E. Duek

Fission and emission of H and He in the reactions of 215 MeV16O with181Ta,208Pb and238U

The reactions of 215 MeV16O with12C,181Ta,208Pb and238U have been studied. Inclusive measurements for4He emission are given from each target, and for fission and1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for4He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of16O with12C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He 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 observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He 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 average4He 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.

GANES: A computer program for the simulation of laboratory spectra of evaporative light particles in heavy ion reactions

Abstract A computer program called GANES has been written to simulate the evaporation of light particles in heavy ion reactions. The program uses the very efficient “weighted Monte Carlo” method. GANES has been designed to treat two-body breakup reactions such as fusion fission and three-body breakup reactions such as sequential fission (of a heavy product) after a primary inelastic collision. GANES calculates energy spectra for light particles evaporated from a composite nucleus before scission, and also from fully accelerated fragments after scission. Mass, energy and angular distributions of the heavy fragments are taken into account. Energy spectra (inclusive and exclusive) of the evaporated particles are calculated by a semiclassical formulation for detectors of finite size at a number of laboratory directions, both in and out of the reaction plane. Detector thresholds and various coincidence gates are also considered. GANES has been used to identify the emission sources of evaporative particles and to determine their multiplicities. It has also been used to study the thermodynamic properties of the emitters, such as effective emission barrier, temperature, moment of inertia and root-mean-square spin.

Products of 3≦Z≦9 from long-lived intermediates in the reactions 336 MeV40Ar+Ag,154Sm and Au

Angular and energy distributions have been measured for products of 3≦Z≦9 from reactions of 336MeV40Ar+107,9Ag/154Sm/197Au. The c.m. differential cross sections and mean energies increase as one moves from ≈100deg. to ≈170deg. similar to those for fission or evaporation-like emission. Most characteristics mimic those observed for4He, but the cross sections are smaller by a factor of as ≈1/300 for each reaction studied. A semiclassical analysis has been used to estimate the effective barriers to emission and root-mean-square spins of the emitters.

New probe of intermediate reaction times: Near-scission emission of 4He for 334 MeV 40Ar + 238U

Abstract We have investigated the emission of 4 He in coincidence with mass-selected fragments from fission for 334 MeV 40 Ar + 238 U. Upper-limit multiplicities are assigned for fragment evaporation and composite-nucleus evaporation, but additional mechanisms are also required. A new low-energy component is found at backward angles; it is sensitive to the fragment masses but does not reflect their full kinematic shifts. Thus it must be due to near-scission emission (prior to full fragment acceleration).

Correlations between fission fragments for 172 MeV20Ne+197Au: A case study for incomplete fusion

Angular and energy correlations have been measured for coincident fission-fragment pairs from the reaction of197Au with 8.6 MeV/u20Ne. These data have been analyzed to test for fission after incomplete fusion and its admixture with fission generally attributed to the relatively pure compound-nucleus mechanism. A Monte Carlo kinematic simulation program has been written to provide a basis for detailed comparisons of the experimental data to the calculations that employ various mechanistic assumptions. We conclude that incomplete fusion is indeed a prominent precursor to fission even for incident energies of less than 10 MeV/u. Similar data from earlier studies have been reanalyzed and shown to be consistent with this conclusion.

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

Multiplicities are compared for4He evaporation in reactions of40Ar and56Fe projectiles (8.5 MeV/u) with withnatAg and238U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like4He emission from the composite nuclei is shown to be large for mean spins ≦100ℏ, and to decrease dramatically with increasing spin. Coincidence measurements between two4He particles have also been studied for40Ar+natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the4He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.

Coincidence studies of heavy fragment pairs in the reaction 343 MeV40Ar+197Au

Measurements are reported for correlated energies and angles of heavy fragments from reactions of 343 MeV40Ar with197Au. The results show a continuous evolution of the mass distributions from fission-like products at angle pairs of ≈60 and 300 degrees to deeply inelastic projectile-like products at ≈45° and 300°. Certain difficulties are emphasized for the use of the “symmetric fragmentation” cross sections in the context of current theoretical models (e.g. the “extra push” theory). The fission-like reactions have been studied in some detail and the results have been compared to a Monte Carlo kinematic simulation. To account for the experimental results forward-peaked light particle emission (n, H, He) must occur in a large fraction of the fission-like reactions.

Blurred images of complete fusion and incomplete fusion: Reaction simulation analysis of fission-fragment angular correlations

We use Monte Carlo reaction simulation calculations to reassess several studies of fissionfragment angular correlations. The reactions chosen are 15–30 MeV/nucleon14N +238U and 14.6 MeV/nucleon20Ne +197Au. Reanalysis by means of these stimulations leads to much lower limits than originally reported for the fraction of complete fusion. We infer that at these energies the probability is very large for the direct emission of at least one light particle prior to fission. This implies that the distinction between “complete fusion” and “incomplete fusion” has become blurred beyond meaningful distinction.