Winifred E. Parker

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.

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.

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.

Studies of Light Charged Particle Emission From Fission and ER Reactions in the System 344 Mev 28Si+121Sb-149Tb (E=240 MeV)

Abstract 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.

The role of reversed kinematics and double kinematic solutions in nuclear reactions studies

Abstract The advantages of reversed kinematics in nuclear reactions studies are discussed, with particular emphasis on the origin of double solutions in the reaction kinematics. This possibility for double solutions does not exist in normal kinematics, and provides the basis for a new method of imposing important experimental constraints on the uniqueness of fitting complex observations. By gating on one or the other of the two solutions, light particle kinematics can be greatly influenced in coincidence measurements. The power of the method is illustrated with data from the reaction 1030 MeV 121 Sb + 27 Al, where charged particle emissions arise from several different sources.

Coincidence correlations between light charged particles in the matched reactions 905 and 1030 MeV and 550 and 750 MeV

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.

Coincidence Correlations Between Light Charged Particles in the Matched Reactions 905 and 1030 MeV121Sb + 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.