Reaction mechanisms in the 40Ar + 197Au collisions at 35 MeV/nucleon

doi:10.1016/0375-9474(86)90372-6

Nuclear Physics A

Volume 456, Issue 1, 4 August 1986, Pages 173-185

https://doi.org/10.1016/0375-9474(86)90372-6 Get rights and content

Abstract

The fission fragment angular correlation has been measured in the Ar + Au reaction at 35 MeV/nucleon. Two components are clearly separated: the sequential fission of the target (σ = 320 ± 80 mb) and a fusion-like process (σ = 480 ± 120 mb). The linear momentum transfer per nucleon for the fusion-like events reaches the limitation of 190 MeV/c observed in earlier studies. This limitation does not reflect a limit to the excitation energy that a nucleus can contain. Heavy residues with energies larger than 0.13 $MeV nucleon$ have been measured in the forward region. This observation of residues with high excitation energies implies the existence of a competition between fission and evaporation at excitation energies as high as ∼800 MeV. Particles and light nuclei have been detected in a forward multidetector in coincidence with two fission fragments or a heavy residue.

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Formation of light particles in nucleus-nucleus collisions at low energies
1995, Progress in Particle and Nuclear Physics
The principal experimental results on the yield of light charged particles in nucleus-nucleus collisions at low and intermediate energies are reviewed. Inclusive spectra of light particles as well as semi-inclusive obtained in coincidence with characteristic KX - rays, γ - rays, neutrons, projectile-like fragments, other light particles, fission fragments, and evaporation residues are analyzed. The main theoretical models used for the description of light particle formation are briefly outlined together with their merits and shortcomings. The unsolved problems of fast light particle formation, in particular, and of nucleus-nucleus interaction dynamics, on the whole, are discussed with the outlooks of new experiments clearing up some of these promlems.
Hot-nuclei formation and decay: The Ar + Ag system at 50 and 70 MeV u
1994, Nuclear Physics, Section A
Hot-nuclei fusion properties have been studied for the Ar + Ag system at 50 and 70 MeV/u bombarding energy. The set up was very well suited to the analysis of sequential evaporation leading to an evaporation residue since the residue was detected in coincidence with all the corresponding light charged particles by use of a 4π- multidetector. An event by event analysis was performed in order to distinguish between particles evaporated from the fusion nucleus and others. It was possible to reconstruct the initial excitation energy of the fusion nucleus by adding the contributions of all the charged decay products and by estimating the neutron contribution. It was found that the excitation energy distributions obtained at 50 and 70 MeV/u were quite similar. Very large values are reached in both cases (∼ 600 MeV). This result means that the standard statistical decay observed at limited excitation is still observed for very hot nuclei. The corresponding temperatures were measured from the kinetic-energy spectra slopes. Slightly larger values were observed at 70 MeV/u. This result is consistent with a slightly smaller mass of the fusion nucleus obtained in this case. In a last aspect of the experiment where many particles events were analysed in the 4π set up, it turned out that the highest involved excitation energies can lead either to a multifragment emission, or to a standard sequential statistical decay.
Evolution of fission and competing decay mechanisms in 40Ar-induced reactions on Au, Th at 27-77 MeV/u of incident energy
1994, Nuclear Physics, Section A
Fission fragments, heavy reaction residues and the multiplicity of all simultaneously evaporated neutrons are observed from the reactions ⁴⁰Ar + Au, Th over a wide range of incident ⁴⁰Ar energy from 27 to 77 MeV/u. The inclusive neutron-multiplicity distributions prove the invariance of energy dissipation with incident energy in these reactions: peripheral collisions with low dissipation and central collisions with high dissipation contribute invariably about half of the reaction cross section. Two fragment correlations and fragment angular distributions show that fission is, for energies in excess of 35 MeV/u, progressively escorted by the emission of lighter complex particles, while the importance of fission generally fades away in favour of very heavy-residue production and, to a smaller extent yet in the considered energy range, eventually also in favour of true multifragmentation.
Reaction-mechanism evolution for the system 20Ne + 60Ni at intermediate energies: from massive transfer to fragmentation
1993, Nuclear Physics, Section A
Mass and charge distributions for heavy residues in the reaction ²⁰Ne + ⁶⁰Ni at 50 MeV/nucleon were measured by in-beam and off-line γ-ray spectrometry. The stacked foil method was used to obtain information about the distribution of the velocity component parallel to the beam direction for target-like residues. The comparison of the data to the predictions of a participant-spectator model indicates that an 8% width for the dissipated energy distribution accounts for the observed projected ranges.
Multifragment decay of highly excited nuclei
1993, Nuclear Physics, Section A
Coincidences between at least three fragments emitted at large angle have been used to study central collisions in the Ar+Au reactions at 30 and 60 MeV/u incident energies. For both energies, the formation of equilibrated very hot nuclei is evidenced. Average excitation energy as high as 1.2 GeV is reached in the reaction at 60 MeV/u. The increase of the excitation energy deposited for the upper bombarding energy is confirmed by the correlated increases of the nuclear temperature and of the evaporated light-particle multiplicity.
Production and deexcitation of hot nuclei in collisions of 27 MeV/nucleon 40Ar with 238U
1990, Nuclear Physics, Section A
Hot nuclei, produced in the reaction 1080 MeV ⁴⁰Ar+²³⁸U, have been studied by means of the light charged particles emitted in coincidence with fission fragments. A dominant fraction of the light charged particles emitted in the backward hemisphere (60%, 75%, 80% and 85% for ^1,2,3H and ⁴He, respectively) comes from evaporation from a composite nucleus prior to scission. Careful analysis of the cross sections and energy spectra of these evaporated particles yields several properties of the hot nuclear emitters: (i) they are thermalized with an average temperature of 4.0–4.6 MeV, (ii) they are quite deformed (mean axis ratio $≈ 21$ ) and are rapidly spinning ( $J_{r.m.s. of} 100–140 h ̵ MeV$ ). Predictions from a dynamical model, based on the Landau-Vlasov equation, are consistent with the experimental results, and give insights into the time evolution of the fused system

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^☆: This experiment has been carried out at the GANIL National Laboratory.

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Nuclear Physics A

Abstract

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Cited by (18)

Formation of light particles in nucleus-nucleus collisions at low energies

Hot-nuclei formation and decay: The Ar + Ag system at 50 and 70 MeV u

Evolution of fission and competing decay mechanisms in <sup>40</sup>Ar-induced reactions on Au, Th at 27-77 MeV/u of incident energy

Reaction-mechanism evolution for the system <sup>20</sup>Ne + <sup>60</sup>Ni at intermediate energies: from massive transfer to fragmentation

Multifragment decay of highly excited nuclei

Production and deexcitation of hot nuclei in collisions of 27 MeV/nucleon <sup>40</sup>Ar with <sup>238</sup>U

Reaction mechanisms in the 40Ar + 197Au collisions at 35 MeV/nucleon☆

Abstract

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Reaction mechanisms in the ⁴⁰Ar + ¹⁹⁷Au collisions at 35 MeV/nucleon☆