, description of nuclear surface, collision mode and hadronisation time are discussed. The problem of the stopping time at which the cascade model gives place to the evaporation model is examined in detail. The introduction of a pre-equilibrium step, intermediate between the cascade and evaporation steps is investigated and shown to bring no significant improvement of the predictions. The production of composites and of intermediate mass fragments is shortly studied, with the help of the standard model, of the three step model mentioned above and of a cascade plus percolation model.
An extensive study of GeV light-ion-induced multifragmentation and its possible interpretation in terms of a nuclear liquid–gas phase transition has been performed with the Indiana Silicon Sphere ...(ISiS)
4
π
detector array. Measurements were performed with 5–15
GeV/
c
p
,
p
¯
, and
π
-
beams incident on
197Au and 2–5
GeV
3He incident on
natAg and
197Au targets. Both the reaction dynamics and the subsequent decay of the heavy residues have been explored. The data provide evidence for a dramatic change in the reaction observables near an excitation energy of
E
*
/
A
=
4
–
5
MeV
/
residue
nucleon
. In this region, fragment multiplicities and energy spectra indicate emission from an expanded/dilute source on a very short time scale (20–50
fm/
c). These properties, along with caloric curve and scaling-law behavior, yield a pattern that is consistent with a nuclear liquid–gas phase transition.
Around 270 medium-mass residual nuclei, formed in spallation reactions induced by 136Xe projectiles impinging on a liquid hydrogen target at 500 A MeV, have been unambiguously identified at GSI using ...the magnetic spectrometer FRS. The individual production cross sections and the longitudinal momentum distributions have been determined with high accuracy. These data represent an important constraint for theoretical models describing spallation reactions.
The “Fermi shuttle” acceleration of electrons in ion–atom collisions, i.e. multiple collision sequences of electrons bouncing off the projectile and target nuclei, can lead to the emission of very ...energetic electrons (i.e. with velocities higher than the binary encounter electrons). We performed measurements of the evolution of the Fermi shuttle electron yield with the induced perturbation and the target atomic number. The yield increases with the perturbation parameter (ratio of projectile charge and projectile velocity q/vP), which was varied from q/vP≈0.2 (weak perturbation) to q/vP≈2 (strong perturbation). We also introduce a more realistic scaling parameter, which accounts for the re-bouncing of the electrons on target and projectile, and show that the yields increase as a function of this parameter. For a given projectile, the Fermi shuttle electron yield increases with the target atomic number. Furthermore, we show that the velocity distribution of the high-energy electrons is exponentially decreasing N(v)∼exp(−nv) and exhibits the same evolution of the slope n with projectile velocity as in the case of Fermi accelerated deuterons.
Ar + Ni and Ni + Ni collisions are investigated between 32 and around 100A MeV incident energy with the 4π multidetector INDRA. Fusion cross-sections are found to decrease from ˜ 180mb at 32A MeV to ...zero above 50A MeV. Other experimental results, for light systems, are compared. Moreover, theoretical works are discussed and fusion cross-sections, calculated from two dynamical simulations based on nuclear Boltzmann equation (Boltzmann-Nordheim-Vlasov and Landau-Vlasov models), are also compared to experimental results.
INDRA, a 4π charged product detection array at GANIL Pouthas, J.; Borderie, B.; Dayras, R. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/1995, Letnik:
357, Številka:
2
Journal Article
Recenzirano
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
3F
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.
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