M. S. Zisman

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.

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.

Studies of4He emission in both fusion-like and inelastic reactions of 340-MeV40Ar+238U

The fission-fragment angular correlation technique has been used in conjunction with a position-sensitive avalanche detector to identify the fusion-like and inelastic collisions of 340-MeV40Ar+238U. For each of these two reaction types, we have characterized the correlated4He emission by measuring three-fold coincidences between two fission fragments and the light charged particle. We find an abundance of both evaporation-like and direct4He emission, each of which is associated with both fusion-fission and sequential-fission processes. Several comparison tests of both the coincidence and singles data strongly indicate that very little4He emission is due to evaporation from accelerated fission fragments in this system. The evaporative4He emission appears instead to have a strong contribution originating from the composite system prior to fission. The forward angle4He emissions, characterized by prominent forward-peaking and relatively high energies, suggest an origin associated with the very early stages of the collision.

Masses of the unbound nucleiNe16,F15, andO12

The (/sup 4/He,/sup 8/He) and (/sup 3/He,/sup 8/Li) reactions have been employed at detection angles near 8/sup 0/ on gas targets of /sup 20/Ne and /sup 16/O to produce and measure the masses of the nuclei /sup 16/Ne, /sup 15/F, and /sup 12/O. The (/sup 4/He, /sup 8/He) reactions were performed at an incident energy of 117 MeV, and the (/sup 3/He, /sup 8/Li) reaction was done at 88 and 75 MeV. The mass excesses of /sup 16/Ne, /sup 15/F, and /sup 12/O were determined to be 23.92 +- 0.08, 16.67 +- 0.18, and 32.10 +- 0.12 MeV, respectively. Estimated ground state decay widths were GAMMA/sub c.m./ = 0.2 +- 0.1, 0.8 +- 0.3, and 0.40 +- 0.25 MeV, for /sup 16/Ne, /sup 15/F, and /sup 12/O, respectively. A d coefficient of 8 +- 5 keV is indicated for the isobaric multiplet mass equation description of the mass-16 multiplet. Consideration of the possible decay mechanisms of /sup 16/Ne and /sup 12/O indicates that both nuclei probably have an appreciable diproton decay width.

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.

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

Abstract Emission of 4 He and 1 H has been studied in reactions of 485 MeV 56 Fe + 197 Au, 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 4 He/ 1 H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed 4 He/ 1 H 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 4 He and 1 H are deduced for each source of emission, and are compared with results from similar systems. The experimental 4 He/ 1 H multiplicity ratios indicate roughly equal probability for 4 He and 1 H evaporation from a given excited source. The observation of pre-scission 4 He/ 1 H 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 4 He/ 1 H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of4He and1H has been measured in coincidence with fission for reactions of 469-MeV56Fe+238U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the4He/1H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for4He and1H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed4He/1H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For4He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For1H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of4He/1H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.