David J. Moses

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.

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

Abstract Emission of 4 He and 1 H has been studied in the reactions 905 and 1030 MeV 121 Sb + 27 Al and 550 and 750 MeV 86 Kr + 63 Cu, which lead to similar composite systems of 149 Tb ∗ (or 148 Gd ∗ ). 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 1 H and 4 He 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.

Surprising properties of the nuclear stratosphere indicated by energy spectra and large-angle correlations between 4He and 1,2,3H or 4He

Abstract Angular and energy distributions are reported for 4 He in correlation with 1,2,3 H and 4 He in the reaction 337 MeV 40 Ar+ nat Ag. Comparisons are made to a statistical model that includes emitter. Energy spectra important barrier reductions compared to those for cold nuclei, ≈25%, 12%, 8% and 0 for 1 H, 4 He, 2 H and 3 H respectively. Emitter deformation alone cannot give an explanation. A transitory nuclear stratosphere is suggested with proton emission favored from its fringes of lowest density.

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.

Particle spectra from reversed kinematics: Powerful probes of hot rotating nuclei

Inclusive1H/4He emission has been studied for three reactions in “reversed kinematics”,40Ar/56Fe/86Kr (8.5MeV/u)+12C. The4He spectra include the heretofore unobserved region of low c.m. energies in the direction of the heavy reactant. A test of forward-backward symmetry is made for the hot-spot, moving source model; the data do not support the model. Effective emission barriers for4He evaporation are found to be within ∼10% of the corresponding fusion barriers, while for1H evaporation the emission barriers are more than 25% lower. It is possible that deformations predicted by the rotating liquid drop model can account for the4He evaporation spectra, but substantial change in the standard evaporation treatment is required to explain the spectra for1H.

Search for ternary fragmentation in the reaction 856 MeV 98Mo+51V: kinematic probing of intermediate-mass-fragment emissions

Abstract A new technique has been applied to coincidence measurements between fission fragments (FF) and intermediate mass fragments (IMF) emitted from the composite system 149 65 Tb at an excitation energy of 224 MeV. The method permits simultaneous observation of IMF emissions along and normal to the FF separation axes. For the integrated total of 0.10± 0.02 IMF emitted per fission, we find no significant correlation with FF direction, suggesting that IMFs associated with fission reactions are predominantly emitted from the system prior to fission.

Coincidence correlations between light charged particles in the matched reactions 905 and 1030 MeV 121Sb + 27Al and 550 and 750 MeV 86Kr + 63Cu

Abstract Coincidence measurements have been made between 4 He 1 H and a second particle ( 4 He 1 H ) produced in the reactions 905 and 1030 MeV 121 Sb + 27 Al and 550 and 750 MeV 86 Kr + 63 Cu . All four of the reactions lead to composite systems of 149Tb∗ or 148Gd∗ which facilitates comparisons of matched reactions. In the two 121 Sb + 27 Al reactions and the 550 MeV 86 Kr + 63 Cu reaction, the particle-particle coincidences are dominated by processes that lead to evaporation residues. In the 750 MeV 86 Kr + 63 Cu reaction, about half of the 1H and 4He are associated with evaporation residue production. These particle-particle coincidences arise from cross sections that are subsets of the total evaporation residue cross sections. The derived individual particle chain lengths are ∼ 2 for both protons and alphas in each of the four reactions. The particle multiplicities and subset cross sections associated with the evaporation residues are quite similar for the matched reactions, in contrast to fission-like processes where spin-dependent entrance channel effects were observed.

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.

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive4He and4H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u238U with238U and−197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For238U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For238+238U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In238U+197Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.