Light charged particle emission in 485 MeV 56Fe + 197Au reactions: Correlations with heavy fragments and relationships to spin and lifetime

doi:10.1016/0375-9474(87)90438-6

Nuclear Physics A

Volume 465, Issue 2, 6 April 1987, Pages 339-364

https://doi.org/10.1016/0375-9474(87)90438-6 Get rights and content

Abstract

Emission of ⁴He and ¹H has been studied in reactions of 485 MeV ⁵⁶Fe + ¹⁹⁷Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the ⁴He/¹H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed ⁴He/¹H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for ₄He and ¹H are deduced for each source of emission, and are compared with results from similar systems. The experimental ⁴He/¹H multiplicity ratios indicate roughly equal probability for ⁴He and ¹H evaporation from a given excited source. The observation of pre-scission ⁴He/¹H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable ⁴He/¹H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

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Light charged particles (LCP) have been measured for the reaction 344 MeV $^{28}$ Si $+^{121}$ Sb in singles and in coincidence with evaporation residues (ER), fusion–fission fragments (FF), and other LCP. A major feature of this experiment was the use of a gas-filled magnetic spectrometer in the forward direction to separate ER from the much more abundant yield of elastically scattered projectiles and projectile-like fragments. The dominant sources of evaporative $^{1}$ H and $^{4}$ He emission are the ER (approximately 75%), with the remainder being largely associated with fission reactions. For these latter reactions, most of the $^{1}$ H and $^{4}$ He can be well accounted for by evaporation from the composite system prior to fission and by evaporation from the postfission fragments. LCP emission cross sections were determined for each identified source, and a comparison has been made to previous studies. From this comparison, indications were found for significant entrance channel effects, with the more asymmetric channels exhibiting much larger LCP cross sections. Statistical model predictions for ER emissions are in good agreement with observed LCP energy spectra, angular distributions, and integrated inclusive and exclusive cross sections, with all calculations using the same unique set of model parameters. This result contrasts strongly with recent reports for light mass systems, where model calculations were unable to simultaneously reproduce all observables.
Mechanisms of light-charged-particle emission in the matched reactions 905 and 1030 MeV 121Sb + 27A1 and 550 and 750 MeV 86Kr + 63Cu
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Emission of ⁴He and ¹H has been studied in the reactions 905 and 1030 MeV ¹²¹Sb + ²⁷Al and 550 and 750 MeV ⁸⁶Kr + ⁶³Cu, which lead to similar composite systems of ¹⁴⁹Tb^∗ (or ¹⁴⁸Gd^∗). Energy spectra and angular distributions were measured for the light charged particles in coincidence with heavy fragments from fusion-fission (FF) and deeply inelastic reactions (DIR). These emissions are well described by three primary sources: the detected and partner (undetected) fragments (FE), and the composite system prior to scission (CE). The multiplicities derived for ¹H and ⁴He in coincidence with fission-like fragments or projectile-like fragments are much smaller than those associated with evaporation residues (ER). The multiplicity data indicate that the probability for CE emission decreases with increasing spin of the emitter. The characteristics of the CE particles detected in coincidence with DIR and FF are consistent with emission from a strongly distorted nuclear system en route toward scission. Strong evidence is presented confirming near-scission emission (NSE) as an additional source of charged particles. Comparisons of particle multiplicities for matched reactions revealed substantial differences in the FF and DIR processes which can be understood as resulting from different spin zones in the entrance channels.
Pre-scission charged-particle multiplicities following the reactions 164,167,170Er + 28Si
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Proton and α-particle multiplicities associated with fission have been measured following the reactions ^164,167,170Er + ²⁸Si at energies between 140 and 185 MeV. The multiplicities at the lowest energies place limits on the level-density parameters. They also increase with energy much more rapidly than those calculated with a statistical model and delays to fission are essential to reproduce the observed behaviour. It is not possible to obtain fits to neutron, proton and alpha-multiplicities using only a pre-saddle delay to fission. However, a consistent picture emerges if a saddle-to-scission delay is also used. The resultant pre-saddle delay is ⩽ 10 × 10⁻²¹ s and implies the fissioning nucleus spends most of the time (∼ 50 × 10⁻²¹ s) at deformation considerably larger than the equilibrium value.
Heavy-ion-induced fission - experimental status
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The process of fission of a composite nucleus produced in heavy-ion collisions is characterized in terms of the experimental kinetic-energy, mass and angular distributions of the fission fragments. Measurements of associated particles, and possibly of γ-rays, emitted prior to scission demonstrate that heavy-ion fission is an inherently slow (~10⁻²⁰ – 10⁻¹⁹ s) process that is dominated by overdamped collective motion in the exit channel. Experimental fission excitation functions are consistent with the fission barriers of the finite-range liquid-drop model. While fission fragment angular distributions are well described in terms of the transition-state model, for relatively small angular momenta and excitation energies, the model is inapplicable for composite nuclei with vanishing effective barriers and for high spins and excitation energies. Angular anisotropies for these latter systems suggest considerable relaxation of the tilting mode.
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^☆: Based in part on the Ph.D. Thesis of D.J.M., Department of Chemistry, Carnegie-Mellon University, 1986.

¹: Present address: Department of Chemistry, Middle East Technical University, Ankara, Turkey.

²: Present address: Instituto di Fisica Sperimentale, Mostra D'Oltremare, Napoli 80125, Italy.

³: Present address: Bhabha Atomic Research Centre, Nuclear Physics Division, Trombay 400085 Bombay, India.

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Nuclear Physics A

Abstract

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Studies of light charged particle emission from fission and ER reactions in the system 344 MeV <sup>28</sup>Si + <sup>121</sup>Sb → <sup>149</sup>Tb (E* = 240 MeV)

Mechanisms of light-charged-particle emission in the matched reactions 905 and 1030 MeV <sup>121</sup>Sb + <sup>27</sup>A1 and 550 and 750 MeV <sup>86</sup>Kr + <sup>63</sup>Cu

Pre-scission charged-particle multiplicities following the reactions <sup>164,167,170</sup>Er + <sup>28</sup>Si

Heavy-ion-induced fission - experimental status

Heavy-ion break-up processes in the Fermi energy range

Light charged particle and intermediate mass fragment emission in the reaction 640 MeV Kr86+63Cu

Light charged particle emission in 485 MeV 56Fe + 197Au reactions: Correlations with heavy fragments and relationships to spin and lifetime☆

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Light charged particle emission in 485 MeV ⁵⁶Fe + ¹⁹⁷Au reactions: Correlations with heavy fragments and relationships to spin and lifetime☆