Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Abstract

Emission of ~He in the reaction 334MeV 4~ has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the 4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed 4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the 4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average 4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the "fast fission" or "quasifission" idea, which has attracted much current attention.

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

“…This fact, together with the results of [12,13] implies that the probability of ternary fission increases with the excitation energy of the fissioning system.…”

“…The geometrical factors were taken from the reaction simulation, as it was done in [12]. The multiplicities calculated for the 12C+197Au system are: McN E=0.025_+0.006 for alpha evaporation from the compound nucleus and MFFE=0.008 0.003 for the fragment emission.…”

Field emission of ? particles during scission of161Ho,169ta compound nuclei

“…From the energetic position of both, the deficit and surplus anomaly and the obvious correlation with the scission axis we infer that both are related and have to be attributed to a near-scission emission process. A surplus anomaly has also been observed in the c~ emission of 8.4 MeV/n 4~ [4] and 10 MeV/n 35Cl+natAg [-5] and was interpreted as near-scission emission.…”

“…From previous experiments [- 1,3,4,5,19] it is known that the spectra are dominated by sequential light particle emission from composite nucleus (CNE) and from fully accelerated fission fragments (FE). In a standard 3-source calculation [5,20,21] the reaction kinematics were defined by the measured velocity distribution of the fragments; the energy distribution of the primary fragments was parametrized by a Gaussian distribution, the mean and standard deviation of which were taken from the measured distributions.…”

Multiplicities for evaporative4He emission in heavy ion reactions: Relationships to spin and lifetime expectancy of the composite nucleus

“…In this paper we report the results of such multiplicity determinations for composite-nucleus evaporation (CE) and fragment evaporation (FE). We examine the behaviour of four reaction systems each with incident laboratory energy of 8.5 MeV/u: 4~ +"atAg [26], 56Fe+natAg [27], 4~ [28], 56Fe -~-238U [29], Our main purpose is to compare the multiplicities for evaporative processes and to preview the kind of information that can be obtained from such investigations. We will only outline the kinematic analysis and defer the details to forthcoming papers [26][27][28][29].…”